Purpose: To effectively spare organs-at-risk (OAR) doses using multi-leaf collimator (MLC)’s optimal segments, volumetric modulated arc therapy (VMAT) using a half-beam technique (HF) was devised for whole pelvic radiation therapy (WPRT). The potential dosimetric benefits of VMAT-HF were suggested for clinical application with statistical analysis.
Methods: Fifteen eligible patients diagnosed with anal, vaginal, and cervical cancer were selected. Dosimetric benefits of VMAT-HF, presented with dose conformity and dose–volume parameters in terms of modulation complex score, were compared to those of VMAT optimized using a fully opened field (FF). The left or right side of FF was alternatively opened in VMAT-HF by using a pair of arcs rotating clockwise and counterclockwise. Consequent normal tissue complication probability (NTCP) by reducing the irradiated volumes and delivered doses for OAR were evaluated as well as dose-volume parameters.
Results: Compared to VMAT-FF, VMAT-HF showed superior conformal dose distribution, where the 75% isodose line compactly surrounded the separated regional lymph nodes. Dose conformity was comparable between VMAT-HF and VMAT-FF; however, VMAT-HF achieved the conformal dose distribution using 60%–70% less intensity modulation complexity than that used by VMAT-FF. The small intestine and colon showed a noticeable reduction in the irradiated volumes of up to 35% and 15%, respectively, at an intermediate dose of 20–45 Gy. The small intestine showed statistically significant dose sparing at the volumes that received a dose every 5 Gy from 15 to 45 Gy. Such a dose reduction for the small intestine and colon in VMAT-HF presented significant NTCP reduction than that in VMAT-FF.
Conclusion: VMAT-HF provided more optimal composition of MLC segments to deliver conformal doses without excessive intensity modulation. It achieved distinguishable dose sparing for gastrointestinal OAR in WPRT without sacrificing the target dose conformity. VMAT-HF can be usefully used in reducing OAR toxicities associated with WPRT.