New IAEA end-to-end on-site IMRT audit methodology: Pilot test results

Introduction Keeping pace with rapid development of radiotherapy technology during recent years, many cancer centres have transitioned, or are about to do so, from 3-dimensional conformal radiation therapy (3D CRT) to intensity modulated radiation therapy (IMRT) including volumetric modulated arc therapy (VMAT). In view of the complexity of advanced radiotherapy techniques, the IAEA has developed an audit methodology to review the physics aspects of IMRT dose delivery. Pilot test results of the proposed methodology are presented in this report. Methodology Procedures were developed for end-to-end, on-site auditing of head and neck (H&N) IMRT treatments using a specially designed anatomical phantom named "Shoulder, Head and Neck, End-to-end" (SHANE) (CIRS, Norfolk, VA). During the audit, the phantom undergoes processes similar to a radiotherapy patient treatment, i.e., it is imaged on a computed tomography (CT) scanner, the irradiation is planned using a treatment planning system (TPS) and the plan is delivered using a linear accelerator. Normally the audit is expected to take two days. The ultimate goal of the audit is to verify the quality of IMRT treatment from a physics perspective by comparing the calculated and measured doses. The SHANE phantom can accommodate: (1) an ionisation chamber in four different locations to measure the dose at selected points associated with planning target volumes (PTVs) and an organ at risk (OAR), and (2) a film in a coronal plane to measure the dose distribution. Prior to the on-site visit, the participating centre is required to provide clinically used data of output factors, small beam profiles, and MLC QA tests. In addition, the centre has to prepare an IMRT plan for a virtual H&N patient represented by CT images of the SHANE phantom along with predefined structures. Specific dose constraints are defined for the planning process. Following the preparatory phase, an on-site visit is scheduled. Initial auditing activities include CT scanning of the phantom, importing the newly acquired CT phantom images into the TPS, transferring of the preliminary H&N plan and structures on these images, and re-optimizing the preliminary plan. The calculated doses at the ionisation chamber positions and in the coronal plane corresponding to the film position are recorded. These activities are followed by measurements of the beam output and subsequent film irradiations in a slab phantom for the film calibration, for checking small field profiles and for MLC QA tests. The local patient-specific QA of the treatment plan is also performed. Next, the SHANE phantom is positioned on the treatment couch using the local treatment position verification protocol and is irradiated according to the plan. The phantom is irradiated four times with an ionisation chamber in four different positions; the film located in the coronal plane is given three fractions. Finally, forms with preliminary results are filled in. In the following weeks, the auditing organization analyzes the results and sends the audit report to the participating radiotherapy centre with the appropriate comments and recommendations. The methodology described above has been pilot-tested in three radiotherapy centres in Europe.