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UBC Theses and Dissertations

Dose painting to combat tumor hypoxia while sparing urethra in prostate IMRT: a biologically based adaptive approach accounting for setup uncertainties and organ motion Yin, Lingshu


Enhanced resistance to radiation could be caused by both chronic hypoxia and acute hypoxic which has been reported in prostate cancer in various studies. Therefore currently used dose prescriptions (70Gy in 35 fractions) for external beam radiation therapy (EBRT) of prostate cancer has been suggested insufficient to provide optimum clinical outcome. In this study, we propose a Biologically Guided Radiation Therapy approach to boost dose in hypoxic prostate tumor regions while sparing the urethra. A previously proposed hypoxia model was modified for prostate cancer and incorporated into treatment plan optimization. The concept of equivalent uniform dose (EUD) was used in the optimization and evaluation of results. CT data from 25 prostate cancer patients who recently received EBRT at the British Columbia Cancer Agency (BCCA) and hypothetical hypoxic regions manually drawn on these CT scans were selected for this study. The results show that our methods could boost dose in target volume to substantially higher levels. EUD of planning target volume increased to more than 80Gy, despite accounting for effects of hypoxia. This increase was achieved with only minor changes in dose in normal tissues, typically less than 5Gy. Notably, urethra sparing was excellent with a EUD around 64Gy. Robustness of the proposed approach is verified against various hypoxic settings. EUD comparison between RT plans in biological guided and conventional approaches using the same RT technique (Volumetric Modulated Arc Therapy) also suggests that biologically guided radiation therapy (BGRT) approach is more suitable for dose painting purposes with the advantage of delivering sufficient dose to hypoxia region in different scenarios and sparing normal tissue better. Furthermore, we also investigated the impact of inter-fraction patient set-up error and intra-fraction organ motion on the high dose gradients achieved with this proposed dose painting method and explored the feasibility of adapting geometrical uncertainties (represented as systematic error and random error) into treatment planning. Image error obtained from EPID images are used to derive systematic uncertainty and random uncertainty. During the geometrical uncertainty adapted optimization, dose matrix in PTV is shifted based on systematic error and convolved with a Gaussian kernel which is pre-calculated using random error. CT sets and organ contours from five patients who enrolled in the previous dose painting i study are selected. For each of them, seven plans are generated using cumulated uncertainty data which was collected after every five fractions. We also present the outcome in terms of equivalent uniform dose (EUD). For four of the patients, EUD history of all seven plans suggests using the proposed optimization method with uncertainty data from the first five fractions, it is possible to achieve the same target coverage of static treatment plans (difference in EUD less than 1Gy). Meanwhile, the elimination of PTV margin also leads to a significant dose reduction (more than 15Gy) in rectum.

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