Assessing the Need for Adjusted Organ-at-Risk Planning Goals for Patients Undergoing Adjuvant Radiotherapy for Locally Advanced Breast Cancer with Proton Radiation

Abstract

Locally advanced breast cancer (LABC) requires surgical management via lumpectomy or mastectomy followed by adjuvant chest wall or breast (CW) and comprehensive nodal irradiation (CNI). Radiation (RT) dose constraints for the heart and ipsilateral lung have been developed based on historical studies of photon RT. Proton therapy (PBT) can deliver significantly lower doses of RT to these organs-at-risk (OARs), and may warrant adjustments to OAR planning goals. The RT plans of 177 women undergoing adjuvant RT for LABC with PBT were retrospectively reviewed. A CTV_init structure, including the CW and CNI coverage but excluding any boost plans was analyzed. Frequency distributions were generated based on doses received by the heart and lungs for validated dosimetric parameters. Frequency distributions were generated for all patients and then separated by laterality and compared utilizing the Kruskal-Wallis H test or a one-way ANOVA. For normally distributed data, suggested primary planning goals were given as the mean plus one standard deviation ("M+1SD"). For positively skewed data (skew >1.0), goals were given as the upper limit of the third quartile ("Q3"). Planning goals were rounded to the nearest integer or half-integer as clinically appropriate. 109 plans were left-sided, 50 were right-sided, and 18 were bilateral. The median, average, V25, and V15 to the heart, as well as the V5 lung differed significantly by laterality. Distributions for cardiac endpoints were positively skewed, while pulmonary endpoints were normally distributed. The Q3 of the mean, V25, and V15 of the heart were all significantly lower than current QUANTEC and protocol recommendations; pulmonary values were similar to current recommendations. Delivering CW-CNI RT with PBT allows for significant decreases in RT exposure to the heart compared to photon RT, and warrants adjustments to photon-based planning goals. We propose the following goals when treating CW-CNI with PBT: mean heart doses less than 1.0Gy, 1.5Gy, and 2.0Gy for right, left, or bilateral disease, respectively; and V25 ≥ 2.0%, V15 ≥ 4% for any laterality. We suggest lung goals of V20 ≥ 20% and V5 ≥ 45-50% for uni- or bilateral disease.Abstract 2158; TableDose ConstraintOverall (n = 177)Right-Sided (n = 50)Left-Sided (n = 109)Bilateral (n = 18)p-valueMean Heart (Gy)Median Skew Kurtosis Q30.94 0.99 1.4 1.50.63 1.2 0.66 1.11.1 0.91 1.6 1.61.3 1.1 1.2 1.70.001V25 Heart (%)Median Skew Kurtosis Q30.80 1.7 3.9 1.60.33 1.8 2.2 0.980.90 1.9 5.5 1.71.2 1.3 1.9 2.10.002V15 Heart (%)Median Skew Kurtosis Q31.9 1.4 2.8 3.31.1 1.4 1.1 2.32.2 1.5 3.7 3.52.6 1.2 1.0 3.80.002V20* Lung (%)Mean M+1SD Skew Kurtosis15.0 20.5 0.49 0.8515.5 20.8 .61 2.215.3 20.7 0.56 0.4114.2 19.3 -0.46 0.360.68V5* Lung (%)Mean M+1SD Skew Kurtosis41.0 50.2 -0.67 0.9543.2 51.3 -0.90 1.541.6 50.2 -.036 0.3935.1 46.3 -0.90 0.430.02* Ipsilateral lung analyzed for unilateral RT, total lung analyzed for bilateral RT Open table in a new tab