Dosimetric evaluation of biaxially rotational dynamic radiation therapy and swing-fixed non-coplanar volumetric modulated arc therapy with OXRAY in prostate cancer: a comparative study

Kouta Hirotaki; Kenji Makita; Kento Tomizawa; Masashi Wakabayashi; Hidehiro Hojo; Masaki Nakamura; Masashi Ito

doi:10.3857/roj.2025.00332

Radiation Oncology Journal > Volume 44(1); 2026 > Article

Hirotaki, Makita, Tomizawa, Wakabayashi, Hojo, Nakamura, and Ito: Dosimetric evaluation of biaxially rotational dynamic radiation therapy and swing-fixed non-coplanar volumetric modulated arc therapy with OXRAY in prostate cancer: a comparative study

Original Article

Radiation Oncology Journal 2026; 44(1): 8-15.

Published online: March 10, 2026

DOI: https://doi.org/10.3857/roj.2025.00332

Dosimetric evaluation of biaxially rotational dynamic radiation therapy and swing-fixed non-coplanar volumetric modulated arc therapy with OXRAY in prostate cancer: a comparative study

Kouta Hirotaki^1,^2,^*, Kenji Makita^3,^4,^*, Kento Tomizawa³

, Masashi Wakabayashi⁵, Hidehiro Hojo³, Masaki Nakamura³, Masashi Ito²

¹Doctoral Program in Medical Sciences, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Tsukuba, Japan

²Department of Radiological Technology, National Cancer Center Hospital East, Kashiwa, Japan

³Department of Radiation Oncology, National Cancer Center Hospital East, Kashiwa, Japan

⁴Department of Radiation Oncology, National Hospital Organization Shikoku Cancer Center, Matsuyama, Japan

⁵Biostatistics Division, Center for Research Administration and Support, National Cancer Center Hospital East, Kashiwa, Japan

Correspondence: Kento Tomizawa Department of Radiation Oncology, National Cancer Center Hospital East, 6-5-1 Kashiwanoha, Kashiwa, Chiba 277-8577, Japan
Tel: +81-4-7133-1111 E-mail: ktomizaw@east.ncc.go.jp

^*These authors contributed equally to this work.

Received June 5, 2025 Revised August 30, 2025 Accepted September 8, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://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

This study evaluated the usefulness of non-coplanar volumetric modulated arc therapy (VMAT) using a novel O-ring-type linear accelerator, OXRAY, in patients with prostate cancer (PCa). The focus was on those with the planning target volume (PTV) adjacent to the small or large intestine.

Materials and Methods

We enrolled 10 consecutive PCa patients with the PTV less than 10 mm from the small or large intestine. These patients underwent conventional coplanar VMAT (Conv-VMAT) using Halcyon, another O-ring-type linear accelerator, at our institute between July 2023 and July 2024. To evaluate the utility of OXRAY, we developed two simulation plans: biaxially rotational dynamic radiation therapy (BROAD-RT) and swing-fixed non-coplanar VMAT (SF-VMAT). A total dose of 63 Gy in 21 fractions was prescribed for all plans. Dosimetric parameters of the PTV and organs at risk were compared among the three plans using paired t-tests.

Results

No significant differences in the dose received to 95% of the volume (D95) of the PTV were observed among the three plans. However, the D95 of the PTV-base, defined as the three cranial slices of the PTV, significantly improved in BROAD-RT compared to Conv-VMAT, adhering to dose constraints of the small or large intestine. On the other hand, the improvement in SF-VMAT was insignificant. BROAD-RT and SF-VMAT significantly improved rectal dose indices compared to Conv-VMAT.

Conclusion

For PCa patients with the PTV close to the small or large intestine, non-coplanar VMAT using OXRAY may improve the PTV-base coverage and decrease rectal dose compared to conventional VMAT plans, while adhering to dose constraints of the small or large intestines.

Keywords: Radiotherapy, Intensity-modulated, Prostatic neoplasms

Introduction

Prostate cancer (PCa) is the most prevalent cancer among men [1]. Radiation therapy (RT) is a valuable treatment option for PCa [2,3]. Intensity-modulated radiation therapy (IMRT), a technique that allows for varying the intensity of irradiation across each beam, has enabled dose escalation to target volumes and reduced dose to organs at risk (OARs) in PCa [4,5].

Gastrointestinal (GI) and genitourinary (GU) toxicities are important adverse events associated with RT for PCa [6-8]. Various dose constraints have been established to reduce severe GI/GU toxicities, including limiting the volume of the small intestine receiving 45 Gy and the dose to 50% of the rectal volume [9,10]. While the IMRT has decreased GI/GU toxicity in PCa compared to three-dimensional conformal RT [11,12], the small or large intestine often constrains the dose to the planning target volume (PTV), particularly in the base region. Therefore, for patients with PCa and the PTV in close proximity to the small or large intestine, a new technology is required to deliver an adequate dose to the base region of the PTV while adhering to the intestinal dose constraints.

Recently, a novel linear accelerator system, OXRAY (Hitachi Ltd., Tokyo, Japan), has been developed. OXRAY features an O-ring-type gantry that rotates in biaxial synchronization, enabling flexible non-coplanar beam trajectories without requiring couch movement. Other ring-gantry systems, such as Halcyon (Varian Medical Systems, Palo Alto, CA, USA), are widely used for their compact design and rapid delivery but are limited to coplanar beam arrangements due to a fixed gantry axis and a treatment couch with only three translational degrees of freedom. In contrast, OXRAY combines a biaxially rotating gantry with a robotic couch capable of five-axis movement, allowing for precise patient positioning and dynamic non-coplanar beam delivery without couch rotation. The multileaf collimator (MLC) width of OXRAY was 2.5 mm at the center and 5.0 mm at other locations, while Halcyon featured a 5.0 mm MLC width. The MLC drive speed was 6.50 cm/sec for OXRAY and 5.00 cm/sec for Halcyon. This distinctive non-coplanar volumetric modulated arc therapy (VMAT) shows promise in improving target coverage in the base region of the prostate, particularly when adjacent to the small or large intestine. Nevertheless, limited research has been conducted to validate the effectiveness of non-coplanar VMAT using OXRAY in patients with PCa.

This study aimed to evaluate the utility of non-coplanar VMAT using OXRAY on dose coverage compared to conventional coplanar VMAT (Conv-VMAT) for PCa. The focus was particularly on patients with PTV in close proximity to the small or large intestine, particularly in the base region of the PTV.

Materials and Methods

1. Patients and imaging datasets

We retrospectively enrolled ten consecutive PCa patients with PTV less than 10 mm from the small or large intestine. These patients underwent Conv-VMAT at National Cancer Center Hospital East between July 2023 and July 2024 (Table 1). All patients were immobilized using a suction brace that covered the knee and heel. Computed tomography (CT) datasets were acquired using Aquilion One (Canon Medical Systems, Tochigi, Japan) with an image slice thickness of 0.2 cm. Patients were instructed to have a comfortably filled bladder (by ingesting 400 mL of water 30–50 minutes before the CT scan) and an empty rectum to ensure reproducibility. Rectal spacers were not utilized for any of the patients.

Although informed consent was not required, the homepage of National Cancer Center Hospital East published the details of this study and allowed patients to decline participation. The study methods, including the investigation procedure and handling of patient information, were approved by the Institutional Review Board of National Cancer Center Hospital East (IRB No. 2018-076).

2. Contouring

The clinical target volume (CTV), PTV, and OARs, including the bladder, small intestine, large intestine, and rectum, were delineated for clinical practice by radiation oncologists. The CTV encompassed the prostate and the proximal 1–2 cm of the seminal vesicles. The PTV was generated by expanding the CTV by 0.7 cm anteriorly, 0.5 cm posteriorly, and 0.6 cm in all other directions. Furthermore, to evaluate the radiation dose distribution for the base and apex regions of the prostate, PTV-base was defined as the superior-most three CT slices of the PTV, while PTV-apex was defined as the inferior-most three CT slices.

3. Treatment planning and clinical goals

A total dose of 63 Gy in 21 fractions was prescribed for all plans. The clinical goals used in treatment planning are shown in Table 2. When dose constraints posed challenges during optimization, those concerning the small or large intestines were prioritized. To evaluate the utility of OXRAY, we created two simulation plan types: biaxially rotational dynamic radiation therapy (BROAD-RT) and swing-fixed non-coplanar VMAT (SF-VMAT). BROAD-RT plans comprised two arcs with continuously modulating ring angles at 11 modulation points using a 6-MV flattened photon beam (Fig. 1A), with the arc trajectory depicted in Fig. 1B. SF-VMAT plans comprised two arcs with fixed ring angles of 20° and 340° using a 6-MV flattened photon beam. These ring angles were selected to avoid potential collisions between the gantry and treatment couch in the OXRAY system. A radiation therapist generated all BROAD-RT and SF-VMAT plans. Both BROAD-RT and SF-VMAT plans were developed using OXRAY and RayStation (RaySearch Laboratories AB, Stockholm, Sweden) with a collapsed cone convolution algorithm.

To compare the BROAD-RT and SF-VMAT plans with Conv-VMAT plans, we utilized VMAT plans employed in clinical practice at our institution. The Conv-VMAT plans were generated using a linear accelerator with an O-ring gantry (Halcyon, Varian Medical Systems). The Conv-VMAT plans consisted of two arcs with collimator angles set at 345° and 15°, utilizing a 6-MV flattening filter-free photon beam and calculated using the Acuros XB algorithm with Eclipse (Varian Medical Systems). All plans created using RayStation and Eclipse were calculated with a grid size of 2 mm.

4. Dosimetric analysis

For the target, we analyzed PTV D98 (DX, dose received to X% of the volume of the structure), D50, D2, PTV-base D95, and PTV-apex D95. Regarding OARs, we examined small and large intestine V45 (VX, the volume received X Gy or more); rectum V30 (VX, the percentage of the volume received X Gy or more), V40, V50, D0.03 cc (the dose received in a volume of 0.03 cm³); bladder V40, V50, D0.03 cc; and penile bulb (PB) D90.

To estimate the impact of dose differences on clinical complications, normal tissue complication (NTCP), including GI and GU toxicities were calculated using the Lyman-Kutcher-Burman model for all plans [14,15]. The parameters used for NTCP calculation were taken from a previous study (Supplementary Table 1).

5. Statistics

Each dose index was summarized using the mean and standard deviation in all plans. A paired t-test was used to compare the dose indices of the three plans. All tests were two-tailed, and a p-value < 0.05 was considered statistically significant. All statistical analyses were performed using the SAS software version 9.4 (SAS Institute Inc., Cary, NC, USA).

Results

1. PTV coverage

In our cohort, the average PTV volume was 90.75 cm³. A comparison of the radiation dose parameters for the targets among the three plans is shown in Table 3. No significant differences were observed in the D98, D95, or D50 values among the three plans. The BROAD-RT plans exhibited a significantly lower D2 than the Conv-VMAT plans (p = 0.042). The SF-VMAT plans did not show a significant decrease in D2 compared to the Conv-VMAT plans (p = 0.115). Both BROAD-RT and SF-VMAT plans showed significantly lower conformity index (CI) than that of Conv-VMAT plans (p < 0.001 and p < 0.001, respectively). On the other hand, no significant differences in homogeneity index were observed among three plans. Concerning the PTV-base, the BROAD-RT plans showed significantly higher D95 than the Conv-VMAT plans (p = 0.047). While the SF-VMAT plans indicated an improvement in D95 compared to the Conv-VMAT plans, the difference was statistically insignificant (p = 0.084). For the PTV-apex, both the SF- and BROAD-RT plans did not significantly improve D95 compared to the Conv-VMAT plans (p = 0.371 and p = 0.161, respectively).

2. OARs

Table 3 presents the dose parameters for OARs in the three plans. No significant differences in V45 for the small or large intestine were observed among the three plans. Concerning the bladder, both the BROAD-RT and SF-VMAT plans did not significantly improve V40, V50, and D0.03 cc compared to the Conv-VMAT plans. For the rectum, both the BROAD-RT and SF-VMAT plans achieved significantly lower V30 values than the Conv-VMAT plan (p < 0.001 and p = 0.033, respectively). Furthermore, the SF-VMAT plans attained a significantly lower V30 in the rectum than the BROAD-RT plans (p = 0.033). The SF-VMAT plans significantly reduced the V40 of the rectum compared to the BROAD-RT and Conv-VMAT plans (p = 0.003 and p = 0.002, respectively). Additionally, the SF-VMAT plans significantly decreased the V50 of the rectum compared to the BROAD-RT and Conv-VMAT plans (p = 0.026 and p = 0.009, respectively). Both the BROAD-RT and SF-VMAT plans significantly reduced D0.03 cc of the rectum compared to the Conv-VMAT plans (p < 0.001 and p < 0.001, respectively). Regarding the PB, both the BROAD-RT and SF-VMAT plans significantly increased the D90 compared to the Conv-VMAT plans (p = 0.041 and p = 0.025, respectively).

3. NTCP

NTCPs for rectum and bladder in the three plans are shown in Supplementary Table 2. Regarding the rectum, the BROAD-RT plans had the lowest mean NTCP, followed by the SF-VMAT and Conv-VMAT plans. In pairwise comparisons, BROAD-RT plans significantly reduced NTCP compared with Conv-VMAT plans (p = 0.037). There were no significant differences between BROAD-RT and SF-VMAT plans, or between SF-VMAT and Conv-VMAT plans. Regarding the bladder, there were no significant differences in NTCP among the three plans.

Discussion and Conclusion

This study examined the utility of two non-coplanar VMAT techniques (BROAD-RT and SF-VMAT) employing OXRAY on dose distribution in patients with PCa near the small or large intestine. BROAD-RT plans enhanced the coverage of the PTV base (the three cranial slices of the PTV) compared to Conv-VMAT plans, adhering to the dose constraints of the small or large intestine. Furthermore, both BROAD-RT and SF-VMAT plans decreased rectal dose in comparison to Conv-VMAT plans.

To improve dose coverage over conventional VMAT, a new non-coplanar irradiation technique, dynamic trajectory radiotherapy (DTRT) has been developed [16]. DTRT involves simultaneous rotation of the gantry and couch during beam-on, allowing for a highly flexible beam trajectory. Studies have indicated that, in the specific case of PCa, DTRT has only shown minor reductions in rectal and bladder doses compared to conventional VMAT [17]. In our study, BROAD-RT plans for ten cases of PCa also showed only minor reductions in rectal and bladder doses compared to the conventional VMAT plans. However, BROAD-RT plans improved coverage of the PTV-base, while adhering to dose constraints for the small or large intestine. These findings suggest that the advantages of non-coplanar VMAT plans may be limited for small, simple-shaped targets such as PCa. Our study focused on PCa patients with tumors in close proximity to the small or large intestine, where non-coplanar VMAT plans could offer benefits in complex situations within nearby OARs. Recently, many patients with PCa have received a hydrogel spacer (HS; SpaceOAR System, Augmenix, Inc., Waltham, MA, USA) injection between the prostate and rectum to mitigate rectal toxicity [18,19]. The HS injection may displace the seminal vesicle, the base of the PTV, in a cranial direction [20]. In such cases, the BROAD-RT plan could be particularly effective. Although the BROAD-RT and SF-VMAT plans showed significantly lower CI values compared with Conv-VMAT plans, this trade-off was deemed acceptable considering the compliance in the dose constraints. The proximity of the target to intestines posed limitations to achieving ideal CI values. Nonetheless, our plans met all clinical dose constraints.

The apex is the most common location of cancer in the prostate [21,22]. Therefore, it is crucial to ensure adequate dose coverage of the apical region during RT for PCa. In this study, the BROAD-RT and SF-VMAT plans showed minor, though not statistically significant improvements in PTV-apex coverage compared to the Conv-VMAT plans. However, the D90 value of the PB increased in the BROAD-RT and SF-VMAT plans compared to that in the Conv-VMAT plan. The observed increase in PB D90 in both BROAD-RT and SF-VMAT plans is considered to result from caudal dose extension associated with non-coplanar beam arrangements. Research on erectile function post-radiation therapy concerning the dose to critical erectile structures primarily focuses on the PB [23-25]. However, the PB dose remained sufficiently low in all three plans. Consequently, the impact of PB irradiation dose on the risk of sexual dysfunction is deemed minimal, and the BROAD-RT and SF-VMAT plans are considered well-tolerated.

The rectum, bladder, and intestine are crucial OARs in radiotherapy for PCa [26-30]. In this study, the rectal dose was reduced in the BROAD-RT and SF-VMAT plans compared to the Conv-VMAT plans. Additionally, BROAD-RT plans significantly reduced the NTCP for the rectum compared with Conv-VMAT plans (Supplementary Table 2). This result appears to reflect the benefits of using non-coplanar beams in the setting of this study. While the number of patients with PCa using HS has increased, some patients decline or are not suitable for HS injection. For these patients, OXRAY seems to be more desirable. However, there was no improvement in bladder and intestine doses (including bladder NTCP values) with the BROAD-RT and SF-VMAT plans compared with Conv-VMAT plans. This lack of improvement may be due to trade-offs during plan optimization or to suboptimal trajectory selection, which could have made reducing doses to GU organs and intestines challenging in this study. Therefore, more optimal non-coplanar beam trajectories may allow simultaneous dose reduction to the rectum, bladder, and intestines. This matter should be explored more thoroughly in subsequent research.

This study has several limitations. First, the Conv-VMAT plans were generated by multiple planners in clinical practice, while the BROAD-RT and SF-VMAT plans were developed by a single individual. Therefore, the planners’ proficiency in optimizing VMAT plans may have influenced the study results. Second, the treatment planning systems utilized in the Conv-VMAT (Eclipse) and BROAD-RT/SF-VMAT (Raystation) plans were different, potentially impacting the study. Third, the optimization of gantry trajectory and ring angle for BROAD-RT and SF-VMAT plans has not been thoroughly explored. Since the ideal gantry trajectory and ring angle are contingent on the anatomical locations of the target and OARs, further validation of the modulation point and ring angle combination is necessary. Fourth, the small sample size (n = 10) limits the generalizability of the findings, and larger studies are warranted to validate these results. Finally, planning time and computational resource usage were not systematically recorded in this study. Future investigations should evaluate these parameters to better assess the clinical feasibility of each technique.

In conclusion, for patients with PCa whose PTV is near the small or large intestine, non-coplanar VMAT plans using OXRAY may improve the PTV-base coverage and decrease rectal dose compared with conventional coplanar VMAT plans, while adhering to dose constraints of the small or large intestines.

Statement of Ethics

The contents of the study, including the investigation procedure and the handling of patient information, were approved by the institutional review board of the National Cancer Center Hospital East (IRB No. 2018-076). Informed consent was not required for this planning study on anonymized patient data.

Conflict of Interest

KH and KT received honoraria for lectures from Hitachi, Ltd (Tokyo, Japan).

Acknowledgments

The authors wish to express sincere gratitude to Yuichi Nagai, general manager of the Radiation Technology Department, for supporting the research facilities and providing the environment for conducting this research. We would like to express our deep gratitude to Takaki Ariji and Hajime Oyoshi, Chief of Radiation Technology, for sharing his knowledge and skills and providing guidance in the treatment planning.

Funding

None.

Author Contributions

Conceptualization, KM; Data curation, HK; Formal analysis, KT, MW; Investigation, KH; Methodology, KM; Project administration, KT; Resources, KM, KT, HH, MN; Supervision, MI; Visualization, KH, KT; Writing of the original draft, KH, KM, KT; Writing of the review and editing, MW, HH, MN, MI.

Data Availability Statement

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Supplementary Materials

Supplementary materials can be found via https://doi.org/10.3857/roj.2025.00332.

Supplementary Table 1.

Parameters used for normal tissue complication evaluation

roj-2025-00332-Supplementary-Table-1.pdf

Supplementary Table 2.

Comparison of normal tissue complication between three plans

roj-2025-00332-Supplementary-Table-2.pdf

Fig. 1.

(A) Example of biaxially rotational dynamic radiation therapy arc design showing 11 modulation points for dynamic ring angle adjustment. (B) Arc trajectory illustrating non-coplanar beam paths used to minimize collision risk and improve planning target volume–base coverage.

Table 1.

Patient characteristics

Patient No.	Age (year)	Gleason score	PSA	T category^a)	PTV (cm³)	Distance of PTV to intestine (cm)
1	84	4 + 4	13.3	2a	112.9	<0.1
2	83	4 + 3	6.3	2a	137.1	0.4
3	86	5 + 4	12.9	3b	85.1	0.7
4	62	3 + 4	4.6	2a	72.7	<0.1
5	68	3 + 4	5.8	2c	74.5	<0.1
6	80	4 + 3	26.8	2a	97.4	0.5
7	59	4 + 4	17.0	3a	96.1	<0.1
8	78	4 + 5	22.6	3a	71.3	0.5
9	84	5 + 5	51.7	2c	76.6	0.2
10	84	4 + 5	6.1	2c	83.8	0.7

PSA, prostate-specific antigen; PTV, planning target volume.

^a)TNM Classification of Malignant Tumours, 8th edition [13].

Table 2.

Clinical goals for treatment planning

Structure	Parameter	Clinical goal
PTV	D99%	>93% prescribe dose
	D95%	>100% prescribe dose
	D50%	<105% prescribe dose
	Maximum dose	<108% prescribe dose
Bladder	V34 Gy	<50%
	V55 Gy	<25%
Rectum	V17 Gy	<50%
	V34 Gy	<35%
	V55 Gy	<17%
	V58 Gy	<10%
	V63 Gy	<3%
Small intestine	V43 Gy	<0.5 cm³
Large intestine	Maximum dose	<55 Gy

PTV, planning target volume; DX%, dose received to X% of the structure's volume; VX%, the percentage of the volume that received XGy or more.

Table 3.

Comparison dose indices between three plans

Dose indices	Mean ± SD			p-value
Dose indices	BROAD-RT	SF-VMAT	Conv-VMAT	BROAD-RT vs. Conv	SF vs. Conv	BROAD-RT vs. SF
PTV
D98 (%)	97.43 ± 0.59	97.87 ± 0.51	97.76 ± 0.61	0.306	0.682	0.140
D95 (%)	99.86 ± 0.45	99.88 ± 0.44	102.61 ± 0.57	0.437	0.383	0.945
D50 (%)	102.95 ± 0.37	102.85 ± 0.13	102.61 ± 0.57	0.125	0.246	0.487
D2 (%)	104.60 ± 0.39	104.76 ± 0.32	105.16 ± 0.56	0.042	0.115	0.323
CI	0.86 ± 0.03	0.84 ± 0.04	0.94 ± 0.03	<0.001	<0.001	0.084
HI	7.17 ± 0.64	6.89 ± 0.79	7.40 ± 1.17	0.603	0.285	0.418
PTV-base
D95 (%)	96.26 ± 4.57	98.17 ± 1.32	94.90 ± 5.78	0.047	0.084	0.187
PTV-apex
D95 (%)	100.53 ± 1.85	100.03 ± 2.10	98.82 ± 3.80	0.161	0.371	0.422
Small and large intestine
V45 (cm³)	0.35 ± 0.84	0.41 ± 0.92	0.98 ± 2.18	0.284	0.270	0.381
Bladder
V40 (%)	24.49 ± 9.90	24.32 ± 9.80	25.73 ± 9.78	0.124	0.081	0.512
V50 (%)	17.40 ± 7.24	17.39 ± 7.26	17.72 ± 6.57	0.605	0.616	0.977
D0.03 cc (Gy)	66.43 ± 0.40	66.55 ± 0.49	66.96 ± 0.55	0.064	0.119	0.466
Rectum
V30 (%)	17.00 ± 6.20	14.84 ± 5.57	19.19 ± 4.84	0.033	<0.001	0.001
V40 (%)	11.71 ± 5.17	10.59 ± 4.72	13.26 ± 4.67	0.062	0.002	0.003
V50 (%)	7.98 ± 4.16	7.44 ± 3.84	8.84 ± 3.67	0.149	0.009	0.026
D0.03 cc (Gy)	63.65 ± 1.60	63.67 ± 1.60	65.33 ± 1.06	<0.001	<0.001	0.902
Penile bulb
D90 (Gy)	4.82 ± 3.01	4.65 ± 2.52	2.72 ± 0.53	0.041	0.025	0.400

SD, standard deviation; BROAD-RT, biaxially rotational dynamic radiation therapy; SF, swing-fixed; VMAT, volumetric modulated arc therapy; Conv, conventional; PTV, planning target volume; DX, dose received by the X% of the volume; CI, conformity index; HI, homogeneity index; VX, the percentage of the organ volume that received X Gy or more; D0.03 cc, dose received by the 0.03 cm³ of the volume.

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