Spot Scanning-Based Proton Radiotherapy for Paraspinal and Skull Base Chordomas and Chondrosarcomas: Presentation of the Next-Generation Proton Delivery System and 5-year Clinical Data from the First Spot Scanning-Based Proton Facility

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Introduction: Proton radiotherapy (PRT) has established itself for mesenchymal tumors and is historically based on passive scattering technology delivering flat and fixed extent dose fields. At the Paul Scherrer Institute (PSI), spot scanning-based technology was developed, was first introduced in 1996, and is now in routine use. Spot Scanning-Based Particle Delivery System: As protons are charged particles, narrow pencil beams can be easily deflected using magnets. The range of the protons can be controlled, thus allowing protons to be scanned three-dimensionally through the tumor. Optimized dose calculations allow for highly conformal dose distributions. Ulitmately, this allows for intensity modulated proton therapy (IMPT), the proton analogy of IMRT with photons. Patient Population and Clinical Results: Sixty-four patients with skull base chordomas and chondrosarcomas were treated (1999 to 2005). The mean fractionated dose for chordomas was 73.8 Gy (RBE) and 68.4 Gy (RBE) for chondrosarcomas. At mean follow-up of 38 months, actuarial 5-year local control rates were 81% for chordomas and 94% for chondrosarcomas, resulting in disease-specific survival of 81% and 100%, respectively. In addition, 40 patients with extracranial chordomas from the axial skeleton were analyzed. With median follow-up time of 43 months, and median total dose of 72 Gy (RBE), 5-year rates of tumor control and disease-specific survival were 80%. High-grade neurotoxicity was observed in 6% of skull base patients. Conclusion: Spot scanning-based PRT represents the most advanced proton delivery system worldwide. Our results establish overall safety and feasibility and compare favorably with passive scattering-based proton data.

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