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Effects of Inter-Fraction Organ Displacement/deformation On the Delivered Doses to the Heart, Esophagus, and Lungs in Patients Receiving Thoracic Radiotherapy

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J Hammers

J Hammers*, J Matney , O Kaidar-Person , T Zagar , L Marks , S Das , P Mavroidis , University North Carolina, Chapel Hill, NC


SU-F-T-516 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To quantitatively assess the effects of inter-fraction changes in organ shape and location on the delivered dose distribution to the organs at risk (OAR) in lung cancer patients.

Methods: This study analyzes treatment data of 10 patients, who were treated to 60Gy in 30 fractions. In each fraction a cone beam CT (CBCT) was acquired. Each CBCT was registered with the planning CT using deformable registration tools within MIM Software. The daily setup shifts were used to translate the planned dose distribution on the deformed planning CT. The structures of lungs, esophagus and heart were re-delineated by a physician on each CBCT. The doses delivered to each OAR, reflecting changes in the position and shape variations, were recomputed. Resultant daily dose volume histograms (DVHs) for OARs were computed and compared to those from the planning CT.

Results: Based on the findings of two patients and 24 CBCTs analyzed so far, higher doses are delivered to the lungs and esophagus compared to the treatment plan. The dose differences per fraction between the delivered doses and those in the treatment plan are: for patient 1, lung mean dose = 5.3±1.3cGy and esophagus mean dose = 3.4±3.5cGy. For patient 2, lung mean dose = 12.0±3.9cGy and esophagus mean dose = 34.2±7.5cGy. Regarding the maximum dose to heart, the results varied (-18.9±22.0cGy for patient1 and 53.0±62.2cGy for patient2).

Conclusion: The dosimetric effects of inter-fractional anatomical variations could be estimated using deformable image registration and manual organ segmentation for each CBCT. A considerable dose distribution variation between fractions was observed for the OARs. These changes are currently not taken into account while treating the patients and these may explain cases with severe side effects even when the treatment plan looks satisfactory. These results suggest the need for automated daily dose tracking and accumulation.

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