EFFECT OF SET-UP ERROR ON THE DOSE ACROSS THE JUNCTION OF MATCHING CRANIAL-SPINAL FIELDS IN THE TREATMENT OF MEDULLOBLASTOMA

Purpose: The effect of systematic and stochastic setup error on the do se delivered to the gap region for the three field radiation treatment of medulloblastoma is studied. The consequences of such setup error i s discussed. Methods and Materials: The treatment of medulloblastoma i s typically a 3 field technique, in which two lateral cranial fields a re matched with a spine field. The x-ray dose delivered to the region between the matched fields depends upon the gap size. The choice of th e gap width between the cranial and spinal fields is controversial. It is currently a compromise between minimizing the risk of dose hot spo ts to the spine, and the associated clinical complications, as well as the magnitude of cold spots (underdosing) across the gap, with the as sociated risk of disease recurrence. In this paper, we examine the eff ect of gap width with a moving junction, refered to as ''field feather ing'', on the dose across the field junction for a 6MV photon beam. In addition, we have studied 129 portal films and 40 simulation films to assess the accuracy and precision of patient setup during treatment w ith a plan involving feathered fields. Selected landmarks observable o n both portal and simulation films were identified and the variation i n the distances to the field edges measured. The distribution of patie nt setup error was convoluted with the beam profiles for a 6MV linac. These convoluted field edges were used to obtain dose profiles across the gap region as a function of gap separation. The consequences for t herapy are discussed. In addition, analysis of patient setup error on an alternative treatment involving beam modifiers to broaden the beam penumbra is discussed. Results: The magnitude of the spatial stochasti c and systematic setup error was determined to be approximately three and two millimeters respectively. The dosimetric consequences of patie nt setup error lead to over and under dosing in the spinal gap region for the three field technique. The degree of under or over dose depend s on the nature and magnitude of the patient setup error. Conclusions: The effect of patient setup error can lead to significant dosimetric errors in the dose to the gap region depending on the magnitude of the setup errors. The effective over and under dose can be compensated by the use beams modifiers such as a beam spoiler or vibrating jaws.