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 (delta D) for breast and LPC objectives (average <1%) were smaller than those for heart and lung (average 28% 31% for D-mean). In general, the delta D of no-kV imaging was approximately that of weekly kV imaging x a factor of 1.2 1.4. Although most dose objectives were not affected, the potential higher heart dose may be of concern. No strong correlation was found between delta D for different kV frequencies and patient anatomy size/distance or the first 3-day couch shift data. Conclusions: Despite resulting in lower imaging dose, time, cost, and similar target coverage, a reduction in kV imaging frequency may introduce higher heart complication risk. Daily kVs are needed more in left-sided breast patients. A less frequent imaging schedule, if considered, cannot be individually optimized using CT anatomic features or early shift data.