TU‐A‐T‐6E‐01: Patient QA

Abstract

The number of patients treated with Intensity Modulated Radiation Therapy (IMRT) has grown dramatically over the past five years. Recent clinical studies have shown that IMRT has the potential to improve outcomes for multiple disease sites and pathologies. However, the clinical successful or failure of each individual institution's IMRT program is dependant on the correct delivery of the 3D dose distributions calculated by the planning system (Planned Dose) to the correct location in the patient (Delivered Dose). There are many sources for errors that can arise during IMRT planning and delivery, but it is ultimately the responsibility of the Medical Physicist to ensure that the Planned Dose “agrees” with the Delivered Dose. Unlike conventional and 3D conformal radiotherapy, IMRT plans must be verified for each individual patient because of the many sources of potential error. The difference between Planned Dose and Delivered Dose (i.e. Error) in IMRT can originate from at least three different sources: 1.) The Treatment Planning Model, 2.) Treatment Delivery Mechanics, and 3.) Time‐Dependant Target/Tissue Positioning. The only method for determining the cumulative effect of all these potential sources of error is to measure the entire delivery sequence on the linear accelerator for each patient. Patient‐Specific IMRT quality assurance (QA) measurements can be classified into three levels. Level I measurements are taken using film, diode arrays, electronic imaging devices, etc. The measurement device(s) are typically placed at depth in water‐equivalent material. Level II measurements are taken with a phantom that moves to simulate intra‐fraction motion (such as respiration motion). These measurements are typically made with film, diodes, and/or ionization chambers. Level III measurements are performed using daily CT imaging acquired on the delivery system (CT‐on‐Rails, Cone‐Beam CT, Megavoltage CT, etc…). Electronic detector data acquired during treatment delivery is then combined with the daily CT images to reconstruct the doses delivered to the patient. This presentation will discuss commercially available dose measurement tools, phantoms, and techniques for performing Level I and II patient‐specific IMRT quality assurance. The current status of research and development in Level III IMRT quality assurance will also be discussed. Issues associate each technique will be discussed and clarified with practical examples. Recommendations will be presented for acceptable tolerances and testing frequencies.Educational Objectives:1. To understand the issues surrounding patient‐specific IMRT quality assurance2. To understand the difference between patient‐specific QA techniques3. To identify potential sources of errors in IMRT quality assurance4. To understand the impact of target localization and patient positioning on IMRT