Target tailoring and proton beam therapy to reduce small bowel dose in cervical cancer radiotherapy

The aim of the study was to investigate the potential clinical benefit from both target tailoring by excluding the tumour-free proximal part of the uterus during image-guided adaptive radiotherapy (IGART) and improved dose conformity based on intensity-modulated proton therapy (IMPT). The study included planning CTs from 11 previously treated patients with cervical cancer with a > 4-cm tumour-free part of the proximal uterus on diagnostic magnetic resonance imaging (MRI). IGART and robustly optimised IMPT plans were generated for both conventional target volumes and for MRI-based target tailoring (where the non-invaded proximal part of the uterus was excluded), yielding four treatment plans per patient. For each plan, the V-15Gy, V-30Gy, V-45Gy and D-mean for bladder, sigmoid, rectum and bowel bag were compared, and the normal tissue complication probability (NTCP) for >= grade 2 acute small bowel toxicity was calculated. Both IMPT and MRI-based target tailoring resulted in significant reductions in V-15Gy, V-30Gy, V-45Gy and D-mean for bladder and small bowel. IMPT reduced the NTCP for small bowel toxicity from 25% to 18%; this was further reduced to 9% when combined with MRI-based target tailoring. In four of the 11 patients (36%), NTCP reductions of > 10% were estimated by IMPT, and in six of the 11 patients (55%) when combined with MRI-based target tailoring. This > 10% NTCP reduction was expected if the V-45Gy for bowel bag was > 275 cm(3) and > 200 cm(3), respectively, during standard IGART alone. In patients with cervical cancer, both proton therapy and MRI-based target tailoring lead to a significant reduction in the dose to surrounding organs at risk and small bowel toxicity.