Target volume shape variation during hypo-fractionated preoperative irradiation of rectal cancer patients

Purpose: To quantify the day-to-day target volume shape variation in rectal-cancer patients treated with preoperative 5 x 5 Gy radiotherapy. Materials and methods: For 27 patients a prone position plan-CT (pCT) and five daily pre-treatment cone-beam-CT (CBCT) scans were acquired. A sub-region of the CTV (MesoRect, anus up to the cranial end of the mesorectal-fascia) was delineated on all scans. The MesoRect deformation was quantified by the distance between pCT- and CBCT-delineations and was stored in surface-maps. Finally, the influence of bladder and rectum filling on MesoRect deformation was evaluated. Data were analyzed for male and female patients separately. Results: A large range of systematic and random deformations, 1-7 mm (1SD), on different areas of the MesoRect were found. The maximum deformations were located at the upper-anterior-side of the MesoRect. For females the errors were up to 3 mm larger than for males. Small correlations, r(2) <= 0.4, were found with changes in bladder volume. Larger correlations, r(2) <= 0.7, were found for rectal volume in a distinctive area in the upper-half of the MesoRect. Conclusions: Substantial and heterogeneous deformations of the MesoRect were found. Therefore different PTV margins in positions along the cranio-caudal axis, in the anterior-posterior direction. Margins should also be larger for female patients compared to male patients. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 92 (2009) 202-209