Single-Institutional Experience Assessing the Role of Quality Assurance (QA) CT Scans and Their Impact on Adaptive Planning with Pencil-Beam Scanning Proton Therapy in the Setting of Cone Beam CT (CBCT)

Abstract

Verification QA CTs are performed at pre-defined time intervals during proton therapy based on disease, patient- and plan-specific parameters. QA CTs are performed in the treatment position and imported into the treatment planning system (TPS). The proton therapy plan is recalculated in the QA CT and evaluated by the treating physician to assess target coverage, dose conformality, homo/heterogeneity and dose to organs at risk (OARs). Based on this analysis an adaptive plan may need to be generated. This process takes significant work effort from dosimetrists, physicists, therapists and physicians. We evaluate the incidence with which a QA CT resulted in an adaptive plan in an effort to improve and optimize clinical workflows. QA CT data was gathered from the inception of the proton center (2/2016-1/2018). During this timeframe, 1528 QA CTs were performed on 785 patients. All patients had at least one QA CT. The QA CT frequency varied from weekly to once during the entire course of therapy. The frequency of QA CTs per patient generally decreased with the clinical implementation of CBCT in 11/2016 and QA CT frequency guidelines were changed in 4/2017 as a result of an interim analysis. When ordered, CBCT use ranged from weekly to daily depending on disease site. CBCT review was utilized to guide QA CT ‘ad-hoc’ scans based on anatomic changes. The majority of QA CTs were performed for: prostate 545 (36%); thoracic 273 (18%); and head and neck 203(13%) cancers. A total of 82 adaptive plans occurred as a result of plan evaluation on a QA CT for a 5% incidence. Disease site breakdown of adaptive plans are as follows: head and neck 30%, lung 23%, breast/esophagus/pelvis 7% each, prostate 5%, craniospinal 4%, other 13%. Nine patients (11%) required two adaptive plans during the course of their therapy and one patient three adaptive plans. When evaluating the work effort associated with the QACT process (ratio of QA CT to adaptive plan), <1% of prostate QA CTs resulted in an adaptive plan; 13% of pelvic cancers; 12% of head and neck cancers; 12% of thoracic tumors; and 8% of breast cancers. The results of this analysis have led us to adapt our clinical workflows and time intervals in which we perform QA CTs, as well as the work efforts associated with the process. Adaptive planning must continue to take into account patient-related and plan-specific parameters, including changes seen during weekly on-treatment visits and on CBCTs to ensure the highest quality care.