T Zhuang, D Parsons*, Y Gonzalez, G Gibbard, M Lin, B Cai, B Hrycushko, D Nguyen, R Reynolds, A Godley, A Pompos, N Desai, S Jiang, University of Texas Southwestern Medical Center, Dallas, TX

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  D Parsons

MO-C930-IePD-F6-5 (Monday, 7/11/2022) 9:30 AM - 10:00 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 6

Purpose: To explore the feasibility of generating high quality on-couch IMRT plan for prostate cancer patients without individualized pre-treatment planning on a cone-beam CT (CBCT)-based on-line adaptive radiotherapy platform.

Methods: Five previously treated prostate cancer patients with 60Gy in 20 fractions were selected. An IMRT preplan with 12 equally-spaced beams was made based upon the CT and contours of one model patient. The PTV margin from prostate was 5mm posteriorly and 7mm in other directions. The 5 mm bladder and rectum wall were the main OARs of concern. Previous simulation CTs were used as the on-couch CBCT data. The Ethos emulator was utilized to conduct virtual treatments of four other patients using the same preplan optimized on the geometry of the model patient. Using adaptive workflow an on-couch treatment plan was generated after contouring the prostate, rectum and bladder, and then compared to original clinical plans.

Results: The range of ratios of prostate volume, anterior-posterior and lateral sizes for four test patients to the model patient were 110.9-385.3%, 114.4-143.1%, and 112.7-125.5%, respectively. The PTV coverage on recalculated on-couch plans was > 95% in three of the four patients. It dropped to 90.2% for the patient with the largest lateral size. The D30% and D50% for both bladder_wall (35.6±2.2Gy / 25.5±2.5Gy) and rectal_wall (33.6±2.6Gy / 24.9±1.9Gy) for the recalculated on-couch plans satisfied our institutional constraints. Compared to the clinical plans, there was no significant difference on D30% for bladder_wall and rectal_wall (p=0.68/0.16) but significantly higher on the D50% (p=0.04/0.03) using paired t-test.

Conclusion: On-couch generated IMRT plans for prostate cancer using online contours from CBCT and adaptive radiotherapy workflow achieved similar quality as if the plans were individually pre-optimized. Further study will be conducted to identify selection criteria for patient anatomy most amenable to this workflow and extend to other sites.

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