(P068) VMAT vs Eight-Field IMRT: Dosimetric Comparison of Pelvic Radiotherapy for High-Risk Prostate Cancer Patients in Terms of Bone Marrow Sparing

Yasemin Bolukbasi, Vildan Alpan, Yucel Saglam, Ugur Selek; American Hospital, MD Anderson Radiation Treatment Center, Istanbul

PURPOSE: Although there is no complete consensus on elective pelvic nodal irradiation for high-risk prostate cancer patients, pelvic radiotherapy with androgen ablation have been more commonly used in many centers. An important part of bone marrow (BM) reserve remains in the pelvic radiation treatment field. We intend to evaluate and compare the intensity-modulated radiotherapy (step and shoot IMRT [ssIMRT]) and volumetric arc radiotherapy (VMAT) techniques for pelvic radiotherapy in terms of pelvic bone marrow doses.

MATERIALS AND METHODS: This study was based on the simulation scan data of 10 prostate cancer patients as 3-mm slice thickness using a full bladder and rectal balloon. The first phase of the treatment planning was prescribed to the pelvic lymphatics, prostate, and seminal vesicles (46 Gy, 2 Gy/fraction); then, the second phase consisted of the seminal vesicles and prostate (32 Gy, 2 Gy/fraction). Planning target volume (PTV) margin was 0.4 cm posteriorly due to the rectum and 0.6 cm in all other directions. Using the same target volumes, ssIMRT with eight angles (225°, 260°, 295°, 330°, 30°, 65°, 100°, and 135°) and double-arc (182°–178° arc angle) VMAT were planned for each patient dataset. The planning objective was to cover the PTV by at least 95% of the prescribed isodose and clinical target volume (CTV) by 98% of the prescribed isodose line. No special dose constraint was given for bone marrow sparing. Each technique was compared by using dose-volume histograms (DVHs) of V5, V10, V20, V30, and V40 of the sacrum BM, iliac BM, ischium, pubis, proximal femora (lower pelvis), and femoral BM; V20, V30, V40, and V70 for the bladder; and V30, V40, V76, and V80 for the rectum, homogeneity index, and the monitor units (MU). Two-sided Wilcoxon’s test was used for statistical analysis (P < .05).

RESULTS: For the same PTV coverage, VMAT and ssIMRT plans had similar dose distributions for femur, iliac, sacrum, and total BM, as well as the other critical structures. However, VMAT plans in comparison with IMRT ensured significantly lower high-dose volumes on the rectum, such as bringing V80 from 1.6% to 0.9% (P = .01), and provided similar homogeneity index with lowered MUs (1,048 vs 1,591; P = .018).

CONCLUSION: In this cohort, VMAT plans without a specific constraint for BM were 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.

Proceedings of the 97th Annual Meeting of the American Radium Society- americanradiumsociety.org