Program Information
Positioning Uncertainty of Shielding Plates Used During Intraoperative Breast Radiotherapy
D Parrott1 , J Harris2*, J Reiland3 , X Kong4 , F Liu5 , D Spitznagel6 , (1) USD Sanford School of Medicine, Sioux Falls, South Dakota, (2) Avera Cancer Instititue, Sioux Falls, South Dakota, (3) Avera Medical Group, Sioux Falls, South Dakota, (4) Avera Cancer Instititue, Sioux Falls, South Dakota, (5) Avera Cancer Instititue, Sioux Falls, South Dakota, (6) Avera Cancer Institute, Sioux Falls, SD
Presentations
PO-BPC-Exhibit Hall-8 (Saturday, March 18, 2017) Room: Exhibit Hall
Purpose: For certain women intraoperative electron radiation therapy (IORT) is being utilized to reduce the standard breast radiotherapy regimen of six to seven weeks to a single dose of radiation to the tumor bed at the time of lumpectomy. In order to achieve this dose at the tumor bed while minimizing dose to the thorax, a PMMA-copper shielding plate is placed in the direct path of the radiotherapy beam in the retro-mammary muscular pocket. It is especially important to determine the uncertainty in the placement of the shielding plate in order to choose the correct shield size when delivering a single high dose. Otherwise, the thorax may receive unintended high doses. We aim to quantify the deviation of the center of the shielding plate from the center of the irradiating beam.
Methods: Intraoperative fluoroscopic images were assessed in image editing software for 10 patients to determine the alignment of the central clip of the lumpectomy cavity, used to center the collimator and ultimately the Mobetron system, with the shielding plate.
Results: The mean deviation from the central clip to the center of the shielding plate was 0.6 cm (95% CI 0.4 to 0.9). The greatest deviation measured was 1.0 cm; for two patients,there was no deviation.
Conclusion: We calculated a small but statistically significant deviation between the center of the shielding plate and the Mobetron. This deviation was less than 1.0 cm for all patients. At our institution, the PMMA-copper shield size is chosen such that it is 2.0cm greater in diameter than the collimator. This margin accounts for misalignment and divergence of the radiation beam.
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