Initial Clinical Experience of CBCT-Based Adaptive Online Radiotherapy for SAbR of Thoracic Malignancies

Abstract

<h3>Purpose/Objective(s)</h3> There has been an increasing trend in the availability and utilization of cone beam computed tomography (CBCT)-based adaptive online radiotherapy (AORT) in radiation oncology clinics. In this study, we report our initial experience of utilizing AORT to deliver stereotactic ablative radiation therapy (SAbR) for either early-stage non-small cell lung cancers (NSCLCs) or metastases to the thorax. <h3>Materials/Methods</h3> We recruited 18 patients treated with daily AORT whose tumors were treated with a total prescribed dose ranging from 33-60Gy treated over 3-5 fractions. During the AORT fraction, the physician contoured an ITV and critical OARs on the CBCT and two plans were created for the physician to choose from, a scheduled plan and an adaptive plan. The scheduled plan was a re-calculation of the reference plan based on the anatomy of the patient on that day's fraction while the adaptive plan was a re-optimization of the reference plan. For each scheduled and adaptive plan, we recorded PTV and ITV coverage (V100%), dose conformality (R50 and R100), and the maximum dose (D_max) received by the esophagus, bronchus, trachea, heart, and spinal cord. Timing data for each fraction from CBCT acquisition to approved plan and patient ready for treatment were also recorded. Dosimetric data between the scheduled and adaptive plans were compared relative to the scheduled plan. Paired t-tests were used to evaluate differences between the scheduled and adaptive plans. <h3>Results</h3> Among the 68 fractions analyzed, there was a reduction in ITV volume over the course of the adaptive treatments in 54.4% of cases (range: 0.7%-82.4%, mean: 24.3%), with physicians selecting the adaptive over scheduled plan 88.2% of the time. Relative to scheduled plans, mean difference in PTV and ITV coverage was 2.99% (p=0.009) and 1.07% (p=0.005) higher, respectively. Dose conformality, R50 and R100, were improved in 62.5% (p=0.05) and 66.7% (p=0.098) of adaptive compared to the scheduled plans. Mean difference in D_max relative to the scheduled plan for critical OARs ranged from -5.62% to 4.03%, only heart and spinal cord showing significance (p=0.004, p=0.023). Timing data demonstrated an average of 9 min spent on contouring, 5 min on plan optimization, and 5 min for online quality assurance. <h3>Conclusion</h3> A significant proportion of the patients in our SAbR-treated CBCT-adaptive therapy cohort demonstrated ITV volume changes during overall course of treatment. AORT-based lung SAbR treatments significantly improved PTV/ITV prescription coverage (compared to scheduled plan deliveries), dose conformity, and OAR sparing, with <15 min added to the overall treatment time for online contour and reoptimization. Overall, CBCT based-AORT is a robust approach to offer high quality online reoptimized plans for SAbR treatment of thoracic malignancies.