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Feasibility of Using Large Plane-Parallel Ionization Chamber to Validate Small Field Dosimetry

P Sabouri , S Mossahebi , U Langner , M Guerrero , B Yi , W D'Souza , S Chen*, university of Maryland School Of Medicine, Baltimore, MD


(Wednesday, 8/1/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 3

Purpose: Commissioning small photon field dosimetry is challenging, particularly in stereotactic radiosurgery (SRS). This study investigates the use of a large plane-parallel chamber to measure the integral dose to validate the commissioning of small field dosimetry.

Methods: A large Bragg-Peak plane-parallel chamber (8cm diameter) is used in this study. The chamber is placed at 10cm depth water phantom and scanned with CT. The CT images are imported and the chamber is contoured in RayStation treatment planning system (TPS). The integral dose to the chamber is defined as a product of the average dose and the volume. A small field (1×1, 1.5×1.5, 2×2, 2.5×2.5, and 3×3cm2) is placed with isocenter at center of front entry window of the chamber in TPS, and the dose is measured in the same setup using 6X Flattening-Filter-Free of a Varian TrueBeam Edge. Since the field size is much smaller than the chamber dimension, a constant calibration factor converting the charge to the integral dose is applied for all field sizes. Field size 3×3cm2 is used as a reference field to calculate the calibration factor, and the calculated integral dose for small field sizes are compared to the measurements.

Results: The TPS calculated integral dose to the plane-parallel chamber is compared to the measurements for field size 1×1, 1.5×1.5, 2×2, and 2.5×2.5cm2, and the difference is -4.7%, -1.4%, -0.2%, and 0.3%, respectively. An excellent agreement is achieved for all field sizes except 1×1cm2. That indicates both output factor and profiles are commissioned accurately in TPS.

Conclusion: A large plane-parallel ion-chamber can be used to validate the accuracy of the small field dosimetry for SRS by measuring the integral dose. As an independent method, it can be used for Quality Assurance in order to avoid coarse errors during SRS commissioning.


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