Acute and long-term complications of therapeutic radiation for skull base tumors

Acute toxicity from radiation of skull base tumors typically resolves spontaneously within days to weeks after completion of therapy. Long-term complications have a latency of months to years after radiation and are usually irreversible. The need to keep the probability of chronic toxicity at an acceptable level has determined the upper limits of radiation doses for the treatment of tumors. The threat of chronic toxicity thus affects the ability to control skull base tumors with radiation. Use of conformal radiation techniques with image guidance, allowing minimization of normal tissue volume receiving significant radiation doses and close regulation of doses to specific structures, provides a low probability of long-term complications. Such conformal techniques may result in improved tumor control by allowing delivery of higher radiation doses to the tumor while maintaining an acceptable level of risk for chronic toxicity. Symptoms resulting from chronic radiation toxicity are indistinguishable from those due to tumor progression. In most cases in which radiation is used to treat tumors throughout the body, symptoms developing months after radiation are secondary to tumor recurrence and not due to toxicity from radiation. Notable exceptions are pituitary hormonal dysfunction after radiation for pituitary adenoma and cranial neuropathy after radiosurgery for vestibular schwannoma. With the recent evolution in radiosurgery techniques involving MR imaging guidance, decreased single doses in SRS, and fractionation of radiation dose in SRT, the probabilities of vestibular schwannoma progression and cranial nerve toxicity are approximately equal. Even in the case of visual field deficit after radiation therapy for a pituitary macroadenoma (for which the probability of tumor mass progression is only 5%-10%), the cause of the deficit is more likely to be progressive tumor than radiation-induced neuropathy. Consequently, diagnosis of a late-term radiation complication must be made only after verifying that the tumor has not progressed.