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Systematic Comparison of Cone-Beam CT and Standard Evaluation CT for Use in Adaptive Proton Therapy Decision-Making for Lung Cancer

B Burgdorf

B Burgdorf*, C Teng , B Teo , UNIVERSITY OF PENNSYLVANIA, Philadelphia, Pennsylvania


WE-RAM3-GePD-JT-5 (Wednesday, August 2, 2017) 10:30 AM - 11:00 AM Room: Joint Imaging-Therapy ePoster Theater

Purpose: Lung cancer patients treated with proton therapy receive periodic on-treatment evaluation CTs (rCTs) to evaluate the dosimetric effect of possible anatomical changes such as tumor shrinkage, pleural effusion, or atelectasis. rCTs can be an inconvenience for both patients and departmental workflow. Cone-beam CTs (CBCTs) offer 3-dimensional anatomical information similar to rCTs without such inconveniences. We investigate the efficacy of using CBCTs in combination with an open-source image processing software to assess dosimetric effects of anatomical change compared to rCTs.

Methods: 5 patients who experienced moderate-to-severe anatomical changes on-treatment were retrospectively analyzed. Treatment plans were recalculated onto patients’ CBCTs and rCTs using image processing software that implements a WET-corrected dose computation. In total, 20 paired scans were analyzed. Since CBCTs do not have accurate HU information, prior to dose computation, deformable image registration (DIR) of the planning CT onto the CBCT is performed to allow for HU-mapping and additional correction in regions where DIR is inaccurate. Dose-volume histogram (DVH) parameter differences between CBCT-calculated and rCT-calculated doses were trended over each patient's treatment course.

Results: Differences between rCT-recomputed and CBCT-recomputed doses were calculated for each scan pair, then averaged across all pairs for each patient’s treatment course. For patients 1 and 3, CBCT-doses agreed very well with rCT-doses, with 20/26 of DVH parameters agreeing within 1Gy/1%Vol and all agreeing within 3Gy/3%Vol. For patients 2, 4, and 5, CBCT-dose showed slight overestimation of some DVH parameters compared to rCT-dose, possibly due to uncertainties in the CBCT HU-mapping process. Patient 2 in particular incurred atelectasis on-treatment causing severe target coverage loss. However, even in these cases, 27/39 parameters agreed within 1Gy/1%Vol, and all parameters agreed within 6Gy/5%Vol.

Conclusion: CBCT can effectively replace on-treatment rCT scans with high accuracy. Additional investigation is warranted to further improve accuracy in some cases with severe anatomical changes.

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