Abstract
The Beatson West of Scotland Cancer Center (BWOSCC) has been treating brain metastasis using LINAC based stereotactic radiosurgery with micromultileaf collimator (Brain lab M3) since 2008. Patients are immobilised using Brainlab fixation and treated with static fixed fields (6MV photon), delivered at conventional dose rates of 400-600 monitor units per minute (MU/min). In August 2012 a TrueBeam STX (Varian Medical Systems) was installed. Patients with localised CNS lesions are now treated with 10MV FFF VMAT. METHOD: Treatment records for 10 for conformal static field patients and 10 VMAT FFF patients were reviewed on Varian Eclipse and Offline Review. Beam on time (BOT), time in room (TIR), and clinical dose rate (CDR) were compared. BOT (minutes:seconds) is the aggregate “beam-on” time for all fields or arcs in a given plan. TIR (minutes:seconds) is measured from the first alignment image to last beam off, inclusive of all pre-treatment imaging studies and/or shifts. CDR (MU/min) is the number of MU divided by the BOT.Static field patients had MV imaging only. VMAT patients had kV imaging and pre-treatment cone beam CT (CBCT). 18 patients had single metastasis, 2 patients had 2 mets. 11 patients were treated with 20Gy, 7: 18Gy and 2: 15Gy. RESULTS: Median Planning Target Volume was 5.41cm3 (min 0.98cm3 max 18.88cm3). 10 patients were treated with 6MV static fields (between 4 and 6 fields). 10 patients with 10MV FFF (All 10 Patients had 2 co-planar) arcs. Beam on time: 6X Static 5.85 (range 4.55-6.6), VMAT FFF 2.60 (range 2.01-3.9). Time In Room: 6X Static 34.17 (range 20.42-58.13) VMAT FFF 17.12 (range 12.42-28.28). Monitor Units: 6X Static 2726 (range 2119-3168) VMAT FFF 6216.5 (range 4472-9340). Clinical Dose Rate: 6X Static 600MU/Min VMAT FFF 2385.5MU/Min. CONCLUSION: VMAT FFF has dramatically reduced the TIR: shorter BOT reduces the likelihood of intrafraction motion. It is envisaged that VMAT FFF treatment times will be reduced further as Radiographer competency and skill increases. This is essential in busy RT departments where treatment slots are at a premium.
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