Abstract
The purpose of this study was to determine the dosimetric impact of density variations observed in water‐equivalent solid slabs. Measurements were performed using two 30 cm×30 cm water‐equivalent slabs, one being 4 cm think and the other 5 cm thick. The location and extent of density variations were determined by computed tomography (CT) scans. Additional imaging measurements were made with an amorphous silicon megavoltage portal imaging device and an ultrasound unit. Dosimetric measurements were conducted with a 2D ion chamber array, and a scanned diode in water. Additional measurements and calculations were made of small rectilinear void inhomogeneities formed with water‐equivalent slabs, using a 2D ion chamber array and the convolution superposition algorithm. Two general types of density variation features were observed on CT images: 1) regions of many centimeters across, but typically only a few millimeters thick, with electron densities a few percent lower than the bulk material, and 2) cylindrical regions roughly 0.2 cm in diameter and up to 20 cm long with electron densities up to 5% lower than the surrounding material. The density variations were not visible on kilovoltage, megavoltage or ultrasound images. The dosimetric impact of the density variations were not detectable to within 0.1% using the 2D ion chamber array or the scanning photon diode at distances 0.4 cm to 2 cm beyond the features. High‐resolution dosimetric calculations using the convolution–superposition algorithm with density corrections enabled on CT‐based datasets showed no discernable dosimetric impact. Calculations and measurements on simulated voids place the upper limit on possible dosimetric variations from observed density variations at much less than 0.6%. CT imaging of water‐equivalent slabs may reveal density variations which are otherwise unobserved with kV, MV, or ultrasound imaging. No dosimetric impact from these features was measureable with an ion chamber array or scanned photon diode. Consequently, they were determined to be acceptable for all clinical use.PACS numbers: 87.55.km, 87.55.Qr
Highlights
232 Litzenberg et al.: Density variation in solid water requires the use of water for reference dosimetry measurements on an annual basis, it does not preclude the use of water-equivalent materials for more frequent consistency checks and quality assurance (QA) verification.[1]
232 Litzenberg et al.: Density variation in solid water requires the use of water for reference dosimetry measurements on an annual basis, it does not preclude the use of water-equivalent materials for more frequent consistency checks and QA verification.[1]. AAPM Task Group 106 allows the use of solid water-equivalent materials for some commissioning measurements for convenience.[2]. The relative convenience and ease of use of solid slabs over water tanks makes the various water-equivalent materials widely used
In this study we examine the dosimetric impact of such density variations observed in Certified Therapy Grade Solid Water 457 water-equivalent slabs
Summary
232 Litzenberg et al.: Density variation in solid water requires the use of water for reference dosimetry measurements on an annual basis, it does not preclude the use of water-equivalent materials for more frequent consistency checks and QA verification.[1]. Since Solid Water 457 is frequently used to comply with TG-106 and TG-51 requirements and for verification of clinical techniques such as IMRT, a higher grade of material, Certified Therapy Grade, is available for situations which may impact dosimetric measurements. These slabs receive a higher level of QA verification by the manufacturer and come with additional data verifying some aspects of the quality of the material. The striking nature of these density variations in CT images raises the question of their dosimetric impact on QA measurements, and the need for user-specific QA of each piece of WE material that will be used for quantitative measurements
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