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The Limit of Resolution and Detectability of the ArcCHECK QA Phantom in small field Volumetric Modulated Arc Therapy and Stereotactic Radiosurgery Quality Assurance

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, Master of Science (MS), University of Toledo, Biomedical Sciences (Medical Physics: Diagnostic Radiology).
Purpose: To determine the limit of detectability and resolution of the ArcCheck QA Phantom (Sun Nuclear, Inc.) for quality assurance of volumetric-modulated arc therapy and stereotactic radiosurgery procedures when used in small field sizes. Methods: Eight different square field sizes (0.6x0.6, 1x1, 2x2, 3x3, 5x5, 7x7, 10x10, 15x15 cm2) were measured on the ArcCheck QA phantom at three different gantry angles: 0, 90, and 270 degrees, using a 6 MV beam at its maximum dose rate of 600 MU/min and a dose computed from a 200 MU beam from the Varian Edge linear accelerator (Varian Medical Systems, Palo Alto, CA) at the University of Toledo Dana Cancer Center. Four different types of errors were introduced into quality-assurance analysis procedures. Measured square field sizes were compared against the same measured square field sizes with induced collimator and MLC errors. Induced collimator errors were defined by an expansion of the jaw-defined field size by 1 mm on all axes, a collimator shift of 1 mm on the X2 and Y2 axes, a table shift by 1 mm vertically and longitudinally at 270 and 90 degrees and a table shift of 1mm laterally and longitudinally for angles of 0 and 180 degrees. MLC induced errors included the addition of one and subsequently two opposing MLC leaves in the center of each square field. Dose distributions for the normal square fields and square fields with induced errors were imported into SNC patient software (Sun Nuclear Corporation, Melbourne, FL) in the form of DICOM RT dose files and measured dose distributions were compared between the normally measured square fields and fields containing induced errors. Percent pass rates were computed using gamma analysis criteria of 2 mm/2% with a threshold value of 20%. Point dose ratios were also analyzed for fields with induced MLC errors and output factors were calculated in order to determine the magnitude of the effect that these induced errors had on output measurements as compared with the ability of gamma criteria analysis in SNC to catch errors. A point dose calibration pertaining to each field size at each photon energy of the TrueBeam and Edge linear accelerators (Varian Medical Systems, Palo Alto, CA) was calculated by measuring a point dose at a range of field sizes at each energy (6 MV, 6 FFF and 10FFF for the Edge and 6 MV, 6FFF, 10 MV, and 18 MV for the TrueBeam) and dividing this number by the treatment planning system calculated point dose (calculated in Pinnacle) to obtain a cGy/MU dose calibration. An Extradin A16 Micropoint chamber (Extradin A1SL, Standard Imaging, Inc., Middleton WI) was placed in the center of the plug insert in the center of the ArcCheck phantom and a CNMC 206 electrometer (CNMC Instruments, Nashville, Tn) reading pertaining to a beam of 200 MU at different field sizes for each energy. The dose calibration factor for each energy was calculated and applied to six different patient-specific point dose QA analyses in order to determine the field size dependence of the dose calibration and to determine if the calibration improved the overall QA pass rate as well as the pass rate for individual fields for SRS QA. Finally, MLC errors were induced into three different patient-specific QA procedures performed on the Edge and TrueBeam linear accelerators. Two opposing MLC leaves were extended into the middle of the field (leaf position 30) at each control point of the first 180-180 degree clockwise field in each of the two patient QAs on the Edge and TrueBeam linear accelerators. The effect of extending the MLC leaves was analyzed using gamma analysis in SNC patient software. A point dose analysis of each QA was also taken into account and compared with the result measured using gamma criteria. Results: Examination of results in SNC patient software between measured normal fields and those with induced jaw field size errors indicate that the gamma criteria percent pass rates decrease significantly when errors are induced in the quality assurance analysis. Pass rates for a table shift and increase in field sizes by 1 mm on all axes of the square field indicate the greatest average errors for all gantry angles measured. Evidence of normal error detection was seen at a field size of 3x3 cm2 for a table shift at a 0-degree gantry angle. The field size at which normal error detection was seen by the ArcCheck was indicated at 2x2 cm2 for the 1mm margin errors induced at 90 degree and 270 degree gantry angles. The field size at which normal error detection was seen by the ArcCheck with MLC error induction into square field sizes was indicated at a field size of 2x2 cm2. Two QA procedures that did not improve by applying the field-size specific calibration factor decreased by an average of 0.44%. Three patient-specific quality assurance procedure dose distributions measured with an induced MLC error indicate that errors in MLC leaf position when applied to all control points of a full 360-degree arc are indicated with a lower percent gamma/DTA criteria pass rate. These pass rates were 77.4% and 96.1% on the Edge and 96.5% on the TrueBeam accelerator, respectively, when a measured normal dose distribution and a dose distribution with an induced MLC error were compared in SNC patient software. Of the six patient-specific quality assurance procedures for which a field-size specific point dose calibration factor was applied, four were improved significantly by an average of 87.6% with the application of a field-size specific calibration factor. Discussion and Conclusion: This work indicates the potential for having the ability to detect potential errors in VMAT quality assurance for small field sizes using the ArcCheck QA phantom. The ability of the ArcCheck to detect uncertainties in quality assurance procedures is based on the size of the field and the position and spacing of the diode detectors. Gamma analysis and point dose measurements indicate a 3x3 cm2 field size as the smallest field size at which accurate quality assurance is analyzed. Pass rates resulting in an induction of MLC errors in square field sizes can be utilized to predict pass rates resulting from the induction of MLC errors in patient-specific quality assurance procedures. It is suggested that a field-size specific CGy/MU calibration factor is utilized in order to more accurately predict patient-specific point dose measurements.
David Pearson (Advisor)
E. Ishmael Parsai (Committee Member)
Dianna Shvydka (Committee Member)

Recommended Citations

Citations

  • Gray, T. (n.d.). The Limit of Resolution and Detectability of the ArcCHECK QA Phantom in small field Volumetric Modulated Arc Therapy and Stereotactic Radiosurgery Quality Assurance [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=mco1438969935

    APA Style (7th edition)

  • Gray, Tara. The Limit of Resolution and Detectability of the ArcCHECK QA Phantom in small field Volumetric Modulated Arc Therapy and Stereotactic Radiosurgery Quality Assurance. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=mco1438969935.

    MLA Style (8th edition)

  • Gray, Tara. "The Limit of Resolution and Detectability of the ArcCHECK QA Phantom in small field Volumetric Modulated Arc Therapy and Stereotactic Radiosurgery Quality Assurance." Master's thesis, University of Toledo. Accessed MARCH 29, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=mco1438969935

    Chicago Manual of Style (17th edition)