A dosemetric and radiobiological impact of VMAT and 3DCRT on lumbosacral plexuses, an underestimated organ at risk in cervical cancer patients

Background: The purpose of this study was to evaluate dosimetric and radiobiological difference between volumetric modulated arc therapy (VMAT) and 3-dimensional conformal radiotherapy (3DCRT) in organ at risk (OAR) lumbosacral plexus (LSP) in cervical cancer patients. Materials and methods: 30 patients of cervical cancer who were treated using 3DCRT or VMAT along with chemotherapy followed by brachytherapy were enrolled. LSP was delineated retrospectively. Dosimetric and radiobiological difference was evaluated. Patients were followed for radiation induced lumbosacral plexopathy (RILSP). Results: Median follow-up was 12 months (3-16 months). 53.3% of patients were treated by 3DCRT and 46.7% by VMAT. The mean (+/- SD) LSP volume: 119.03 +/- 15 cm(3). The mean volume percentages (%) of the LSP: V5, V10, V20, V30, V40, V50, V55, and V60 were 100%, 99.8%, 99.2%, 94.3%, 84.03%, 59.7%, 0%, 0%, respectively. All patients received doses to the LSP in excess of 50 Gy, one patient received 55 Gy. A statistically significant difference was observed in the median value of V20, V30, V40, V50, D50, P2, P4, P7, P8, P9, and P10 across two different techniques of radiotherapy - VMAT and 3DCRT. None of the patients presented with RILSP. NTCP value was less in VMAT plans compared to 3DCRT, which is also statistically significant. Conclusion: RILSP is a rare and often refractory complication of pelvic radiotherapy. Advance radiotherapy technique with proper OAR delineation and constraint can prevent the occurrence of RILSP. VMAT has potential benefits for the probability of dose reduction in LSP. Further studies are required focusing on dose distribution in LSP-OAR and radiotherapy modality.