A Monte Carlo-based Inverse-planning System for Advanced Mixed Beam Radiation Therapy

Jinsheng Li, Ph.D., Sotirios Stathakis, Ph.D., Lihui Jin, Ph.D. and Chang-Ming Ma, Ph.D.

Fox Chase Cancer Center, Philadelphia, PA 19111, USA

Purpose: To develop a Monte Carlo-based inverse-planning system for advanced mixed beam radiation therapy (MBRT) combining energy- and intensity-modulated electron beams and intensity-modulated photon beams.

Methods and Material: MBRT is suitable for shallow tumor treatment such as the disease sites of post-mastectomy chest wall, breast and head and neck etc. Feasibility studies on this topic have been performed. Monte Carlo beam simulations and dose calculations are required to provide accurate dose distributions. A fast and effective optimization method based on Monte Carlo calculated dose-deposition coefficients (DDC) is used for plan generation with dose-volume-histogram (DVH) or equivalent-uniform-dose (EUD) based objective functions. The current exiting photon MLCs with smaller source-surface-distances (SSD) (60–70cm) are recommended for electron beam delivery. Beam delivery accuracy and efficiency for MBRT are evaluated with phantom measurements.

Results: The MBRT plans generated by the Monte Carlo based inverse planning system for head-neck patients and breast patients provided excellent dose coverage to the target and normal tissue sparing than photon IMRT plans. The MBRT plans were delivered successfully using the exiting photon MLC on a Siemens accelerator. The calculated dose distributions using the Monte Carlo method and the results of film and chamber measurements in a solid water phantom were compared and the differences were within 3%/2mm.

Conclusion: A Monte Carlo-based inverse planning system has been developed for MBRT of shallow tumor treatment with much improved target dose conformity and significant reduction in the dose to the adjacent normal tissues and critical structures.