Dosimetric Impact of Immobilization Board and Couch Structures in Prone Breast Radiation
Lau, Amy Jessica
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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. The ClearVue™ prone breast board consists of a removable insert on which the contralateral breast rests and a base which is indexed to the couch. As the medial tangential beam may pass through the immobilization board and couch structures, there is concern regarding attenuation and potential bolus effects. Methods: Transmission factors (TF) were measured at various gantry angles using a Farmer chamber at 4 cm depth in a solid water phantom for the top insert and board base. An acrylic cylindrical phantom was used to measure the TFs from the couch top and rails. Treatment plans were also calculated in the Eclipse™ treatment planning system (TPS) for comparison. A parallel plate chamber at a phantom surface was used to quantify the bolus effect. Film measurements were done to further study the bolus effect from the top insert. Clinical treatment plans were recalculated with external structures contoured to determine the clinical impact for selected patients from a patient pool of 44. A new plan was generated to see the possibility of having a clinically acceptable plan incorporating the immobilization board and couch structures. Results: Measured insert TFs ranged 0.956-0.977 for 6 MV and 0.977-0.998 for 23 MV. TPS values were in agreement within 1.4%. Base TFs ranged 0.979-0.985 and 0.989-0.998 for 6 and 23 MV, respectively. TPS values agreed within 0.5%. Measured couch top and rail TFs were ranged from 0.837-0.997. 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 10 cm air gap between the breast and base, close contact increased surface dose by 25-30% for both energies. Recalculated plans have demonstrated that there is reduced target coverage up to 4-5%. It is also feasible to generate a new plan to include immobilization and couch structures. 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.