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This study aimed to identify factors associated with strut-adjusted volume implant (SAVI) displacement in accelerated partial breast irradiation (APBI) using a SAVI device. We retrospectively analyzed computed tomography scans taken at the time of treatment planning and immediately before treatment in 61 patients (median age; 55 years, range; 40–85) treated with SAVI and determined the amount of SAVI displacement that occurred between the time from planning to the treatment. The displacement was calculated for the CT axis and SAVI axis, which is related to the SAVI structure. To investigate the cause of the displacement, multivariate analysis was performed on the calculated standard deviation and the insertion angle of SAVI with respect to the sternum in each cross-section, breast density, amount of air around the SAVI, and SAVI length inside the patient to obtain the β coefficient (p -value). On the CT coordinate system, positive correlations were observed between the SAVI insertion angle and air volume in the lateral (β coefficient:0.255–0.483) and rotational directions (β coefficient:0.341). On the SAVI coordinate system, positive correlations were observed between the SAVI insertion angle and air volume in all lateral (β coefficient:0.270–0.354) and rotational directions (β coefficient:0.294). A negative correlation was observed between the SAVI length inside the patient and the rotational direction (β coefficient: −0.262). SAVI insertion angle, the amount of the air outside SAVI and SAVI insertion length are factors which affect the displacement of the applicator. From the results, the applicator displacement and rotation must be < 3 mm and 10o in order to meet all the dose criteria. Thus, we should be aware of these factors during insertion of the device to avoid the problem in treatment delivery for the APBI. [ABSTRACT FROM AUTHOR]

There remains wide variation in the use of chest wall boluses for postmastectomy radiotherapy, which may result from the need for 2 treatment plans with the commonly used half-time tissue-equivalent 5-mm-thick bolus to achieve a full surface dose. To establish a bolus method requiring one treatment plan, we assessed the surface dose of a thinner daily bolus for all treatment fractions and compared it against the half-time 5-mm-thick bolus. In this basic study, we specifically investigated dosage achieved when using the following: (1) the existing bolus protocol, a half-time 5-mm-thick tissue-equivalent Clearfit bolus (Fujidenolo Inc., Aichi, JP); (2) no bolus; and (3) daily 1-, 2-, and 3-mm-thick Clearfit boluses at 4 and 6 MV photons. Dosimetric measurements were then taken in an anthropomorphic phantom to study the effect of each regimen on the surface doses, and the mean surface doses of the daily thinner boluses were compared with the existing bolus protocol by the Welch 2-sample t test. The mean surface doses for the existing bolus protocol, no bolus, and daily 1-, 2-, and 3-mm-thick boluses were 68% (range, 59% to 77%), 53% (41% to 66%), 73% (60% to 83%), 77% (66% to 87%), and 82% (75% to 91%) of the prescription dose at 4 MV, respectively; the corresponding values at 6 MV were 71% (63% to 79%), 50% (39% to 60%), 72% (56% to 83%), 81% (68% to 90%), and 89% (80 to 97%) of the prescription dose. The mean surface doses were comparable between the existing bolus protocol and the 1-mm-thick daily bolus at 4 and 6 MV. In conclusion, the mean surface dose of a 1-mm-thick bolus approximate that of a half-time 5-mm-thick bolus at 4 and 6 MV. As such, we have started a prospective clinical study on the safety and efficacy of a 1-mm-thick bolus for postmastectomy radiotherapy. [ABSTRACT FROM AUTHOR]