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Session: Therapy: External Beam: treatment planning Optimization [Return to Session]

Characterization and Optimization of GRID Treatment Planning

Y Cho*, S Chao, B Guo, J Suh, Cleveland Clinic Foundation, Cleveland, OH

Presentations

MO-IePD-TRACK 5-3 (Monday, 7/26/2021) 5:30 PM - 6:00 PM [Eastern Time (GMT-4)]

Purpose: Spatially fractionated radiation therapy or GRID is known to be effective to treat bulky mass in breast and head & neck tumors. Biological mechanism includes abscopal effect, escalated damage to endothelial cells and immune system activation. Difference in radiation damage to healthy tissue and tumor cell due to the heterogeneous dose distribution in GRID treatment can be evaluated using therapeutic ratio (TR) defined by the surviving fraction (SF) in normal tissue with GRID dose distribution to the equivalent uniform dose (EUD) with which SF of tumor is maintained. In this study, the efficacy of GRID dose distribution is characterized as a function of collimator design, beam energy, depth of tumor, dose fraction size and tumor type.

Methods: Dose distribution in water phantom was acquired in the treatment planning system, Pinnacle, using three different GRID collimators of 10mm, 12mm, and 14mm with separation of 21mm as well as fluence pattern of 10mm by 10mm segments with 17mm separation using MLCs. Three different beam energies of 6MV, 10MV and 15MV were used with the GRID field size of 10cm x 10cm. Dose was prescribed at depth of 5cm. Dose volume histogram (DVH) in the phantom was collected in every 1cm depth. Three different tumor type (radio-resistance, semi-resistance, and radiosensitive) were considered.

Results: The maximum TR was found at the depth of dose maximum in each beam energy for all collimator design. For the same prescription dose of 15Gy, TR at depth of 5cm increased by 12%-21% and 13-26% from 6MV-to-10MV and 6MV-to-15MV, respectively. EUD for tumor also increased by 22%-32% and 24%-33%. To maintain the same EUD of 5.5Gy for tumor, the low energy photon of 6MV required 27%-59% and 28%-65% more prescription dose compared to 10MV and 15MV, respectively. TR increased with the ratio of collimator opening in the order of 10mm, 12mm, 14mm and MLCs.

Conclusion: The TR and EUD of tumor is characterized to optimize the efficacy of GRID planning.

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