Answer: Patient-specific IMRT QA can be done in two ways. Fieldwise verification, using a 2D detector (film or electronic detector) with some build-up with perpendicular incident beam. The second option is to use the film or electronic detector placed in a phantom. In both cases the dose to the patient is not verified directly, but a hybride plan using the original patient’s fields applied to the phantom geometry and composition is created. Therefore, the verification does not use the original patient anatomy as defined in the planning CT. Dose computation does not include the effect of inhomogeneity, nor can it evaluate the effect of delivery discrepancies to the target and organ at risk structures.
COMPASS can determine the 3D dose distribution in the patient anatomy, based on the measured beam intensity. Therefore, it directly addresses the expected clinical consequences of delivery discrepancies, which are evaluated (among other representations) as DVH for target and OAR structures.
Answer: Primarily, COMPASS determines the fluence for all segments in a beam. As this quantity cannot be directly measured, COMPASS does first a calculation of the expected response (=electrical signal) for each segment and detector pixel, based on LINAC and detector models. After the measurement, expected and delivered responses are compared. The residual response (=response difference) is then used for a computation of the really delivered fluence.
The dose computation in COMPASS is a second, independent step in which the resulting dose to the patient is determined based on a collapsed-cone algorithm.
Answer:
Answer: We have to distinguish between MatriXXEvolution based COMPASS and Transmission Detector based COMPASS. For the MatriXXEvolution based version, the commissioning is equivalent to a TPS commissioning, and usually data from TPS commissioning can be used (profiles, depth dose curves, output factors, absolute calibration). A list of recommended input data is given in Appendix A.
The commissioning of COMPASS with the Transmission Detector is more demanding, taking 2 steps. Step 1 is equivalent to the COMPASS and MatriXXEvolution commissioning. Step 2 requires data measured with a commissioned Transmission Detector and the beam model from step 1. This refinement of beam model commissioning is needed to describe the response of the Transmission Detector with BU much lower than full build-up and in the vicinity of collimator, where electrons and scattered photons contribute considerably to the response. The measurements are carried out defining a plan (containing a set of geometric fields) delivered to a homogeneous phantom (e.g. stack of water equivalent or PMMA slabs).
Answer: COMPASS uses a collapsed cone superposition algorithm for 3D dose determination.
Answer: COMPASS uses the collapsed cone superposition algorithm, but in an implementation which is different from any TPS. The primary quantity determined by COMPASS is the fluence for each segment. Discrepancies in the delivery can be visualized as differences in the response pattern. The fluence determined in COMPASS is then used (together with the planning CT) as input for the dose computation with the collapsed cone algorithm, whose accuracy can be seen at the same level as state-of-the-art TPS algorithms.
A comparison of COMPASS and TPS algorithms (and commissioning) can be done by a pure computational dose determination in COMPASS, just assuming nominal delivery. This dose distribution (as well as the reconstructed dose distribution) can be exported as 3D DICOM dose cube for analysis with other tools.
Answer: COMPASS accepts the following file formats:
Answer: For each treatment, COMPASS needs 4 data sets (all DICOM):
Answer: The Transmission Detector is attached to the accessory holder of the LINAC. When mounted, it can be slided out to allow the usage of the light field. Gantry holders are available for LINACs from Elekta, Varian and Siemens.
Answer: There are 2 versions of gantry mount with SSD = 762 mm (1 cm resolution in isocenter, 31 x 31 cm2 max field size) and SSD = 1000 mm (0.762 cm resolution in isocenter, 23.6 x 23.6 cm2 max field size) . An additional build-up (30 x 30 cm2 plates) is placed on the detector, recommended 20 mm WE. No backscatter is needed.
Answer: Yes. COMPASS 2.0 supports MatriXXEvolution with the Gantry Angle Sensor. Measured frames are recorded together with the measured angle. The 3D dose calculation takes the actually measured gantry angle into account. Gantry angle measurements in combination with the Transmission Detector are in preparation.
Answer: For each detector used together with COMPASS (MatriXX, MatriXXEvolution, Transmission Detector) a software interface is needed which contains a detector modeling and control functions.
For verification of rotational plans, the MatriXX has to be upgraded to MatriXXEvolution functionalities in addition. This upgrade includes the Gantry Angle Sensor.
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