Inaccurate Positioning Might Introduce Significant Calibration Errorsto a Detector-Array QA Device for Flatten Filter Free Beams

Research Article

Austin J Radiat Oncol & Cancer. 2015;1(2): 1006.

Inaccurate Positioning Might Introduce Significant Calibration Errorsto a Detector-Array QA Device for Flatten Filter Free Beams

Wang S, Chao KSC and Chang J *

Department of Radiation Oncology, Weill Cornell Medical College, New York, USA

*Corresponding author: Chang J, Department of Radiation Oncology, Weill Cornell Medical College, 525 E 68 St, Box 575, NY 10065, New York

Received: October 09, 2014; Accepted: February 27, 2015; Published: March 02, 2015


Purpose: This study investigated the calibration error caused by misalignment of the beam with the MapCheck2 due to inaccurate positioning. We hypothesized the calibration for the Flatten-Filter-free (FFF) beam is more vulnerable to the positioning error than the conventional flattened beam.

Materials and Methods: A MapCheck2 was calibrated for the 10MV conventional and FFF beams with perfect alignment and with an introduced 1-cmpositioning error. The effect of positioning error was modeled as a detector independent multiplication factor to predict the calibration error for misalignment up to 1 cm. The calibrated sensitivities of both beams were compared to evaluate their dependence on the beam type.

Results: The 1-cm positioning error lead to 0.39% and 5.24% local calibration error for the conventional and FFF beams, respectively. After propagating to the edges of the MapCheck2, the calibration errors became 6.5% and 57.7%. The propagation error increased almost linearly with the positioning error. The percentage difference of sensitivities between the conventional and FFF beams was small (0.11±0.49%) without positioning error but was significantly larger (-32.2±17.8%) with1-cmmisalignment.

Conclusion: The positioning error is not handled by the current commercial calibration algorithm. The calibration errors for the FFF beam are ~9 times greater than that for the conventional beam with identical positioning error, and a small 1mm positioning error might lead up to 8% calibration error. Since the sensitivities depend weakly on the beam type, it is recommended to cross-check the calibrated sensitivities between the conventional and FFF beams to detect potential calibration errors.

Keywords: IMRT QA; Calibration error; 2D Detector array


(FFF): Flattening Filter Free; (2D): Two-Dimensional; (IMRT): Intensity-Modulated Radiotherapy (QA): Quality Assurance; (CAX): Central Axis; (LINAC): Linear Accelerator; (MU): Monitor Unit


MapCheck (Sun Nuclear Corporation, Melbourne, FL, USA) is a popular dosimetry device for radiotherapy [1-4]. It consists of a Two-Dimensional (2D) array of detectors that can be used for quick dosimetric check of radiation beams, particularly, the Intensity- Modulated Radiotherapy (IMRT) beams. Like many other dosimetry devices, the accuracy of this device depends on how well the detectors are calibrated. Since the sensitivities (detector responses to unit radiation fluence) of the detectors are not identical, a universal calibration factor does not apply and each detector requires individual calibration instead. A few calibration procedures [5-11] have been developed for dosimetry of 2D-array detectors, most of which can be applied to MapCheck. A successful calibration procedure should calibrate the radiation sensitivities of individual detectors with high fidelity so that they respond to incoming radiation uniformly. In addition, the calibration shouldn’t be over sensitive to uncertainty introduced during the calibration, e.g., machine output fluctuation, minor positioning errors…

For MapCheck calibration, the vendor recommends a patented algorithm [5] that the sensitivities of a row of detectors are calculated iteratively from the ratio of wide open-field images acquired with the MapCheck aligned with the beam Central Axis (CAX) and shifted laterally. Output variation is estimated from an image acquired with MapCheck rotated 180 degree, and removed from the iteratively calculated sensitivities. Although this calibration algorithm is efficient, it highly depends on the reproducibility of repeatedly delivered wide fields. Any local fluence fluctuation such as machine output variation will propagate to peripheral detectors due to the iterative nature of this algorithm and should be taken into consideration in the calibration algorithm.

Since the MapCheck is shifted manually during the calibration, a positioning error is inevitable. In this study, the positioning error is quantified as the displacement of the MapCheck from the required nominal calibration position. Since the detector sensitivity is calibrated based on the local fluence, a displacement of the MapCheck causes beam misalignment, which leads to a local variation of the expected fluence and ultimately results in a calibration error. This error shouldn’t be very significant for a flattened beam. Since the beam profile is relatively flat, a slight shift of the beam profile does not cause a significant change to the local fluence. For Flattening- Filter-Free (FFF) beams [12-16], on the other hand, the beam misalignment might lead to a higher calibration error because the beam profile varies considerably. Therefore, we hypothesized that the misalignment-induced calibration error depends on the gradient of the beam profile, and is more prominent for the FFF beam than the conventional flattened beam.

In the study, we investigated the errors of calibrated sensitivities for the conventional and FFF beams due to inaccurate positioning of the MapCheck during the calibration. A model was developed to predict the calibration errors using the calibration measurements with and without 1 cm displacement of the MapCheck. The calibration errors as a function of displacement were calculated and compared for the conventional and FFF beams.

Materials and Methods

Experiments of this study were conducted on a MapCheck2 device of our department commissioned for Quality Assurance (QA) of clinical IMRT beams. The MapCheck2 consists of an array of 1527 diode detectors distributed in a 26×32 cm2(row by column) area, but there are no detectors in the four 7×7 cm2 corner triangles. Detectors on each row have a 1 cm lateral spacing and the row-to-row interval is 0.5 cm. There is also a 0.5 cm lateral position difference between neighboring rows, leading to a diagonal detector-to-detector distance of about 0.71 cm. The diode detector of the MapCheck2has a photonenergy range from Co-60 to 25 MV, and an electron energy range from 6 MeV to 25 MeV.

MapCheck2 calibration

Figure 1 illustrates how the MapCheck2 is calibrated and how the positioning error might affect the calibration results. The calibration procedure is described in details in the patent by Simon et al. [5] And will be briefly summarized here. In Figure 1, R(X,Y) is the relative 2D in-detector beam fluence normalized to the center of the beam profile (the green profile) and S(X,Y) is the sensitivity for detector (X,Y) when the Central Axis (CAX) of the beam is aligned with the center of the MapCheck2. The detector reading is D0(X,Y)= R(X,Y)×S(X,Y), where the subscript 0 z. Please note that the beam shift is actually achieved by shifting the detector to the opposite side.