(Sunday, 4/8/2018) 8:00 AM - 10:00 AM
Room: Marquis Ballroom 5-8
Measurement-Based Patient-Specific IMRT QA: Fact and Fiction
Patient-specific IMRT QA measurement is an important component of the process designed to identify discrepancies between calculated and delivered radiation doses. Several professional organizations (AAPM, ACR, ASTRO) have strongly recommended patient-specific IMRT QA be employed as part of the clinical IMRT process. While the value of patient-specific IMRT QA has been debated among medical physicists, especially whether computational methods can replace physical measurements, measurement-based patient-specific IMRT QA methods are still considered the gold standard. They are widely used and are the core element of most IMRT QA programs. In many centers, a QA measurement is routinely performed after a patient’s IMRT plan is created and approved by the radiation oncologist. The calculations and measurements are compared and approved or rejected using the institution’s criteria for agreement. If the agreement is deemed acceptable, then one infers that the delivered patient plan will be accurate within clinically acceptable tolerances. Understanding the details of methods used to evaluate the dose distributions are essential to identify and interpret the discrepancies between measurements and calculations.
Learning Objectives:
1. To review published data on the achieved agreement between measurements and calculations
2. To review commonly used measurement methods and discuss the advantages and disadvantages associated with each method
3. To discuss IMRT QA verification metrics, their use and vendor-implementation, including practical considerations when performing IMRT QA analysis
Is it safe, non-measurement based Patient Specific IMRT QA?
Purpose: Patient Specific QA (PSQA) of IMRT is to verify if beams are delivered as planned and to assure the delivered dose is the same as planned. This abstract is to review the methods of non-measurement based PSQA and its advantages.
Methods: PSQA may check 3 components; (1) If all of the plan parameters such as gantry, collimator, field size, MLC positions and MU are transferred correctly from the planning system to the treatment machine, (2) Beams are delivered as planned and (3) Dose distributions are identical within tolerance range. As an example of the non-measurement based PSQA, a University of Maryland method will be presented. UM has developed and adopted the non-measurement based PSQA which consists of a home-grown software to compare plan parameters between the planning system and the record and verify system, a comprehensive weekly MLC QA program and a Monte Carlo dose calculation system (currently, it is changed to a commercially available secondary dose calculation system). It is essential to measure enough number of cases and to compare the results to the planning system and the secondary calculation system when commissioning.
Results: The non-measurement based PSQA has 5 advantages; (1) not only confirms that the right plan which has been approved is transferred, but also plan parameters are compared with two decimal point precision, (2) compares dose volume histograms and 3-D dose distributions, (3) can detect dose variations from various clinical situations, such as very large patient dimension, irregular contour and heterogeneity correction, while a measurement-based method with a single phantom cannot, (4) can deliver treatments to any machines with same beam characteristics and same comprehensive QA program and (5) QA time is shorter and can be done even in office hour.
Conclusion: The non-measurement based PSQA provides more information and has many advantages over measurement based methods, while not compromising quality. It is safe to use non-measurement based PSQA.
Learning Objectives:
1. To review what are to be confirmed for IMRT QA,
2. To review the solutions of the secondary calculation,
3. To review what should be done to implement the non-measurement based IMRT QA.
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