VMAT vs Eight Field Imrt: Dosimetric Comparison of Pelvic Radiotherapy for Patients with High-Risk Prostate Cancer in Terms of Bone Marrow Sparing

Abstract

Although there is no complete consensus on elective pelvic nodal irradiation for patients with high-riskprostate 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 studyaimed to evaluate and compare the intensity modulated (step-and-shoot IMRT: ssIMRT) and volumetricmodulated arc radiotherapy (VMAT) techniques for pelvic radiotherapy in terms of pelvic BM doses.METHODSThis study was based on the simulation scan data of 10 patients with prostate cancer as 3-mm slicethickness using a full bladder and rectal balloon. The first phase of the treatment planning prescribed topelvic 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, ssIMRT with eight angles (225°, 260°,295°, 330°, 30°, 65°, 100°, 135°) and double arc (182°, -178° arc angle) VMAT were planned for eachpatient data set. The planning objective was to cover the PTV by at least 95% of the prescribed isodoseand 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, V40of the sacral BM (SBM), iliac BM (IBM), and ischium, pubis, and proximal femora (lower pelvis) andfemoral BM (FBM). In addition, V20 V30, V40, and V70 for bladder, and V30, V40, V76, and V80 forthe rectum, homogeneity index and the monitor units (MU) were evaluated. The two-sided Wilcoxon’stest was used for statistical analysis (p