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Session: Therapy General ePoster Viewing [Return to Session]

The Use of Daily Transit Dosimetry to Monitor Anatomy Changes and Setup Errors in Prone Breast Radiotherapy

X Shen1*, S Hsu1,2, D Mynampati1, P Brodin1,2, A Basavatia1, J Fox1,2, W Tome1,2, (1) Montefiore Medical Center, Bronx, NY, (2) Albert Einstein College of Medicine, Bronx, NY,


PO-GePV-T-240 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To investigate the use of daily transit dosimetry to monitor anatomy changes and setup errors in prone breast radiotherapy.

Methods: Five breast cancer patients treated with 3D tangential fields were retrospectively studied. All patients were treated prone and received 42.4-Gy in 16 fractions to the whole breast. Transit dosimetry and kVCBCT were acquired at each fraction. Using the field border structure on planning CT, PTV_CBCT was generated for each fraction same way as clinical planning PTV. Plans were recalculated on CBCT scans and the mean doses to PTV_CBCT were recorded. A reference PTV_CBCT was selected from the first three fractions, and absolute mean dose difference (AMDD) was calculated for each fraction. Transit doses were compared to the reference fraction dose using mean absolute error (MAE) in percentage within a region of interest defined by the planning PTV with 3 cm expansion to account for the daily breast motion. The AMDD in PTV_CBCT was evaluated as a function of MAE in transit dosimetry.

Results: The median AMDD from all fractions ranges from 0.8% to 6.2% for five cases. While one case overall has larger AMDD ranging from 1.1% to 9.6%, other cases have AMDD within 3%. The median MAE for all cases ranges from 3.4% to 11.3%. The same case mentioned above also has the largest MAE with a range of 6.6% - 16.2%, when other cases have MAE within 11%. In general, AMDD increases as MAE increases with a R2 value of 0.6102 using a linear regression fitting on all cases.

Conclusion: Our study demonstrated a potential of using daily transit dosimetry to track anatomy changes and setup errors in prone breast radiotherapy. Current results suggest a threshold of 10% in MAE indicates setup position check or adaptive planning. Future work will include more patients including supine breast cases.



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