VMAT vs eight field IMRT: dosimetric comparison of pelvic radiotherapy for patients with high-risk prostate cancer in terms of bone marrow sparing

Abstract

Objective: although there is no complete consensus on elective pelvic nodal irradiation for patients with - risk prostate cancer, pelvic radiotherapy with androgen ablation has been more commonly used in many centers. An important part of bone marrow (BM) reserve remains in the pelvic radiation treatment field. This study aimed to evaluate and compare the intensity modulated (step-and-shoot IMRT: ssIMRT) and volumetric modulated arc radiotherapy (VMAT) techniques for pelvic radiotherapy in terms of pelvic BM doses. Methods: this study was based on the simulation scan data of 10 patients with prostate cancer as 3-mm slice thickness using a full bladder and rectal balloon. The first phase of the treatment planning prescribed to pelvic lymphatic (46 Gy/2 Gy/fraction). The second phase consisted of the seminal vesicles and prostate (32 Gy/2 Gy/fraction). The PTV margin was 0.4 cm posteriorly due to rectum and 0.6 cm in all other (including PTVlymphatic) directions. Using same target volumes, ssIMR'l with eight angles (225 degrees, 260 degrees, 295 degrees, 330 degrees, 30 degrees, 65 degrees, 100 degrees, 135 degrees) and double arc (182 degrees, -178 degrees arc angle) VMAT were planned for each patient data set. The planning objective was to cover the PTV by at least 95% of the prescribed isodose and CTV by 98% of the prescribed isodose line. No special dose constraint was given for BM sparing. Each technique was compared by using dose volume histograms (DVH) of V5, V10, V20, V30, V40 of the sacral BM (SBM), iliac BM (IBM), and ischium, pubis, and proximal femora (lower pelvis) and femoral BM (IBM). In addition, V20 V30, V40, and V70 for bladder, and V30, V40, V76, and V80 for the rectum, homogeneity index and the monitor units (MU) were evaluated. The two-sided Wilcoxon's test was used for statistical analysis (p<0.05). Results: for the same PTV coverage, VMAT and ssIMRT plans had similar dose distributions for FBM, IBM, SBM, and total BM as well as the other critical structures. However, VMAT plans in comparison to IMRT ensured significantly lower high dose volumes on rectum such as bringing V80 from 1.6% to 0.9% (p=0.01), and provided similar homogeneity index with lowered monitor units (1048 vs. 1591, p=0.018). Conclusion: in this cohort, VMAT plans without a specific constraint for BM are not found to be superior to ssIMRT in terms of BM reserve irradiation; while VMAT could be encouraged for patients with higher rectum doses such as V80.