Comprehensive QA for IMRT: The role of internal and external audits

Full text: Intensity modulated radiation therapy (IMRT) is becoming a routine practice at many modern radiation oncology facilities. The treatment planning and beam delivery processes in IMRT can be very complex and require special attention with regard to quality assurance (QA). At our institution, a commercially available inverse planning system is used together with dynamic multileaf collimation (DMLC) to plan and deliver IMRT. In this work we describe the overall implementation of a comprehensive QA programme for the clinical implementation of IMRT with emphasis on QA auditing. The QA programme covers four major areas: delivery hardware QA, treatment plan specific measurements, internal annual audit of IMRT cases, and participation in clinical trials requiring an external audit of procedures leading to institutional credentialing. The four main aspects of the programme are outlined below. Delivery hardware QA refers to the procedures designed to test the performance of the DMLC treatment delivery, in terms of MLC leaf position, leaf acceleration and speed, and MLC position reproducibility. The procedures are usually carried out in solid phantoms using radiographic film. Treatment plan specific measurements must be performed for every treatment plan. The complex nature of dynamically delivered IMRT beams makes it impossible to verify monitor unit calculations and field portal geometry in a simple fashion. A dose distribution based on the patient's beam intensities and treatment geometry is calculated on a QA phantom and measurements with an ionization chamber at one or more points in the phantom are made and compared with the calculated values. A planar distribution is measured with film and also compared to calculated values. The annual internal audit is a complete review of thirty IMRT cases from the previous calendar year, and is comprised of three components: an assessment of the treatment plan with respect to plan evaluation parameters, verification of the patient's daily treatment record, and a review of the physics QA tests and documentation for each patient. Quantitative acceptability criteria, based on relevant endpoints, are formulated for each parameter investigated. For each patient, up to 21 parameters are evaluated and a score of 1 is assigned for compliance or 0 for non-compliance with the acceptability criteria. A total score is calculated for each patient, and an overall score is computed for the entire data set. Participation in multi-institutional collaborative trials and protocols such as those endorsed by the Radiation Therapy Oncology Group (RTOG) may require credentialing of the entire IMRT planning, treatment, and QA process by an external agency such as the Radiological Physics Center (RPC) or the Quality Assurance Review center (QARC). The credentialing process may include the completion of a facility questionnaire, submission of 'dry run' plans, approval of the institution's QA process, and a phantom experiment requiring CT scanning, planning, and delivery of an IMRT treatment on a phantom containing dose measuring devices. Additionally, once credentialed for protocol participation, all patients enrolled in the study have every aspect of their treatment scrutinized by an external agency. In summary, a comprehensive quality assurance programme specific to IMRT is a necessity. The programme should include components addressing the delivery hardware and plan specific dosimetric measurements. Auditing should also play a key role in the implementation of IMRT. Internal departmental audits are a valuable tool for retrospective analysis of patient cases and QA procedures. External audits are important because they are usually conducted by an unbiased observer objectively, and ensure that the institution is complying with international standards. Moreover, even more importantly, participation in these trials and credentialing processes ensures an appropriate standard of practice for all participating institutions. (author)