Residual Set-up Error for Canine Brain Radiation Therapy

Abstract

When targeting a structure in three-dimensional space, the repositioning accuracy of the patient on the treatment table should be consistent among all radiation treatment sessions. Immobilization devices are used reposition the patients, and imaging systems built into the radiation machine are used to correct the patient’s position prior to treatment. Radiation oncologists usually treat a margin of normal tissue around the tumor called planning target volume (PTV) to account for interfraction (set-up errors) and intrafraction motion (such as motion due to respiration). The size of the PTV margin is an estimate of the targeting accuracy that can be achieved using immobilization devices and image guidance. The PTV margin reported for stereotactic radiation therapy (SRT) and stereotactic radiosurgery (SRS) to treat canine brain tumors ranges from 0-3 millimeters (Kelsey, Gieger, and Nolan 2018; Griffin et al. 2014; Dolera et al. 2017). An ideal margin would be a narrow margin to minimize the chance of toxicity to the normal brain and wide enough to cover the entire tumor target. The size of the PTV margin also depends on the radiation therapy delivery technique and the fractionation scheme (dose of radiation for each treatment session) planned for determined tumor types and locations. Accurate patient set-up and adequate PTV margin selection are required to minimize the chance of radiation side effects to the normal tissue surrounding the tumor region and to maximize tumor control. This is particularly relevant for SRS and SRT treatments, as higher doses of radiation are used to treat cancers compared to conventional radiation protocols.