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Effective and Efficient Grid Therapy Using High Dose Rate Flattening Filter Free Beam and Multileaf Collimator

M Liu

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


SA-B-BRD-4 (Saturday, March 5, 2016) 10:30 AM - 12:30 PM Room: Grand Ballroom D

Purpose:Treating bulky tumors with grid therapy (GT) has demonstrated high response rates in combination with conventionally fractionated external beam radiotherapy. MLC-based GT (MLC-GT) is becoming more popular because of its technical convenience and comparable dosimetric properties with the Cerrobend GT block. However, long delivery time (>15min), with consequent increased risk of intrafraction motion, is a major disadvantage of conventional MLC-GT. In this study, by taking the advantage of high dose rate of flattening filter free (FFF) beams and the beam automation feature of Varian Truebeam system, we developed a GT technique with similar dosimetric characteristics, but more efficient delivery compared to conventional MLC-GT.

Methods:Two-hundred 5mm X 5mm grids were shaped by 40 central 5mm leaves of a Millennium120 MLC. The distance between adjacent grids was 1cm. A field-in-field 2D plan with 5 fields (3cm X 20cm field size) was generated using 10XFFF beam (dose rate=2400MU/min) in the Eclipse treatment planning system (AAA-v11) with prescription of 15Gy to the grid at 1.5cm depth. Each field contained two rows of grids. Doses were verified at depths of 1.5cm, 5cm and 10cm with calibrated Gafchromic EBT3 films in a 20cm think, 30cm X 30cm solid water phantom. The measured doses were compared to the Eclipse planar doses. Ten points were selected randomly to quantify the difference between the delivered and calculated doses.

Results:The valley-to-peak dose ratio at the 3 depths was approximately 20%, which are very similar to published results. Film dosimetry revealed good agreement between the delivered and calculated dose. The overall gamma passing rates were >95% (5%, 1mm). The point dose differences were 8.7%±2.5%, 5.8%±1.4% and 4.9%±1.3% at depths of 1.5cm, 5cm and 10cm, respectively. The delivery time was 7 min.

Conclusion:FFF beam combined with MLC and automation can provide effective and efficient GT for the treatment of bulky tumors.

Funding Support, Disclosures, and Conflict of Interest: The work was partially supported by a research scholar grant, RSG-15-137-01-CCE from the American Cancer Society.

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