Volumetric-modulated arc radiotherapy for carcinomas of the anal canal: A treatment planning comparison with fixed field IMRT

A treatment planning study was performed to compare volumetric-modulated arc radiotherapy against conventional fixed field IMRT.CT datasets of 10 patients affected by carcinoma of the anal canal were included and five plans were generated for each case: fixed beam IMRT, single (RA1)- and double (RA2)-modulated arcs with the RapidArc technique. Dose prescription was set according to a simultaneous integrated boost strategy to 59.4 Gy to the primary tumour PTVI (at 1.8 Gy/fraction) and to 49.5 Gy to risk area including inguinal nodes, PTVII. Planning objectives for PTV were minimum dose95%, maximum dose107%; for organs at risk (OARs): bladder (mean45 Gy, D(2%)56 Gy, D(30%)35 Gy), femurs (D(2%)47 Gy), small bowel (mean30 Gy, D(2%)56 Gy). MU and delivery time scored treatment efficiency.All techniques fulfilled objectives on maximum dose. Some deviations were observed on minimum dose for PTV. Uniformity (D(5)-D(95)) on PTVI resulted 6.6+/-1.4% for IMRT and ranged from 5.7+/-0.3% to 8.1+/-0.8% for RA plans (+/-1 standard deviation). Conformity index (CI(95%)) was 1.3+/-0.1 (IMRT) and 1.4+/-0.1 (all RA techniques). Bladder: all techniques resulted equivalent above 40 Gy; V(30 Gy) approximately 57% for the double arcs, approximately 61% for RA1 and approximately 65% for IMRT. Femurs: maximum dose was of the order of 41-42 Gy for all RA plans and approximately 45 Gy for IMRT. Small bowel: all techniques respected planning objectives. The number of computed MU/fraction was 1531+/-206 (IMRT), 468+/-95 (RA1), and 545+/-80 (RA2) leading to differences in treatment time: 9.4+/-1.7 min for IMRT vs. 1.1+/-0.0 min for RA1 and 2.6+/-0.0 min for double arcs.RapidArc showed improvements in organs at risk and healthy tissue sparing with uncompromised target coverage when double arcs are applied. Optimal results were also achieved anyway with IMRT plans.