Room: Davidson Ballroom A
Purpose: Stereotactic radiosurgery (SRS) has become increasingly used in patients with multiple brain metastasis; with advancing technology the ability to perform linear accelerator based SRS to multiple sites with a single isocenter (IMRS) has the added convenience to shorten treatment time and treatment fractions. We seek to investigate the dosimetric differences of single isocenter multiple target SRS compared to traditional single isocenter per target SRS (tSRS).
Methods: Thirty-Five spherical targets were contoured on an SRS phantom in sets of 7 (4mm, 8mm, 12mm, 16mm, and 20mm targets). Subsequent SRS plans were created to 20Gy at 95% target coverage, in a single fraction, using Raystation version 220.127.116.11 with traditional SRS techniques (single isocenter per target) and an IMRS technique using a 0.1x0.1x0.1cm3 dose grid. Each single target plan was planned using 5-7 non-coplanar VMAT arcs, the multiple target single isocenter plan was planned using 9 non-coplanar VMAT arcs. Dosimetric evaluations comparing conformity and gradient indices, mean brain dose, as well as volume of brain receiving 5Gy(V5), 10Gy(V10), 12Gy(V12), and 15Gy(V15).
Results: Preliminary results with 8mm targets show: the composite of tSRS plans yielded a Paddick conformity index of 0.79 and Paddick gradient index of 4.04 with the whole brain receiving: V15 of 4.7cc, V12 of 7.1cc, V10 of 9.7cc and V5 of 39.8cc and a mean dose of 1.62Gy. The IMRS plan yielded a Paddick conformity index of 0.56 and a Paddick gradient index of 5.89 with the whole brain receiving: V15 of 5.2cc, V12 of 8.9cc, V10 of 13.8cc, and V5 of 196.2cc, and a mean dose of 3.24Gy.
Conclusion: Preliminary data suggests a dosimetric advantage to tSRS compared to IMRS for smaller targets. At >50% prescription dose, relatively small differences between IMRS and tSRS plans exist. However, significant advantages at <50% dose and subsequently mean brain dose favor tSRS.
Not Applicable / None Entered.