2Department of Radiation Oncology, American Hospital-Koç University Hospital, İstanbul-Turkey DOI : 10.5505/tjo.2017.1591 OBJECTIVE
Although there is no complete consensus on elective pelvic nodal irradiation for patients with high-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, ssIMRT with eight angles (225°, 260°,
295°, 330°, 30°, 65°, 100°, 135°) and double arc (182°, -178° 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 (FBM). 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.