Adaptive radiotherapy for long course neo-adjuvant treatment of rectal cancer

Abstract: Purpose: To quantify the potential margin reduction with adaptive radiotherapy (ART) during neo-adjuvant treatment of locally-advanced rectal cancer. Methods and materials: Repeat CT scans were acquired for 28 patients treated with 25×2Gy, daily during the first week, and followed by weekly scans. The CTV was delineated on all scans, and shape variation was estimated. Five ART strategies were tested, consisting of an average CTV over the planning CT and one to five repeat CTs. Required PTV margins were calculated for adapted and non-adapted treatment. The strategy with the least PTV volume over the whole treatment was selected and bowel area dose reduction was estimated. Results: Substantial systematic and random shape variation demanded for a PTV margin up to 2.4cm at the upper-anterior part of the CTV. Plan adaptation after fraction 4 resulted in a maximum 0.7cm margin reduction and a significant PTV reduction from 1185 to 1023cc (p <0.0001). The bowel area volume receiving 15, 45, and 50Gy was reduced from 436 to 402cc, 111 to 81cc, and 49 to 29cc, respectively (p <0.0001). Conclusions: With adaptive radiotherapy, maximum required PTV margins can be reduced from 2.4 to 1.7cm, resulting in significantly less dose to the bowel area. [Copyright &y& Elsevier]