Target volume shape variation during irradiation of rectal cancer patients in supine position

Purpose To quantify the inter-fraction shape variation of the mesorectum for rectal cancer patients treated with 5 × 5 Gy in supine position and compare it to variation in prone position. Methods and materials For 28 patients a planning CT (pCT) and five daily cone-beam-CT (CBCT) scans were acquired in supine position. The mesorectal part of the CTV (MesoRect) was delineated on all scans. The shape variation was quantified by the distance between the pCT- and the CBCT delineations and stored in surface maps after online setup correction. Data were analyzed for male and female patients separately and compared to prone data. Results A large range of systematic, 1–8 mm (1SD), and random, 1–5 mm, shape variation was found, comparable to prone patients. Random-shape variation was comparable for male and female patients, while systematic variation was 3 mm larger for female patients. Conclusions Shape variation of the MesoRect is substantial, heterogeneous and different between male and female patients. Differences between supine and prone orientation, however, are small. Clinical margins should be differentiated in position along the cranio–caudal axis, in anterior–posterior direction and for gender. Margins should also be increased, even when online setup correction is used. Due to the small margin differences between prone and supine treatments, the setup choice should be determined on dose to the organs at risk.