To evaluate the large and complicated irradiated field, patients with inguinal lymph node in the irradiation field were enrolled in present study. A total of ten patients with a diagnosis of cervical, anal, and vaginal cancer was selected. Cervical, anal, and vaginal cancer were 4, 4, and 2, respectively. The four patients with cervical cancer were involved to lower 1/3 vagina. Treatment decision and radiotherapy protocol reflected National Comprehensive Cancer Network guidelines (29, 30). The patient and tumor characteristics were summarized in Table 1. For cervical and vaginal cancer patients, WPRT was followed by high-dose rate brachytherapy.
All patients underwent a computed tomography simulation in a supine position with arms on the chest. This retrospective dosimetric study was approved by the Institutional Review Board of the Dongguk University Medical Center (110757-201711-HR-02-01), and informed consent in writing was obtained from individual patient. All information was anonymous prior to analysis.
Table 1. Characteristics of patients, tumors, and treatment regimen selected for whole pelvic radiation therapy.
No Sex/Age Origin Stage Pathology Treatment aim
WPRT Prescribed Dose (Gy)
Chemotherapy
1 M/59 Anus T2N0 SCC Adjuvant 50 -
2 F/75 Anus Recurrent SCC Adjuvant 50 -
3 F/78 Anus T2N1 SCC Definitive 50 FMC
4 M/88 Anus T2N0 SCC Definitive 41.4 FMC
5 F/51 Vagina T2N0 AC Definitive 45 -
6 F/61 Vagina T1N0 SCC Definitive 45 -
7 F/67 Cervix T3aN0 SCC Definitive 45 WC
8 F/71 Cervix T3bN0 SCC Definitive 45 WC
9 F/89 Cervix T3aN0 AC Definitive 45 -
10 F/86 Cervix T3aN1 SCC Definitive 45 -
WPRT, whole pelvis radiotherapy; SCC, squamous cell carcinoma; FMC, 5-fluorouracil, mitomycin C; AC, adenocarcinoma; WC,weekly cisplatin.
B. Target delineation
Clinical target volume (CTV) and pelvic lymph nodes were delineated according to the consensus guidelines (31-33). Treatment volume for gynecological cancer patients includes the tumor involving the lower third of the vagina as well as tumor bed, parametria, uterosacral ligaments, and pelvic lymph nodes. Obturator, presacral, and internal iliac node chains come under the pelvic lymph nodes in all cases. External and perirectal nodes are added for gynecological and anal cancer patients, respectively. The PTV was created by adding a 5 mm margin to the CTV. Small bowel, bladder, rectum, and femur heads, were contoured as OAR. Each PTV received a prescribed dose of 50 Gy in 25 fractions. Dose constraints were applied for OAR as per Radiation Therapy Oncology Group (RTOG) protocols for each treatment site (9, 10, 32).
C. Volumetric modulated arc therapy planning
All VMAT plans consisted of four full arcs alternately rotating clockwise and counterclockwise in Eclipse (ver. 10.0, Varian Medical Systems, Palo Alto, CA) and the normal tissue optimization. The collimator angles less than ± 40° were used for each arc to improve dose conformity as well as to minimize the interleaf leakage and the tongue-and-groove effect in VMAT. Meanwhile, dose distributions of the VMAT plans were optimized by using different field sizes. As shown in figure 1, an optimized VMAT plan (FF-VMAT) is created with a field size (FF) which is enough to fully cover an entire PTV, but less than 15 cm with X-jaws in consideration of the maximum leaf span of the Millenium MLC. The opening of the FF in a superior-inferior direction was adjusted to sufficiently include the PTV plus a margin less than 1cm. The field size was reviewed using beam’s eye views at all different gantry angles of the arcs. A new VMAT plan (HF-VMAT) is optimized with half size of the FF. The optimized dose was delivered by opening the half and the other half of the FF for two arcs rotating clockwise and counter clockwise, respectively, as in Fig.1(c). In addition, a modified field size (MFF) 2 cm reduced from the each X-jaw of the FF is employed for a VMAT plan (MFF-VMAT) to evaluate dose conformity depending on MLC segments and sequences. Dosimetric benefits among three VMAT plans were analyzed, when a 95% of the PTV was covered by the prescribed dose with the same dose
constraints applied for OAR. The dose distributions for each treatment site were calculated by Analytical Anisotropic Algorithm (ver. 10.0, Varian Medical Systems) and progressive resolution optimizer (ver. 10.0, Varian Medical Systems).
Figure 1. Beam's eye views of three plans. (a) FF-VMAT, an optimized VMAT plan using a beam fully open to cover the planning target volume (b) MFF-VMAT, an optimized VMAT plan using a 2-cm reduced field from each side of Jaw rather than the fully open field used for FF-VMAT (c) HF-VMAT, an optimized VMAT plan using a half beam.
D. Analysis of dosimetric factors
As the dose conformity for the PTV are comparable among the VMAT plans optimized with different field sizes, dose sparing for OAR was evaluated with dose-volume histogram (DVH) as well as relevant dose-volume parameters associated with estimating complications for individual OAR. The normal organ dosimetric parameters for small bowel, bladder, rectum and femur head were compared according to radiotherapy planning technique. The DVH of all plans obtained, and dosimetric parameters, such as, Vdose and mean dose, were calculated from DVH. The Vdose was defined as the percentage volume that received at least the dose. Maximum (Dmax) and mean (Dmean) doses, and the dose (D2cc) delivered to the most irradiated 2cc volume were analyzed for OAR. For small bowel and colon, each volume receiving the doses from 45 Gy to 5 Gy at a dose interval of 10 Gy was evaluated, especially including the V15Gy regarded as an important parameter to predict gastrointestinal toxicities (34). Because rectum is included in the treatment volume of anal cancer, rectal dose was analyzed in 6 patients except anal cancer. In addition, to evaluate the potential radiobiological impacts on OAR depending on dose distributions in WPRT, equivalent uniform dose (EUD) and normal tissue complication probability (NTCP) were calculated using Emami-Burman parameters. To present the sensitivities and variability of NTCP in terms of analytic models, Lyman-Kutcher-Burman (LKB) and EUD-based log-logistic models were adopted (35). The effect of varying alpha-beta ratios for acute and late complications on EUD and NTCP were discussed accordingly for individual OAR. The alpha-beta ratios and required biological parameters to calculated NTCP were summarized in Table 2 (35-40).
Table 2. Radiobiological parameters to calculate the equivalent uniform dose and normal tissue complication probabilities for organs at risk in whole pelvic radiation therapy
LKB, Lyman-Kutcher-Burman; TD50: The 50% tolerance dose of the whole organ
E. Beam complexity of volumetric modulated arc therapy plans
The traveling distances between control points and segment shapes of the VMAT arcs can affect complexity of the beam intensity modulation. The modulation complexity score (MCS) using variabilities of leaf sequences (LSV) and segment area (AAV) was adopted to comprehensively present the plan complexity across all segments (34). It is formulated using equation [1] by reflecting each segment weight to the corresponding relative arc weight,
𝑴𝑪𝑺𝑽𝑴𝑨𝑻 =
∑𝑵𝒂𝒓𝒄=𝟏∑(𝒏−𝟏)𝒄𝒑=𝟏 [(𝑨𝑨𝑽𝒄𝒑𝒂𝒓𝒄+𝑨𝑨𝑽𝟐 𝒄𝒑+𝟏𝒂𝒓𝒄 ) × (𝑳𝑺𝑽𝒄𝒑𝒂𝒓𝒄+𝑳𝑺𝑽𝟐 𝒄𝒑+𝟏𝒂𝒓𝒄 ) × ((𝑴𝑼𝒄𝒑+𝟏𝒂𝒓𝒄𝑴𝑼−𝑴𝑼𝒂𝒓𝒄 𝒄𝒑𝒂𝒓𝒄))]
eq. [1]
The parameters of n and N mean the total number of control points per each arc and the total number of arcs used in each VMAT plan. The LSVcp and the AAVcp for each control point are calculated using Eq. [2] and [3], respectively, where m is the number of MLC leaves which move underneath the unblocking portion of the field defined by X and Y jaws for each control point.
𝐿𝑆𝑉𝑐𝑝=∑𝑚𝑖=1(𝑝𝑜𝑠𝐿− |(𝑝𝑜𝑠𝑖+1𝐿 − 𝑝𝑜𝑠𝑖𝐿)|) (𝑚 − 1) × 𝑝𝑜𝑠𝐿𝑐𝑝
×∑𝑚𝑖=1(𝑝𝑜𝑠𝑅 − |(𝑝𝑜𝑠𝑖+1𝑅 − 𝑝𝑜𝑠𝑖𝑅)|)
(𝑚 − 1) × 𝑝𝑜𝑠𝑅𝑐𝑝 eq. [2]
𝐴𝐴𝑉𝑐𝑝 = ∑𝑚𝑖=1(𝑝𝑜𝑠𝑖𝐿− 𝑝𝑜𝑠𝑖𝑅)
𝑚 × (𝑝𝑜𝑠𝐿𝑎𝑟𝑐− 𝑝𝑜𝑠𝑅𝑎𝑟𝑐) eq. [3]
The 𝑝𝑜𝑠𝑖𝐿presents the i-th leaf position of the MLC at the left bank. The 𝑝𝑜𝑠𝐿𝑐𝑝 and 𝑝𝑜𝑠𝐿𝑎𝑟𝑐mean the furthest position of the MLC leaf from the isocenter among the all MLC leaves constituting a shape of the individual segment and across the all the control points of an individual arc. The R denotes the MLC leaves on the right bank. Thus, while the LSV presents variability of traveling distances of the MLC leaf sweeping the each set of control points relative to the maximum lateral separation from the isocenter for the each side, the AAV presents the complexity of the separation of each pair of the MLC leaves relative to the maximum separation created among all of MLC leaves across all segments consisting of the arc.
F. Dose homogeneity evaluation
The conformity number (CN), and homogeneity index (HI) were evaluated to know the dose homogeneity.
The CN was defined as: CN = (VT,RI/VT)/(VT,RI/VRI). VT,RI is the target volume covered by the reference isodose. VT is the total target volume, and VRI is the corresponding volume to the reference isodose (41).
To distinguish statistically significant dosimetric effects by the different planning techniques for WPRT using VMAT, Wilcoxon signed rank test was performed using statistical analysis software (SPSS, version 20, SPSS Inc., Chicago, IL). The p-value less than 0.05 were considered statistically significant.
III. Results
A. Normal organ dosimetry
When the PTV has met the same goal for the dose coverage in VMAT plans using three different techniques, dose sparing for the bladder and the small bowel was noticeable in an axial and a coronal view (figure 2).
Figure 2. Comparison of irradiated isodose distributions according to three plans. (a) FF-VMAT (b) MFF-VMAT (c) HF-VMAT.
1. Small bowel
Mean small bowel dose of HF-VMAT plan was significantly lower than FF-VMAT plan (29.57 vs 32.89, p<0.05). V30 and V40 to small bowel were also significantly lower (V30:
46.43 vs 62.38, V40: 21.44 vs 28.72, p<0.05). Although it was no statistically significant between HF-VMAT and MFF-VMAT plan, HF-VMAT plan showed the improved dosimetric values of small bowel. V10 to small bowel was 91.74, 91.61, and 91.76 in FF-VMAT, MFF-FF-VMAT, and HF-VMAT plan, respectively. The small bowel presented a 13-14% reduced volume at V35 and V25. Even if the volume difference was little at V15, HF-VMAT plan brought about smaller volumes as compared to the other HF-VMAT plans. The mean value of 10 patients according to three VMAT plans is shown in figure 3. And, more detailed DVH values are shown in table 3.
Figure 3. The mean dose-volume histogram of small bowel.
Table 3. Comparison of dose-volume parameters for small bowel in VMAT plans using three different techniques. The mean dose (Dmean), maximum dose (Dmax), dose delivered to 2cc of the most irradiated organ volume (D2cc) were evaluated for small bowel.
Variable FF-VMAT MFF-VMAT HF-VMAT p-valuea) p-valueb) p-valuec)
Dmean (Gy) 32.89±3.69 30.68±3.57 29.57±2.28 0.005 0.005 0.074 Dmax (Gy) 55.29±0.56 55.12±34.44 55.39±4.52 0.508 0.114 0.074 D2cc (Gy) 53.91±0.84 53.89±0.29 53.74±17.01 0.169 0.760 0.445 V5 (%) 97.39±30.86 96.56±2.56 96.59±2.41 0.066 0.008 0.953 V10 (%) 91.74±5.09 91.61±5.24 91.76±5.07 0.646 0.173 0.683 V15 (%) 89.16±6.10 89.03±6.29 88.37±5.56 0.017 0.074 0.074 V20 (%) 85.50±8.44 83.35±8.90 79.95±5.62 0.005 0.013 0.037 V25 (%) 78.10±12.60 70.28±12.48 65.01±7.13 0.005 0.005 0.173 V30 (%) 62.38±14.16 50.77±13.88 46.43±9.17 0.009 0.005 0.285 V35 (%) 45.29±11.50 34.36±10.17 31.10±6.65 0.005 0.005 0.074 V40 (%) 28.72±7.51 24.27±8.68 21.44±4.94 0.005 0.047 0.059 V45 (%) 18.72±5.89 15.70±5.18 14.18±3.44 0.009 0.005 0.047
Values are presented as mean ± standard deviation, a) Comparison of FF-VMAT and HF-VMAT, b) Comparison of FF-VMAT and MFF-VMAT, c) Comparison of MFF-VMAT and HF-VMAT
2. Bladder
Mean bladder dose of HF-VMAT plan was significantly lower than FF-VMAT and MFF-VMAT plan (33.64 vs 38.63 vs 37.21, p<0.05). V30 and V40 to bladder were also significantly lower (V30: 62.57 vs 89.15 vs 86.20, p<0.05, V40: 25.69 vs 37.74 vs 31.27).
V10 to bladder was 100 in all plans. The bladder showed highest volume reduction of 12%
at V40 and 27% at V30. Although MVMAT plan showed better results than the FF-VMAT plan at > 35Gy, it was not as good as HF-FF-VMAT plan. The mean value of 10 patients according to three VMAT plans is shown in figure 4. More detailed DVH values are shown in table 4.
Figure 4. The mean dose-volume histogram of bladder.
Table 4. Comparison of dose-volume parameters for bladder in VMAT plans using three different techniques. The mean dose (Dmean), maximum dose (Dmax), dose delivered to 2cc of the most irradiated organ volume (D2cc) were evaluated for bladder.
Variable FF-VMAT MFF-VMAT HF-VMAT p-valuea) p-valueb) p-valuec)
Dmean (Gy) 38.63±2.25 37.21±2.21 33.64±3.86 0.005 0.017 0.005 Dmax (Gy) 53.85±1.18 53.41±1.42 53.53±1.52 0.721 0.203 0.646 D2cc (Gy) 51.35±1.87 50.88±1.87 50.15±2.14 0.005 0.019 0.013
V5 (%) 100 100 100 - - -
V10 (%) 100 100 99.44±1.76 0.317 - 0.317
V15 (%) 100 100 97.86±5.12 0.068 - 0.068
V20 (%) 100 100 91.69±9.99 0.018 - 0.018
V25 (%) 99.87±0.35 99.53±0.97 82.54±17.25 0.018 0.273 0.018 V30 (%) 89.15±12.24 86.20±9.29 62.57±23.62 0.011 0.260 0.017 V35 (%) 66.61±16.61 55.98±14.48 42.13±13.20 0.005 0.022 0.005 V40 (%) 37.74±12.40 31.27±11.10 25.69±10.68 0.005 0.013 0.005 V45 (%) 20.11±10.04 17.41±8.86 14.28±7.81 0.005 0.013 0.005
Values are presented as mean ± standard deviation, a) Comparison of FF-VMAT and HF-VMAT, b) Comparison of FF-VMAT and MFF-VMAT, c) Comparison of MFF-VMAT and HF-VMAT
3. Colon
Mean colon dose of HF-VMAT plan was significantly lower than FF-VMAT plan (23.45 vs 25.27, p<0.05). V30 and V40 to small bowel were also significantly lower (V30: 34.85 vs 40.01, V40: 20.22 vs 26.34, p<0.05). Colon showed highest volume reduction at V35up to 8%. The HF-VMAT plan achieved a 4-5% volume decrease for colon at V45 and V35.
Dose sparing for the volumes at the same percentage led to 2 Gy less Dmean for colon.
MFF-VMAT plan showed better results than the FF-VMAT plan at > 25Gy and it was statistically significant. However the MFF-VMAT plan was not as good as HF-VMAT plan.
More detailed DVH values are shown in table 5.
Table 5. Comparison of dose-volume parameters for colon in VMAT plans using three different techniques. The mean dose (Dmean), maximum dose (Dmax), dose delivered to 2cc of the most irradiated organ volume (D2cc) were evaluated for colon.
Variable FF-VMAT MFF-VMAT HF-VMAT p-valuea) p-valueb) p-valuec)
Dmean (Gy) 25.27±6.81 23.98±6.34 23.45±6.29 0.005 0.005 0.037 Dmax (Gy) 55.11±0.97 55.12±0.34 54.95±0.46 0.959 0.508 0.959 D2cc (Gy) 52.85±16.73 53.89±0.29 52.31±1.21 0.285 0.114 0.799 V5 (%) 85.98±15.11 85.87±15.18 84.95±15.17 0.074 0.799 0.074 V10 (%) 74.20±15.81 72.49±16.40 71.51±16.10 0.047 0.721 0.074 V15 (%) 65.37±16.89 64.71±17.19 63.12±18.32 0.114 0.333 0.059 V20 (%) 57.91±18.21 55.71±18.56 53.90±2011 0.028 0.093 0.285 V25 (%) 48.45±19.39 44.35±19.99 43.74±18.81 0.005 0.005 0.386 V30 (%) 40.01±19.54 36.60±17.87 34.85±16.17 0.005 0.013 0.093 V35 (%) 35.32±19.65 34.36±10.17 27.68±12.48 0.005 0.005 0.037 V40 (%) 26.34±12.33 20.89±9.27 20.22±8.79 0.005 0.005 0.093 V45 (%) 16.80±8.83 15.70±5.18 13.16±5.98 0.005 0.005 0.333
Values are presented as mean ± standard deviation, a) Comparison of FF-VMAT and HF-VMAT, b) Comparison of FF-VMAT and MFF-VMAT, c) Comparison of MFF-VMAT and HF-VMAT
4. Rectum
Because rectum is included in the treatment volume of anal cancer, six patients except anal cancer were analyzed. Mean rectal dose of HF-VMAT plan was significantly lower than FF-VMAT plan (38.84 vs 40.02, p<0.05). V30 and V40 to small bowel were also significantly lower (V30: 82.84 vs 88.04, V40: 43.82 vs 49.69, p<0.05). HF-VMAT and MFF-VMAT plans showed improved OAR compared to FF-VMAT plan, and it was statistically significant differences at > 30Gy. However the difference was small. More detailed DVH values are shown in table 6.
Table 6. Comparison of dose-volume parameters for rectum in VMAT plans using three different techniques. The mean dose (Dmean), maximum dose (Dmax), dose delivered to 2cc of the most irradiated organ volume (D2cc) were evaluated for rectum.
Variable FF-VMAT MFF-VMAT HF-VMAT p-valuea) p-valueb) p-valuec) Anal cancer
Values are presented as mean ± standard deviation, a) Comparison of FF-VMAT and HF-VMAT, b) Comparison of FF-VMAT and MFF-VMAT, c) Comparison of MFF-VMAT and HF-VMAT
B. Normal tissue complication probability
The NTCP values of small bowel, colon, bladder and rectum were compared according to three VMAT plans. In small bowel and colon, HF-VMAT and MFF-VMAT plans showed improved NTCP compared to FF-VMAT plan, and it was statistically significant.
HF-VMAT plan reduced the NTCP for small bowel toxicity from 12 - 13% to 8 - 9% than FF-VMAT plan. And, colon toxicity was reduced from 3.2% to 1.8%. Although HF-VMAT plan showed improved NTCP compared to MFF-VMAT plan, the difference was small.
More detailed NTCP values are shown in table 7.
Table 7. Evaluation of normal tissue complication probabilities and equivalent-uniform dose (EUD) with statistical significance. The NTCP was evaluated using two analytic models including Lyman-Kutcher-Burman (LKB) and log-logistic model.
Variable Variable FF-VMAT MFF-VMAT HF-VMAT p-valuea) p-valueb) p-valuec)
Small bowel LKB (%) 12.81±4.24 10.25±3.69 8.98±2.42 0.007 0.005 0.047
Values are presented as mean ± standard deviation, a) Comparison of FF-VMAT and HF-VMAT, b) Comparison of FF-VMAT and MFF-VMAT, c) Comparison of MFF-VMAT and HF-VMAT
C. Target coverage
HF-VMAT plan showed superiority compared with FF-VMAT plan in CN (0.90 vs 0.86, p<0.05). The CN of MFF-VMAT plan was also increased than FF-VMAT (0.89 vs 0.86, p<0.05). And, it was similar between HF-VMAT and MFF-VMAT (0.90 vs 0.89). There was no statistically significant in the HI. The mean values and comparison are provided in Table 8. And, the DVH of PTV to three VMAT plans is shown in figure 5.
Table 8. Mean conformity number (CN) and homogeneity index (HI) according to three plans.
Variable FF-VMAT MFF-VMAT HF-VMAT p-valuea) p-valueb) p-valuec)
CN 0.86±0.01 0.89±0.02 0.90±0.03 0.007 0.160 0.005
HI 35.90±2.20 34.87±2.64 36.07±4.31 0.575 0.052 0.139
Values are presented as mean ± standard deviation, a) Comparison of FF-VMAT and HF-VMAT, b) Comparison of FF-VMAT and MFF-VMAT, c) Comparison of MFF-VMAT and HF-VMAT
Figure 5. The mean dose-volume histogram of planning target volume.
D. Modulation index
HF-VMAT and MFF-VMAT plans showed improved MI compared to FF-VMAT plan, it showed a statistically significant differences. The MI values of FF-VMAT and HF-VMAT plans were respectively 366.95 and 306.31, which was about 20% lower. The MI value of MFF-VMAT plan was similar with HF-VMAT plan. The mean values and comparison are provided in Table 9.
Table 9. Mean modulation index (MI) according to three plans.
Variable FF-VMAT MFF-VMAT HF-VMAT p-valuea) p-valueb) p-valuec)
MI (MCS) 366.95±92.28 307.88±74.11 306.31±62.95 0.017 0.005 0.959
Values are presented as mean ± standard deviation, a) Comparison of FF-VMAT and HF-VMAT, b) Comparison of FF-VMAT and MFF-VMAT, c) Comparison of MFF-VMAT and HF-VMAT
IV. Discussion
The elective WPRT including pelvic lymph node irradiation is regarded as a standard treatment regimen for intermediate or high-risk rectal and gynecological cancers. Although the benefits of WPRT has been controversial for the locally advanced prostate cancer patients (43), its role and gain have been reexamined with consideration of interaction between RT and adjuvant androgen deprivation therapy (ADT) according to the duration and timing of the hormone therapy and the field sizes of RT. Long-term outcomes were suggested in terms of improved free from progression and biochemical failure in ADT plus RT. In addition, recent results as per a new protocol were reported treatment gains in short-term ADT plus pelvic lymph node irradiation using IMRT (44, 45).
WPRT can bring out clinical merits with other treatment modalities, or clever radiation dose delivery techniques such as simultaneously integrated boost using a different dose scheme can be useful (46-48). However, acute and late radiation-induced complications have been significant concerns due to the use of a large size of the field (16-18). When large and irregular irradiated field is required to cover target volumes and regional lymph nodes, the radiation exposure of normal organ is inevitable. To achieve the target dose coverage with normal organ sparing in large and irregular anatomical geometries, VMAT is one of the useful dose delivery methods (21-23). Optimal fluence, which is created by MLC segments at individual numerous beam angles going through the arc trajectories in VMAT, can effectively reduce the radiation doses of normal organs.
The present study was conducted to overcome the limitation of MLC motion in wide field VMAT plan. In order to compensate for MLC leakage and non-blocking phenomena, HF-VMAT and MVMAT plans were designed and compared with conventional FF-VMAT. Anal cancer and vaginal cancer patients were enrolled due the benefit of HF-VMAT plan was expected to be more pronounced in WPRT patients with inguinal field.
Studies about HF-VMAT plan were already published, however they were very rare. Lai et al. reported that HF-VMAT plan was more effective for normal organ saving and dose distribution than FF-VMAT plan in breast cancer irradiation. HF-VMAT plan showed
improved dosimetry in ipsilateral lung and heart than FF-VMAT plan (27). There is also a study that the HF-VMAT plan showed an improved dose distribution in radiotherapy of scalp and lower neck node (49, 50). Previous studies have shown that RT field is large and irregular, like breast cancer or head and neck cancer. In present study, we analyzed the effect of VMAT plan in patients undergoing WPRT with inguinal node, and the HF-VMAT plan showed some dosimetric benefit compared to FF-HF-VMAT plan. We also compared MFF-VMAT plan with HF-VMAT and FF-VMAT plan in present study. The MFF-VMAT plan is a plan that X field size is fixed at the maximum equipment size (14 cm), in order to optimize the MLC modulation. Most of the parameters were not as good as HF-VMAT plan, but showed better results than FF-VMAT plan. This means that the adjustment of irradiation field size is possible to improve the dose distribution by the increasing of MLC modulation.
The small bowel requires careful treatment planning, because it is an organ that can cause fatal side effects, such as perforation, even at low radiation dose (51, 52). Especially, for gynecologic malignancies, the possibility of side effects may be increased because the small bowel can come down to the empty space after hysterectomy. Even in older patients, there is a high possibility that the small bowel may become deformed due to the elasticity of the abdominal wall or muscle weakness. Ahamad et al. analyzed the effect of IMRT in cervical cancer patient undergoing postoperative radiotherapy, and significantly less small bowel was irradiated by IMRT than conformal radiotherapy for doses greater
The small bowel requires careful treatment planning, because it is an organ that can cause fatal side effects, such as perforation, even at low radiation dose (51, 52). Especially, for gynecologic malignancies, the possibility of side effects may be increased because the small bowel can come down to the empty space after hysterectomy. Even in older patients, there is a high possibility that the small bowel may become deformed due to the elasticity of the abdominal wall or muscle weakness. Ahamad et al. analyzed the effect of IMRT in cervical cancer patient undergoing postoperative radiotherapy, and significantly less small bowel was irradiated by IMRT than conformal radiotherapy for doses greater