Computed Tomography Guided Online Adaptive Stereotactic Body Radiation Therapy for Lymph Node Oligometastases: Impact on Dose to Target and Organs at Risk.

Abstract

Purpose: This study investigates the radiation dose in abdominal and pelvic lymph node oligometastases (A-P LN) and gastrointestinal organs (GIOs) of patients undergoing computed tomography (CT) guided online adaptive stereotactic body radiation therapy in a phase 2 study. The study aimed to increase the dose to the target iso-toxically.

Methods and materials: Patients with oligometastatic A-P LN received 45 Gy in 5 fractions on the CyberKnife. Each patient had 3 plans created using a pretreatment planning CT scan: plan A, standard of care (SOC) plan based on the planning CT; plan B, adaptive plan with GIO contours from a diagnostic CT; plan C, adaptive plan with 45 Gy prescribed to the 80% isodose-line. After a prefraction in-room CT scan, the radiation therapy technologist used a decision tree to select the plan with the highest target coverage without exceeding organs at risk constraints. Dose volume histogram parameters were extracted from the original planning CT, the fraction CT with the selected library plan and the fraction CT with the SOC plan.

Results: In total, 52 patients were included, and 55 online adaptive treatments were performed. An adaptive plan was chosen in 58% of fractions and in 78% of the patients and resulted in a significant higher D_mean compared with the planned dose (100.6 biologically effective dose (BED₁₀) vs 95 BED₁₀; P < .001). The GIO D_0.5cc was 2.7 Gy EQD₂³ higher for the SOC plan on the fraction CT compared with the SOC on the original planning CT (P = .009). There was no significant difference in GIO D_0.5cc between the SOC plan on the fraction CT and the selected plan on the fraction CT.

Conclusions: CT-based online adaptive stereotactic body radiation therapy for A-P LN oligometastases, using a library of plans, led to an adaptive plan selection in the majority of patients. This workflow enabled a mean BED₁₀ 5 Gy isotoxic dose escalation to the gross tumor volume.