Abstract P4-12-25: Dosimetric comparison of breast and pan nodal radiotherapy techniques to estimate the risk of developing a second primary cancer in the upper inner quadrant of the contralateral breast
Authors: Settatree Sarah , Brand Douglas , Ranger Alison , Dunlop Alex , Kirby Anna 
Published: March 2020

Abstract
Purpose: To quantify estimates of risk in young patients (&lt;40 years) of an upper inner quadrant (UIQ) contralateral breast (CB) cancer after breast and locoregional lymph node radiotherapy that includes internal mammary chain. Comparison of two optimal photon techniques with proton beam therapy (PBT).
Methods and Materials: In 14 patients’ CT datasets from the HeartSpare Plus dosimetry study(1) the CB was further subdivided into quadrants. Three radiotherapy techniques for breast and pan nodal radiotherapy were analysed: Wide Tangents in voluntary Breath Hold (WT-vBH), Volumetric-Modulated Arc Radiotherapy in vBH (VMAT-vBH) and PBT in Free Breathing. (PBT-FB). Mean dose to the whole CB, and contralateral UIQ for each patient across three techniques was calculated. The statistical significance of differences between the mean doses to UIQ across the three techniques were analysed using Friedman’s test followed by Wilcoxon signed rank comparison.
Based on estimates of CB cancer risks from the WECARE study (2) and assuming the patients were under 40 years of age, the dose to UIQ was converted to Rate Ratios (RR), both overall and with &gt;5 years follow up.
Results: The mean dose and range to CB UIQ (Gy) for WT-vBH was 4.7 (0.5-11.0), VMAT-vBH was 3.9 (0.5-6.8) and PBT-FB 0.5 (0.1-1.7) p value &lt;0.01. PBT-FB achieves a statistically significantly lower dose than either WT-vBH or VMAT-vBH. For a patient under 40 years followed for &gt; 5 years this translates into a Rate Ratio (RR) with 95% confidence interval (CI) of 6.9 (1.5-19.9) for WT-vBH, 6.3 (1.4 – 18.0) for VMAT-vBH and 1.2 (0.5-3.0) for PBT-FB.
WT-vBHVMAT-vBHPBT-FBDose to whole CB (Gy) mean (and range)1.6 (0.3-3.2)1.5 (0.3-3.5)0.2 (0.1-0.8)Dose to UIQ CB (Gy) mean (and range)4.7 (0.5-11.0)3.9 (0.5 – 6.8)0.5 (0.1 –1.7)RR (with 95% CI) of UIQ CB cancer In woman &lt;404.8 (1.7-10.4)4.4 (1.7-9.4)1.1 (0.6 –1.9)RR (with 95% CI) of UIQ CB cancer in woman &lt;40 followed for &gt;5 years6.9 (1.5 -19.9)6.3 (1.4-18.0)1.2 (0.5 –3.0)
Conclusion: Women &lt;40 years of age have an elevated long term risk of a second primary cancer in the UIQ of CB which is higher for optimal photon techniques than proton beam therapy.
Refernces
1. Ranger A, Dunlop A, Hutchinson K, et al. A Dosimetric Comparison of Breast Radiotherapy Techniques to Treat Locoregional Lymph Nodes Including the Internal Mammary Chain. Clin. Oncol. 2018;30:346–353.
2. Stovall M, Smith SA, Langholz BM, et al. Dose to the Contralateral Breast From Radiotherapy and Risk of Second Primary Breast Cancer in the WECARE Study. Int. J. Radiat. Oncol. Biol. Phys. 2008;72:1021–1030.
Citation Format: Sarah Settatree, Douglas Brand, Alison Ranger, Alex Dunlop, Anna Kirby. Dosimetric comparison of breast and pan nodal radiotherapy techniques to estimate the risk of developing a second primary cancer in the upper inner quadrant of the contralateral breast [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-12-25.