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Rapid Delivery of GRID Therapy for Deep-Seated Bulky Tumors: A Novel 3D MLC-Based Forward Planning Treatment Technique Via 10MV-FFF Beam

D Pokhrel*, L Critchfield, M Halfman, M Bernard, W St Clair, M Randall, M Kudrimoti, University of Kentucky, Lexington, KY

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To present our novel 3D MLC-based forward-planning technique for rapid delivery of GRID therapy via flattening filter free (FFF) beams, providing ablative doses to bulky tumors using a brachytherapy-like dose-tunneling for the target and sparing adjacent critical structures.

Methods: Ten patient plans (4 head/neck, 3 chest, 1 para-spinal, 1 pelvis, 1 liver) with 6.5-13.0 cm tumors were created. Standard Millenium120 MLC leaves fitted to gross tumor volume (GTV) generated 1 cm diameter holes and 2 cm center-to-center distance (at isocenter), using an in-house algorithm. For a single-dose of 15 Gy, plans use 6-coplanar differentially-weighted 10MV-FFF (2400 MU/min) beams at 60-degrees spacing with 90-degree collimator rotation, generated dose-tunneling distributions without post-processing GTV-contours with Acuros-based dose calculations. We evaluated GTVD50%, GTVD10%, GTV dose heterogeneities (peak-to-valley dose ratio, PVDR), dose to adjacent critical organs, maximum dose 2 cm away from the GTV (D2cm), treatment planning and delivery time.

Results: 10X-FFF GRID therapy plans exhibited high mean GTVD50%, GTVD10% for 15Gy of 8.2±0.7Gy (range: 7.2–9.9Gy) and 13.5±0.7Gy (range: 12.4–14.6Gy), respectively, escalating central tumor dose to 18 Gy. Average PVDR and D2cm was 3.1±0.3 (range: 2.7–3.8) and 69.1±10.6% (range: 58.4–87%), respectively. Averaged maximum and dose to 5 cc of skin were 11.4±3.2Gy (range: 6.7–14.7Gy) and 7.2±2.9Gy (range: 1.2–10.4Gy), respectively. Spinal cord (<6.0Gy), heart (<5.5Gy) and small bowel (<4.5Gy) were spared. Average monitor units and beam-on time was 2306±174 and 1.01±0.1 min, respectively. Planning time was under one hour.

Conclusion: Our 10X FFF-beam rapid GRID-therapy enhanced target dose for bulky tumors, with low adjacent organ dose. This simple and fast FFF-GRID treatment can be easily adopted. It provides detailed dosimetric information and could provide same day treatment for bulky masses by eliminating longer inversely-optimized planning and physics quality-assurance time. Clinical validation of 10X-FFF GRID therapy and radiobiological response of delivering rapid treatment is under investigation.

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Keywords

3D, X Rays

Taxonomy

TH- External Beam- Photons: treatment planning/virtual clinical studies

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