SU-F-T-568: QA of a Multi-Target Multi-Dose VMAT SRS.

PURPOSE: To, experimentally, corroborated the prescribed doses utilizing dosimeters (e.g. films and TLDs) that can provide high spatial resolution, allow dose measurement of multiple targets at once, and provide accurate dosimetric results.

METHODS: A single-isocenter 6FFF SRS VMAT plan consisting of one 358° arc at 0° couch angle and four 179° arcs at 30°, 60°, 330° and 300° couch angles respectively, was generated in ECLIPSE v.11 using a Rando-Alderson anthropomorphic head phantom CT study. This plan was a reproduction of a clinical plan generated for a stage-IV melanoma patient diagnosed with 19 intracranial lesions. The phantom was loaded with axially mounted (between phantom slabs) Gafchromic EBT3 film and TLDs strategically positioned within various target volumes. Film and TLDS were calibrated according to established protocols. Target prescription doses were 16 Gy (3cc≤, 3 lesions), 18 Gy (∼1-3cc, 10 lesions) and 20 Gy (≤1cc, 6 lesions). Phantom setup was verified through CBCT imaging prior to irradiation. Gafchromic films were scanned in transmission mode and TLDs were read, respectively, ∼24 hrs after irradiation.

RESULTS: Dose calibrated Gafchromic film data were compared to the ECLIPSE calculated data using a 3% / 3mm gamma function analysis. Results for the gamma values were 96-99% in agreement with the calculated data and with 84-90% of the film pixels within the 3% dose difference. TLD data showed a dose difference of 0.4-8% while the film data for those same locations yielded a difference of 0.4-4%. It was observed that the highest dose discrepancies correlated with the location of the small volume targets.

CONCLUSION: Overall this study corroborated that a VMAT SRS treatment, employing various treatment table rotations and arcs, to multiple intracranial lesions with multiple dose prescriptions can be delivered accurately with the existing radiotherapy technology.