Analysis of head motion prior to and during proton beam therapy

Purpose: We report on the use of a noninvasive patient motion monitoring sy stem to evaluate the amount of head motion prior to and during proton radia tion therapy sessions. Methods and Materials: Two optical displacement sensors, placed close to th e patient's head, were used for online monitoring of the head position, wit h submillimeter accuracy. Motion data, including the difference between sta rt and end position (Dx) and the maximum displacement during the recorded s ession (Dx-max), were acquired for pretreatment sessions to analyze alignme nt radiographs, and for treatment sessions. We have recorded 102 pretreatme nt and 99 treatment sessions in 16 patients immobilized with a thermoplasti c mask, and JJ pretreatment and 56 treatment sessions in 13 patients immobi lized with vacuum-assisted dental fixation, To avoid incorrect data analysi s due to replicate observations, only 1 pretreatment and 1 treatment sessio n per patient were selected at random for statistical comparison of mean or median motion parameters in different subgroups. Results: Both techniques showed similar immobilization efficiencies. The me dian Dr and Dr-max values were 0.18 mm and 0.46 mm, respectively, for 16 tr eatment sessions with mask immobilization, and 0.22 mm and 0.50 mm, respect ively, for 13 treatment sessions with dental immobilization, Motion paramet ers for pretreatment and treatment sessions were not statistically differen t. Conclusion: Online verification of patient's head motion is feasible and pr ovides valuable data for confirmation of proper treatment delivery in indiv idual patients, as well as for the evaluation of different immobilization m ethods. (C) 2000 Elsevier Science Inc.