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

Area of Science:

• Radiation Oncology
• Medical Imaging
• Breast Cancer Treatment

Background:

• Accelerated partial breast irradiation (APBI) offers limited incremental benefit over whole breast irradiation (WBI) or no radiotherapy in favorable early-stage breast cancer (ESBC).
• Accurate target coverage and minimal toxicity are paramount for justifying APBI.
• Precise tumor delineation is challenging, particularly in the prone position, with scarce data on accuracy.

Purpose of the Study:

• To evaluate the impact of an indicator clip marking tumor depth on precision and accuracy in APBI.
• To assess the added value of a preoperative CT scan in the treatment position for improving target delineation accuracy.

Main Methods:

• Four radiation oncologists delineated the clinical target volume (CTV) in 12 patients using standard guidelines (CTVstandard).
• Additional delineations included CTVclip (expansion around an indicator clip) and CTVclip_CT (expansion based on clip and preoperative GTV).
• Precision was measured by Jaccard index (JIpairs); accuracy was assessed by the overlap rate between the gross tumor volume (GTV) and CTVs.

Main Results:

• Interobserver precision improved significantly with the indicator clip (JIpairs 0.75) compared to standard delineation (JIpairs 0.38).
• Accuracy, measured as GTV-CTV overlap, improved with the clip (0.67) and was highest with the preoperative CT (0.88).
• Preoperative CT in the treatment position yielded the most optimal high-dose coverage (D95, D90) of the GTV.

Conclusions:

• An indicator clip enhances accurate target coverage for prone APBI but does not fully replace the benefits of a preoperative CT.
• Omitting preoperative CT risks missing the target, especially for small tumors, potentially compromising the APBI concept.
• Increasing target volumes to mitigate missing the target raises questions about the fundamental principles of APBI.