UBC Theses and Dissertations
IMRT treatment planning for arteriovenous malformations : patient stratification and dosimetric quality assurance Sonier, Marcus
Stereotactic Radiosurgery is a treatment of choice for Arteriovenous Malformations (AVMs) in anatomical locations associated with a risk of surgical complications. However, SRS has a risk of toxicity due to radiation injury to brain tissue. Therefore, use of intensity-modulated radiotherapy (IMRT) has been advocated because, compared to 3D Conformal Radiotherapy (3DCRT), it leads to improved PTV conformity and Normal Tissue (NT) sparing. The aim of this study was: 1) to develop stratification rules for AVM patients based on benefits they receive from IMRT; 2) to assess optimized dose distributions against prospectively collected data for symptomatic radiation injury; 3) to test and benchmark IMRT QA procedures for patient applications with the iPlan system. Thirty-one AVM patients previously treated with 3DCRT were replanned using static gantry IMRT for BrainLab microMLC using the iPlan system, with the 3DCRT plans as a reference. First, PTV constraints were applied and the conformity of the prescription dose to the PTV was compared between the treatment techniques. Next, NT constraints were introduced into the IMRT plans at the 7 and 12Gy isodoses. These constraints were manipulated to achieve maximum NT sparing while maintaining PTV coverage. Then, NT volumes receiving 7 and 12Gy were compared between the plan types. Finally, ion chamber and film dose verification were performed to scrutinize the accuracy of the IMRT improvements and determine the clinical validity of each plan. Examination of conformity index, NT max dose, and 7 and 12Gy isodose volumes showed a separation of patients into those who did and did not benefit from IMRT for two plan types: PTV Only and OAR Low. For PTV Only, each subset of patients received improvements of 0.10-0.68, 4.0-12.3%, 0-7.072cc, and 0.5-4.496cc, respectively, while, for OAR Low, patients received improvements of 0.10-0.58, 0-6.5%, 1.0-7.952cc, and 0.5-3.704cc, respectively. The 12Gy volume results translated to a decrease in the probability of symptomatic injury by 0.3-11.2% and 0.3-9.3% for PTV Only and OAR Low IMRT. In conclusion, this work indicates the potential for significant patient improvements when treating AVMs and provides rules to predict which patients are likely to benefit from IMRT.
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