Limitation of conventional two dimensional radiation therapy planning in nasopharyngeal carcinoma.

Abstract Purpose : To assess the adequacy of coverage of gross tumor volume (GTV) with traditional two dimensional (2D) radiation therapy (RT) planning in patients with nasopharyngeal cancer (NPC). Materials and methods : The study comprised 94 of 179 patients treated with definitive RT between 1993 and 1997. The inclusion requirement was the availability of a digitally archived pretreatment magnetic resonance imaging. The digital images were used to record the precise location of the GTV in the sagittal plane. As a separate procedure, sagittal dose distributions for each treatment phase were created by digitizing the simulation field parameters into treatment planning software without knowledge of the GTV. The location of the GTV and dose distribution in the sagittal plane were superimposed on each other and GTV coverage by the 50, 90 and 95% isodose lines determined for each phase of treatment. Results : The 1997 tumour node metastasis (TNM) stage distribution was: 7 (8%) stage I, 16 (16%) stage II, 30 (32%) stage III and 41 (44%) stage IV. Median follow-up was 4.4 years. Median primary dose was 66 Gy. The actuarial 5-year overall survival, disease free survival and local relapse free rates were 88, 54 and 66%, respectively. The GTV was covered by the 50, 90 and 95% isodose lines for all phases of the multiphase plan in only 53, 20 and 9% of patients, respectively. The GTV was more likely to be undercovered in the latter phases of the plan particularly in those patients with advanced T category. Conclusion : 2D RT planning has significant limitations in achieving adequate GTV coverage in NPC. We strongly recommend 3D planning using either conformal techniques of dose delivery or intensity modulated radiation therapy for the treatment of these patients.