Developing reference plans for evaluating global clinical trials credentialing and PSQA systems.

Purpose: To develop a practical framework for creating a diverse set of validated reference plans (varying in complexity) and implement a workflow to introduce beam modeling, calibration, and delivery errors into the reference cohort to test and compare various dosimetry audit methodologies.

Methods: Sixteen IMRT and VMAT reference plans were created, using RayStation software, for four phantom geometries based on established credentialing cases from participating Global Harmonization Group (GHG) members. These reference plans were first validated in a multi-ion-chamber phantom. Nine dosimetric errors (perturbations) were introduced into the plans by modifying beam model and/or delivery parameters (MLC-offset, MLC-transmission, leaf-tip-width, PDD, beam calibration, and MLC-position) based on documented community distributions of errors; this produced 144 plans. The dose impact on the clinical target volume (CTV) and organs at risk (OARs) was determined, and a range of classifications was developed to determine if the perturbed plan should pass or should fail an audit.

Results: Introducing errors into the reference plans impacted each plan differently. Dose perturbations ranged from <1% to >10% in the mean dose to the CTV and <10% to >30% in the near maximum dose to OAR (D0.03). The 144 plans included clear "acceptable" and "unacceptable" scenarios, with significant changes in dose (relative to baseline reference values), as well as near pass/fail threshold results. Plan complexity was found to have a strong impact on dose deviation, and the mean MLC Gap metric was found to best capture this relationship.

Conclusion: This study presented a framework to develop a set of reference plans and perturbations that can be used to assess and compare various audit and PSQA methodologies. The GHG has developed this framework as part of our ongoing work to test the comparability of their audit systems; this framework supports our work of aligning international dosimetry audits across the globe.