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 condition	Value
Delivery mode	VMAT
Calculation algorithm	Monte Carlo
Photon energy	10 MV
Number of arcs	2
Calculation grid	3 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.

Organ	Dose–volume constraint (% of total volume)
Rectum	V45 Gy,<35%
	V30 Gy<50%
Bladder	V55 Gy,<70%
	V40 Gy<85%
Femoral heads	V25 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).

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, D_2%, (the dose inhabiting 2% or less of PTV volume in the dose–volume histogram, D_98%, the dose inhabiting >98% of PTV volume), and D_50%, 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.

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 size		Monitor units	Calculated time (s)	PTV coverage (%)	D35% of bladder (Gy)	D25% of rectum (Gy)	Homogeneity
360°	Average	854.7	153.4	92.3	58.8	49.0	0.06312
	Std	53.9	5.4	1.1	12.6	7.1	0.00300
330°	Average	889.0	144.1	92.8	58.5	48.9	0.06285
	Std	60.1	6.5	1.7	12.7	7.4	0.00339
300°	Average	907.1	131.0	92.5	60.2	48.8	0.06315
	Std	58.7	8.6	2.1	11.7	7.8	0.00237
270°	Average	899.4	116.1	92.3	58.6	48.8	0.06675
	Std	67.3	6.6	2.2	12.7	7.4	0.00384
240°	Average	900.5	103.6	93.3	58.1	51.3	0.06545
	Std	33.1	7.0	1.2	13.5	8.3	0.00215
210°	Average	870.2	86.8	92.9	58.8	51.9	0.06848
	Std	59.2	5.4	1.7	12.8	8.0	0.00305
180°	Average	853.3	73.2	92.7	60.9	49.1	0.07040
	Std	54.4	2.3	1.8	10.7	7.7	0.00227

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 authors have nothing to disclose.

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.