Quantitative Analysis of the Treatment Set Up Reliability of a Prone Breast Boost for Early Stage Breast Cancer

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Adjuvant whole-breast radiation and a tumor bed boost for early-stage breast cancer can be delivered in the supine or prone position. Delivering the boost in the prone position has the advantage of removing the supine simulation CT and the capability of obtaining the total treatment dose distribution. However, the reliability of daily prone boost set up is largely unknown, and imaging is used for treatment verification. Here we evaluate the reliability of a daily clinical setup protocol for a prone mini-tangent boost, using daily cone beam CT (CBCT) to verify the adequacy of target coverage. We retrospectively reviewed 13 patients treated with a prone mini-tangent boost. The target (CTV) was the tumor bed without further expansion. Distance from CTV to block edge was 1.5cm. The boost prescription was 10Gy in 5 fractions. Coverage goals were the treating physician’s discretion, However, in all cases the planned dose to 90% of the CTV (D90) > 90% of prescription. Patients were positioned daily using a clinical setup with visual verification. Port films were taken to verify day 1 setup and when there was concern about the daily clinical setup. A CBCT was then acquired. If needed, additional shifts were made based on the CBCT, often followed by port films. Retrospectively, the simulation CT was independently registered to the daily CBCTs by a physician, and a daily tumor bed contour was generated. The treatment plan was transferred to the daily CBCT and delivered dose was calculated. To assess the reliability of clinical setup prior to CBCT acquisition, we assessed how often the daily D90 was < 90% of prescription dose (D90 < 90%) and how often the daily volume receiving full prescription dose (V100daily) did not receive the planned V100 (V100plan) by > 10% (V100plan-V100daily > 10%). Twelve patients with full imaging details were analyzed. Of 60 analyzable daily fractions, D90 < 90% for 3/60 fractions (5%), and V100plan-V100daily > 10% for 11/60 fractions (18%). Post-CBCT shifts were made for 1 fraction with D90 < 90% and for 2 fractions with V100plan-V100daily > 10%. Fractions for which coverage did not meet reliability metrics most commonly involved superficial tumor beds that extended into the region of subcutaneous dose build-up. In all such cases, allowing a portion of the target to receive less than prescription dose was deemed acceptable by the treating physician. We found that a daily clinical treatment setup for a prone boost was reliable in most cases. Addition of a CBCT was helpful to identify clinical setup variability. Treatment of a superficial tumor bed may be more sensitive to small variations in daily treatment setup due to the dose gradient across the target. When superficial target coverage is critical, supine boost positioning may be considered.