Fractionated Photon Radiation Therapy for Skull Base Chordomas and Chondrosarcomas

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Introduction

The fundamental challenge to effectively managing chordomas and chondrosarcomas arising in the base of the skull is the anatomic location of the tumor. The standard treatment is initially surgery, typically followed by postoperative radiation therapy; this combination is advocated because the proximity of critical structures such as the brainstem, optic tracts, pituitary gland, and temporal lobes precludes complete resection of tumor. Recurrence rates of at least 58% from surgery alone are typical. Furthermore, chordomas are relatively resistant to standard doses of radiation. Treating skull base chordomas with 60 Gy radiation has yielded control rates of 23%–39%. Increasing the total dose to 66.6 Gy raises the rate of local tumor control to only 50%. Clearly, higher doses must be delivered to the tumor to increase the rate of tumor control, but in doing so, critical structures must be spared exposure beyond tolerance.

Because heavy charged particle techniques, such as the proton beam, can deliver high total doses to tumor while sparing nearby critical structures, they have historically been chosen for skull base chordomas and chondrosarcomas. To reduce the risk of late radiation effects in normal tissue, high-dose radiation is usually highly fractionated. In the last decade, stereotactic radiosurgery, in one to five fractions of photons or charged particles, has also been evaluated for selected small tumors distant from critical structures. Current research seeks more advanced techniques of planning and delivery of fractionated photon irradiation able to match the dosimetry achieved with particles.

Strategies for Delivering Fractionated Photon Radiation Therapy