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Dosimetric Impact of Immobilization Board and Couch Structures in Prone Breast Radiation

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A Lau

A Lau1*, K Salerno2 , T Ma1,2 , I Wang1,2 , (1) University at Buffalo, Buffalo, NY, (2) Roswell Park Cancer Institute, Buffalo, NY


SU-I-GPD-T-506 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The prone breast technique, initially introduced for treatment of pendulous breasts to minimize skin effects, may provide improved dose homogeneity and reduced doses to the heart and lung compared to supine in breast conservation patients. As the medial tangential beam may pass through the immobilization board and couch structures, there is concern regarding attenuation and potential bolus effects.

Methods: The ClearVue™ prone breast board used consisted of an insert on which the contralateral breast rests and a base indexed to the couch. Transmission factors (TF) were measured at various gantry angles using a Farmer chamber at 4cm depth in a solid water phantom. Treatment plans were also calculated in the Eclipse™ treatment planning system (TPS) for comparison. A parallel plate chamber at phantom surface was used to quantify the bolus effect.

Results: Measured TFs through the insert ranged 95.6-97.7% for 6 MV and 97.7-99.8% for 23 MV. TPS values were in agreement within 1.4%. The board base TFs ranged 97.9-98.5% and 98.9-99.8% for 6 and 23 MV, respectively. TPS values agreed within 0.5%. Measured couch rail TFs were 96.3% and 97.9% for 6 and 23 MV, respectively. The insert increased the surface dose by 62-93% for 6 MV and 115-153% for 23 MV. The board base increased the surface dose by 39-55% for 6 MV and 65-83% for 23 MV. Compared to a 10cm air gap between the breast and base, close contact increased surface dose by 25-30% for both energies.

Conclusion: Results demonstrated possible reduced coverage by attenuating external structures and an increased surface dose from the breast board. Proper modelling of immobilization devices and couch structures in the TPS may allow more accurate dose computation in treatment planning as per AAPM TG176 recommendations. Further study is needed regarding the clinical impact and significance of these findings.

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