Abstract
BackgroundTo investigate the setup uncertainties and to establish an optimal imaging schedule for the prone-positioned whole breast radiotherapy.MethodsTwenty prone-positioned breast patients treated with tangential fields from 2015 to 2017 were retrospectively enrolled in this study. The prescription dose for the whole breast treatment was 266 cGy × 16 for all of the patients and the treatments were delivered with the SSD setup technique. At every fraction of treatment, patient was firstly set up based on the body localization tattoos. MV portal imaging was then taken to confirm the setup; if discrepancy (> 3 mm) was found between the portal images and corresponding plan images, the patient positioning was adjusted accordingly with couch movement. Based on the information acquired from the daily tattoo and portal imaging setup, three sets of data, named as weekly imaging guidance (WIG), no daily imaging guidance (NIG), and initial 3 days then weekly imaging guidance (3 + WIG) were sampled, constructed, and analyzed in reference to the benchmark of the daily imaging guidance (DIG). We compared the setup uncertainties, target coverage (D95, Dmax), V5 of the ipsilateral lung, the mean dose of heart, the mean and max dose of the left-anterior-descending coronary artery (LAD) among the 4 imaging guidance (IG) schedules.ResultsRelative to the daily imaging guidance (IG) benchmark, the NIG schedule led to the largest residual setup uncertainties; the uncertainties were similar for the WIG and 3 + WIG schedules. Little variations were observed for D95 of the target among NIG, DIG and WIG. The target Dmax also exhibited little changes among all the IG schedules. While V5 of the ipsilateral lung changed very little among all 4 schedules, the percent change of the mean heart dose was more pronounced; but its absolute values were still within the tolerance. However, for the left-sided breast patients, the LAD dose could be significantly impacted by the imaging schedules and could potentially exceed its tolerance criteria in some patients if NIG, WIG and 3 + WIG schedules were used.ConclusionsFor left-side whole breast treatment in the prone position using the SSD treatment technique, the daily imaging guidance can ensure dosimetric coverage of the target as well as preventing critical organs, especially LAD, from receiving unacceptable levels of dose. For right-sided whole breast treatment in the prone position, the weekly imaging setup guidance appears to be the optimal choice.
Highlights
To investigate the setup uncertainties and to establish an optimal imaging schedule for the prone-positioned whole breast radiotherapy
It is evident that the no daily imaging guidance (NIG) schedule led to the most frequent setup uncertainties in all of the 3 groups of error magnitude, followed by weekly imaging guidance (WIG) and 3 + WIG
The results show that the dosimetric coverage of the Clinical target volume (CTV) and Dmax were little impacted by the setup certainties for all the imaging guidance schedules
Summary
To investigate the setup uncertainties and to establish an optimal imaging schedule for the prone-positioned whole breast radiotherapy. The whole breast radiation treatment is delivered with patient in the supine position. There is evidence to show that some patients with large breast may benefit from radiation treatment delivered in the prone position in terms of reduced doses to lung and heart [4,5,6]. Since it is more difficult to immobilize patient in the prone position, it is more challenging to reproduce the patient setup positioning based on the tattoo location alone [7, 8]. Daily MV portal imaging guidance is considered to be needed for the accuracy and reproducibility of treatment setup, at the expense of added imaging doses to patient [9]. An alternative approach is to increase the beam margin (> 7 mm) to ensure the target coverage [10], which leads to inclusion of additional normal tissues into the treatment fields
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