The Radiation Exposure To The Contra Lateral Breast From Portal Images During The Course Of Breast Radiotherapy

To evaluate the radiation dose to the contra-lateral breast from portal images during the course of conventional whole breast Irradiation (WBI) and partial breast irradiation (PBI). The dose to the contra-lateral breast from portal images was measured in a female anthropomorphic phantom with medium-sized breast. Twelve thermoluminescent dosimeters (TLDs) were placed in the contra-lateral (right) breast near the midline of the phantom. For WBI, the plan was designed to treat the left breast of phantom with 2 tangential fields.opposed coplanar tangent fields were designed on the left breast of the phantom. For PBI, an actual patient plan of a PBI treatment of a left breast with 5 conformal treatment fields was used on the phantom. Double exposure technique was used to create portal images for each treatment field. In addition, orthogonal portals were also taken for the PBI case. In order to obtain the measurable dose to the TLDs, in WBI, 2000 MU was delivered to each of four fields (i.e. two opposed coplanar tangent fields and two corresponding open fields). In PBI, the dose to the contra-lateral breast was measured from setup verification orthogonal portals and the treatment fields' portals respectively. The anterior-posterior as well as lateral field received 500 MU each. However, 1500 MU was delivered to each of ten fields (i.e. five treatment fields and five open fields). The average contra-lateral breast dose from portal imaging per MU (ACDMU) was calculated as measured total dose divided the total MU delivered to the ipsilateral breast. Twenty WBI patients and nine PBI patients were randomly selected to obtain the average total MU (ATMU), maximum and minimum MU used to obtain portal images. The estimated radiation dose from portal imaging to contra-lateral breast was calculated as ATMU multiplied by ACDMU. Table 1 lists the statistics of estimated radiation dose to the contra-lateral breast from portal images for the twelve positions monitored in this study. The second and the third columns show the mean dose and the dose range for the 20 WBI and the 9 PBI patients respectively. In general, the PBI portal image techniques consistently gave a significantly higher dose to the contra-lateral breast. For PBI, the orthogonal portals contribute over 99% of the dose to the contra-lateral breast for all the points. When acquiring portal images for breast radiotherapy, both the number of images and corresponding exposure should be considered without compromising the quality of treatment delivery.Table 1Radiation dose to the contra-lateral breast from weekly portal imagesWBIPBIPointMean (max-min) (cGy)Mean (max-min) (cGy)122.95 (43.39–15.06)15.39 (27.36–6.73)21.64 (3.09–1.07)9.44 (16.80–4.12)32.42 (4.58–1.59)11.13 (19.80–4.86)42.32 (4.38–1.52)14.30 (25.44–6.25)52.56 (4.83–1.68)14.05 (24.99–6.13)61.58 (2.98–1.03)11.12 (19.78–4.85)73.00 (5.68–1.97)13.99 (24.88–6.10)81.72 (3.25–1.13)11.39 (20.26–4.97)98.40 (15.87–5.51)15.10 (26.85–6.60)102.04 (3.86–1.34)11.25 (20.02–4.91)111.36 (2.56–0.89)9.47 (16.84–4.13)121.78 (3.36–1.17)11.3 (20.10–4.93) Open table in a new tab