Purpose: To quantify the day-to-day target volume shape variation in rectal-cancer patients treated with preoperative 5x5Gy 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, 1-7mm (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 3mm 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.