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Pencil Beam Scanning Vs. Passive Scattered Proton Therapy for Pediatric Thoracic/pelvic Cancers

M Moteabbed

M Moteabbed*, T Yock, H Paganetti, Massachusetts General Hospital, Boston, MA

TU-A-108-10 Tuesday 8:00AM - 9:55AM Room: 108

Purpose: To investigate if and under what conditions pencil beam scanning (PBS) will offer therapeutic benefits compared to passive scattered proton therapy for pediatric patients with tumors in the thorax/pelvis.

Methods: A cohort of six pediatric patients with thorax/pelvic solid tumors treated with passive scattered protons was selected. The tumor types included prostate/bladder rhabdomyosarcoma and spinal/pelvic ewing sarcoma. Five PBS plans were created and optimized for each patient with variable beam spot sizes and utility of beam shaping devices namely apertures and range compensators. These included 12, 8 and 3 mm spot sigma at the isocenter, 12 mm spot with the use of apertures and 12 mm spot with both aperture and compensator. The conformity index for all plans was calculated to ensure sufficient tumor coverage. Dose volume histograms were analyzed and equivalent uniform dose (EUD) for each organ was computed. PBS and PSPT were compared by obtaining the % differences between the two methods for individual organs and for each patient and scanning parameter setting.

Results: The results clearly indicate that PBS with 12 mm sigma spots size does not offer dosimetric or clinical advantage over PSPT. However, adding apertures or both apertures and compensators could ensure equivalent or better treatment quality. On average the EUD difference was 0.2% of the prescribed dose for the larger spot size and -3.9% when adding both beam shaping components. Decreasing the spot size further improved all plans, providing better dosimetric quality than PSPT, while alleviating the need for beam shaping devices.

Conclusion: The added benefit of using PBS largely depends on the delivery configurations. We recommend the use of beam shaping components in facilities limited to larger spot size (>10mm sigma), in order to sharpen the dose penumbra and reduce the organ at risk toxicity in children with thoracic/pelvic cancers.

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