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The main aim of the current study was to assess the dosimetric impact on intensity modulated stereotactic radiotherapy (IMSRT) for locally persistent nasopharyngeal carcinoma (NPC) due to the recalculation from the Anisotropic Analytical Algorithm (AAA) to the recently released Acuros XB (AXB) algorithm. The dosimetric accuracy of using AXB in predicting air∕tissue interface doses from an open single small field in a simple geometric phantom and intensity modulated small fields in an anthropomorphic phantom was also investigated.The central axis percentage depth doses (PDD) of a rectangular phantom containing an air cavity were calculated by both AAA and AXB from 6 MV beam with small field sizes (2 × 2 to 5 × 5 cm(2)). These data were compared to PDD measured by thin thermoluminescent dosimeters (TLDs) and Monte Carlo simulations. The doses predicted by AAA and AXB near air∕tissue interfaces from five different IMSRT plans were compared to the TLD measured doses in an anthropomorphic phantom. The PTV coverage, conformity and doses to organs at risk (OARs) calculated by AAA and AXB were compared for 12 patients, using identical beam setup, leaves movement and monitor units.Testing using the simple rectangular phantom demonstrated that AAA and AXB overestimated the PDD at the air∕tissue interfaces by up to 41% and 6%, respectively, from a 2 × 2 cm(2) field. The secondary build-up curves predicted by AXB caught up well with the measured data at around 2 mm beyond the air cavity. Testing using the anthropomorphic phantom showed that AAA overestimated the doses by up to 10%, while the measured doses matched those of the AXB to within 3%. Using AAA, the planning target coverage represented by 100% of the reference dose was estimated to be 4% higher than using AXB. The averaged minimum dose to the PTV predicted by AAA was about 4% higher and OARs doses 3% to 6% higher compared to AXB.AXB should be used whenever possible as the standard reference for IMSRT boost of NPC cases. The more accurate AXB indicating lower target coverage and lower minimum target dose compared to AAA should be noted.

Gafchromic XRCT, radiochromic film is assessed over a broad energy range, from kilovoltage to megavoltage x rays for variations in reflected optical density to dose response. A large energy dependence was found with reflected optical density output for the same delivered dose varying from 7.8 +/- 0.35 at 25.5 keV (50 kVp) peaking at 12.1 +/- 0.5 at 54 keV (125 kVp) to 0.975 +/- 0.03 at 2300 keV (10 MV) when normalized to 1 at 1400 keV (6 MV) energy. The response is constant (within 3%) in the 36-69 keV equivalent photon energy range, which corresponds to x-ray tube generating potentials of approximately 100-150 kVp. This matches well with beam qualities for diagnostic computed topography applications.

High energy x-ray beam penumbra are measured using Gafchromic EBT film. Gafchromic EBT, due to its limited energy dependence and high spatial resolution provide a high level of accuracy for dose assessment in penumbral regions. The spatial resolution of film detector systems is normally limited by the scanning resolution of the densitometer. Penumbral widths (80%/20%) measured at Dmax were found to be 2.8, 3.0, 3.2, and 3.4 mm (+/- 0.2 mm) using 5, 10, 20, and 30 cm square field sizes, respectively, for a 6 MV linear accelerator produced x-ray beam. This is compared to 3.2 mm +/- 0.2 mm (Kodak EDR2) and 3.6 mm +/- 0.2 mm (Kodak X-Omat V) at 10 cm x 10 cm measured using radiographic film. Using a zero volume extrapolation technique for ionization chamber measurements, the 10 cm X 10 cm field penumbra at Dmax was measured to be 3.1 mm, a close match to Gafchromic EBT results. Penumbral measurements can also be made at other depths, including the surface, as the film does not suffer significantly from dosimetric variations caused by changing x-ray energy spectra. Gafchromic EBT film provides an adequate measure of penumbral dose for high energy x-ray beams.

Three photon interaction processes, namely, the photoelectric effect, Compton effect, and pair production, can occur when materials with high atomic numbers are irradiated by the high- and low-energy bremsstrahlung photons from a linear accelerator. A dose enhancement, due to the photoelectric effect and pair production, near targets with platinum implants (with a high atomic number) in radiosurgery cannot be predicted by the XKnife radiosurgery treatment planning system. In the present work, Monte Carlo simulations using PRESTA EGS4 were employed to investigate the resulting dose enhancements from 4, 6, and 10 MV energies commonly used in the stereotactic radiosurgery system. Dose enhancements from 32% to 68% were observed close to the platinum implant for the above energies when using a 12.5 mm collimator. Comparatively higher dose enhancements were observed when using smaller collimators. It was found that this dose enhancement increased with beam energy but decreased as beam size increased.

This research aimed to quantitatively evaluate the differences in percentage dose of maximum for 6MV and 18MV x-ray beams within the first 1 cm of interactions. Thus provide quantitative information regarding the basal, dermal and subcutaneous dose differences achievable with these two types of high-energy x-ray beams. Percentage dose of maximum build up curves are measured for most clinical field sizes using 6MV and 18MV x-ray beams. Calculations are performed to produce quantitative results highlighting the percentage dose of maximum differences delivered to various depths within the skin and subcutaneous tissue region by these two beams. Results have shown that basal cell layer doses are not significantly different for 6MV and 18MV x-ray beams. At depths beyond the surface and basal cell layer there is a measurable and significant difference in delivered dose. This variation increases to 20% of maximum and 22% of maximum at 1 mm and 1 cm depths respectively. The percentage variations are larger for smaller field sizes where the photon in phantom component of the delivered dose is the most significant contributor to dose. By producing graphs or tables of % dose differences in the build up region we can provide quantitative information to the oncologist for consideration (if skin and subcutaneous tissue doses are of importance) during the beam energy selection process for treatment.

Dose in the build up region for high energy x-rays produced by a medical linear accelerator is affected by the x-ray source to patient surface distance (SSD). The use of isocentric treatments whereby the tumour is positions 100 cm from the source means that depending of the depth of the tumour and the size of the patient, the SSD can vary from distances of 80 cm to 100 cm. To achieve larger field sizes, the SSD can also be extended out to 120 cm at times. Results have shown that open fields are not significantly affected by SSD changes with deviations in percentage dose being less than 4% of maximum dose for SSD's from 80 cm to 120 cm SSD. With the introduction of beam modifying devices such as Perspex blocking trays, the effects are significant with a deviation of up to 22% measured at 6MV energy with a 6 mm Perspex tray for SSD's from 80 cm to 120 cm. These variations are largest at the skin surface and reduce with depth. The use of a multi leaf collimator for blocking removes extra skin dose caused by the Perspex block trays with decreasing SSD.

The effects of chamber polarity have been investigated for the measurement of 6MV and 18MV x-ray surface dose using a parallel plate ionization chamber. Results have shown that a significant difference in measured ionization is recorded between polarities at 6MV and 18MV at the phantom surface. A polarity ratio ranging from 1.062 to 1.005 is seen for 6MV x-rays at the phantom surface for field sizes 5 cm x 5 cm to 40 cm x 40 cm when comparing positive to negative polarity. These ratio's range from 1.024 to 1.004 for 18MV x-rays with the same field sizes. When these charge reading are compared to the Dmax readings of the same polarity it is found that these polarity effects are minimal for the calculation of percentage dose results with variations being less than 1% of maximum.

This note investigates in-vivo dosimetry using a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) for radiotherapy treatment at superficial and orthovoltage x-ray energies. This was performed within one fraction of the patients treatment. Standard measurements along with energy response of the detector are given. Results showed that the MOSFET measurements in-vivo agreed with calculated results on average within +/- 5.6% over all superficial and orthovoltage energies. These variations were slightly larger than TLD results with variations between measured and calculated results being +/- 5.0% for the same patient measurements. The MOSFET device provides adequate in-vivo dosimetry for superficial and orthovoltage energy treatments with the accuracy of the measurements seeming to be relatively on par with TLD in our case. The MOSFET does have the advantage of returning a relatively immediate dosimetric result after irradiation.