Factors Influencing Eligibility for Breast Boost Using Non-Invasive Image-Guided Breast Brachytherapy

Abstract

The intact breast presents a challenge for targeting of the tumor bed (TB) due to patient, breast, and respiratory motion. Non-invasive image-guided breast brachytherapy (NIBB) allows for accurate targeting of the TB for breast boost by utilizing breast immobilization and image-guidance. However, not all patients are ideal candidates for this technique. We evaluated our clinical experience using NIBB to indentify patient and anatomic factors which predict suitability for this technique. Consecutive patients with early stage breast cancer treated with breast conserving therapy were evaluated from April to November 2013. Only patients who were candidates for boost were included in this analysis. Patients with very small breast size (cup size A or smaller) for whom breast immobilization could not be achieved using NIBB were treated with en face electrons. All other patients underwent simulation for NIBB boost. NIBB was delivered using the AccuBoost System (Advanced Radiation Therapy, Inc., Billerica, MA). Patients who were found to be ineligible for NIBB boost on simulation underwent boost treatment with en face electrons or 3D conformal photons. The rate of eligibility for NIBB, reasons for ineligibility, and related patient and anatomic factors were analyzed. Fifty-two patients were evaluated of which 6 patients were ineligible for NIBB due to small breast size. Of the remaining patients who underwent simulation for NIBB boost, 33 patients (72%) were treated with NIBB. The most common reasons for ineligibility were absence of identifiable TB (n=5), inability to position the patient/breast to adequately target the TB (n=4), posterior TB location (n=3), and discomfort during breast compression (n=1). The likelihood of being eligible for NIBB boost was dependent on breast size and breast quadrant. The eligibility for NIBB according to breast cup size was: ≤ A (0%), B (50%), C (71%), D-DD (77%), and >DD (80%), p=0.002. The eligibility based on breast quadrant was: retroareolar/central (100%), upper outer (72%), upper inner (67%), lower outer (38%), lower inner (25%), p=0.07. For patients with a posterior TB, located along the chest wall, 73% were good candidates for NIBB. This was not statistically different compared to a non-posterior location, p=0.2. For patients without clips defining the TB only 45% were candidates for NIBB as compared with 79% of patients who had clips, p=0.05. NIBB boost is feasible in the majority of patients. Patients with larger breast size and a tumor bed located in the central breast or upper outer quadrant are more likely to be good candidates. Posterior TB location can be challenging for NIBB but the majority of patients are still candidates. Surgical clips are very helpful in defining the tumor bed for boost and greatly increase the likelihood of eligibility for NIBB.