Dosimetric effects of the kV based image‐guided radiation therapy of prone breast external beam radiation: Towards the optimized imaging frequency

Purpose For prone breast treatment, daily image‐guided radiation therapy (IGRT) allows couch shifting to correct breast position relative to the treatment field. This work investigates the dosimetric effect of reducing kV imaging frequencies and the feasibility of optimizing the frequency using patient anatomy or their first 3‐day shifts. Method Thirty‐seven prone breast patients who had been treated with skin marker alignment followed by daily kV were retrospectively analyzed. Three IGRT schemes (daily‐kV, weekly‐kV, no‐kV) were simulated, assuming that fractions with kV imaging deliver a dose distribution equivalent to that in computed tomography (CT) planning, whereas other fractions yield a dose distribution as recreated by shifting the CT plan isocenter back to its position before the couch shift was applied. Treatment dose to targets (breast and lumpectomy cavity [LPC]) and organs at risks (OAR)s (heart, ipsilateral lung) in different schemes were calculated. Patient anatomy information on CT plans and first 3‐day couch shift data were analyzed to investigate whether these factors could guide imaging scheme optimization. Results When kV imaging frequency was reduced, the percentage dose changes (δD) for breast and LPC objectives (average