Postoperative radiation therapy for malignant tumors of minor salivary glands. Outcome and patterns of failure.

Background: In the treatment of major salivary gland cancers, the addition of adjuvant postoperative radiation therapy for patients with high risk features has been shown to reduce the incidence of local failure. This retrospective study was done to determine the effectiveness of this approach for minor salivary gland cancers, to document patterns of failure, and to define prognostic variables for treatment outcome.
Methods: Between 1961 and 1990, 160 patients received postoperative radiation at the University of Texas M. D. Anderson Cancer Center (UTMDACC) after gross total removal of their tumors. These operations ranged from excisional biopsies to craniofacial resections with orbital exenterations depending on the original site and size of the tumor. The primary tumor site was in the oral cavity-oropharynx in 111 patients and in the nasal cavity or paranasal sinuses in 46 patients. The most prevalent histologic type was adenoid cystic carcinoma (71%). Microscopic positive margins were present in 64 (40%) patients. Half of the patients had pathologic evidence of perineural invasion. Radiation therapy techniques varied, depending on the site and extent of disease and the era of treatment. Doses ranged from 50 to 75 Gy (median, 60 Gy; mean 59.2 Gy). Follow-up for surviving patients ranged from 24 to 270 months (median, 110 months).
Results: Fifty-seven (36%) patients experienced disease relapse. Nineteen (12%) patients had a local recurrence: 6 within 5 years of treatment, 8 between 5 and 10 years, and 5 after 10 years. Regional failures occurred in 3 of 13 patients with initially node-positive disease but were uncommon (less than 5%) in patients with node-negative disease, regardless of elective neck treatments. Distant metastases developed in 43 patients, mostly (79%) within 5 years of treatment. Actuarial overall survival rates at 5, 10, and 15 years were 81%, 65%, and 43%, respectively. Complications occurred in 51 patients and were of three predominate types: hearing loss (26 patients), ocular injury (15 patients), and bone exposure/necrosis (12 patients). Improved techniques, including better immobilization, customized beam shaping, and treating multiple fields per day, have substantially reduced the risk of serious complications during the past decade.
Conclusions: Postoperative radiation therapy is effective in preventing local recurrence in most patients with minor salivary gland tumors after gross total excision. When local failure occurs, it tends to be a late event. For most patients, the authors recommend a postoperative dose of 60 Gy in 30 fractions to the operative bed; if there is named nerve invasion, the path of the nerve is treated electively to its ganglion.