Program Information
Incorporating Prostate Interfractional Motion Into Gunma Heavy Ion Medical Center Carbon Ion Radiation Therapy
D Bridges1*, K Fukata12 , T Kanai12 , (1) Gunma University, Japan,(2) Gunma University Heavy Ion Medical Center, Japan
Presentations
TU-RPM-GePD-J(A)-6 (Tuesday, August 1, 2017) 3:45 PM - 4:15 PM Room: Joint Imaging-Therapy ePoster Lounge - A
Purpose: Our prostate carbon ion radiation therapy (CIRT) has excellent local control and low toxicity. Because we use two-dimensional (2D) bony matching for patient positioning, further clarification of these results is desired. For this reason and for improved TCP and NTCP estimation, we investigate the robustness of our treatment planning clinical margins and the acceptability of our PTV margins as a function of interfractional motion.
Methods: 1666 daily table shifts of 85 IMXT prostate cancer patients at Gunma University Hospital were exported to MATLAB. These patients lie on the table, CBCT images are acquired, the CBCT is automatically rigidly registered to the planning CT via bony anatomy, and lastly the technologist manually shifts the soft tissue target to isocenter by visual inspection of the CBCT. The difference of these shifts (interfractional motion) were used to derive a 3D probability function and resulting dose distribution for 24 CIRT prostate patients in MATLAB. These doses were written to DICOM files and imported to MIM for DVH analysis.
Results: Mean prostate displacement was (0.0, 0.36, -0.44) with standard deviation (1.0, 2.1, 1.8) millimeters from simulated position in patient’s left, posterior, and superior directions, respectively. The mean distance shifted was 2.0 mm with standard deviation 2.3 mm. Posterior-superior movement correlation was found, possibly due to technologists (radiation therapists) manually aligning the anterior rectal wall. An empirical cumulative distribution function suggests a PTV margin of 6.3 mm around the CTV will ensure planned dose for 95% of treatment fractions. Rectum ≤8mm posterior to prostate is spared 0.5-2Gy(RBE), corroborating reported low toxicity confirmed elsewhere.
Conclusion: This rigid dose-shifting method neglects seminal vesicle deformation. Our probabilistic dose distributions support the conclusion that even with 2D AP-and-lateral bony-matching our planning target margins ensure dose delivery to the CTV of prostate CIRT patients while establishing rectal sparing.
Funding Support, Disclosures, and Conflict of Interest: Daniel Bridges receives a scholarship from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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