Quasi-IMAT study with conventional equipment to show high plan quality with a single gantry arc.

BACKGROUND AND
PURPOSE: : Nowadays, intensity-modulated arc therapy (IMAT) for clinical use is mostly based on forward planning. The aim of this work is to investigate the potential of a step-and-shoot quasi-IMAT (qIMAT) technique to improve plan quality. MATERIAL AND
METHOD: : qIMAT plans with 18 and 36 beams were generated with a total number of 36 segments. Additionally, the number of segments was increased to 72, in order to investigate if the quality of the plans improves with the number of beams and segments. A conventional six-field intensity-modulated radiation therapy (IMRT) plan was used as a reference. The beam setup was applied to the CarPet phantom and to five prostate cancer patients.
RESULTS: : In the phantom case, the dose received by the organ at risk (OAR) decreased considerably by using qIMAT. At the same time, coverage and homogeneity of planning target volume (PTV) remained unaffected. For the prostate cases, a good dose coverage was accomplished inside the PTV. Rectum and bladder were better spared with qIMAT. When increasing the number of segments, only a slight improvement of the plan quality was observed.
CONCLUSION: : The study showed that qIMAT improves the sparing of OARs while keeping the uniformity within the PTV, when compared with conventional IMRT. The more concave the PTV, the more noticeable is this behavior. The qIMAT technique has the advantage that it can be realized with a conventional equipment. The plan quality is high even with a single gantry arc and one segment per beam direction.