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A Novel Equivalent Squares Formalism for use in Small Field Dosimetry
Başlık:
A Novel Equivalent Squares Formalism for use in Small Field Dosimetry
Yazar:
Qureshi, Aleem, author.
ISBN:
9780438098923
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (80 pages)
Genel Not:
Source: Masters Abstracts International, Volume: 57-06M(E).
Committee members: David Pearson, PhD; Diana Shvydka, PhD; Nicholas Sperling, PhD.
Özet:
With advancements in Linear Accelerators and other therapeutic radiation delivery systems, the use of highly modulated treatments (IMRT and VMAT) has become more common. Consequently, the use of high dose, hypo-fractionated treatments (Stereotactic Radio Surgery a.k.a SRS) for small lesions is also becoming increasingly common. Due to the hypo-fractionated and high dose nature of SRS the accuracy of small field dose calculation is of utmost importance. Additionally, the optimization of Intensity Modulated Radio Therapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT) plans can result in the use of small fields. Accurate calculation of dose in small fields is dependent upon the certainty and precision of small field dose measurement, and subsequently the accurate determination of machine output based upon these measurements.
Each of the three Treatment Planning Systems (TPS) (Phillps' Pinnacle 3, Varian's Eclipse, and Raysearch's Raystation), available at the University of Toledo, make use of Output Factors (OF) to characterize machine output as well as to simplify the commissioning process. Each TPS makes use of Equivalent Square Fields (ESF) to minimize the number of fields for which percent depth dose curves, and inline & crossline profiles that must be measured. The use of ESFs allows the TPSs to interpolate and/or extrapolate output factors for fields which were not measured, thus simplifying the commissioning process.
First, the traditional formalism for ESF must be evaluated for use in small fields. This is accomplished by measuring a series of small fields ranging from 0.6x0.6cm2 to 3.0x3.02 with three different types of detectors designed for use in small field dosimetry (Sun Nuclear Edge Diode, Standard Imaging Exradin A26, and the Standard Imaging Exradin W1 Plastic Scintillator) at 6MV and 6FFF photon beam energies along Central Axis (CAX) at 100cm Source to Surface Distance (SSD) and 10cm depth in a large water phantom.
Next the dose calculation of each of the three TPSs was evaluated by comparison to measurements made with the Standard Imaging W1 Plastic Scintillator horizontally oriented. The same fields, varying from 0.6x0.6cm2 to 3.0x3.032, were calculated in each of the three TPSs with a 1mm2 dose grid, along CAX at 100cm SSD and 10cm depth in a simulated large water phantom.
Finally, data acquired using the Standard Imaging W1 Plastic Scintillator was used to inform a novel formalism for Equivalent Squares which more accurately describes the output of the Varian Edge at 6MV and 6FFF. First the actual equivalent square was determined by assigning each unique output factor to a unique field size by determining a best fit equation from the output factors of the square fields. Both an experimental fit and a correction factor approach were used to determine the equivalent square field equation (function of X and Y jaw dimension) based on the newly determined equivalent square field sizes.
It was determined that the traditional formalism for equivalent squares is inaccurate in the calculation of small equivalent square fields, with errors as high as 9% at 6MV and 8.4% at 6FFF. The Standard Imaging W1 Plastic Scintillator was found to be the most consistent and accurate dosimeter in the evaluation of equivalent square fields. Additionally, it was shown that the Pinnacle 3 TPS was the most accurate in the calculation of small field dose because of its limitation to interpolation between commissioned output factors. While the experimental fit used to determine the new equation for equivalent square fields was more accurate, the more practical formalism for equivalent squares involves the use of correction factors. It was also determined that a machine specific, and quality specific correction factor should be used in the calculation of equivalent square fields.
Notlar:
School code: 0232
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
---|---|---|---|
XX(696393.1) | 696393-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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