Medical Physics Seminar – Monday, March 25, 2019
Advancements in Dosimetry for Modulated Clinical Linear Accelerator Treatments
Vimal Desai (student of Dr. Wesley Culberson)
Research Assistant, Department of Medical Physics, UW-School of Medicine & Public Health, Madison, WI - USA
There is a significant disconnect between the static, broad beam, calibration reference conditions in AAPM TGâ€51 and the small, highly modulated, clinical fields commonly encountered in modern intensity modulated radiation therapy treatment (IMRT) plans. In these instances, additional planâ€specific correction factors are required to convert a radiation detector’s reading to an absorbed dose to water. Most institutions lack the time and resources necessary to ac-count for these detector†and planâ€specific correction factors, and a blanket correction is sometimes used based on simplified calculations or most likely ignored altogether. Because composite IMRT treatments are comprised of various MLCâ€defined fields, it cannot always be assumed that the dose to water calculated in a clinical field using a radiation detector is accurate based on its reference field calibration, or that a single correction factor could be applicable for every IMRT plan measured with a given detector. To maintain a high degree of dosimetric accuracy and precision, it is therefore important to investigate both the magnitude and variability of the correction factors across many different treatment plans to determine the accuracy of the detectorâ€reported absorbed dose to water.
Strategies for standardizing dosimetry involving modulated clinical fields are investigated using quantitative plan complexity metrics. A large number of detector-specific corrections were also determined using optimized Monte Carlo simulations and measurements with cutting-edge small field detectors. Finally, modelling strategies are presented to simplify the determination of beam- and detector-specific corrections with the goal of focusing calibration efforts for modulated clinical plans.