Dose comparisons for conformal, IMRT and VMAT prostate plans

Volumetric-modulated arc therapy (VMAT) is a relatively new treatment technique in radiation therapy. A comparison study of conformal, intensity-modulated radiation therapy (IMRT) and single- and double-arc VMAT plans was undertaken to evaluate the dosimetric impact of this new technology in prostate cases. The research questions were as follows: how does VMAT dosimetry compare with IMRT and conformal plans?; does VMAT increase the volume of bowel receiving lower doses?; are one or two VMAT arcs required for standard prostate cases?

Eight prostate cancer and post-prostatectomy patients were randomly selected for this study. Conformal, IMRT and single and double Arc VMAT plans were generated and dosimetric evaluations were performed. Each plan was prescribed a total of 75.6 Gy over a course of 42 fractions to the planning target volume (PTV).

The Healthy Tissue Conformity Index and the conformation number results revealed the IMRT and two VMAT techniques to have superior dosimetry to the PTV compared with the conformal plans. The maximum dose delivered to the PTV was significantly higher with the single-arc VMAT technique compared with the conformal or double-arc VMAT plans. There were no significant differences between the planning techniques for the bladder and small bowel dosimetry. However, IMRT and VMAT plans delivered less radiation to the rectum and femoral heads, and a single-arc VMAT plan was optimal for the right femoral head and the two VMAT techniques were optimal to the IMRT plans for the left femoral head.

Single- and double-arc VMAT consistently resulted in favourable or slightly superior dosimetry when compared with static gantry IMRT for prostate cases. Both the VMAT techniques and static gantry IMRT resulted in superior critical tissue sparing when compared with conformal plans.