Poster — Thur Eve — 25: In vivo dosimetric verification of intensity‐modulated radiation therapy

Dosimetric verification of patient treatment plans has become increasingly important due to the widespread use of complicated delivery techniques. IMRT and VMAT treatments are typically verified prior to start of the patient's course of treatment, using a point dose and/or a film measurement. Pre‐treatment verification will not detect patient or machine‐related errors; therefore, in vivo dosimetric verification is the only way to determine if the patient's treatment was delivered correctly. Portal images were acquired throughout the course of five prostate and six head‐and‐neck patient IMRTtreatments. The corresponding predicted images were calculated using a previously developed portal doseimage prediction algorithm, which combines a versatile fluence model with a patient scatter and EPIDdose prediction model. The prostate patient image agreement was found to vary day‐to‐day due to rectal gas pockets and the effect of adjustable support rails on the patient couch. The head‐and‐neck patient images were observed to be more consistent daily, but an increased measured dose was evident at the periphery of the patient, likely due to patient weight loss. The majority of the fields agreed within 3% and 3 mm for greater than 90% of the pixels, as established by the χ‐comparison. This work demonstrates the changes in patient anatomy that are detectable with the portal doseimage prediction model. Prior to clinical implementation, the effect of the couch must be incorporated into the model, the image acquisition must be automatically scheduled and routine EPID QA must be undertaken to ensure the collection of high‐quality EPIDimages.