Room: Exhibit Hall | Forum 5
Purpose: For prone breast treatment, kV imaging at the treatment angle allows to correct breast tissue and chest wall positions relative to the treatment field, at the cost of more patient on-couch time and imaging dose. This work investigates the influence of different kV imaging frequencies for prone breast IGRT.
Methods: 10 prone breast treatment courses for low risk of secondary cancer were included The daily couch shift data for kV IGRT after skin marker alignment were used to simulate three kV IGRT schemes: 1) daily-kV, 2) weekly-kV, 3) no-kV. These schemes were compared in terms of a)dose to target and normal tissues b)imaging time, and c)imaging dose. To ascertain a), it is assumed that each fraction with kV imaging delivers a dose distribution same as CT plan, while other fraction yields a dose distribution as recreated on the CT plan by shifting isocenter according to couch shift data. The correlation of dose changes of different schemes were also analyzed.
Results: Between these schemes, small dose changes to breast and lumpectomy cavity(averageâ‰¤0.3%) were observed, while maximum dose to heart, ipsilateral lung, and chest wall could increase significantly (up to 144%-174%) when kV imaging was omitted. (Still, dose constraints including heart V30 and lung V20 are within tolerance.) Compared to daily kV imaging, dose changes to all structures were magnified from weekly-kV to no-kV by a factor of 1.2-1.41 (correlation coefficient>0.96); Patient on-couch time was shortened by 15%(weekly-kV) and 20%(no-KV), and total imaging dose was reduced by 2.4 cGy(weekly-kV) and 3.2 cGy(no-kV).
Conclusion: A reduction in kV imaging frequency increases dose deviation from the treatment plan, while imaging dose and time are lowered. The increased dose to normal structures varies significantly between patients, but is below the limit. Further investigation on patient characteristics will be conducted to predict optimal imaging frequency.