A review of image-guided intensity-modulated radiotherapy for spinal tumors

A new paradigm for the radiotherapeutic management of paraspinal tumors has emerged. Intensity-modulated radiotherapy (IMRT) has gained wide acceptance as a way of delivering highly conformal radiation to tumors. IMRT is capable of sparing sensitive structures such as the spinal cord of high-dose radiation even if only several millimeters away from the tumor. Image-guided treatment tools such as cone beam computed tomography coupled with IMRT have reduced treatment errors associated with traditional radiotherapy, making highly accurate and conformal treatment feasible.

This review discusses the physics of image-guided radiotherapy, including immobilization, the radiobiological implications of hypofractionation, as well as outcomes. Image-guided technology has improved the accuracy of IMRT to within 2 mm of error. Thus, the marriage of image guidance with IMRT (IG IMRT) has allowed the safe treatment of spinal tumors to a high dose without increasing the risk of radiation-related toxicity. With the use of near real-time image-guided verification, very-high-dose radiation has been given for tumors in standard fractionation, hypofractionated, and single fraction schedules to doses beyond levels traditionally believed safe in terms of spinal cord tolerance.

Clinical results, in terms of treatment-related toxicity and tumor control, have been very favorable. With follow-up periods extending beyond 30 months, tumor control rates with single fraction IG IMRT (1800-2400 cGy) are in excess of 90%, regardless of histology, and without serious sequelae such as radiation myelopathy. Patients also report correspondingly high rates of palliation. Excellent results, both in terms of tumor control and minimal toxicity, have been consistently reported in the literature.

IG IMRT represents a significant technological advance. Paraspinal IG IMRT is proof of principle, making it possible to give very-high-dose radiation within close proximity to the spinal cord. By reducing treatment-related uncertainties, margins around tumors can be shortened, thereby reducing the volume of normal tissue that must be irradiated to tumoricidal doses, reducing the likelihood of toxicity. Similarly, higher doses of radiation can be administered safely, improving the likelihood of eradication. Dose escalation can be done to increase the likelihood of tumor cell kill without increasing the dose given to nearby sensitive structures.