Abutment region dosimetry for sequential arc IMRT delivery

Are-based intensity modulated radiation therapy (IMRT) planning and delivery is available as a commercial product (Nomos Corp.). The dose distribution is delivered to 1.68 cm thick regions, and the patient moved in a precise manner between treatments. Assuming accurate patient positioning, the abutment region dose distribution near the gantry isocentre is delivered with no undesired dose heterogeneities. However, for regions far from the isocentre, the dose distribution may exhibit high- or low-dose regions due to uncompensated beam divergence for are treatments of less than 360 degrees gantry angle length. A study has been initiated to characterize abutment region dose distribution heterogeneities for sequential are IMRT delivery. Five dose distributions were optimized, each using 8 cm diameter target volumes at different distances from the isocentre, and the are delivery limited to 290 degrees symmetric about the vertical axis. The target lengths were sufficient to require a treatment consisting of five couch positions, yielding four abutment regions. The dose within the abutment regions was measured using film and analysed as a function of off-axis position along both the vertical and horizontal directions. Little dependence on the dose heterogeneity was seen along the horizontal axis passing through the isocentre. However, the abutment regions along the vertical axis contained 15% low and 7% high doses at 7 cm above and below the isocentre respectively. This dose heterogeneity is not predicted by the current clinical release of the treatment planning software due to limitations of the dose calculation algorithm. The intensity of dose heterogeneity is considered sufficient to warrant further study.