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Can We Use a Decay Plan for GammaPod APBI Treatments?

M Guerrero*, B Zhang, E Nichols, S Becker, Univ. of Maryland School Of Medicine, Baltimore, MD


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The GammaPod is a novel radiation therapy device for prone stereotactic breast treatments. The key elements are a novel vacuum-assisted breast cup for immobilization and localization and a large number of non-coplanar beams from twenty-five Co-60 sources. The current workflow for a 5-fraction partial breast (APBI) schema, involves placement of the breast cup, daily CT, and re-plan for each fraction, posing a significant time burden for the patient and the staff. This work investigates the possibility of using decay plans (original plan adjusted by radioactive decay of the sources) to treat APBI patients after the first fraction.
Methods: We retrospectively reviewed ten previously treated GammaPod patients who were treated with a 5-fraction APBI approach. All CT images and contours along with each fraction’s dose were imported into a 3rd party TPS. CT images of days 2-5 were rigidly registered to the 1st day’s CT image. We evaluated target contours overlap, PTV D95% and CTV D99% for each day as if the 1st day treatment had been used for all days as a decay plan. The registration was done using the breast cup as the main area of interest.
Results: The PTV-PTV overlap between fractions averaged 82% (range 57.2%-95.2%). The target dosimetry showed a significant degradation for most patients. Of the 28 decay plans analyzed, only 9 plans (32%) had clinically acceptable PTV D95% and CTV D99% (larger than 95% of prescription dose). Patients with lumpectomy cavities close to the chest wall showed the largest degradation in target dosimetry.
Conclusion: Preliminary results show that in most cases it is not possible to use a decay plan for APBI treatments due to unacceptable dosimetry of the target volumes. However, a subset of patients based on target location and size may be identified that can benefit from the use of decay plans.


Co-60, Breast, Stereotactic Radiosurgery


TH- External Beam- Photons: Development (new technology and techniques)

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