Preoperative Stereotactic Partial Breast Irradiation: An In Silico Dosimetric Comparison of a Novel Breast-Specific Stereotactic Radiation Therapy System and a Frameless Robotic Radiosurgery System

Abstract

Partial breast irradiation (PBI) has held the promise of shortened courses of radiotherapy with reduced exposure of normal tissues. However, initial postoperative results with PBI have been met with higher than expected rates of fair to poor cosmesis. Preoperative approaches increase patient eligibility, reduce target volumes, and improve dosimetry. At our institution, a novel breast-specific stereotactic radiotherapy (BSRT) device has been developed which employs a dynamic dose-painting technique with 36 noncoplanar Co-60 beams that rotate around the breast in the prone position. We sought a dosimetric comparison of BRST to a frameless robotic radiosurgery system (FRRS). Ten previously treated breast cancer patients were enrolled on an IRB-approved protocol and underwent CT simulation in the prone position with the BSRT patented vacuum-assisted breast immobilization cup. The preoperative gross tumor volume was simulated with a spherical target of equal diameter to each patient’s pathologic tumor size inside the lumpectomy cavity. Clinical (15mm) and planning (PTV) (3mm) target volumes were based upon recent trials and the device’s measured localization uncertainty. BSRT and FRRS plans were generated for each case requiring similar PTV coverage with maximal sparing of surrounding normal tissue. For FRRS planning, the CT image was rotated to simulate supine position as FRRS technique is unable treat prone. The dose to the normal ipsilateral breast, skin, heart, lung, and chest wall were recorded for several percentages of the prescribed dose (Vx%). All volumes and plans were generated by a trained physician/physicist team. Wilcoxon rank sum tests were utilized for statistical comparison. Despite the geometric advantage provided to the FRRS by prone breast anatomy and the vacuum-assisted breast cup immobilization, the BSRT device provided substantially improved sparing of nearly all normal tissues. On average, the BSRT especially reduced the normal breast tissue V5%/V20%/V50%/V80%/V100% by a relative 37.0%/40.8%/24.5%/32.3%/48.8% (p≤0.013) (Table 1), metrics which have been directly linked to cosmetic outcomes [Hepel et al. 2009]. BSRT also relatively reduced the heart max by 46.1% (p=0.005) and the skin V15% by 52.8% (p=0.005). Low and moderate doses to the lung and chest wall were similarly very low with both techniques (p>0.05). This novel BSRT device offers substantial and clinically meaningful dose reductions to the uninvolved normal breast tissue and the majority of organs at risk as compared to FRRS. These differences would be even further magnified under conventional set up for FRRS. The benefits of BSRT will be tested clinically in the pre-operative setting early in 2018.Abstract 3720; Table 1Mean dosimetric parameters BSRT vs. FRRSBSRTFRRSV5% Breast41.3%65.7%V20% Breast17.1%28.9%V50% Breast8.0%10.6%V80% Breast4.2%6.2%V100% Breast2.1%4.1%V15% Skin11.6%24.5%Heart Max7.6%14.1% Open table in a new tab