Dosimetric comparative study of 3 different postoperative radiotherapy techniques (3D-CRT, IMRT, and RapidArc) for II-III stage rectal cancer.

Postoperative radiotherapy is critical for reducing local relapse for advanced rectal carcinoma but has many side effects. Our study compared the dose distribution of target volumes, protection of normal organs at risk (OAR), and monitor unit (MU) for 3 radiotherapy techniques (3-dimensional conformal radiation therapy [3D-CRT], intensity-modulated radiation therapy [IMRT], and RapidArc (Varian Medical Systems, Inc., Palo Alto, CA, USA)). The results advocate for the clinical application of RapidArc technique in the future.Thirty postoperative patients with rectal cancer were enrolled. The 3 radiotherapy plans mentioned above were designed for each patient. The target volume coverage indicators included average dose, conformity index (CI), and homogeneity index (HI) of planning tumor volume (PTV). OAR included the bladder, small intestine, colon, and bilateral proximal femurs. The 30 patients were divided into 3 groups (10 cases in each group) for postoperative radiotherapy with the 3D-CRT, IMRT, or RapidArc technique, respectively.Both the IMRT and RapidArc plans have a significantly higher average PTV dose and better CI and HI (P < 0.01) than 3D-CRT. IMRT and RapidArc result in significantly lower doses of irradiation for all the OAR examined. Both the IMRT and RapidArc plans have a significantly lower V40 of the bladder, small intestine, and colon than 3D-CRT (P < 0.01). The IMRT and RapidArc plans can also reduce the maximum dose (Dmax) for the left proximal femur, V30, and V40 of bilateral proximal femurs compared with 3D-CRT (P < 0.01). Compared with IMRT, RapidArc can further reduce the Dmax of the small intestine, the Dmax and V30 of the bilateral proximal femurs, and the V40 of the right proximal femur (P < 0.01). RapidArc reduces MU remarkably compared with IMRT (P < 0.01). Regarding acute side effects, IMRT and RapidArc can greatly reduce the incidence of grade 3 radiation-induced cystitis and grade 2 enteritis.Both IMRT and RapidArc are better than 3D-CRT regarding PTV coverage and OAR protection. Furthermore, RapidArc is superior to IMRT regarding protection of the small intestine and bilateral proximal femurs and requires a reduced treatment time. RapidArc could be widely applied for postoperative radiotherapy for patients with ΙΙ-ΙΙΙ stage rectal cancer.