Radiotherapy planning study comparing volumetric modulated arc therapy (VMAT) with intensity modulated radiotherapy (IMRT) in the treatment of bladder and pelvic lymph nodes.

Abstract

320 Background: VMAT is increasingly used as an alternative to IMRT and has been shown to reduce treatment time and monitor units delivered. We report a radiotherapy (RT) planning study of bladder and pelvic lymph node (LN) RT comparing dosimetric outcomes of VMAT and IMRT techniques. Methods: 8 patients with/at high risk of LN+ bladder cancer were treated with bladder/pelvic LN IMRT. 4 clinical target volumes (CTVs) were defined: Whole bladder (CTV1), Pelvic LN (CTV2), Involved Bladder (CTV3) and Involved LNs (CTV4). Margins were applied to create 4 corresponding PTVs. IMRT plans were compared with VMAT plans in order to assess planning target volume (PTV) and organ at risk (OAR) coverage. The same PTV/OAR volumes and doses were used for each technique. Results: The mean dose statistics were compared for each dosimetric parameter for both techniques. The Wilcoxon signed-rank test was used to compare techniques with statistical significance assumed as p<0.05. Both techniques met prescription goals for PTV coverage. Comparison of conformity indices revealed no significant difference between techniques. VMAT achieved significantly better homogeneity in coverage of PTV2, although this finding was not replicated in the other PTVs (Table). Homogeneity index (HI) was defined as HI = 100x(D2-D98)/ Dp, where Dp = prescribed dose. VMAT resulted in significantly larger volumes of bowel (4.7%) and rectum (4.8%) receiving low dose radiation (15 Gy) than IMRT, although there was no significant difference seen at higher dose levels. Comparison with 3D conformal radiotherapy (3D-CRT) showed that both techniques resulted in a large reduction in bowel irradiation to 45Gy (IMRT = 123cc, VMAT = 145cc and 3D-CRT = 218cc). Conclusions: VMAT offers an attractive alternative to IMRT with similar conformality. Whilst increased low dose RT to OARs was seen with VMAT, it is of doubtful significance relative to the higher doses received by these structures. [Table: see text]