Stereotactic Radiation for Skull Base Recurrences of Salivary Carcinomas: Implications for Tumor Targeting

Background: Recurrences occur in approximately 10 to 25% of salivary carcinomas and often involve the skull base because of the ability of these tumors to track along nerves. Depending on prior treatment and location, skull base recurrences may be treated with stereotactic radiation therapy, either with conventional fractionation or radiosurgery. Here, we report our institutional experience on the treatment of these recurrent lesions at the skull base with stereotactic radiation.

Methods: A total of 14 patients were given 21 treatments (including six salvage treatments after further local/regional failure at the skull base) with stereotactic radiation therapy from 1995 to 2013 for skull base recurrences of salivary gland cancer. Primary histologies included eight adenoid cystic, two adenocarcinoma, two acinic cell, one mucoepidermoid, and one salivary ductal. Male-to-female ratio was 1:1. Initial treatment involved surgical excision with radiation therapy ± chemotherapy for appropriate indications. Many of the skull base recurrences occurred at the superior border of the initial PTV at the medial skull base whereas two recurrences were more distal along the meninges. Two patients had distant metastases in the lungs at time of initial treatment for skull base recurrence. The median age of patients at the time of recurrence was 52 years (range,21–86 years), and the median time since initial diagnosis was 55 months (range,13–248 months). Of the 21 lesions, 10 were treated with conventional fractionation and 11 were treated with radiosurgery. All treatments were delivered with stereotactic guidance on either the Radionics Xknife or the Novalis BrainLab platform. Local failure was defined as radiologic progression of disease either within the treatment field or at the margin. Median overall survival and time to local failure were calculated for our patient population.

Results: Median overall survival from diagnosis of primary tumor was 92 months (range,22–315 months); median survival from diagnosis of first skull base recurrence was 32 months (range,6–112months). Symptoms of skull based recurrence included palpable fullness, dizziness, vision disturbances, or facial/head pain. All the patients who presented with symptoms experienced symptomatic response with treatment. Median time to first local failure post radiation was 17 months (range,2–101months). There were 13 local/regional treatment failures (including one salvage failure); five of these failures were in the treated field with eight being at the margin of the treated volume. Of the eight marginal failures, six were along the meninges with all intracranial failures being along the meningeal surface. For the five patients given salvage therapy, treatment extended time to final failure by 15 months (range, 0–41 months). Allowing for salvage, 7 of the 14 patients were locally controlled until death or last follow-up. One patient experienced symptomatic radiographic necrosis from stereotactic radiosurgery after a second salvage treatment.

Conclusion: Stereotactic radiation is a well-tolerated treatment that effectively palliates symptomatic patients and may provide long-term control in some patients. Long-term local control remains a difficult problem. Because of the pattern of failure along the meningeal surface with marginal spread of tumor, we suggest considering additional elective margin along the meningeal surfaces when treating these patients.