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

PurposeTo quantify the potential margin reduction with adaptive radiotherapy (ART) during neo-adjuvant treatment of locally-advanced rectal cancer.Methods and materialsRepeat CT scans were acquired for 28 patients treated with 25 × 2 Gy, 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.ResultsSubstantial systematic and random shape variation demanded for a PTV margin up to 2.4 cm at the upper-anterior part of the CTV. Plan adaptation after fraction 4 resulted in a maximum 0.7 cm margin reduction and a significant PTV reduction from 1185 to 1023 cc (p < 0.0001). The bowel area volume receiving 15, 45, and 50 Gy was reduced from 436 to 402 cc, 111 to 81 cc, and 49 to 29 cc, respectively (p < 0.0001).ConclusionsWith adaptive radiotherapy, maximum required PTV margins can be reduced from 2.4 to 1.7 cm, resulting in significantly less dose to the bowel area.