Briefly, for Stage 1 of the study, 20 breast cancer patients undergoing adjuvant radiotherapy after breast conserving surgery with a visible SC (cavity visualization score 3 - 5) will be recruited. Skin markers will be placed on the skin around the SC during CT simulation in the manner specified in the RAPID trial. During the first week of their treatment, patients will return to the CT scanner on days 1 and 5 to undergo repeat planning CT. The patients will be set up simulating the actual treatment set up at the linear accelerator. These repeat scans will be compared with the original scan to identify any change in the relationship between the skin markings and the SC. Five oncologists trained in SC identification will then contour the SC on the original as well as the repeat CT scans. These images will be fused for each CT scan to form a representative SC volume (RSCV). The RSCV in planning and subsequent CTs will be related to skin marks to evaluate the reliability of patient positioning. Using these skin marks the beams will be placed on the patient in the CT simulation software package.