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Importance of heterogeneity correction for prostate therapy planning as it relates to prostate motion




Baeshen, Arwa Omar, author
Kraft, Susan, advisor
LaRue, Susan, advisor
Custis, Jamie, committee member
Zhang, Dongqing, committee member
Burton, Jenna, committee member

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Prostate adenocarcinoma is the most common cancer among men and second leading cause of mortality of men in the United States. External beam radiotherapy (RT) is often used for local prostate tumor control as part of multimodality therapy. Dosimetric treatment planning for RT is based on complex calculations made by computerized planning software, which are designed to achieve a target prescribed dose to the prostate while not exceeding normal tissue constraints. Those RT planning calculations are made from an initial pre-treatment computed tomographic (CT) scan, which provides the location, volume and density of the prostate and critical normal tissues. The calculation step applies Heterogeneity Correction (HC) during RT planning, which adjusts the delivered radiation fields according to regional tissue densities such as the presence of bone in the anatomic region of interest. Inter-fraction and intra-fraction prostate movement are both known to occur during the course of radiotherapy. Current standards of practice utilize ways to track and account for prostatic movement in order to maintain accurate delivery to that organ. However, those methods do not adjust for the HC that was already applied during the original treatment plan calculations. The use of HC for prostate cancer RT is therefore of particular importance because prostate movement relative to the pelvic skeleton might result in dosimetric inaccuracies, since the HC used in initial RT planning is based on the original prostate position. This project was part of a larger research study in which intact normal male dogs received hypofractionated stereotactic radiation to the prostate, as a translational animal model for human prostate cancer. In this study, inter-fraction prostate motion was evaluated and then those data were used to examine the impact of this movement on the use of heterogeneity correction (HC) on stereotactic body radiation therapy (SBRT) of the prostate, by evaluating the dose received by the planned target volume (PTV) and surrounding tissue during prostate RT planning. In Aim 1, cone beam CT (CBCT) images from ten dogs were evaluated retrospectively to estimate typical inter-fraction prostate movement. Organs of interest were contoured on each daily treatment CBCT data set, and those images were registered (fused) to the original planning CT. Prostate motion was quantified by determining the displacement of each isocenter relative to the original radiotherapy planning CT. For Aim 2, CT scans acquired during the course of SBRT were used to prospectively calculate new treatment plans that incorporated prostate displacement from four dogs, with and without HC. Organs of interest were contoured on each CT data set, and images were registered (fused) to the original planning CT. As above, prostate motion was quantified by measuring the isocenter movement in three axes relative to original RT planning CT. An optimal original planning CT was run twice for each CT, with and without HC, while adjusting the prostatic isocenter. Dosimetric data for organs of interest were evaluated using dose volume histograms (DVH) and comparing doses to previously defined constraint values. Results indicated a wide range of inter-fraction prostate displacement in both Aims 1 and 2, slightly greater in magnitude than similar human prostate movement data. The greatest prostate displacement was in the y axis (anteroposterior). No statistically significant differences were seen in target or normal tissue doses, with or without HC, suggesting that even in the presence of marked prostate motion, potential inaccuracies caused by HC may not have a great impact on the prostate RT planning. As expected, without HC there was a trend for the dose to the most organs of interest to increase slightly. In terms of how displacement affected tissue doses, maximum displacement of prostate was associated with adjacent tissues exceeding the known normal tissue tolerance. In particular, caudal and left displacement led to large doses exceeding the constraint limits for the posterior rectal wall. Those data indicate the importance of continued tracking or other methods to counteract prostate motion. The results provide a more informed approach for using HC relative to prostate motion during treatment of prostate cancer, as well as providing data relevant to tumor control, acute and late toxicities associated with inter-fraction movement of prostate RT.


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