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Tracking Dosimetric Changes Due to Lung Patient Physical Changes During Proton Therapy Treatment

M Bennett

M Bennett*, B Hoppe, Z Li, S Flampouri, Florida Proton Therapy Institute, Jacksonville, FL

SU-E-J-212 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: It is normal for the bodies of lung cancer patients to change during treatment due to tumor shrinkage, pleural effusion and other treatment related physical changes. The purpose of this study is to determine the differences between the original planned radiation therapy dose and the actual delivered dose given to patients with initially unresectable stage III non-small cell lung cancer (IRB approved study) and to ensure that protocol dosimetric constraints are not violated.

Methods:Target and critical structures are contoured on the original time averaged simulation CT. Patients receive weekly 4DCT scans during the course of treatment. These scans are snapshots of the patients changing bodies. The dose to tumor and anatomical structures actually delivered during that week based on the original plan is determined based on the weekly scan. Approximately 25% of the patients required plan modifications during the course of treatment. MIMvista deformable registration is used to deform the actual dose given each week back to the original planned dose. Dose volume histograms are used to compare the actual dose delivered to the critical structures based on the changing patient body to that given based on the original plan.

Results:Results are analyzed for 20 patients, including 5 patients with plan modifications. The mean change for all structures is 1.5% +/- 0.8%, suggesting insignificant dosimetric effects due to the changing patient body. Maximum changes of 6.8% for the contralateral lung dose and upto 9.2Gy for the maximum absolute dose to the spinal cord occurred, but these were still within dosimetric constraint goals.

Conclusion:Proton lung patients in our institution are imaged weekly and their treatments are adapted when necessary. All of the patients delivered dose accumulated from the weekly scans using deformable registration did not differ significantly from the planned dose and no critical constraints were violated.

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