THE EFFECT OF SETUP UNCERTAINTIES ON THE TREATMENT OF NASOPHARYNX CANCER

Purpose: As radiation treatment techniques become more complicated, th e need to understand the effect of uncertainties on dose distributions increases. This study investigates the effect of positional uncertain ities for patients with nasopharynx carcinoma treated with a multiple field conformal boost technique. Three dimensional setup errors were m easured for six patients and the effect on patient dose was evaluated using dose volume histograms. Methods and Materials: A method is prese nted for determining 3-dimensional translational and rotational setup errors by identifying anatomical landmarks on two treatment field imag es and their corresponding simulation images. Measurements were made o n a daily basis for six patients undergoing conformal treatment. Resul ts: The average magnitude of the translational errors was between 1.5 and 3 mm while the average distance between simulation and treatment i socenters was 5 mm. Both systematic and random setup errors were obser ved. Dose volume histograms incorporating these uncertainties for stan dard parallel opposed and conformal techniques were generated for pati ents experiencing random and systematic setup errors. Conclusion: The data imply that positional uncertainties effect the daily dose distrib utions for target and critical structures differently and that the eff ect may be treatment technique dependent. These results demonstrate th e need to measure setup uncertainties for all sites and to develop tec hniques for incorporating dose uncertainties in treatment plans.