External partial breast irradiation in prone position: how to improve accuracy?

Introduction: In view of the limited incremental benefit between whole breast irradiation (WBI), accelerated partial breast irradiation (APBI) and omission of radiotherapy in favorable early-stage breast cancer (ESBC), APBI can only be justified if it combines adequate target coverage with the lowest achievable toxicity. Interobserver exercises demonstrated the difficulty of precise target delineation, especially in prone position; information on accuracy is even scarcer. We tested the impact of inserting an additional indicator clip, marking the depth of the tumor in the breast, and the added value of a preoperative CT in treatment position on precision and accuracy.
Material and Methods: In 12 patients, tumor bed delineation was performed by four radiation oncologists, with CTV _standard (clinical target volume) based on standard delineation guidelines, CTV _clip resulting from a 1-2-cm symmetrical expansion with the indicator clip as center and CTV _clip_CT expanding from the midpoint between the indicator clip and preoperative gross tumor volume (GTV) as center. Precision was measured as the mean pairwise Jaccard index (JI _pairs ) between observers, accuracy as the mean overlap between GTV and respective CTVs.
Results: JI _pairs was 0.38 for CTV _standard , 0.75 for CTV _clip and 0.59 for CTV _clip_CT . Overlap rate of GTV with CTVs was respectively 0.48, 0.67 and improved further to 0.88 for CTV _clip_CT . High-dose coverage of GTV (D95 and D90) improved with an indicator clip, but the most optimal result was reached when preoperative CT was added.
Conclusions: If EB-APBI in prone position is aimed for, an indicator clip intended to mark the depth of the tumor increases the probability of accurate target coverage, but cannot entirely replace the added value of a preoperative CT in treatment position. Avoiding the cost and effort of such CT implies a risk of missing the target, especially when small volumes are aimed for. Increasing target volumes to reduces this risk, questions the concept of APBI.