ABUTMENT REGION DOSIMETRY FOR SEQUENTIAL ARC IMRT DELIVERY

Are-based intensity modulated radiation therapy (IMRT) planning and de livery is available as a commercial product (Nomos Corp.). The dose di stribution is delivered to 1.68 cm thick regions, and the patient move d in a precise manner between treatments. Assuming accurate patient po sitioning, the abutment region dose distribution near the gantry isoce ntre is delivered with no undesired dose heterogeneities. However, for regions far from the isocentre, the dose distribution may exhibit hig h- or low-dose regions due to uncompensated beam divergence for are tr eatments of less than 360 degrees gantry angle length. A study has bee n initiated to characterize abutment region dose distribution heteroge neities for sequential are IMRT delivery. Five dose distributions were optimized, each using 8 cm diameter target volumes at different dista nces from the isocentre, and the are delivery limited to 290 degrees s ymmetric about the vertical axis. The target lengths were sufficient t o require a treatment consisting of five couch positions, yielding fou r 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 i socentre. However, the abutment regions along the vertical axis contai ned 15% low and 7% high doses at 7 cm above and below the isocentre re spectively. This dose heterogeneity is not predicted by the current cl inical release of the treatment planning software due to limitations o f the dose calculation algorithm. The intensity of dose heterogeneity is considered sufficient to warrant further study.