Comprehensive end-to-end dosimetry audit for stereotactic body radiotherapy in spine, lung, and soft tissue

To create and conduct a comprehensive onsite end-to-end dosimetry audit to assess treatment accuracy of spine, lung, and soft tissue Stereotactic Body Radiotherapy (SBRT) across Australian and New Zealand (ANZ) radiotherapy centers.

The Australian Clinical Dosimetry Service (ACDS) anthropomorphic thorax phantom underwent a CT scan, planning, and treatment delivery according to local techniques at 128 facilities. Target volumes and dose constraints for spine, lung, and soft tissue were defined by the ACDS. Each plan was measured using Gafchromic EBT3 film and PTW 60019 microDiamond detector. A total of 782 plans were measured on 159 treatment machines of various classes and vendors. Audit results with the measured dose calculated as dose-to-medium, in medium (D_m,m) or dose-to-scaled density water, in water (D_w,w) were reported for all measurements, including those made in bone and lung equivalent materials.

The overall audit pass rate was 96% (271/281 plans) for the soft tissue case, 90% (215/238) for the spine, and 90% (236/263) for the lung. The average gamma pass rate for 5%/2mm criteria was 98.7% (soft tissue), 96.5% (spine), and 96.5% (lung). The average point dose difference was -1.0% (± 2.3%), 0.1% (± 3.8%), and -0.3% (± 3.2%) for the soft tissue, spine, and lung cases, respectively. The most common failure modes were in-target dose differences (41.6%) and Image Guided Radiation Therapy (IGRT) mismatches (36.7%).

High pass rates were seen for soft tissue, spine, and lung SBRT, indicating safe implementation of practice in the ANZ region. The modes of failure were assessed for suboptimal results, with the most frequent error due to IGRT mismatches, followed by dose differences in field, either underdosing or overdosing.