Lung SBRT Credentialing in the Canadian LUSTRE Randomized Trial

Abstract

OCOG-LUSTRE is an ongoing phase III randomized trial (NCT01968941) comparing stereotactic body radiation therapy (SBRT) versus conventionally hypofractionated radiation for stage I medically inoperable non-small cell lung cancer (NSCLC). As part of this trial, an extensive credentialing process was undertaken due to the variability in Canadian lung SBRT practice. Here we report the final results from this initiative. A total of 15 Canadian centers participated in the SBRT credentialing process - 8 centers had previous SBRT experience (≥ 2 years) and 7 centers did not. Each center was required to satisfy three credentialing requirements: a) Completion of an institutional survey describing appropriate treatment, dosimetry, and image guidance equipment and parameters; b) Demonstration of acceptable delivery accuracy using the Imaging and Radiation Oncology Core (IROC) thoracic phantom; and c) Successful contouring and completion of standardized test cases for one central and one peripheral NSCLC. All contours and plan metrics were compared to the host institution as a nominal gold standard. All centers have successfully completed surveys and IROC phantom testing. Of the 30 SBRT test cases evaluated, 10 required a re-plan due to major deviations, mainly due to contouring errors. There were no differences in re-planning rates between established and new centers (37.5% vs 28.6%, p=0.61). Of the finally submitted plans, mean contouring errors (calculated as the distance between corresponding contours) were highest for brachial plexus in the central (C) case (12.6±6.6mm), and for the vessels in the peripheral (P) case (13.0±12.6mm). Other highly variable contours included ribs (4.9±4.9mm for C, 7.2±3.4mm for P), and proximal bronchial tree (PBT) (2.8±0.8 for C, 3.3±1.1 for P). Mean dice coefficients were similarly lowest for plexus (0.37±0.20 for C, 0.37±0.14 for P), PBT (0.77±0.06 for C, 0.75±0.09 for P), and vessels (0.69±0.29 for C, 0.64±0.31 for P), as well as esophagus (0.74±0.04 for C, 0.76±0.04 for P). Target volumes (ITV/PTV) and spinal cord contours demonstrated high concordance. All plans achieved per-protocol constraints for PTV coverage and maximum/volumetric constraints to organs-at-risk, although variations existed in dose gradients outside the target. For example, the standard deviation of the mean D2cm was 5.2 Gy (C), and 5.1 Gy (P), and the ratio of the 50% isodose volume/PTV increased by a factor of approximately 1.6 from its minimum to maximum. Our credentialing process demonstrates that small but non-negligible differences exist in both contouring and planning with lung SBRT, regardless of center experience. These variations were more likely to affect organs where concerns regarding late toxicity exist. Therefore, efforts to minimize differences through robust peer review and appropriate avenues of feedback are necessary to improve overall plan quality.