Repeat CT assessed CTV variation and PTV margins for short- and long-course pre-operative RT of rectal cancer

Abstract Purpose To quantify the inter-fraction shape variation of the CTV in rectal-cancer patients treated with 5×5 (SCRT) and 25×2Gy (LCRT) and derive PTV margins. Methods and materials Thirty-three SCRT with daily repeat CT scans and 30 LCRT patients with daily scans during the first week followed by weekly scans were included. The CTV was delineated on all scans and local shape variation was calculated with respect to the planning CT. Margin estimation was done using the local shape variation to assure 95% minimum dose for at least 90% of patients. Results Using 482 CT scans, systematic and random CTV shape variation was heterogeneous, ranging from 0.2cm close to bony structures up to 1.0cm SD at the upper-anterior CTV region. A significant reduction in rectal volume during LCRT resulted in an average 0.5cm posterior shift of the upper-anterior CTV. Required margins ranged from 0.7cm close to bony structures up to 3.1 and 2.3cm in the upper-anterior region for SCRT and LCRT, respectively. Conclusions Heterogeneous shape variation demands anisotropic PTV margins. Required margins were substantially larger in the anterior direction compared to current clinical margins. These larger margins were, however, based on strict delineated CTVs, resulting in smaller PTVs compared to current practice.