title: Attenuation of radiation dose by the skull base bone in patients with nasopharyngeal carcinoma: clinical importance.
abstract: OBJECTIVE
  To measure the degree of attenuation of radiation dose by the skull base bone in patients with nasopharyngeal carcinoma (NPC) and to study its clinical importance.
  
  
  METHODS
  Isodose distribution in 11 patients with NPC who received bilaterally opposed large-field irradiation (1.8 Gy per fraction) was studied with a three-dimensional treatment planning system with tissue inhomogeneity correction. Also studied were the sites of local tumor recurrence in 37 patients with NPC and skull base destruction (>/=0.5 cm) or intracranial invasion treated with radiation therapy from January 1989 to December 1992. Regression analyses were performed.
  
  
  RESULTS
  In the dosimetric study, the low-dose areas (<1.65 Gy) were located at the level of the skull base in all 11 patients. A significantly positive correlation between the maximum width of the skull base bone and the low-dose volume (<1.65 Gy) 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, logistic regression).
  
  
  CONCLUSIONS
  Herein demonstrated is the inadequacy of the radiation dose over the skull base due to attenuation by the skull base bone. The relationship between width of skull base bone and skull base tumor recurrence also is established.
authors.affiliation: Departments of Radiation Oncology, Chang Gung Memorial Hospital, 123, Ta-Pei Rd, Niao Sung Hsian, Kaohsiung Hsien, Taiwan. hsiungcy@hotmail.com
authors.forename: C Y
authors.initials: CY
authors.lastname: Hsiung
authors.forename: J M
authors.initials: JM
authors.lastname: Wu
authors.forename: C J
authors.initials: CJ
authors.lastname: Wang
authors.forename: S C
authors.initials: SC
authors.lastname: Kuo
authors.forename: S A
authors.initials: SA
authors.lastname: Yeh
authors.forename: H C
authors.initials: HC
authors.lastname: Hsu
authors.forename: E Y
authors.initials: EY
authors.lastname: Huang
mesh_terms: D000328:Adult; D002277:Carcinoma / Q000453:epidemiology / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D009303:Nasopharyngeal Neoplasms / Q000453:epidemiology / Q000532:radiotherapy*; D009364:Neoplasm Recurrence, Local / Q000453:epidemiology; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D019291:Skull Base