Tumor volume and local control in primary radiotherapy of nasopharyngeal carcinoma

An investigation of the effect of tumor volume and total dose on local cont rol following primary radiotherapy for nasopharyngeal carcinoma was carried out in order to estimate the radiation dose necessary to control a specifi c tumor volume. Between 1983 and 1996 a total of 104 patients underwent rad iation therapy for nasopharyngeal carcinoma at the Department of Radiation Oncology of the University of Wuerzburg. Total doses of between 8 and 80 Gy (5 fractions per week) were administered. Complete CT-data on primary tumo r size, total tumor dose (calculated by 3D- or quasi 3D-CT-based radiation planning computer) and on local control status in the follow-up period were available for 63 patients. Lymph node metastases were present in 38 of the se patients and they were also entered into the study. Thus this study is b ased on a total of 101 tumor regions. A Poisson probability-based model was used for calculating the dose-response relationship. Assuming. a correlati on between tumor volume and the total dose necessary to obtain local contro l, the individual tumor volumes were rescaled to a 1 ml volume by introduci ng a volume-dependent modification factor for the applied dose. in order ro eliminate the influence of different individual tumor volumes. All dose va lues given are based on a fractionation scheme of 2 Gy single dose, 5 fract ions per week. Nineteen tumors and 11 lymph nodes were considered locally u ncontrolled or recurrent. Without dose-volume modification, a weak dose-res ponse correlation was found and a typical shallow dose-response curve was c alculated with a 50% response dose (RD50) of 60.2 Gy and a normalized dose- response gradient (gamma(50)) of 3.2 +/- 0.62. After dose-volume modificati on and rescaling to a 1 ml tumor volume, a steep dose-response curve with a n RD50 of 40.9 Gy and gamma(50) of 8.2 was found. Tumor volume is a very im portant factor influencing local control in nasopharyngeal carcinoma. The r escaling procedure to a reference volume of 1 ml used in this study reveale d a very steep dose-response relationship. This result suggests that the cl inically observed smooth dose-response relationships may be explained by in terindividual tumor volume heterogeneity. The additional dose necessary to control a tumor of the double volume is close to 5 Gy. With a total dose of 72 Gy (5 x 2 Gy/week), tumor volumes larger than 64 ml are unlikely to be controlled.