검색
검색 팝업 닫기

Ex) Article Title, Author, Keywords

Article

Split Viewer

Original Article

Progress in Medical Physics 2022; 33(4): 101-107

Published online December 31, 2022 https://doi.org/10.14316/pmp.2022.33.4.101

Copyright © Korean Society of Medical Physics.

Minimization of Treatment Time Using Partial-Arc Volumetric Modulated Arc Therapy with Bladder Filling Protocol for Prostate Cancer

Hojeong Lee1 , Dong Woon Kim1 , Ji Hyeon Joo1,2 , Yongkan Ki1,2 , Wontaek Kim2,3 , Dahl Park3 , Jiho Nam3 , Dong Hyeon Kim2,3 , Hosang Jeon1

1Department of Radiation Oncology and Research Institute for Convergence of Biomedical Science and Technology, Pusan National University Yangsan Hospital, 2Department of Radiation Oncology, Pusan National University School of Medicine, Yangsan, 3Department of Radiation Oncology, Pusan National University Hospital, Busan, Korea

Correspondence to:Hosang Jeon
(hjeon316@gmail.com)
Tel: 82-55-360-2693
Fax: 82-55-360-3449

Received: November 11, 2022; Revised: December 9, 2022; Accepted: December 12, 2022

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: Radiotherapy after bladder filling protocol (BFP) is known to enhance treatment quality and reduce side effects in prostate cancer, a common male solid cancer globally. However, due to the need to hold back urine during treatment, patients frequently complain of discomfort, and treatment is frequently suspended when patients urinate during treatment and urine penetrates the treatment device, causing malfunction. Therefore, the effect of minimizing treatment time when partial-arc volumetric modulated arc therapy (VMAT) was used instead of full-arc was assessed in this study.
Methods: A total of 70 plans were created in 10 patients using 7 different arc sizes, and the treatment time for each plan was calculated.
Results: Reduced arc size by half resulted in a 54.4% decrease in mean treatment duration, with a proportional tendency observed. Furthermore, the effect of VMAT arc size reduction on target dose homogeneity was significantly limited, and the effect on surrounding organs at risk (OAR) was negligible. It should be noted, however, that when the arc size decreases by >40%, the dose increases in the area without OAR around the target.
Conclusions: The results of this study demonstrated that partial-arc VMAT for enhancing treatment convenience and efficacy of prostate cancer patients undergoing BFP can achieve a considerable reduction in treatment time while preserving treatment quality, and it is expected to be useful for partial-arc VMAT plan design and implementation in practice.

KeywordsBladder filling protocol, Prostate cancer, Partial-arc volumetric modulated arc therapy, Treatment time

In Europe and the United States, prostate cancer is the second most prevalent solid tumor in men [1]. Furthermore, radiation therapy is known to be very effective, with a 91% probability that no clinical side effects will be observed 5 years after treatment [2]. However, because radiation-sensitive organs at risk (OAR) such as the rectum, bowel, and bladder are anatomically close to the prostate, clinicians now prefer using precise radiation therapy techniques such as intensity-modulated radiation therapy or volumetric modulated arc therapy (VMAT) [3,4] that treat the prostate while protecting nearby OARs.

Our facility employs the VMAT technique for prostate cancer treatment, and the bladder filling protocol (BFP) is performed before treatment to further improve treatment accuracy and OAR protection. BFP is used to keep bladder volume constant during each daily treatment by allowing patients to drink a fixed amount of water at a fixed time before treatment. This is a well-known strategy for effectively administering radiation treatment to prostate cancer patients [5-9]. This step is conducted for two primary reasons. First, because the bladder volume remains constant during each treatment, the position of the surrounding organs remains constant, which can improve radiation treatment accuracy. Second, the bladder with a specified volume can protect the OARs from excessive exposure by pushing adjacent OARs, like the bowel, out of the radiation treatment area.

Despite these benefits, the most significant disadvantage of BFP during treatment is the limited treatment time. VMAT is conducted at our institution with the Gantry rotating 360° to maximize the OAR protection effect, and the procedure takes about 10 minutes, including preparation for treatment, cone-beam computed tomography to confirm patient positioning and radiation treatment delivery. Therefore, elderly patients frequently complain of discomfort caused by holding back urine when the bladder is more than 150 mL full. Furthermore, changes in the patient’s posture may impair treatment accuracy. Furthermore, treatment suspension or machine failure has frequently occurred when urine–from patients who were unable to control urination during treatment–penetrates the Gantry passing beneath the patient.

Therefore, in this study, we sought to compensate for the shortcomings of BFP by introducing partial-arc VMAT, i.e., we removed some of the arcs as opposed to a full-arc VMAT that uses the entire 360° range. This method is likely to have two effects. First, treatment time can be reduced by reducing the arc range used for VMAT; second, by preferentially removing the arc located under the patient, the possibility of treatment interruption due to patients’ urine penetration into the treatment device can be fundamentally avoided. In this study, VMAT plans with different arc sizes were generated using the Radiation Treatment Planning (RTP) system, and the clinical efficacy of partial-arc VMAT combined with BFP was assessed using a comparative analysis.

1. Plan conditions

In this study, the total prescribed dose in prostate cancer cases was 70 Gy, and 10 patients (mean age, 70 y) who were recommended 2 Gy per fraction were chosen. BFP was conducted in all patients to fill the bladder with 150 mL before treatment daily, and an enema was used to empty the rectum before treatment. All treatment plans were generated using the same RTP system (MONACO v5.11.03; Elekta, Stockholm, Sweden), and all other conditions were applied identically except for the VMAT arc size (Table 1).

Table 1 Plan conditions

Plan conditionValue
Delivery modeVMAT
Calculation algorithmMonte Carlo
Photon energy10 MV
Number of arcs2
Calculation grid3 mm

VMAT, volumetric modulated arc therapy.


Regarding VMAT arc size, 7 different arcs with a range of 180°–360° were chosen and applied to 10 patients. Therefore, the total number of plans employed in this study was 70. Fig. 1 depicts the size of the arc used in each plan.

Figure 1.Size of arcs applied to different plans. VMAT, volumetric modulated arc therapy.

To determine the planning target volume (PTV), optimized conditions to deliver ≥99.7% (69.8 Gy) of the prescription dose to the entire volume and not surpass 101.4% (71 Gy) were applied. Furthermore, regarding dose-limiting conditions of the rectum, bladder, and femoral area near the PTV, we followed our internal recommendations per tolerance doses and volumes of the OARs proposed by QUANTEC [10] as tabulated in Table 2.

Table 2 VMAT dose constraints for organs at risk

OrganDose–volume constraint (% of total volume)
RectumV45 Gy,<35%
V30 Gy<50%
BladderV55 Gy,<70%
V40 Gy<85%
Femoral headsV25 Gy<40%

VMAT, volumetric modulated arc therapy.


2. Evaluation of treatment time

Eq. 1 depicts the predicted treatment time for each plan that was evaluated using the monitor unit value and mean dose rate value provided by RTP. Furthermore, to contrast the predicted treatment time with the actual treatment time, the actual treatment time was recorded after each plan was executed using a linear accelerator for treatment (VersaHD, Elekta).

Estimated treatment time min= Total MU Mean dose rateMUmin 

3. Analysis of plan quality

To assess the effect of the VMAT arc size change on the PTV dose, the PTV dose coverage and homogeneity index (HI) of the prescribed dose were measured in each plan. PTV dose coverage is the ratio of the volume delivered above the prescribed dose in the PTV, and for HI, D2%, (the dose inhabiting 2% or less of PTV volume in the dose–volume histogram, D98%, the dose inhabiting >98% of PTV volume), and D50%, the dose inhabiting 50% of PTV volume, were extracted and calculated as in Eq. 2. Therefore, the HI value can assess the homogeneity of the dose distribution within the PTV, with 0 being the most ideal value.

HI= D2 % D98 %D50 %

Furthermore, the dose distributions of various plans were compared to assess the impact of changes in the VMAT arc on the change in the dose distribution near the PTV, and the degree of dose change according to the change in the VMAT arc was evaluated for the rectum and bladder, which are major OARs.

1. Evaluation of treatment time

Fig. 2a shows a linear correlation (P=0.9749) between the size of the VMAT arc used in each plan and the required time calculated from RTP. Furthermore, the treatment time lowered by an average of 54.4% when the size of the arc was reduced by 50%, i.e., from 360° to 180°. When the RTP calculation time was compared to the beam-on time actually performed in the therapeutic linear accelerator, a linear proportional relationship was confirmed, as shown in Fig. 2b, with the actual time being 9.6% longer on average than the RTP calculation time.

Figure 2.Calculated treatment time by RTP in comparison with (a) sizes of VMAT arcs and (b) actual treatment time. RTP, Radiation Treatment Planning; VMAT, volumetric modulated arc therapy.

2. Plan quality analysis

Fig. 3a demonstrated that the change in VMAT arc size had almost no effect on PTV dose coverage. However, HI tended to slightly rise with decreasing VMAT arc size, and as shown in Fig. 3b, HI increased by 11.6% when treatment time was reduced by 50%.

Figure 3.Calculated treatment time by RTP in comparison with (a) PTV dose coverage and (b) homogeneity index. PTV, planning target volume; RTP, Radiation Treatment Planning.

Fig. 4 depicts the effect of change in the VMAT arc size on the dose distribution around the PTV. The leaked dose in the direction where there were no OARs seemed to rise noticeably when the arc size decreased by >40% (see white arrows in Fig. 4). Fig. 5 demonstrated that the doses of the rectum and bladder, where dose limits were applied during the VMAT optimization process, had no discernible tendency to vary with change in VMAT arc size. All data were tabulated in Table 3.

Table 3 Data on treatment time and plan quality concerning different arc sizes

Arc sizeMonitor
units
Calculated
time (s)
PTV
coverage (%)
D35% of
bladder (Gy)
D25% of
rectum (Gy)
Homogeneity
360°Average854.7153.492.358.849.00.06312
Std53.95.41.112.67.10.00300
330°Average889.0144.192.858.548.90.06285
Std60.16.51.712.77.40.00339
300°Average907.1131.092.560.248.80.06315
Std58.78.62.111.77.80.00237
270°Average899.4116.192.358.648.80.06675
Std67.36.62.212.77.40.00384
240°Average900.5103.693.358.151.30.06545
Std33.17.01.213.58.30.00215
210°Average870.286.892.958.851.90.06848
Std59.25.41.712.88.00.00305
180°Average853.373.292.760.949.10.07040
Std54.42.31.810.77.70.00227

PTV, planning target volume; Std., standard deviation.


Figure 4.Dose distribution near PTV concerning VMAT arc sizes of (a) 360°, (b) 270°, (c) 210°, and (d) 180°. White arrows indicate dose leakage outside PTV. The displayed dose scale is between 75% and 110%.

Figure 5.Calculated treatment time by RTP in comparison with (a) rectum dose at 25% volume and (b) bladder dose at 35% volume. RTP, Radiation Treatment Planning.

The treatment time calculated from RTP is nearly linearly proportional to the size of the VMAT arc used in the plan, and it is thought to provide valuable information when generating a partial-arc VMAT plan for prostate cancer treatment. Furthermore, it is expected to improve the convenience of patients who underwent BFP. The reason why the actual treatment time was about 10% longer than the RTP calculation time is believed to be the additional time needed for Gantry movement and multi-leaf collimator alignment.

The effect of reducing the VMAT arc on treatment quality was also considerably limited, and quantitatively, halving the size of the arc increased the HI by an average of 11.6%, which slightly reduced the PTV dose homogeneity. However, because the maximum dose of PTV in all plans created in this study was less than 110% of the prescribed dose, the effect of increased HI on treatment quality was surprisingly small. Furthermore, the PTV coverage was hardly impacted by the arc size. Furthermore, OAR doses looked to be primarily determined by anatomical location characteristics of individual patients rather than the effect of arc size. However, when the arc size is reduced close to 180°, the dose leakage in the direction where there are no OARs should be considered, as shown in Fig. 4. Therefore, we recommend using an arc size of around 270 which lessens treatment time by 25% while maintaining treatment quality.

There are different limitations to this study. First, because the study was based on a specific RTP, the same evaluation targeting RTPs with different VMAT characteristics is required. Second, several plan parameters that can affect VMAT precision–including arc number and control point number–were fixed to specific values within the manufacturer’s recommended range and thus their influence was ignored. Third, because other procedures not covered in this study, including imaging for patient positioning or equipment operation preparation, account for a significant portion of total treatment time, optimization of the overall treatment procedure is thought to be necessary.

This study confirmed that partial-arc VMAT can achieve a significant reduction in treatment time while maintaining treatment quality in prostate cancer patients undergoing BFP, and it is expected to be useful for partial-arc VMAT plan design and implementation in practice.

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean government (2020R1C1C1013300) and a 2022 research grant from Pusan National University Yangsan Hospital.

The data that support the findings of this study are available on request from the corresponding author.

Conceptualization: Hosang Jeon and Hojeong Lee. Data curation: Dong Woon Kim and Hojeong Lee. Formal analysis: Dong Woon Kim and Dahl Park. Funding acquisition: Yongkan Ki and Jiho Nam. Investigation: Ji Hyeon Joo and Yongkan Ki. Methodology: Hosang Jeon and Dong Woon Kim. Project administration: Hosang Jeon. Resources: Yongkan Ki and Wontaek Kim. Software: Hojeong Lee and Dong Hyeon Kim. Supervision: Wontaek Kim and Yongkan Ki. Validation: Hosang Jeon and Hojeong Lee. Visualization: Ji Hyeon Joo and Hojeong Lee. Writing-original draft: Hosang Jeon. Writing-review & editing: Hosang Jeon and Yongkan Ki.

  1. Zaorsky NG, Palmer JD, Hurwitz MD, Keith SW, Dicker AP, Den RB. What is the ideal radiotherapy dose to treat prostate cancer? A meta-analysis of biologically equivalent dose escalation. Radiother Oncol. 2015;115:295-300.
    Pubmed CrossRef
  2. Dearnaley D, Syndikus I, Mossop H, Khoo V, Birtle A, Bloomfield D, et al. Conventional versus hypofractionated high-dose intensity-modulated radiotherapy for prostate cancer: 5-year outcomes of the randomised, non-inferiority, phase 3 CHHiP trial. Lancet Oncol. 2016;17:1047-1060. Erratum in: Lancet Oncol. 2016;17:e321.
    Pubmed KoreaMed CrossRef
  3. Fischer-Valuck BW, Rao YJ, Michalski JM. Intensity-modulated radiotherapy for prostate cancer. Transl Androl Urol. 2018;7:297-307.
    Pubmed KoreaMed CrossRef
  4. Hoffmann M, Pacey J, Goodworth J, Laszcyzk A, Ford R, Chick B, et al. Analysis of a volumetric-modulated arc therapy (VMAT) single phase prostate template as a class solution. Rep Pract Oncol Radiother. 2019;24:92-96.
    Pubmed KoreaMed CrossRef
  5. Stam MR, van Lin EN, van der Vight LP, Kaanders JH, Visser AG. Bladder filling variation during radiation treatment of prostate cancer: can the use of a bladder ultrasound scanner and biofeedback optimize bladder filling? Int J Radiat Oncol Biol Phys. 2006;65:371-377.
    Pubmed CrossRef
  6. Yoon HI, Chung Y, Chang JS, Lee JY, Park SJ, Koom WS. Evaluating variations of bladder volume using an ultrasound scanner in rectal cancer patients during chemoradiation: is protocol-based full bladder maintenance using a bladder scanner useful to maintain the bladder volume? PLoS One. 2015;10:e0128791.
    Pubmed KoreaMed CrossRef
  7. Smith GA, Dunlop A, Barnes H, Herbert T, Lawes R, Mohajer J, et al. Bladder filling in patients undergoing prostate radiotherapy on a MR-linac: the dosimetric impact. Tech Innov Patient Support Radiat Oncol. 2022;21:41-45.
    Pubmed KoreaMed CrossRef
  8. O'Doherty UM, McNair HA, Norman AR, Miles E, Hooper S, Davies M, et al. Variability of bladder filling in patients receiving radical radiotherapy to the prostate. Radiother Oncol. 2006;79:335-340.
    Pubmed CrossRef
  9. Braide K, Kindblom J, Lindencrona U, Månsson M, Hugosson J. The value of a bladder-filling protocol for patients with prostate cancer who receive post-operative radiation: results from a prospective clinical trial. Acta Oncol. 2019;58:463-468.
    Pubmed CrossRef
  10. Michalski JM, Gay H, Jackson A, Tucker SL, Deasy JO. Radiation dose-volume effects in radiation-induced rectal injury. Int J Radiat Oncol Biol Phys. 2010;76(3 Suppl):S123-S129. Erratum in: Int J Radiat Oncol Biol Phys. 2019;104:1185.
    Pubmed CrossRef

Article

Original Article

Progress in Medical Physics 2022; 33(4): 101-107

Published online December 31, 2022 https://doi.org/10.14316/pmp.2022.33.4.101

Copyright © Korean Society of Medical Physics.

Minimization of Treatment Time Using Partial-Arc Volumetric Modulated Arc Therapy with Bladder Filling Protocol for Prostate Cancer

Hojeong Lee1 , Dong Woon Kim1 , Ji Hyeon Joo1,2 , Yongkan Ki1,2 , Wontaek Kim2,3 , Dahl Park3 , Jiho Nam3 , Dong Hyeon Kim2,3 , Hosang Jeon1

1Department of Radiation Oncology and Research Institute for Convergence of Biomedical Science and Technology, Pusan National University Yangsan Hospital, 2Department of Radiation Oncology, Pusan National University School of Medicine, Yangsan, 3Department of Radiation Oncology, Pusan National University Hospital, Busan, Korea

Correspondence to:Hosang Jeon
(hjeon316@gmail.com)
Tel: 82-55-360-2693
Fax: 82-55-360-3449

Received: November 11, 2022; Revised: December 9, 2022; Accepted: December 12, 2022

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.

Abstract

Purpose: Radiotherapy after bladder filling protocol (BFP) is known to enhance treatment quality and reduce side effects in prostate cancer, a common male solid cancer globally. However, due to the need to hold back urine during treatment, patients frequently complain of discomfort, and treatment is frequently suspended when patients urinate during treatment and urine penetrates the treatment device, causing malfunction. Therefore, the effect of minimizing treatment time when partial-arc volumetric modulated arc therapy (VMAT) was used instead of full-arc was assessed in this study.
Methods: A total of 70 plans were created in 10 patients using 7 different arc sizes, and the treatment time for each plan was calculated.
Results: Reduced arc size by half resulted in a 54.4% decrease in mean treatment duration, with a proportional tendency observed. Furthermore, the effect of VMAT arc size reduction on target dose homogeneity was significantly limited, and the effect on surrounding organs at risk (OAR) was negligible. It should be noted, however, that when the arc size decreases by >40%, the dose increases in the area without OAR around the target.
Conclusions: The results of this study demonstrated that partial-arc VMAT for enhancing treatment convenience and efficacy of prostate cancer patients undergoing BFP can achieve a considerable reduction in treatment time while preserving treatment quality, and it is expected to be useful for partial-arc VMAT plan design and implementation in practice.

Keywords: Bladder filling protocol, Prostate cancer, Partial-arc volumetric modulated arc therapy, Treatment time

Introduction

In Europe and the United States, prostate cancer is the second most prevalent solid tumor in men [1]. Furthermore, radiation therapy is known to be very effective, with a 91% probability that no clinical side effects will be observed 5 years after treatment [2]. However, because radiation-sensitive organs at risk (OAR) such as the rectum, bowel, and bladder are anatomically close to the prostate, clinicians now prefer using precise radiation therapy techniques such as intensity-modulated radiation therapy or volumetric modulated arc therapy (VMAT) [3,4] that treat the prostate while protecting nearby OARs.

Our facility employs the VMAT technique for prostate cancer treatment, and the bladder filling protocol (BFP) is performed before treatment to further improve treatment accuracy and OAR protection. BFP is used to keep bladder volume constant during each daily treatment by allowing patients to drink a fixed amount of water at a fixed time before treatment. This is a well-known strategy for effectively administering radiation treatment to prostate cancer patients [5-9]. This step is conducted for two primary reasons. First, because the bladder volume remains constant during each treatment, the position of the surrounding organs remains constant, which can improve radiation treatment accuracy. Second, the bladder with a specified volume can protect the OARs from excessive exposure by pushing adjacent OARs, like the bowel, out of the radiation treatment area.

Despite these benefits, the most significant disadvantage of BFP during treatment is the limited treatment time. VMAT is conducted at our institution with the Gantry rotating 360° to maximize the OAR protection effect, and the procedure takes about 10 minutes, including preparation for treatment, cone-beam computed tomography to confirm patient positioning and radiation treatment delivery. Therefore, elderly patients frequently complain of discomfort caused by holding back urine when the bladder is more than 150 mL full. Furthermore, changes in the patient’s posture may impair treatment accuracy. Furthermore, treatment suspension or machine failure has frequently occurred when urine–from patients who were unable to control urination during treatment–penetrates the Gantry passing beneath the patient.

Therefore, in this study, we sought to compensate for the shortcomings of BFP by introducing partial-arc VMAT, i.e., we removed some of the arcs as opposed to a full-arc VMAT that uses the entire 360° range. This method is likely to have two effects. First, treatment time can be reduced by reducing the arc range used for VMAT; second, by preferentially removing the arc located under the patient, the possibility of treatment interruption due to patients’ urine penetration into the treatment device can be fundamentally avoided. In this study, VMAT plans with different arc sizes were generated using the Radiation Treatment Planning (RTP) system, and the clinical efficacy of partial-arc VMAT combined with BFP was assessed using a comparative analysis.

Material and Methods

1. Plan conditions

In this study, the total prescribed dose in prostate cancer cases was 70 Gy, and 10 patients (mean age, 70 y) who were recommended 2 Gy per fraction were chosen. BFP was conducted in all patients to fill the bladder with 150 mL before treatment daily, and an enema was used to empty the rectum before treatment. All treatment plans were generated using the same RTP system (MONACO v5.11.03; Elekta, Stockholm, Sweden), and all other conditions were applied identically except for the VMAT arc size (Table 1).

Table 1 . Plan conditions.

Plan conditionValue
Delivery modeVMAT
Calculation algorithmMonte Carlo
Photon energy10 MV
Number of arcs2
Calculation grid3 mm

VMAT, volumetric modulated arc therapy..



Regarding VMAT arc size, 7 different arcs with a range of 180°–360° were chosen and applied to 10 patients. Therefore, the total number of plans employed in this study was 70. Fig. 1 depicts the size of the arc used in each plan.

Figure 1. Size of arcs applied to different plans. VMAT, volumetric modulated arc therapy.

To determine the planning target volume (PTV), optimized conditions to deliver ≥99.7% (69.8 Gy) of the prescription dose to the entire volume and not surpass 101.4% (71 Gy) were applied. Furthermore, regarding dose-limiting conditions of the rectum, bladder, and femoral area near the PTV, we followed our internal recommendations per tolerance doses and volumes of the OARs proposed by QUANTEC [10] as tabulated in Table 2.

Table 2 . VMAT dose constraints for organs at risk.

OrganDose–volume constraint (% of total volume)
RectumV45 Gy,<35%
V30 Gy<50%
BladderV55 Gy,<70%
V40 Gy<85%
Femoral headsV25 Gy<40%

VMAT, volumetric modulated arc therapy..



2. Evaluation of treatment time

Eq. 1 depicts the predicted treatment time for each plan that was evaluated using the monitor unit value and mean dose rate value provided by RTP. Furthermore, to contrast the predicted treatment time with the actual treatment time, the actual treatment time was recorded after each plan was executed using a linear accelerator for treatment (VersaHD, Elekta).

Estimated treatment time min= Total MU Mean dose rateMUmin 

3. Analysis of plan quality

To assess the effect of the VMAT arc size change on the PTV dose, the PTV dose coverage and homogeneity index (HI) of the prescribed dose were measured in each plan. PTV dose coverage is the ratio of the volume delivered above the prescribed dose in the PTV, and for HI, D2%, (the dose inhabiting 2% or less of PTV volume in the dose–volume histogram, D98%, the dose inhabiting >98% of PTV volume), and D50%, the dose inhabiting 50% of PTV volume, were extracted and calculated as in Eq. 2. Therefore, the HI value can assess the homogeneity of the dose distribution within the PTV, with 0 being the most ideal value.

HI= D2 % D98 %D50 %

Furthermore, the dose distributions of various plans were compared to assess the impact of changes in the VMAT arc on the change in the dose distribution near the PTV, and the degree of dose change according to the change in the VMAT arc was evaluated for the rectum and bladder, which are major OARs.

Results

1. Evaluation of treatment time

Fig. 2a shows a linear correlation (P=0.9749) between the size of the VMAT arc used in each plan and the required time calculated from RTP. Furthermore, the treatment time lowered by an average of 54.4% when the size of the arc was reduced by 50%, i.e., from 360° to 180°. When the RTP calculation time was compared to the beam-on time actually performed in the therapeutic linear accelerator, a linear proportional relationship was confirmed, as shown in Fig. 2b, with the actual time being 9.6% longer on average than the RTP calculation time.

Figure 2. Calculated treatment time by RTP in comparison with (a) sizes of VMAT arcs and (b) actual treatment time. RTP, Radiation Treatment Planning; VMAT, volumetric modulated arc therapy.

2. Plan quality analysis

Fig. 3a demonstrated that the change in VMAT arc size had almost no effect on PTV dose coverage. However, HI tended to slightly rise with decreasing VMAT arc size, and as shown in Fig. 3b, HI increased by 11.6% when treatment time was reduced by 50%.

Figure 3. Calculated treatment time by RTP in comparison with (a) PTV dose coverage and (b) homogeneity index. PTV, planning target volume; RTP, Radiation Treatment Planning.

Fig. 4 depicts the effect of change in the VMAT arc size on the dose distribution around the PTV. The leaked dose in the direction where there were no OARs seemed to rise noticeably when the arc size decreased by >40% (see white arrows in Fig. 4). Fig. 5 demonstrated that the doses of the rectum and bladder, where dose limits were applied during the VMAT optimization process, had no discernible tendency to vary with change in VMAT arc size. All data were tabulated in Table 3.

Table 3 . Data on treatment time and plan quality concerning different arc sizes.

Arc sizeMonitor
units
Calculated
time (s)
PTV
coverage (%)
D35% of
bladder (Gy)
D25% of
rectum (Gy)
Homogeneity
360°Average854.7153.492.358.849.00.06312
Std53.95.41.112.67.10.00300
330°Average889.0144.192.858.548.90.06285
Std60.16.51.712.77.40.00339
300°Average907.1131.092.560.248.80.06315
Std58.78.62.111.77.80.00237
270°Average899.4116.192.358.648.80.06675
Std67.36.62.212.77.40.00384
240°Average900.5103.693.358.151.30.06545
Std33.17.01.213.58.30.00215
210°Average870.286.892.958.851.90.06848
Std59.25.41.712.88.00.00305
180°Average853.373.292.760.949.10.07040
Std54.42.31.810.77.70.00227

PTV, planning target volume; Std., standard deviation..



Figure 4. Dose distribution near PTV concerning VMAT arc sizes of (a) 360°, (b) 270°, (c) 210°, and (d) 180°. White arrows indicate dose leakage outside PTV. The displayed dose scale is between 75% and 110%.

Figure 5. Calculated treatment time by RTP in comparison with (a) rectum dose at 25% volume and (b) bladder dose at 35% volume. RTP, Radiation Treatment Planning.

DISCUSSION

The treatment time calculated from RTP is nearly linearly proportional to the size of the VMAT arc used in the plan, and it is thought to provide valuable information when generating a partial-arc VMAT plan for prostate cancer treatment. Furthermore, it is expected to improve the convenience of patients who underwent BFP. The reason why the actual treatment time was about 10% longer than the RTP calculation time is believed to be the additional time needed for Gantry movement and multi-leaf collimator alignment.

The effect of reducing the VMAT arc on treatment quality was also considerably limited, and quantitatively, halving the size of the arc increased the HI by an average of 11.6%, which slightly reduced the PTV dose homogeneity. However, because the maximum dose of PTV in all plans created in this study was less than 110% of the prescribed dose, the effect of increased HI on treatment quality was surprisingly small. Furthermore, the PTV coverage was hardly impacted by the arc size. Furthermore, OAR doses looked to be primarily determined by anatomical location characteristics of individual patients rather than the effect of arc size. However, when the arc size is reduced close to 180°, the dose leakage in the direction where there are no OARs should be considered, as shown in Fig. 4. Therefore, we recommend using an arc size of around 270 which lessens treatment time by 25% while maintaining treatment quality.

There are different limitations to this study. First, because the study was based on a specific RTP, the same evaluation targeting RTPs with different VMAT characteristics is required. Second, several plan parameters that can affect VMAT precision–including arc number and control point number–were fixed to specific values within the manufacturer’s recommended range and thus their influence was ignored. Third, because other procedures not covered in this study, including imaging for patient positioning or equipment operation preparation, account for a significant portion of total treatment time, optimization of the overall treatment procedure is thought to be necessary.

This study confirmed that partial-arc VMAT can achieve a significant reduction in treatment time while maintaining treatment quality in prostate cancer patients undergoing BFP, and it is expected to be useful for partial-arc VMAT plan design and implementation in practice.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean government (2020R1C1C1013300) and a 2022 research grant from Pusan National University Yangsan Hospital.

Conflicts of Interest

The authors have nothing to disclose.

Availability of Data and Materials

The data that support the findings of this study are available on request from the corresponding author.

Author Contributions

Conceptualization: Hosang Jeon and Hojeong Lee. Data curation: Dong Woon Kim and Hojeong Lee. Formal analysis: Dong Woon Kim and Dahl Park. Funding acquisition: Yongkan Ki and Jiho Nam. Investigation: Ji Hyeon Joo and Yongkan Ki. Methodology: Hosang Jeon and Dong Woon Kim. Project administration: Hosang Jeon. Resources: Yongkan Ki and Wontaek Kim. Software: Hojeong Lee and Dong Hyeon Kim. Supervision: Wontaek Kim and Yongkan Ki. Validation: Hosang Jeon and Hojeong Lee. Visualization: Ji Hyeon Joo and Hojeong Lee. Writing-original draft: Hosang Jeon. Writing-review & editing: Hosang Jeon and Yongkan Ki.

Fig 1.

Figure 1.Size of arcs applied to different plans. VMAT, volumetric modulated arc therapy.
Progress in Medical Physics 2022; 33: 101-107https://doi.org/10.14316/pmp.2022.33.4.101

Fig 2.

Figure 2.Calculated treatment time by RTP in comparison with (a) sizes of VMAT arcs and (b) actual treatment time. RTP, Radiation Treatment Planning; VMAT, volumetric modulated arc therapy.
Progress in Medical Physics 2022; 33: 101-107https://doi.org/10.14316/pmp.2022.33.4.101

Fig 3.

Figure 3.Calculated treatment time by RTP in comparison with (a) PTV dose coverage and (b) homogeneity index. PTV, planning target volume; RTP, Radiation Treatment Planning.
Progress in Medical Physics 2022; 33: 101-107https://doi.org/10.14316/pmp.2022.33.4.101

Fig 4.

Figure 4.Dose distribution near PTV concerning VMAT arc sizes of (a) 360°, (b) 270°, (c) 210°, and (d) 180°. White arrows indicate dose leakage outside PTV. The displayed dose scale is between 75% and 110%.
Progress in Medical Physics 2022; 33: 101-107https://doi.org/10.14316/pmp.2022.33.4.101

Fig 5.

Figure 5.Calculated treatment time by RTP in comparison with (a) rectum dose at 25% volume and (b) bladder dose at 35% volume. RTP, Radiation Treatment Planning.
Progress in Medical Physics 2022; 33: 101-107https://doi.org/10.14316/pmp.2022.33.4.101

Table 1 Plan conditions

Plan conditionValue
Delivery modeVMAT
Calculation algorithmMonte Carlo
Photon energy10 MV
Number of arcs2
Calculation grid3 mm

VMAT, volumetric modulated arc therapy.


Table 2 VMAT dose constraints for organs at risk

OrganDose–volume constraint (% of total volume)
RectumV45 Gy,<35%
V30 Gy<50%
BladderV55 Gy,<70%
V40 Gy<85%
Femoral headsV25 Gy<40%

VMAT, volumetric modulated arc therapy.


Table 3 Data on treatment time and plan quality concerning different arc sizes

Arc sizeMonitor
units
Calculated
time (s)
PTV
coverage (%)
D35% of
bladder (Gy)
D25% of
rectum (Gy)
Homogeneity
360°Average854.7153.492.358.849.00.06312
Std53.95.41.112.67.10.00300
330°Average889.0144.192.858.548.90.06285
Std60.16.51.712.77.40.00339
300°Average907.1131.092.560.248.80.06315
Std58.78.62.111.77.80.00237
270°Average899.4116.192.358.648.80.06675
Std67.36.62.212.77.40.00384
240°Average900.5103.693.358.151.30.06545
Std33.17.01.213.58.30.00215
210°Average870.286.892.958.851.90.06848
Std59.25.41.712.88.00.00305
180°Average853.373.292.760.949.10.07040
Std54.42.31.810.77.70.00227

PTV, planning target volume; Std., standard deviation.


References

  1. Zaorsky NG, Palmer JD, Hurwitz MD, Keith SW, Dicker AP, Den RB. What is the ideal radiotherapy dose to treat prostate cancer? A meta-analysis of biologically equivalent dose escalation. Radiother Oncol. 2015;115:295-300.
    Pubmed CrossRef
  2. Dearnaley D, Syndikus I, Mossop H, Khoo V, Birtle A, Bloomfield D, et al. Conventional versus hypofractionated high-dose intensity-modulated radiotherapy for prostate cancer: 5-year outcomes of the randomised, non-inferiority, phase 3 CHHiP trial. Lancet Oncol. 2016;17:1047-1060. Erratum in: Lancet Oncol. 2016;17:e321.
    Pubmed KoreaMed CrossRef
  3. Fischer-Valuck BW, Rao YJ, Michalski JM. Intensity-modulated radiotherapy for prostate cancer. Transl Androl Urol. 2018;7:297-307.
    Pubmed KoreaMed CrossRef
  4. Hoffmann M, Pacey J, Goodworth J, Laszcyzk A, Ford R, Chick B, et al. Analysis of a volumetric-modulated arc therapy (VMAT) single phase prostate template as a class solution. Rep Pract Oncol Radiother. 2019;24:92-96.
    Pubmed KoreaMed CrossRef
  5. Stam MR, van Lin EN, van der Vight LP, Kaanders JH, Visser AG. Bladder filling variation during radiation treatment of prostate cancer: can the use of a bladder ultrasound scanner and biofeedback optimize bladder filling? Int J Radiat Oncol Biol Phys. 2006;65:371-377.
    Pubmed CrossRef
  6. Yoon HI, Chung Y, Chang JS, Lee JY, Park SJ, Koom WS. Evaluating variations of bladder volume using an ultrasound scanner in rectal cancer patients during chemoradiation: is protocol-based full bladder maintenance using a bladder scanner useful to maintain the bladder volume? PLoS One. 2015;10:e0128791.
    Pubmed KoreaMed CrossRef
  7. Smith GA, Dunlop A, Barnes H, Herbert T, Lawes R, Mohajer J, et al. Bladder filling in patients undergoing prostate radiotherapy on a MR-linac: the dosimetric impact. Tech Innov Patient Support Radiat Oncol. 2022;21:41-45.
    Pubmed KoreaMed CrossRef
  8. O'Doherty UM, McNair HA, Norman AR, Miles E, Hooper S, Davies M, et al. Variability of bladder filling in patients receiving radical radiotherapy to the prostate. Radiother Oncol. 2006;79:335-340.
    Pubmed CrossRef
  9. Braide K, Kindblom J, Lindencrona U, Månsson M, Hugosson J. The value of a bladder-filling protocol for patients with prostate cancer who receive post-operative radiation: results from a prospective clinical trial. Acta Oncol. 2019;58:463-468.
    Pubmed CrossRef
  10. Michalski JM, Gay H, Jackson A, Tucker SL, Deasy JO. Radiation dose-volume effects in radiation-induced rectal injury. Int J Radiat Oncol Biol Phys. 2010;76(3 Suppl):S123-S129. Erratum in: Int J Radiat Oncol Biol Phys. 2019;104:1185.
    Pubmed CrossRef
Korean Society of Medical Physics

Vol.35 No.1
March 2024

pISSN 2508-4445
eISSN 2508-4453
Formerly ISSN 1226-5829

Frequency: Quarterly

Current Issue   |   Archives

Stats or Metrics

Share this article on :

  • line