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Evaluation of the Robustness of Proton Therapy Compared with IMRT Regarding Interfractional Variations for Prostate Cancer

M Moteabbed

M Moteabbed*, A Trofimov, G Sharp, H Lu, Massachusetts General Hospital, Boston, MA

MO-G-137-3 Monday 5:15PM - 6:00PM Room: 137

Purpose: To evaluate the impact of interfractional changes on the delivered dose in proton therapy (PT) and intensity modulated radiation therapy (IMRT) for low-intermediate risk prostate cancer.

Methods: A subset of prostate patients participating in a randomized clinical trial at our institution also took part in a complementing serial imaging study. For patients randomized to receive 79.2 Gy in 44 fractions (9 weeks) of PT or IMRT, respectively, CT or Cone Beam CT scans were acquired on a weekly basis. The images were rigidly co-registered to the planning CT to simulate the set-up procedure. For each patient, both PT and IMRT plans were applied to each of the weekly image sets. Doses were normalized to the number of fractions to approximate the total dose received in a specific week. The weekly doses were then accumulated on the planning CT image using deformable image registration. Dose volume histograms were analyzed and compared to the original plans. The equivalent uniform doses (EUD) were calculated for targets and adjacent organs.

Results: Comparison based on a representative case indicated no significant changes to the prostate dose (boost target). However, the dose to seminal vesicles was 3.7 and 5.4% lower than planned, for proton therapy and IMRT respectively. The maximum difference between the planned and accumulated EUDs for all organs at risk was below 5.5 Gy (7% of the prescribed dose), observed in the bladder. On average, the organ EUD was different from planned by 2.5 and 2.8% for protons and IMRT respectively.

Conclusion: Despite the use of immobilization devices and bladder filling instruction per protocol, interfractional organ variability was observed. The data acquisition and analysis are ongoing, as patients are accrued to the trial. To date, the OAR dose changes were found to be small with no significant differences between protons and IMRT.

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