Dose reduction from loss of backscatter in superficial x-ray radiation therapy with the Pantak SXT 150 unit.

Loss of backscatter has been measured for a Pantak Therapax SXT 150 superficial x-ray therapy unit. Ionisation chamber measurements at the surface were performed in solid water with and without underlying lead to confirm the applicability of previous published measurements. Measurements of loss of backscatter with and without lead allowed the effect of underlying lead to be separated from the effect of loss of backscatter. It was found that the difference between the effect of lead and that of air on surface dose is minimal for solid water phantom thickness exceeding 3 cm. Differences up to 3% in dose reduction at the surface were found between this work and previous published results with different x-ray units.

Output factor measurements for a kilovoltage X-ray therapy unit.

Output factors at the surface for treatment cones and lead cut-outs have been measured for a Pantak Therapax SXT 150 superficial therapy unit with x-ray beam qualities from 1 to 13 mm A1 HVL. A variety of phantom materials and two ionisation chambers were tested for their suitability in output factor and percentage depth dose measurement. Solid water proved a useful water-equivalent phantom material with discrepancies between measurements in water and solid water less than 2.3% for percentage depth dose and less than 0.6% for output factors. Larger measurement discrepancies were found for Plastic Water and Perspex. A PTW Markus chamber was found to compare well with a NE 2532/3 low energy chamber in percentage depth dose measurement, but discrepancies arose between the chambers in output factor measurements, up to 5% for small field sizes. Measurements indicated that the Markus chamber had an energy dependent response in the kilovoltage range, which could account for the discrepancy in output factor measurement.

Tin foil as bolus material for therapeutic electron beams from the Varian Clinac 2100C/D.

Tin foils of sub-millimetre thickness have been investigated as bolus material for therapeutic electron beams from the Varian Clinac 2100C/D linear accelerator. Measurements with ionisation chamber and radiographic film in Plastic Water or water were performed under tin foil bolus to determine surface dose, therapeutic ranges, output factor correction, penumbra and dose outside the field edge. Appropriate thicknesses of tin foil for 90% dose at the surface were found to be approximately 0.3 mm for 6 MeV, and 0.4 mm for 9 MeV and 12 MeV. Enhanced therapeutic interval with tin foil bolus over water-equivalent bolus has previously been reported, but was found not to be evident for 12 MeV and for a small (4 x 4 cm2) 9 MeV field. The penumbra width of fields with tin foil and water-equivalent bolus were found to be within 2 mm, while the doses at 1 cm outside the field edge were within 1.5% of peak dose. Output factor corrections for fields with tin foil were measured as within 2% of unity. Air gaps between the tin foil and phantom surface up to 5 mm were observed to have minimal effect on output correction factor, relative surface dose, and therapeutic range.

Radiological properties of normoxic polymer gel dosimeters.

The radiological properties of the normoxic polymer gel dosimeters MAGIC, MAGAS, and MAGAT [methacrylic and ascorbic acid in gelatin initiated by copper; methacrylic acid gelatine gel with ascorbic acid; and methacrylic acid gelatine and tetrakis (hydroxymethyl) phosphonium chloride, respectively] have been investigated. The radiological water equivalence was determined by comparing the polymer gel macroscopic photon and electron interaction cross sections over the energy range from 10 keV to 20 MeV and by Monte Carlo modeling of depth doses. Normoxic polymer gel dosimeters have a high gelatine and monomer concentration and therefore mass density (kg m(-3)) up to 3.8% higher than water. This results in differences between the cross-section ratios of the normoxic polymer gels and water of up to 3% for the attenuation, energy absorption, and collision stopping power coefficient ratios through the Compton dominant energy range. The mass cross-section ratios were within 2% of water except for the mass attenuation and energy absorption coefficients ratios, which showed differences with water of up to 6% for energies less than 100 keV. Monte Carlo modeling was undertaken for the polymer gel dosimeters to model the electron and photon transport resulting from a 6 MV photon beam. The absolute percentage differences between gel and water were within 1% and the relative percentage differences were within 3.5%. The results show that the MAGAT gel formulation is the most radiological water equivalent of the normoxic polymer gel dosimeters investigated due to its lower mass density measurement compared with MAGAS and MAGIC gels.

Effect of saccharide additives on response of ferrous-agarose-xylenol orange radiotherapy gel dosimeters.

Glucose, sucrose, starch, and locust bean gum have been used as additives to the ferrous-agarose-xylenol orange (FAX) gel dosimeter. The saccharide enhanced dosimeters were found to have a higher dose sensitivity over a standard FAX gel as measured by both optical density change and magnetic resonance imaging (MRI). With optical density measurement, OD-dose sensitivity increases were up to 55% for glucose, 122% for sucrose and 43% for starch, while locust bean gum did not give a consistent response. With MRI, R1-dose sensitivity increases were up to 178% with sucrose addition. The FAX gel with sucrose was studied in greatest detail. The OD-dose sensitivity dependence on cooling rate was reduced for the sucrose FAX gel over the standard FAX gel, which has significant implications for uniform dose sensitivity in large gel phantoms. The thermal oxidation rate in the sucrose FAX gel was up to 2.3 times higher than in the standard gel. The OD-dose sensitivity of oxygenated sucrose FAX gels was 4.3 times greater than standard FAX gels, while continued enhancement in OD-dose sensitivity with increased sucrose concentrations beyond 2.0 g/l was found only for the oxygenated sucrose FAX gels. Both the molar absorption coefficient of the ferric ion-xylenol orange complex at 543 nm and gel pH were not affected by the presence of sucrose, with the implication that the higher OD-dose sensitivity of gels with saccharides is due to increased chain reaction production of ferric ions.

Development of a CCD array imaging system for measurement of dose distributions in doped agarose gels.

An imaging system for agarose gel sections has been investigated for applications in rapid two-dimensional radiation dosimetry. The imaging system, with white light illumination and CCD camera detection, was designed for measurement of the radiation-induced optical density changes in iron- and xylenol orange dye-doped agarose gels. The performance of the imaging system was compared with that of a laser scanning system for the gels and with the accepted dosimetry standard, the ionization chamber. In measurement of beam profiles of two therapeutic radiation fields, relative dose values from the CCD camera imaging system were on average within 3% ranging from 0.005% to 7.5%) of values recorded with a parallel plate ionization chamber. In comparison with the laser scanner, the CCD camera imaging system provided comparable spatial resolution and an increased rate of data acquisition, although a consistently reduced signal to noise ratio was observed. Suggestions for improving the camera imaging technique include noise reduction through camera cooling and further frame averaging.

Testing random number generators for Monte Carlo applications.

Central to any system for modelling radiation transport phenomena using Monte Carlo techniques is the method by which pseudo random numbers are generated. This method is commonly referred to as the Random Number Generator (RNG). It is usually a computer implemented mathematical algorithm which produces a series of numbers uniformly distributed on the interval [0,1). If this series satisfies certain statistical tests for randomness, then for practical purposes the pseudo random numbers in the series can be considered to be random. Tests of this nature are important not only for new RNGs but also to test the implementation of known RNG algorithms in different computer environments. Six RNGs have been tested using six statistical tests and one visual test. The statistical tests are the moments, frequency (digit and number), serial, gap, and poker tests. The visual test is a simple two dimensional ordered pair display. In addition the RNGs have been tested in a specific Monte Carlo application. This type of test is often overlooked, however it is important that in addition to satisfactory performance in statistical tests, the RNG be able to perform effectively in the applications of interest. The RNGs tested here are based on a variety of algorithms, including multiplicative and linear congruential, lagged Fibonacci, and combination arithmetic and lagged Fibonacci. The effect of the Bays-Durham shuffling algorithm on the output of a known "bad" RNG has also been investigated.