Abstract Purpose/Objective(s): The purpose of this paper is to report the dosimetric effects of conscious sparing of contralateral lobe of thyroid gland in carcinoma breast patient receiving locoregional radiation by tangential beam Intensity Modulated radiotherapy technique. Materials/Methods: Treatment plans of 20 Ca Breast patients, who received post mastectomy adjuvant locoregional radiotherapy, were evaluated. Since October 2021 we adopted a department protocol of conscious sparing of contralateral thyroid in breast radiotherapy. 10 of these had conscious sparing of contralateral thyroid (group A) whereas 10 of old plans were evaluated when C/L thyroid were not given constraints & are being passively spared due to location on contralateral/non treatment side of neck (group B). Treatment planning was done using tangential beam (6 MV) IMRT in Eclipse 13.7 TPS in all of these patients. All of these patients received treatment to chest wall & SCF. RTOG contouring guidelines were followed in all the patients. Prescription Dose was 40Gy in 15# for both groups. The best treatment plans were selected ensuring 95% PTV dose coverage and acceptable dose to OARs. Uniform contouring, plan optimization & evaluation protocols were followed for all of these patients. Results: The mean thyroid volume in group A is 4.30cc & in group B is 3.99cc which is not statistically different (P value -0.639) The mean dose of thyroid in group A is 7.98Gy whereas in group B it is 17.00Gy which is spastically significant (P value – 0.000003). Comparison of mean dose to contralateral lobe (in Gy) between the two groups Groups No. Mean ± SD ‘t’ value P value Thyroid dose with constraint 10 7.98±1.46 -6.702, df=18 0.000003* Thyroid dose without constraint 10 17.00±3.99 Unpaired ‘t’ test applied. P value = 0.000003, Significant Conclusion: OARs present on contralateral side are passively spared due to the location. This often results in a tendency of not actively prescribing constraints to these. This study emphasizes that conscious sparing of contralateral Thyroid lobe can further reduce the dose to significant levels. This reduction can preserve thyroid function further. Further clinical trials are required to corroborate this dosimetric gain with clinical thyroid function preservation. Citation Format: Manish Chomal, Minaal Iyer, Ananth Kalimurthy, Joshua Sahay, Sushanta Banik. Concious sparing of contralateral thyroid lobe in Ca Breast patients receiving post mastectomy radiotherapy by IMRT technique – A single institution dosimetric study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-10-15.