Attenuation of radiation dose by the skull base bone in patients with nasopharyngeal carcinoma: Clinical importance

PURPOSE: To measure the degree of attenuation of radiation dose by the skul l base bone in patients with nasopharyngeal carcinoma (NPC) and to study it s clinical importance. MATERIALS AND METHODS: Isodose distribution in 11 patients with NPC who rec eived bilaterally opposed large-field irradiation (1.8 Cy per fraction) was studied with a three-dimensional treatment planning system with tissue inh omogeneity correction. Also studied were the sites of local tumor recurrenc e in 37 patients with NPC and skull base destruction (greater than or equal to0.5 cm) or intracranial invasion treated with radiation therapy from Jan uary 1989 to December 1992. Regression analyses were performed. RESULTS: In the dosimetric study, the low-dose areas (<1.65 Gy) were locate d at the level of the skull base in all 11 patients. A significantly positi ve correlation between the maximum width of the skull base bone and the low -dose volume (<1.65 Cy) was demonstrated (P = .003, linear regression). In the clinical study, local tumor recurrence was noted in 18 patients (49%). The sites of local recurrence included skull base in 16 patients (43%) and nasopharynx in six patients (16%). Wider skull base bone was a significant predictor of skull base recurrence after radiation therapy (P = .03, logist ic regression). CONCLUSION: Herein demonstrated is the inadequacy of the radiation dose ove r the skull base due to attenuation by the skull base bone. The relationshi p between width of skull base bone and skull base tumor recurrence also is established.