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In post-mastectomy right-sided breast cancer patients undergoing adjuvant radiotherapy, the liver is also incidentally irradiated due to its location at the caudal border of the chest wall irradiation field. This study evaluates the three accepted caudal border protocols: 0 cm, 1 cm, and 2 cm from the inframammary fold of the contralateral breast, regarding its correlation with the liver mean dose and volume of liver irradiated among right-sided breast cancer patients during free breathing chest wall radiotherapy.
Forty-eight (48) right-sided breast cancer cases that underwent radiotherapy in our institution were randomly selected from our 2017-2018 census. Three plans were created by the medical physicist for each case by applying the 3 different caudal border protocols generating 144 values for analysis. The liver volume, liver mean dose and liver volume irradiated at 50% isodose of each protocol of each case was obtained for analysis. Liver mean doses were also assessed if they are below tolerance dose limits of liver irradiation as set by QUANTEC (Quantitative Analyses of Normal Tissue Effects in the Clinic). Liver volumes were also assessed if they correlate with liver mean dose or volume of liver irradiated.
All liver mean doses generated in the study were below the tolerance dose of <30 Gy (for healthy liver) and <28 Gy (for liver with hepatocellular carcinoma). A weak, yet highly statistically significant, direct linear relationship was seen between an increasing caudal border distance from the inframammary fold and liver mean dose (p = 0.000, r = 0.302). The liver mean dose was not significantly different between the 0 cm caudal border and the 1 cm border, however a significant increase was observed between the 0 cm caudal border and the 2 cm caudal border (p = 0.001). When using tangential fields in right sided chest wall irradiation, the average liver mean doses at the 0 cm, 1 cm, and 2 cm caudal border were found to be 305.56 cGy, 359.16 cGy, and 413.38 cGy, respectively, while the volume of the liver irradiated at 50% isodose and their corresponding percentage are 29.89 cGy (2.6%), 38.78 cGy (3.38%), and 48.66 cGy (4.23%) respectively. Liver volume was not observed to correlate with the liver mean dose (r = -0.026, p = 0.862,), however, a significant correlation was observed between the amount of liver volume and the volume of liver irradiated at 50% isodose using the 2 cm caudal border as reference (r = 0.316, p = 0.029).
Using the 0 cm, 1 cm, and 2 cm caudal border protocol will ensure that liver doses are within tolerance dose limits in right sided chest wall radiotherapy for post mastectomy breast cancer patients. However, an increasing linear relationship is observed in the liver mean dose as the caudal border increases its distance inferiorly from the contralateral inframammary fold. A significant increase in liver mean dose is noted between the 0 cm and 2 cm protocol, but not when comparing the 0 cm and 1 cm protocol. Volume of liver irradiated at 50% isodose is generally low even when using the 2 cm caudal and was found to be correlated to the liver volume. Reducing the caudal border of the chest wall irradiation field within recommended protocol may be used to reduce liver dose in breast irradiation.