On the feasibility of treating to a 1.5 cm PTV with a commercial single-entry hybrid applicator in APBI breast brachytherapy

Purpose: To evaluate and determine whether 30 patients previously treated with the SAVI^TM device could have been treated to a PTV_EVAL created with a 1.5 cm expansion. This determination was based upon dosimetric parameters

derived from current recommendations and dose-response data.

Material and methods: Thirty patients were retrospectively planned with PTV_EVALs generated with a 1.5 cm

expansion (PTV_EVAL_1.5). Plans were evaluated based on PTV_EVAL_1.5 coverage (V90, V95, V100), skin and rib maximum doses (0.1 cc maximum dose as a percentage of prescription dose), as well as V150 and V200 for the PTV_EVAL_1.5. The treatment planning goal was to deliver ≥ 90% of the prescribed dose to ≥ 90% of the PTV_EVAL_1.5. Skin and rib maximum doses were to be ≤125% of the prescription dose and preferably ≤ 100% of the prescription dose. V150 and V200 were not allowed to exceed 52.5 cc and 21 cc, respectively. Plans not meeting the above criteria were recomputed with a 1.25 cm expanded PTV_EVAL and re-evaluated.

Results: Based on the above dose constraints, 30% (9/30) of the patients evaluated could have been treated with a 1.5 cm PTV_EVAL. The breakdown of cases successfully achieving the above dose constraints by applicator was: 0/4 (0%) 6-1, 6/15 (40%) 8-1, and 3/11 (27%) 10-1. For these PTV_EVAL_1.5 plans, median V90% was 90.3%, whereas the maximum skin and rib doses were all less than 115.2% and 117.6%, respectively. The median V150 and V200 volumes were 39.2 cc and 19.3, respectively. The treated PTV_EVAL_1.5 was greater in volume than the PTV_EVAL by 41.7 cc, and 60 cc for the 8-1, and 10-1 applicators, respectively. All remaining plans (17) successfully met the above dose constraints to be treated with a 1.25 cm PTV_EVAL (PTV_EVAL_1.25). For the PTV_EVAL_1.25 plans, V90% was 93.7%, and the maximum skin and rib doses were all less than 109.2% and 102.5%, respectively. The median V150 and V200 volumes were 41.2 cc and 19.3, respectively. The treated PTV_EVAL_1.25 was greater in volume than the PTV_EVAL by 16 cc, 24.9 cc, and 33.5 cc for the 6-1, 8-1 and 10-1 applicators, respectively.

Conclusions: It is dosimetrically possible to treat beyond the currently advised 1.0 cm expanded PTV_EVAL. Most patients should be able to be treated with a 1.25 cm PTV_EVAL and a select group with a 1.5 cm PTV_EVAL. Applicator size appears to determine the ability to expand to a 1.5 cm PTV_EVAL, as smaller devices were not as propitious in this regard. Further studies may identify additional patient groups that would benefit from this approach.