$bf{Purpose:}$ The goal of this study was (i) to use artificial intelligence to automate the traditionally labor-intensive process of manual segmentation of tumor regions in pathology slides performed by a pathologist and (ii) to validate the use of a well-known and readily available deep learning architecture. Automation will reduce the human error involved in manual delineation, increase efficiency, and result in accurate and reproducible segmentation. This advancement will alleviate the bottleneck in the workflow in clinical and research applications due to a lack of pathologist time. Our application is patient-specific microdosimetry and radiobiological modeling, which builds on the contoured pathology slides. $bf{Methods:}$ A U-Net architecture was used to segment tumor regions in pathology core biopsies of lung tissue with adenocarcinoma stained using hematoxylin and eosin. A pathologist manually contoured the tumor regions in 56 images with binary masks for training. Overlapping patch extraction with various patch sizes and image downsampling were investigated individually. Data augmentation and 8-fold cross-validation were used. $bf{Results:}$ The U-Net achieved accuracy of 0.91$pm$0.06, specificity of 0.90$pm$0.08, sensitivity of 0.92$pm$0.07, and precision of 0.8$pm$0.1. The F1/DICE score was 0.85$pm$0.07, with a segmentation time of 3.24$pm$0.03 seconds per image, achieving a 370$pm$3 times increased efficiency over manual segmentation. In some cases, the U-Net correctly delineated the tumors stroma from its epithelial component in regions that were classified as tumor by the pathologist. $bf{Conclusion:}$ The U-Net architecture can segment images with a level of efficiency and accuracy that makes it suitable for tumor segmentation of histopathological images in fields such as radiotherapy dosimetry, specifically in the subfields of microdosimetry.
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