MPC is an application to verify geometry and beam performances of Truebeam Linac, through automated checks based on their kV-MV imaging systems. Data acquisition in MPC comprises a series of 39 images (12 with kV and 27 with MV detector) acquired at predefined positions without and with the IsoCal phantom in the beam and with particular MPC pattern settings.
MPC performs geometric and dosimetric checks. The geometric checks intend to test the treatment isocenter size and its coincidence with imaging devices, the positioning accuracy of the imaging systems, the collimator, the gantry, the jaws, the MPC leaves and the couch position. The dosimetric checks refer to a reference MV image and give the beam output, uniformity and center change relative to the reference.
FEATURE OF MPC
The Machine Performance Check (MPC) is a new Truebeam major mode, designed to evaluate the machines geometric performance in five minutes. It employs a fully automated measurement sequence that uses the kV and MV imaging systems and the proven IsoCal phantom. The IsoCal phantom is a hollow cylinder 23 cm in diameter and length with 16 tungsten-carbide bearing balls (each 4 mm in diameter). For imager system calibration it uses a collimator plate attached to an accessory slot, having a steel pin in its center. For MPC implementation, the IsoCal phantom does not use such as a collimator plate and it is mounted to the couch top using a dedicated holder (figure a).
The operator enters into the MPC mode at the Truebeam console (figure b) and just initiates the procedures that takes place automatically. MPC automatically acquires a series of MV and KV images, moving the machine and imaging systems in the pre-defined position. The two detector panels are positioned at a distance of 150 cm from the source. The images are immediately processed and the result displayed for a quick evaluation, indicating whether the values are within system specifications (figure c), exported and reported.
The predefined acquisitions consist of a set of 39 images, 12 acquired by kV (XI system) with the IsoCal phantom in the field, 27 with MV (detected on the portal vision system) of which 20 are with and 7 without the IsoCal phantom in the field. The 39 images are the input for the machine performance parameter evaluation.
MPC has been here evaluated for the five photon energies available on our Truebeam machine: 6, 10 and 15 MV and 6,10 MV FFF.
For most of the MPC checks, an independent control has been performed at the same time of the acquisition of the MPC to evaluate the agreement of the two methods. For the independent checks, the here used procedures, phantoms and detectors were those available in the department and routinely used for quality assurance.
DISADVANTAGE OF MPC
Clearly MPC has contribute a major role in verifying geometry and beam performance of TrueBeam Linacs,by showcasing its reliableness,fast and easy to use method.But somehow, the MPC contains drawbacks of its own. These disadvantages includes the absence of linearity check in MPC and the fact that MPC is not a true QA tool.
- Absence of Linearity Check in MPC
Linearity check in MPC is a must have as it contributes to determine the linear reportable range for the TrueBeam. The MPC evaluates the machines geometric performance. It employs a fully automated measurement sequence that uses the KV and MV imaging systems and the proven IsoCal phantom. This process gives out a list of data. Everything that affects the data, for good or bad, is reflected one way or another on the table and the graph to be plotted.
The Linearity of the graph indicates higher accuracy and precision. Once the linearity check has been performed, then only the calibration verification can be executed in accordance with the Clinical Laboratory Improvement Amendments (CLIA) to verify the continued acceptable performance.
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MPC is not a true QA tool
Quality assurance (QA) tool is a tool used to measure and assure the quality of a product and quality control, the process of ensuring products and services meet consumer expectations. MPC is not considered as a true QA tool as it does not replace the need to perform a QA routine. MPC just acts as an application to verify geometry and beam performances of TrueBeam Linacs, through automated checks based on their KV-MV imaging systems but never influence the quality of the product produced.
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| IMPACT OF NOT USING MPC |
Other Application Similar with (MPC)
- Flattening Filter Free (FFF)
However, there are some differences of Machine Performance Check (MPC) with other application stated above.
MPC evaluated five different photon energies for different parameters which is 6MV, 10MV, 15MV, 6MV FFF and 10MV FFF. Compared to other application of Truebeam, they evaluated different ranges of photon energies of different parameters.
Treatment of IsoCentre of MPC determined by the IsoCal phantom gives the accuracy and detail result in the review workspace where the result of each test is possible to be analysed and trended. While the other application such as Catalyst HD may display IsoCentre error because of the IsoCentre is sometimes established by moving planned coordinates alone while Catalyts is running.
Results are shown in the review workspace in very detail. Detailed test results on the left side and acquired images and analysis on the right.
MPC measure the position accuracy of different couch axes using a reference position that was established as fixed room coordinate system using KV and MV images with IsoCal phantom. Whereas, other application such as Catalyst HD on Truebeam uses different mode setting which is cPosition and cMotion in order to positioning the patient correctly on the couch using the only one-beam-before-MV images.
The IsoCal phantom mounted on the couch top
