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Development and Commissioning of the Effective and Efficient Grid Therapy Using High Dose Rate Flattening Filter Free Beam and Multileaf Collimator

N Wen

M Liu , N Wen*, C Beyer , F Siddiqui , I Chetty , B Zhao , Henry Ford Health System, Detroit, MI


SU-F-T-506 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Treating bulky tumors with grid therapy (GT) has demonstrated high response rates. Long delivery time (~15min), with consequent increased risk of intrafraction motion, is a major disadvantage of conventional MLC-based GT (MLC-GT). The goal of this study was to develop and commission a MLC-GT technique with similar dosimetric characteristics, but more efficient delivery.

Grid plan was designed with 10X-FFF (2400MU/min) beam and MLC in a commercial treatment planning system (TPS). Grid size was 1cm by 1cm and grid-to-grid distance was 2cm. Field-in-field technique was used to flatten the dose profile at depth of 10cm. Prescription was 15Gy at 1.5cm depth. Doses were verified at depths of 1.5cm, 5cm and 10cm. Point dose was measured with a plastic scintillator detector (PSD) while the planar dose was measured with calibrated Gafchromic EBT3 films in a 20cm think, 30cmx30cm solid water phantom. The measured doses were compared to the doses calculated in the treatment planning system. Percent depth dose (PDD) within the grid was also measured using EBT3 film. Five clinical cases were planned to compare beam-on time.

The valley-to-peak dose ratio at the 3 depths was approximately 10-15%, which is very similar to published result. The average point dose difference between the PSD measurements and TPS calculation is 2.1±0.6%. Film dosimetry revealed good agreement between the delivered and calculated dose. The average gamma passing rates at the 3 depths were 95% (3%, 1mm). The average percent difference between the measured PDD and calculated PDD was 2.1% within the depth of 20cm. The phantom plan delivery time was 3.6 min. Average beam-on time was reduced by 66.1±5.6% for the 5 clinical cases.

An effective and efficient GT technique was developed and commissioned for the treatment of bulky tumors using FFF beam combined with MLC and automation.

Funding Support, Disclosures, and Conflict of Interest: The Department of Radiation Oncology at Henry Ford Health System receives research support from Varian Medical Systems and Philips Health Care

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