Ex) Article Title, Author, Keywords
Ex) Article Title, Author, Keywords
Progress in Medical Physics 2024; 35(4): 89-97
Published online December 31, 2024
https://doi.org/10.14316/pmp.2024.35.4.89
Copyright © Korean Society of Medical Physics.
Hyojun Park1,2 , Jin Jegal1,2
, Yoonsuk Huh1,2
, Inbum Lee1,2
, Sung Hyun Lee1,2,3
, Chang Heon Choi1,2,3
, Jung-In Kim1,2,3
, Seonghee Kang1,2,3
Correspondence to:Seonghee Kang
(kangsh012@gmail.com)
Tel: 82-2-2072-2099
Fax: 82-2-765-3317
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Purpose: This study investigated the dose perturbation according to the size of the sensitive volume in the dosimeter in small-field dosimetry.
Methods: The dose profiles with different field sizes were measured using three different dosimeters: the CC13, Razor ion chamber, and Edge solid-state detector. Both the open and wedged beams with different field sizes were employed in the measurement. The profiles were measured in a water phantom at maximum dose depths of 5, 10, and 20 cm. The penumbra and width of the open-beam profiles were compared according to the types of the dosimeters and beam. The dose fall-off between the peak and 20% dose was evaluated for the wedged beam profiles.
Results: In the open-beam measurement, the fall-off of the profile was steeper with the Edge detector, which has the smallest sensitive volume. Meanwhile, the dose in the out-of-field region was the smallest with the Edge detector. The widths of the penumbra were 6.10, 4.47, and 4.03 mm for the profile of the 3×3 cm2 field measured by the CC13 chamber, Razor chamber, and Edge detector, respectively. The width of the profile was not changed even if different dosimeters were used in the measurement. The wedged beam profiles showed more clear peaks at the field edge when a smaller dosimeter was used.
Conclusions: The results demonstrate the necessity of dosimeters with a small sensitive volume for measuring a small-field beam or a steep dose gradient.
KeywordsSmall-field dosimetry, Dose gradient measurement, Ion chamber, Solid-state dosimeter
In modern radiotherapy, precise dose delivery is essential in the delivery of a conformal dose to the tumor while maximizing the sparing of normal tissues. Small fields are composed of several beam segments in advanced treatment techniques such as intensity-modulated radiotherapy, volumetric-modulated arc therapy, and stereotactic body radiotherapy. These techniques focus on a high dose on the tumor and minimize the dose to nearby normal tissues, improving the treatment outcomes. The importance of accurate dose calculation increases in the treatment planning system (TPS), particularly for small-field beams [1-4]. To establish an accurate treatment plan consisting of small beam segments, the dosimetric characteristics of the small-field beams should be accurately defined [1,4,5]. The commissioning data imported into the TPS ensures that the patients received appropriate treatments as planned. The accuracy of the commissioning data should be guaranteed for the reliable calculation of dose distributions with small beam segments or steep dose gradients [1,2].
Ion chambers are commonly used to measure the percentage depth dose (PDD) and lateral dose distribution. For the PDD, the size of the chamber volume does not affect the dosimetry results in the small field more than that in the large field because of scattering radiation [6]. Conversely, the shape of the lateral dose distribution, or lateral dose profile, is much affected by the size of the sensitive volume of the chamber in small-field beams than in larger ones. Ion chambers collect electronic signals generated within the sensitive volume and regard the sum of the signal as the point ionization, which is further converted to the dose. This induces unwanted discrepancies from the actual amount of the point ionization–volume-averaging effect [7]. This effect is dominant when the sensitive volume is large compared with the dose gradient being measured. Typically, the size of their sensitive volumes ranges from 0.12 to 0.60 cm3, whereas the entire volume becomes considerably larger as the chamber includes components such as the wall, electrode, and build-up cap. The measured dose distribution can be perturbed with a relatively large sensitive volume when these dosimeters are used in the dosimetry of small-field beams or beams with steep dose gradients [8-10]. Therefore, specialized ion chambers with a small sensitive volume should be utilized. Compared with ion chambers, solid-state dosimeters provide advantages for small-field dosimetry because of their smaller and denser construction, with higher electron density, resulting in better spatial resolution and sensitivity for photon measurement [6]. In addition, compared with gas-filled detectors, solid-state detectors require lower energies to produce an electron/hole pair, referring to the larger intensity of the detector signal than the gas detectors [11-13]. These detectors are suitable for small-field dosimetry with high spatial resolution [14,15].
This study aimed to evaluate the effect of the ion chamber volume not only on small-field dosimetry but also on the measurement of high-dose gradients by comparing the dose profiles of small-field beams from the Harmony Pro linear accelerator (Elekta). The investigations were carried out by comparing the measured dose distributions of the open and wedged beams according to different dosimeters of different sizes. The shape of the dose fall-off in each profile was compared among the CC13 and Razor ion chambers (IBA Dosimetry GmbH) and the Edge detector (Sun Nuclear).
Fig. 1 presents the devices used in dose profile measurements with different dosimeters. A water phantom (SMARTSCAN; IBA dosimetry GmbH) that provides 3-dimensional scanning over 48×48×41 cm3 was used. The positioning resolution of the scanner was 0.10 mm, and its positioning accuracy/reproducibility was ±0.10 mm. The phantom and scanner were connected to a common control unit electrometer, operating with dedicated software (myQA Accept; IBA dosimetry GmbH). Three different dosimeters were used: a CC13 ion chamber, a Razor (CC01) ion chamber, and an Edge detector whose sensitive volumes were 0.13, 0.01, and 0.19 mm3, respectively.
The setup for measuring the dose profiles is explained in Fig. 1. Two beam types were measured, namely, open and wedged beams. The sensitive volume effect on the small-field dosimetry was evaluated throughout the open-beam measurement. For the open-beam measurement, both inline and crossline profiles were measured for field sizes of 3×3, 5×5, and 10×10 cm2. The source-to-surface distance (SSD) was 90 cm. The photon energies were 6 MV, 6 MV-flattening filter free (FFF), and 10 MV. The measurement depths were the depth at the maximum dose (dmax) , 5, 10, and 20 cm. The dmax was defined as 1.50, 1.60, and 2.10 cm for the beam energies of 6 MV, 6MV-FFF, and 10 MV, respectively. The dosimeters were located considering the effective point of measurement for each dosimeter. The scan speed was 4 mm/s, with a step size of 1 mm. A faster scan speed and small step size led to an increment of fluctuations in the profiles. Meanwhile, the data can be insufficient when a very large step size is applied. The scanning setup was optimized based on these considerations. The penumbra in the profile, which was defined as the region ranging from 80% to 20% of the dmax, was compared between the profiles of different dosimeters because the volume-averaging effect was dominant in that region, whereas it was negligible in the flattened area. In addition, the measured profiles for the 3×3 cm2 field were compared with the golden beam data (GBD), which refer to the reference profiles for the evaluation of dose measurement accuracy, being provided by the vendor. The width of the penumbra was calculated using the myQA Accept software with a linear function for quantitative comparison. The field width was also evaluated in terms of the full width at half maximum (FWHM) of the profile calculated using the same software.
The dose distribution was intentionally modified to include a sudden and high-dose gradient to investigate the sensitive volume effect on the dosimetry of the steep dose gradient by applying the wedge to the beam field. The 60-degree motorized wedge (Elekta), which is a physical wedge integrated into the gantry head, was used. Most measurement setups were identical to those in the open-beam measurement, except for the beam energy and field size, which were fixed at 6 MV and 10×10 cm2, respectively. The CC13 and Razor ion chambers were used in the measurement. The inline profile with the dose gradient was measured and compared according to the dosimeters. The dose distribution at the edge of the profile was compared because of a peak with a high-dose gradient, which can be affected by the volume-averaging effect. The dose gradient was evaluated in the penumbra located at the region where the thinner part of the wedge was positioned. The dose gradient was compared between the CC13 and Razor chambers by calculating the slope of the dose distributions in the region from the point of the dmax to that of the 20% dose. The parameters were calculated through the linear regression of the profiles.
Figs. 2–4 show the fall-off regions in the dose profiles of different field sizes with comparison between the dosimeters. The width of the penumbra and FWHM of the profiles are listed in Tables 1–3 according to beam energies, field sizes, and dosimeters. The inline profiles showed a steeper dose fall-off than the crossline profiles.
Table 1 Analysis of the profiles of the 6 MV beams measured by different dosimeters
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 5.95 | 4.63 | 3.80 | 3.50 | 27.75 | 27.65 | 27.50 | 27.60 | |
5×5 | 6.15 | 4.78 | 3.88 | − | 45.90 | 45.95 | 46.00 | − | ||
10×10 | 6.38 | 5.03 | 4.10 | − | 92.25 | 92.05 | 91.65 | − | ||
5 | 3×3 | 6.28 | 5.05 | 4.13 | 3.80 | 28.90 | 28.85 | 28.65 | 28.80 | |
5×5 | 6.68 | 5.35 | 4.38 | − | 47.75 | 47.80 | 47.70 | − | ||
10×10 | 7.08 | 5.70 | 4.70 | − | 95.80 | 95.60 | 95.15 | − | ||
10 | 3×3 | 6.68 | 5.45 | 4.50 | 4.20 | 30.50 | 30.45 | 30.25 | 30.30 | |
5×5 | 7.18 | 5.90 | 4.90 | − | 50.40 | 50.45 | 50.30 | − | ||
10×10 | 8.05 | 6.80 | 5.65 | − | 100.95 | 100.80 | 100.25 | − | ||
20 | 3×3 | 7.28 | 6.15 | 5.15 | 4.75 | 33.70 | 33.60 | 33.35 | 33.45 | |
5×5 | 8.10 | 6.98 | 5.78 | − | 55.65 | 55.75 | 55.50 | − | ||
10×10 | 10.00 | 9.10 | 7.50 | − | 110.70 | 110.60 | 110.55 | − |
Table 2 Analysis of the profiles of 6 MV-flattening filter free beams measured by different dosimeters
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 6.08 | 4.60 | 3.58 | 3.50 | 27.30 | 27.30 | 27.25 | 27.50 | |
5×5 | 6.28 | 4.80 | 3.95 | − | 45.45 | 45.45 | 45.60 | − | ||
10×10 | 6.68 | 5.08 | 4.20 | − | 91.40 | 91.45 | 91.35 | − | ||
5 | 3×3 | 6.53 | 5.08 | 4.03 | 3.90 | 28.35 | 28.40 | 28.30 | 28.60 | |
5×5 | 6.88 | 5.38 | 4.48 | − | 47.15 | 47.15 | 47.25 | − | ||
10×10 | 7.48 | 5.90 | 4.88 | − | 94.80 | 94.85 | 94.75 | − | ||
10 | 3×3 | 7.03 | 5.60 | 4.45 | 4.30 | 29.85 | 29.90 | 29.80 | 30.15 | |
5×5 | 7.58 | 6.08 | 5.13 | − | 49.80 | 49.65 | 49.85 | − | ||
10×10 | 8.60 | 7.05 | 5.90 | − | 99.80 | 99.90 | 99.75 | − | ||
20 | 3×3 | 7.83 | 6.35 | 5.18 | 4.95 | 32.85 | 32.90 | 32.75 | 33.30 | |
5×5 | 8.75 | 7.23 | 6.13 | − | 54.65 | 54.70 | 54.80 | − | ||
10×10 | 10.98 | 9.35 | 8.08 | − | 109.75 | 109.90 | 109.75 | − |
Table 3 Analysis of the profiles of 10 MV beams measured by different dosimeters
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 6.60 | 5.33 | 4.38 | 3.70 | 28.10 | 27.95 | 27.85 | 28.30 | |
5×5 | 6.88 | 5.60 | 4.63 | − | 46.60 | 46.40 | 46.25 | − | ||
10×10 | 7.13 | 5.90 | 4.90 | − | 92.90 | 92.85 | 92.60 | − | ||
5 | 3×3 | 6.98 | 5.73 | 4.75 | 4.10 | 29.05 | 28.90 | 28.70 | 29.15 | |
5×5 | 7.33 | 6.03 | 5.10 | − | 48.05 | 47.90 | 47.80 | − | ||
10×10 | 7.70 | 6.45 | 5.35 | − | 95.80 | 95.75 | 95.45 | − | ||
10 | 3×3 | 7.30 | 6.15 | 5.15 | 4.45 | 30.70 | 30.50 | 30.25 | 30.75 | |
5×5 | 7.85 | 6.63 | 5.60 | − | 50.70 | 50.50 | 50.30 | − | ||
10×10 | 8.63 | 7.43 | 6.13 | − | 100.85 | 100.90 | 100.50 | − | ||
20 | 3×3 | 8.05 | 6.93 | 5.83 | 4.95 | 33.85 | 33.65 | 33.35 | 33.80 | |
5×5 | 8.85 | 7.68 | 6.45 | − | 55.95 | 55.70 | 55.35 | − | ||
10×10 | 10.43 | 9.40 | 7.75 | − | 111.15 | 111.10 | 110.70 | − |
The profiles of the 3×3 cm2 field measured by the Edge detector showed the smallest discrepancies from those of the golden beam. The dose fall-off was the steepest with the Edge detector, whereas the region of the dose >80% of the dmax was broader than that of the other dosimeters. Meanwhile, the profiles of the CC13 ion chamber showed the highest dose in the out-of-field region among the dosimeters. The width of the penumbra was the smallest with the profiles measured by the Edge detector among the dosimeters (Tables 1–3). The widths of the penumbra were 6.68, 5.45, and 4.50 mm for the 3×3 cm2 profiles of the 6 MV X-rays at a depth of 10 cm measured by the CC13, rotary chambers, and Edge detector, respectively. The maximum differences in the penumbra width from the profile measured by the CC13 chamber were 1.63 and 2.90 mm, compared with those measured by the Razor chamber and Edge detector, respectively. However, the FWHM of the profile was nearly identical among the profiles regardless of the field size and dosimeter (Table 1). The maximum difference in the FWHM was 1.67% in the measurement of the 3×3 cm2 fields between the CC13 chamber and the Edge detector. The effect of the dosimeter volume was dominantly observed near the point where the dose gradient rapidly changed. The results imply that the Edge detector is the most suitable for measuring small-field beams among the investigated dosimeters.
The profiles of the wedged beams were measured using the CC13 and Razor ion chambers (Fig. 5). Considerable differences were found near the field edge where the dose peaked because the thickness of the wedge was the smallest. The dose at the field edge was higher in the Razor chamber measurement than in those in the CC13 chamber. Table 4 shows the slopes of the dose gradient at the field edge where the thinner part of the wedge was located. The slope was the largest at the dmax, and the difference between the chambers was also the largest. Similar to the open-beam measurement, the dose fall-off after the peak was steeper with the profiles of the Razor chamber than with those of the CC13 chamber. The slope of the dose gradient at dmax differed by 25.41% from the profile of the CC13 chamber to that of the Razor chamber. The results at other depths also showed remarkable differences in the slope of the dose gradient. Meanwhile, the dose in the wedged area was nearly identical between the profiles measured by the different ion chambers.
Table 4 Slope of the dose gradient between the points of the maximum and 20% doses
Depth (cm) | CC13 | Razor |
---|---|---|
dmax | −10.26 | −12.87 |
5 | −9.00 | −10.99 |
10 | −8.39 | −10.09 |
20 | −6.48 | −6.66 |
In this study, the dose perturbation by volume-averaging over the sensitive volume of the dosimeter was investigated. The slope of the dose-fall region was higher with the inline profiles than with the crossline profiles. This was because the field along the inline direction was produced by the diaphragm, whereas the multileaf collimator (MLC) produced the crossline field. The leaf of the MLC has rounded edges, which blurs the dose distribution. The dosimeter with a smaller sensitive volume more accurately described the steep dose gradient than that with a larger sensitive volume. Among the dosimeters, the profiles measured by the Edge detector showed the steepest dose fall-off because it has the smallest sensitive volume. This was due to its small sensitive volume because the dose was averaged over the volume of the dosimeter. Meanwhile, the CC13 chamber obtained a broader dose fall-off in the profiles than the others. The flattened area including the dose >80% of the dmax was even larger with the dosimeter with a small sensitive volume than with the larger dosimeter. This implies that the out-of-field dose can be overestimated, whereas the dose at the edge in the flattened area can be underestimated for not only the small field but also the large field. In addition, the width of the beam in the high-dose region can be different from the dose distribution measured by the ion chamber, particularly in small-field beams. Similarly, the dmax was higher with the smaller ion chamber in the wedged measurement. The slope of the dose fall-off at the wedge hill was larger with that at the smaller chamber, whereas the dose was similar in the region of the wedge toe because of small dose changes detectable with the larger chamber. As the dosimeter’s sensitive volume becomes smaller, the spatial resolution of the chamber improves, enhancing the detectability of the steep dose gradient. The current radiation treatment employs small-sized beam segments to produce a conformal dose distribution. Therefore, accurate dosimetry for the small-sized beam is required, and the use of small-sized dosimeters can accomplish reliable small-field dosimetry. The penumbra of the beam fields can also be cross-checked by comparing the measurements between the small and large size dosimeters.
In the wedged field measurement, only ion chambers were used rather than the Edge detector. The diode detectors have overresponse to low-energy scattered photons because of the photoelectric effect in silicon [16-18]. The scattering material above the diode detector affects its response [16]. The hard wedge produces large amounts of scatter as it interacts with the X-rays [19-21]. Because it can affect the response of the Edge detector during the measurement, CC13 and Razor ion chambers were used to measure the wedged beam profiles.
Despite the advantages of small-sized ion chambers and SSDs for small-field dosimetry, limitations remain on employing them for routine dosimetry in the clinic. The sensitivity of the ion chamber lower than the size of the sensitive volume reduces because of the low density of air. The signal collected by the ion chamber for the same number of dose reduces; subsequently, the results of the dosimetry further fluctuated. This fluctuation can affect the accuracy of dose distribution measurements, so the measuring time should be extended to reduce it. In SSDs, they can be damaged by the irradiation of high-energy ionizing radiations, resulting in changes in the detector response. In addition, compared with larger devices, small-sized dosimeters are difficult to accurately align to the beam center. Therefore, both the ion chambers of different sizes and the SSDs should be employed to their specified usage.
This study investigated the effect of the dosimeter volume on the dosimetry of the small-field and steep gradient beam. The dosimeter with a small sensitive volume accurately measures the small beam and the steep dose gradient. However, these types of dosimeters can be selectively used because of their physical restrictions, such as radiation-induced damage, lower sensitivity, and difficulty in accurate beam alignment. The combined use of small and large dosimeters can improve the accuracy of radiation beam dosimetry, which can be employed in the beam modeling in the TPS.
This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20227410100040, Development of patch-type flexible personal dosimeter and real-time remote monitoring system using high-performance inorganic perovskite).
Chang Heon Choi and Jin Jegal are members of the editorial board of the
All relevant data are within the paper and its Supporting Information files.
Conceptualization: Seonghee Kang. Data curation: Hyojun Park, Yoonsuk Huh, Jin Jegal, Inbum Lee, Sung Hyun Lee, Seonghee Kang. Formal analysis: Seonghee Kang, Chang Heon Choi, Jung-In Kim. Funding acquisition: Seonghee Kang. Investigation: Hyojun Park, Yoonsuk Huh, Jin Jegal, Inbum Lee, Sung Hyun Lee, Seonghee Kang. Project administration: Seonghee Kang, Chang Heon Choi, Jung-In Kim. Visualization: Hyojun Park. Writing – original draft: Hyojun Park, Writing – review & editing: Hyojun Park, Sung Hyun Lee, Seonghee Kang, Chang Heon Choi, Jung-In Kim.
Progress in Medical Physics 2024; 35(4): 89-97
Published online December 31, 2024 https://doi.org/10.14316/pmp.2024.35.4.89
Copyright © Korean Society of Medical Physics.
Hyojun Park1,2 , Jin Jegal1,2
, Yoonsuk Huh1,2
, Inbum Lee1,2
, Sung Hyun Lee1,2,3
, Chang Heon Choi1,2,3
, Jung-In Kim1,2,3
, Seonghee Kang1,2,3
1Department of Radiation Oncology, Seoul National University Hospital, Seoul, 2Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, 3Biomedical Research Institute, Seoul National University Hospital, Seoul, Korea
Correspondence to:Seonghee Kang
(kangsh012@gmail.com)
Tel: 82-2-2072-2099
Fax: 82-2-765-3317
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Purpose: This study investigated the dose perturbation according to the size of the sensitive volume in the dosimeter in small-field dosimetry.
Methods: The dose profiles with different field sizes were measured using three different dosimeters: the CC13, Razor ion chamber, and Edge solid-state detector. Both the open and wedged beams with different field sizes were employed in the measurement. The profiles were measured in a water phantom at maximum dose depths of 5, 10, and 20 cm. The penumbra and width of the open-beam profiles were compared according to the types of the dosimeters and beam. The dose fall-off between the peak and 20% dose was evaluated for the wedged beam profiles.
Results: In the open-beam measurement, the fall-off of the profile was steeper with the Edge detector, which has the smallest sensitive volume. Meanwhile, the dose in the out-of-field region was the smallest with the Edge detector. The widths of the penumbra were 6.10, 4.47, and 4.03 mm for the profile of the 3×3 cm2 field measured by the CC13 chamber, Razor chamber, and Edge detector, respectively. The width of the profile was not changed even if different dosimeters were used in the measurement. The wedged beam profiles showed more clear peaks at the field edge when a smaller dosimeter was used.
Conclusions: The results demonstrate the necessity of dosimeters with a small sensitive volume for measuring a small-field beam or a steep dose gradient.
Keywords: Small-field dosimetry, Dose gradient measurement, Ion chamber, Solid-state dosimeter
In modern radiotherapy, precise dose delivery is essential in the delivery of a conformal dose to the tumor while maximizing the sparing of normal tissues. Small fields are composed of several beam segments in advanced treatment techniques such as intensity-modulated radiotherapy, volumetric-modulated arc therapy, and stereotactic body radiotherapy. These techniques focus on a high dose on the tumor and minimize the dose to nearby normal tissues, improving the treatment outcomes. The importance of accurate dose calculation increases in the treatment planning system (TPS), particularly for small-field beams [1-4]. To establish an accurate treatment plan consisting of small beam segments, the dosimetric characteristics of the small-field beams should be accurately defined [1,4,5]. The commissioning data imported into the TPS ensures that the patients received appropriate treatments as planned. The accuracy of the commissioning data should be guaranteed for the reliable calculation of dose distributions with small beam segments or steep dose gradients [1,2].
Ion chambers are commonly used to measure the percentage depth dose (PDD) and lateral dose distribution. For the PDD, the size of the chamber volume does not affect the dosimetry results in the small field more than that in the large field because of scattering radiation [6]. Conversely, the shape of the lateral dose distribution, or lateral dose profile, is much affected by the size of the sensitive volume of the chamber in small-field beams than in larger ones. Ion chambers collect electronic signals generated within the sensitive volume and regard the sum of the signal as the point ionization, which is further converted to the dose. This induces unwanted discrepancies from the actual amount of the point ionization–volume-averaging effect [7]. This effect is dominant when the sensitive volume is large compared with the dose gradient being measured. Typically, the size of their sensitive volumes ranges from 0.12 to 0.60 cm3, whereas the entire volume becomes considerably larger as the chamber includes components such as the wall, electrode, and build-up cap. The measured dose distribution can be perturbed with a relatively large sensitive volume when these dosimeters are used in the dosimetry of small-field beams or beams with steep dose gradients [8-10]. Therefore, specialized ion chambers with a small sensitive volume should be utilized. Compared with ion chambers, solid-state dosimeters provide advantages for small-field dosimetry because of their smaller and denser construction, with higher electron density, resulting in better spatial resolution and sensitivity for photon measurement [6]. In addition, compared with gas-filled detectors, solid-state detectors require lower energies to produce an electron/hole pair, referring to the larger intensity of the detector signal than the gas detectors [11-13]. These detectors are suitable for small-field dosimetry with high spatial resolution [14,15].
This study aimed to evaluate the effect of the ion chamber volume not only on small-field dosimetry but also on the measurement of high-dose gradients by comparing the dose profiles of small-field beams from the Harmony Pro linear accelerator (Elekta). The investigations were carried out by comparing the measured dose distributions of the open and wedged beams according to different dosimeters of different sizes. The shape of the dose fall-off in each profile was compared among the CC13 and Razor ion chambers (IBA Dosimetry GmbH) and the Edge detector (Sun Nuclear).
Fig. 1 presents the devices used in dose profile measurements with different dosimeters. A water phantom (SMARTSCAN; IBA dosimetry GmbH) that provides 3-dimensional scanning over 48×48×41 cm3 was used. The positioning resolution of the scanner was 0.10 mm, and its positioning accuracy/reproducibility was ±0.10 mm. The phantom and scanner were connected to a common control unit electrometer, operating with dedicated software (myQA Accept; IBA dosimetry GmbH). Three different dosimeters were used: a CC13 ion chamber, a Razor (CC01) ion chamber, and an Edge detector whose sensitive volumes were 0.13, 0.01, and 0.19 mm3, respectively.
The setup for measuring the dose profiles is explained in Fig. 1. Two beam types were measured, namely, open and wedged beams. The sensitive volume effect on the small-field dosimetry was evaluated throughout the open-beam measurement. For the open-beam measurement, both inline and crossline profiles were measured for field sizes of 3×3, 5×5, and 10×10 cm2. The source-to-surface distance (SSD) was 90 cm. The photon energies were 6 MV, 6 MV-flattening filter free (FFF), and 10 MV. The measurement depths were the depth at the maximum dose (dmax) , 5, 10, and 20 cm. The dmax was defined as 1.50, 1.60, and 2.10 cm for the beam energies of 6 MV, 6MV-FFF, and 10 MV, respectively. The dosimeters were located considering the effective point of measurement for each dosimeter. The scan speed was 4 mm/s, with a step size of 1 mm. A faster scan speed and small step size led to an increment of fluctuations in the profiles. Meanwhile, the data can be insufficient when a very large step size is applied. The scanning setup was optimized based on these considerations. The penumbra in the profile, which was defined as the region ranging from 80% to 20% of the dmax, was compared between the profiles of different dosimeters because the volume-averaging effect was dominant in that region, whereas it was negligible in the flattened area. In addition, the measured profiles for the 3×3 cm2 field were compared with the golden beam data (GBD), which refer to the reference profiles for the evaluation of dose measurement accuracy, being provided by the vendor. The width of the penumbra was calculated using the myQA Accept software with a linear function for quantitative comparison. The field width was also evaluated in terms of the full width at half maximum (FWHM) of the profile calculated using the same software.
The dose distribution was intentionally modified to include a sudden and high-dose gradient to investigate the sensitive volume effect on the dosimetry of the steep dose gradient by applying the wedge to the beam field. The 60-degree motorized wedge (Elekta), which is a physical wedge integrated into the gantry head, was used. Most measurement setups were identical to those in the open-beam measurement, except for the beam energy and field size, which were fixed at 6 MV and 10×10 cm2, respectively. The CC13 and Razor ion chambers were used in the measurement. The inline profile with the dose gradient was measured and compared according to the dosimeters. The dose distribution at the edge of the profile was compared because of a peak with a high-dose gradient, which can be affected by the volume-averaging effect. The dose gradient was evaluated in the penumbra located at the region where the thinner part of the wedge was positioned. The dose gradient was compared between the CC13 and Razor chambers by calculating the slope of the dose distributions in the region from the point of the dmax to that of the 20% dose. The parameters were calculated through the linear regression of the profiles.
Figs. 2–4 show the fall-off regions in the dose profiles of different field sizes with comparison between the dosimeters. The width of the penumbra and FWHM of the profiles are listed in Tables 1–3 according to beam energies, field sizes, and dosimeters. The inline profiles showed a steeper dose fall-off than the crossline profiles.
Table 1 . Analysis of the profiles of the 6 MV beams measured by different dosimeters.
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 5.95 | 4.63 | 3.80 | 3.50 | 27.75 | 27.65 | 27.50 | 27.60 | |
5×5 | 6.15 | 4.78 | 3.88 | − | 45.90 | 45.95 | 46.00 | − | ||
10×10 | 6.38 | 5.03 | 4.10 | − | 92.25 | 92.05 | 91.65 | − | ||
5 | 3×3 | 6.28 | 5.05 | 4.13 | 3.80 | 28.90 | 28.85 | 28.65 | 28.80 | |
5×5 | 6.68 | 5.35 | 4.38 | − | 47.75 | 47.80 | 47.70 | − | ||
10×10 | 7.08 | 5.70 | 4.70 | − | 95.80 | 95.60 | 95.15 | − | ||
10 | 3×3 | 6.68 | 5.45 | 4.50 | 4.20 | 30.50 | 30.45 | 30.25 | 30.30 | |
5×5 | 7.18 | 5.90 | 4.90 | − | 50.40 | 50.45 | 50.30 | − | ||
10×10 | 8.05 | 6.80 | 5.65 | − | 100.95 | 100.80 | 100.25 | − | ||
20 | 3×3 | 7.28 | 6.15 | 5.15 | 4.75 | 33.70 | 33.60 | 33.35 | 33.45 | |
5×5 | 8.10 | 6.98 | 5.78 | − | 55.65 | 55.75 | 55.50 | − | ||
10×10 | 10.00 | 9.10 | 7.50 | − | 110.70 | 110.60 | 110.55 | − |
Table 2 . Analysis of the profiles of 6 MV-flattening filter free beams measured by different dosimeters.
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 6.08 | 4.60 | 3.58 | 3.50 | 27.30 | 27.30 | 27.25 | 27.50 | |
5×5 | 6.28 | 4.80 | 3.95 | − | 45.45 | 45.45 | 45.60 | − | ||
10×10 | 6.68 | 5.08 | 4.20 | − | 91.40 | 91.45 | 91.35 | − | ||
5 | 3×3 | 6.53 | 5.08 | 4.03 | 3.90 | 28.35 | 28.40 | 28.30 | 28.60 | |
5×5 | 6.88 | 5.38 | 4.48 | − | 47.15 | 47.15 | 47.25 | − | ||
10×10 | 7.48 | 5.90 | 4.88 | − | 94.80 | 94.85 | 94.75 | − | ||
10 | 3×3 | 7.03 | 5.60 | 4.45 | 4.30 | 29.85 | 29.90 | 29.80 | 30.15 | |
5×5 | 7.58 | 6.08 | 5.13 | − | 49.80 | 49.65 | 49.85 | − | ||
10×10 | 8.60 | 7.05 | 5.90 | − | 99.80 | 99.90 | 99.75 | − | ||
20 | 3×3 | 7.83 | 6.35 | 5.18 | 4.95 | 32.85 | 32.90 | 32.75 | 33.30 | |
5×5 | 8.75 | 7.23 | 6.13 | − | 54.65 | 54.70 | 54.80 | − | ||
10×10 | 10.98 | 9.35 | 8.08 | − | 109.75 | 109.90 | 109.75 | − |
Table 3 . Analysis of the profiles of 10 MV beams measured by different dosimeters.
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 6.60 | 5.33 | 4.38 | 3.70 | 28.10 | 27.95 | 27.85 | 28.30 | |
5×5 | 6.88 | 5.60 | 4.63 | − | 46.60 | 46.40 | 46.25 | − | ||
10×10 | 7.13 | 5.90 | 4.90 | − | 92.90 | 92.85 | 92.60 | − | ||
5 | 3×3 | 6.98 | 5.73 | 4.75 | 4.10 | 29.05 | 28.90 | 28.70 | 29.15 | |
5×5 | 7.33 | 6.03 | 5.10 | − | 48.05 | 47.90 | 47.80 | − | ||
10×10 | 7.70 | 6.45 | 5.35 | − | 95.80 | 95.75 | 95.45 | − | ||
10 | 3×3 | 7.30 | 6.15 | 5.15 | 4.45 | 30.70 | 30.50 | 30.25 | 30.75 | |
5×5 | 7.85 | 6.63 | 5.60 | − | 50.70 | 50.50 | 50.30 | − | ||
10×10 | 8.63 | 7.43 | 6.13 | − | 100.85 | 100.90 | 100.50 | − | ||
20 | 3×3 | 8.05 | 6.93 | 5.83 | 4.95 | 33.85 | 33.65 | 33.35 | 33.80 | |
5×5 | 8.85 | 7.68 | 6.45 | − | 55.95 | 55.70 | 55.35 | − | ||
10×10 | 10.43 | 9.40 | 7.75 | − | 111.15 | 111.10 | 110.70 | − |
The profiles of the 3×3 cm2 field measured by the Edge detector showed the smallest discrepancies from those of the golden beam. The dose fall-off was the steepest with the Edge detector, whereas the region of the dose >80% of the dmax was broader than that of the other dosimeters. Meanwhile, the profiles of the CC13 ion chamber showed the highest dose in the out-of-field region among the dosimeters. The width of the penumbra was the smallest with the profiles measured by the Edge detector among the dosimeters (Tables 1–3). The widths of the penumbra were 6.68, 5.45, and 4.50 mm for the 3×3 cm2 profiles of the 6 MV X-rays at a depth of 10 cm measured by the CC13, rotary chambers, and Edge detector, respectively. The maximum differences in the penumbra width from the profile measured by the CC13 chamber were 1.63 and 2.90 mm, compared with those measured by the Razor chamber and Edge detector, respectively. However, the FWHM of the profile was nearly identical among the profiles regardless of the field size and dosimeter (Table 1). The maximum difference in the FWHM was 1.67% in the measurement of the 3×3 cm2 fields between the CC13 chamber and the Edge detector. The effect of the dosimeter volume was dominantly observed near the point where the dose gradient rapidly changed. The results imply that the Edge detector is the most suitable for measuring small-field beams among the investigated dosimeters.
The profiles of the wedged beams were measured using the CC13 and Razor ion chambers (Fig. 5). Considerable differences were found near the field edge where the dose peaked because the thickness of the wedge was the smallest. The dose at the field edge was higher in the Razor chamber measurement than in those in the CC13 chamber. Table 4 shows the slopes of the dose gradient at the field edge where the thinner part of the wedge was located. The slope was the largest at the dmax, and the difference between the chambers was also the largest. Similar to the open-beam measurement, the dose fall-off after the peak was steeper with the profiles of the Razor chamber than with those of the CC13 chamber. The slope of the dose gradient at dmax differed by 25.41% from the profile of the CC13 chamber to that of the Razor chamber. The results at other depths also showed remarkable differences in the slope of the dose gradient. Meanwhile, the dose in the wedged area was nearly identical between the profiles measured by the different ion chambers.
Table 4 . Slope of the dose gradient between the points of the maximum and 20% doses.
Depth (cm) | CC13 | Razor |
---|---|---|
dmax | −10.26 | −12.87 |
5 | −9.00 | −10.99 |
10 | −8.39 | −10.09 |
20 | −6.48 | −6.66 |
In this study, the dose perturbation by volume-averaging over the sensitive volume of the dosimeter was investigated. The slope of the dose-fall region was higher with the inline profiles than with the crossline profiles. This was because the field along the inline direction was produced by the diaphragm, whereas the multileaf collimator (MLC) produced the crossline field. The leaf of the MLC has rounded edges, which blurs the dose distribution. The dosimeter with a smaller sensitive volume more accurately described the steep dose gradient than that with a larger sensitive volume. Among the dosimeters, the profiles measured by the Edge detector showed the steepest dose fall-off because it has the smallest sensitive volume. This was due to its small sensitive volume because the dose was averaged over the volume of the dosimeter. Meanwhile, the CC13 chamber obtained a broader dose fall-off in the profiles than the others. The flattened area including the dose >80% of the dmax was even larger with the dosimeter with a small sensitive volume than with the larger dosimeter. This implies that the out-of-field dose can be overestimated, whereas the dose at the edge in the flattened area can be underestimated for not only the small field but also the large field. In addition, the width of the beam in the high-dose region can be different from the dose distribution measured by the ion chamber, particularly in small-field beams. Similarly, the dmax was higher with the smaller ion chamber in the wedged measurement. The slope of the dose fall-off at the wedge hill was larger with that at the smaller chamber, whereas the dose was similar in the region of the wedge toe because of small dose changes detectable with the larger chamber. As the dosimeter’s sensitive volume becomes smaller, the spatial resolution of the chamber improves, enhancing the detectability of the steep dose gradient. The current radiation treatment employs small-sized beam segments to produce a conformal dose distribution. Therefore, accurate dosimetry for the small-sized beam is required, and the use of small-sized dosimeters can accomplish reliable small-field dosimetry. The penumbra of the beam fields can also be cross-checked by comparing the measurements between the small and large size dosimeters.
In the wedged field measurement, only ion chambers were used rather than the Edge detector. The diode detectors have overresponse to low-energy scattered photons because of the photoelectric effect in silicon [16-18]. The scattering material above the diode detector affects its response [16]. The hard wedge produces large amounts of scatter as it interacts with the X-rays [19-21]. Because it can affect the response of the Edge detector during the measurement, CC13 and Razor ion chambers were used to measure the wedged beam profiles.
Despite the advantages of small-sized ion chambers and SSDs for small-field dosimetry, limitations remain on employing them for routine dosimetry in the clinic. The sensitivity of the ion chamber lower than the size of the sensitive volume reduces because of the low density of air. The signal collected by the ion chamber for the same number of dose reduces; subsequently, the results of the dosimetry further fluctuated. This fluctuation can affect the accuracy of dose distribution measurements, so the measuring time should be extended to reduce it. In SSDs, they can be damaged by the irradiation of high-energy ionizing radiations, resulting in changes in the detector response. In addition, compared with larger devices, small-sized dosimeters are difficult to accurately align to the beam center. Therefore, both the ion chambers of different sizes and the SSDs should be employed to their specified usage.
This study investigated the effect of the dosimeter volume on the dosimetry of the small-field and steep gradient beam. The dosimeter with a small sensitive volume accurately measures the small beam and the steep dose gradient. However, these types of dosimeters can be selectively used because of their physical restrictions, such as radiation-induced damage, lower sensitivity, and difficulty in accurate beam alignment. The combined use of small and large dosimeters can improve the accuracy of radiation beam dosimetry, which can be employed in the beam modeling in the TPS.
This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20227410100040, Development of patch-type flexible personal dosimeter and real-time remote monitoring system using high-performance inorganic perovskite).
Chang Heon Choi and Jin Jegal are members of the editorial board of the
All relevant data are within the paper and its Supporting Information files.
Conceptualization: Seonghee Kang. Data curation: Hyojun Park, Yoonsuk Huh, Jin Jegal, Inbum Lee, Sung Hyun Lee, Seonghee Kang. Formal analysis: Seonghee Kang, Chang Heon Choi, Jung-In Kim. Funding acquisition: Seonghee Kang. Investigation: Hyojun Park, Yoonsuk Huh, Jin Jegal, Inbum Lee, Sung Hyun Lee, Seonghee Kang. Project administration: Seonghee Kang, Chang Heon Choi, Jung-In Kim. Visualization: Hyojun Park. Writing – original draft: Hyojun Park, Writing – review & editing: Hyojun Park, Sung Hyun Lee, Seonghee Kang, Chang Heon Choi, Jung-In Kim.
Table 1 Analysis of the profiles of the 6 MV beams measured by different dosimeters
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 5.95 | 4.63 | 3.80 | 3.50 | 27.75 | 27.65 | 27.50 | 27.60 | |
5×5 | 6.15 | 4.78 | 3.88 | − | 45.90 | 45.95 | 46.00 | − | ||
10×10 | 6.38 | 5.03 | 4.10 | − | 92.25 | 92.05 | 91.65 | − | ||
5 | 3×3 | 6.28 | 5.05 | 4.13 | 3.80 | 28.90 | 28.85 | 28.65 | 28.80 | |
5×5 | 6.68 | 5.35 | 4.38 | − | 47.75 | 47.80 | 47.70 | − | ||
10×10 | 7.08 | 5.70 | 4.70 | − | 95.80 | 95.60 | 95.15 | − | ||
10 | 3×3 | 6.68 | 5.45 | 4.50 | 4.20 | 30.50 | 30.45 | 30.25 | 30.30 | |
5×5 | 7.18 | 5.90 | 4.90 | − | 50.40 | 50.45 | 50.30 | − | ||
10×10 | 8.05 | 6.80 | 5.65 | − | 100.95 | 100.80 | 100.25 | − | ||
20 | 3×3 | 7.28 | 6.15 | 5.15 | 4.75 | 33.70 | 33.60 | 33.35 | 33.45 | |
5×5 | 8.10 | 6.98 | 5.78 | − | 55.65 | 55.75 | 55.50 | − | ||
10×10 | 10.00 | 9.10 | 7.50 | − | 110.70 | 110.60 | 110.55 | − |
Table 2 Analysis of the profiles of 6 MV-flattening filter free beams measured by different dosimeters
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 6.08 | 4.60 | 3.58 | 3.50 | 27.30 | 27.30 | 27.25 | 27.50 | |
5×5 | 6.28 | 4.80 | 3.95 | − | 45.45 | 45.45 | 45.60 | − | ||
10×10 | 6.68 | 5.08 | 4.20 | − | 91.40 | 91.45 | 91.35 | − | ||
5 | 3×3 | 6.53 | 5.08 | 4.03 | 3.90 | 28.35 | 28.40 | 28.30 | 28.60 | |
5×5 | 6.88 | 5.38 | 4.48 | − | 47.15 | 47.15 | 47.25 | − | ||
10×10 | 7.48 | 5.90 | 4.88 | − | 94.80 | 94.85 | 94.75 | − | ||
10 | 3×3 | 7.03 | 5.60 | 4.45 | 4.30 | 29.85 | 29.90 | 29.80 | 30.15 | |
5×5 | 7.58 | 6.08 | 5.13 | − | 49.80 | 49.65 | 49.85 | − | ||
10×10 | 8.60 | 7.05 | 5.90 | − | 99.80 | 99.90 | 99.75 | − | ||
20 | 3×3 | 7.83 | 6.35 | 5.18 | 4.95 | 32.85 | 32.90 | 32.75 | 33.30 | |
5×5 | 8.75 | 7.23 | 6.13 | − | 54.65 | 54.70 | 54.80 | − | ||
10×10 | 10.98 | 9.35 | 8.08 | − | 109.75 | 109.90 | 109.75 | − |
Table 3 Analysis of the profiles of 10 MV beams measured by different dosimeters
Depth (cm) | Field size (cm2) | Penumbra width (mm) | Field width (mm) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
CC13 | Razor | Edge | GBD | CC13 | Razor | Edge | GBD | |||
dmax | 3×3 | 6.60 | 5.33 | 4.38 | 3.70 | 28.10 | 27.95 | 27.85 | 28.30 | |
5×5 | 6.88 | 5.60 | 4.63 | − | 46.60 | 46.40 | 46.25 | − | ||
10×10 | 7.13 | 5.90 | 4.90 | − | 92.90 | 92.85 | 92.60 | − | ||
5 | 3×3 | 6.98 | 5.73 | 4.75 | 4.10 | 29.05 | 28.90 | 28.70 | 29.15 | |
5×5 | 7.33 | 6.03 | 5.10 | − | 48.05 | 47.90 | 47.80 | − | ||
10×10 | 7.70 | 6.45 | 5.35 | − | 95.80 | 95.75 | 95.45 | − | ||
10 | 3×3 | 7.30 | 6.15 | 5.15 | 4.45 | 30.70 | 30.50 | 30.25 | 30.75 | |
5×5 | 7.85 | 6.63 | 5.60 | − | 50.70 | 50.50 | 50.30 | − | ||
10×10 | 8.63 | 7.43 | 6.13 | − | 100.85 | 100.90 | 100.50 | − | ||
20 | 3×3 | 8.05 | 6.93 | 5.83 | 4.95 | 33.85 | 33.65 | 33.35 | 33.80 | |
5×5 | 8.85 | 7.68 | 6.45 | − | 55.95 | 55.70 | 55.35 | − | ||
10×10 | 10.43 | 9.40 | 7.75 | − | 111.15 | 111.10 | 110.70 | − |
Table 4 Slope of the dose gradient between the points of the maximum and 20% doses
Depth (cm) | CC13 | Razor |
---|---|---|
dmax | −10.26 | −12.87 |
5 | −9.00 | −10.99 |
10 | −8.39 | −10.09 |
20 | −6.48 | −6.66 |
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