Dosimetry using a PMMA phantom was performed in 15 and 29 MeV proton beams from the Birmingham cyclotron, with a Markus parallel-plate ionization chamber and GafChromic EBT and MD-V2-55 film. Simulations of the depth-dose curves were performed with FLUKA 2008.3 and MCNPX 2.5.0, which agreed almost perfectly with each other in range and only differed by 2% in the Bragg peak (BP) region. FLUKA was also used to calculate k(Q) factors for Markus chamber measurements as an improvement to the IAEA TRS-398 values in low-energy beams. FLUKA depth-dose simulations overestimate the BP height measured by ion chamber by about 10%, where the initial proton energy spread was estimated by fitting to the slope of the measured BP distal edge. Both GafChromic films showed an under-response in the BP compared to ion chamber; however, EBT exhibits this effect at lower energies than MD-V2-55. A possible reason for this is attributed to the shape and arrangement of the monomer particles being different in the active components of EBT and MD-V2-55. Relative effectiveness (RE) of both films is presented as functions of residual range R(res) in water and peak proton energy determined by FLUKA, with considerations for the spatial separation of the two active layers in each film. The proton energies at which RE reduces to 90% of maximum film response are 6.7 and 3.2 MeV for MD-V2-55 and EBT, respectively. Additionally, a beam quality correction factor (g(Q,Q0)) is suggested for both GafChromic films, involving water-to-film stopping power ratios evaluated using ICRU recommendations, and a polymer yield factor G(Q0)/G(Q). RE in this work is equated to the reciprocal of the polymer yield factor. The calculated values of (S(w,film))Q/(S(w,film))Q0 are constant within 2.1% and 1.2% across the proton energy range of 1-300 MeV for EBT and MD-V2-55, respectively, so it is concluded that the polymer yield factor is the dominant factor causing the LET quenching effect.

Solid-state detectors offer high sensitivity, stability and resolution and are frequently the dosimeter of choice for on-line dosimetry and small field therapies such as stereotactic radiosurgery. The departure from tissue equivalence of many solid-state devices, including diodes and MOSFETs, has to be carefully considered at lower energies and for Compton scattered radiation where the strongly Z-dependent photoelectric effect is significant. A modification of Burlin cavity theory is proposed that treats primary and scatter photon spectra separately and this has been applied to determine the correction factors for diode detector measurements of 6 and 15 MV linear accelerator beams. Uncorrected, an unshielded diode overestimates the dose at depth by as much as 15% for the 6 MV beam. The model predicts the effect to within 1% for both energies offering a basis for the correction of diodes for use in routine dosimetry.

A method of reading exposed radiochromic film is described which has significant advantages over conventional densitometry. The method employs a document scanner and associated software for imaging the film. The resulting images are easily analysed using standard software to yield high-resolution dose maps. A calibration was performed which relates scanner signal to dose, allowing for the determination of dose at any point on an exposed film. Results obtained using a broad-band densitometer are compared to those where the scanner has been used. The technique was used to measure the dose distribution around a COMS-type ophthalmic applicator.

Objective: Cone beam CT (CBCT) images contain more scatter than a conventional CT image and therefore provide inaccurate Hounsfield units (HUs). Consequently, CBCT images cannot be used directly for radiotherapy dose calculation. The aim of this study is to enable dose calculations to be performed with the use of CBCT images taken during radiotherapy and evaluate the necessity of replanning. Methods: A patient with prostate cancer with bilateral metallic prosthetic hip replacements was imaged using both CT and CBCT. The multilevel threshold (MLT) algorithm was used to categorize pixel values in the CBCT images into segments of homogeneous HU. The variation in HU with position in the CBCT images was taken into consideration. This segmentation method relies on the operator dividing the CBCT data into a set of volumes where the variation in the relationship between pixel values and HUs is small. An automated MLT algorithm was developed to reduce the operator time associated with the process. An intensitymodulated radiation therapy plan was generated from CT images of the patient. The plan was then copied to the segmented CBCT (sCBCT) data sets with identical settings, and the doses were recalculated and compared.Results: Gamma evaluation showed that the percentage of points in the rectum with g , 1 (3%/3 mm) were 98.7% and 97.7% in the sCBCT using MLT and the automated MLT algorithms, respectively. Compared with the planning CT (pCT) plan, the MLT algorithm showed 20.46% dose difference with 8 h operator time while the automated MLT algorithm showed 21.3%, which are both considered to be clinically acceptable, when using collapsed cone algorithm. Conclusion: The segmentation of CBCT images using the method in this study can be used for dose calculation. For a patient with prostate cancer with bilateral hip prostheses and the associated issues with CT imaging, the MLT algorithms achieved a sufficient dose calculation accuracy that is clinically acceptable. The automated MLT algorithm reduced the operator time associated with implementing the MLT algorithm to achieve clinically acceptable accuracy. This saved time makes the automated MLT algorithm superior and easier to implement in the clinical setting.

No abstract

No abstract

No abstract

Ongoing controversy surrounds the adverse health effects of the use of depleted uranium (DU) munitions. The biological effects of gamma-radiation arise from the direct or indirect interaction between secondary electrons and the DNA of living cells. The probability of the absorption of X-rays and gamma-rays with energies below about 200 keV by particles of high atomic number is proportional to the third to fourth power of the atomic number. In such a case, the more heavily ionizing low-energy recoil electrons are preferentially produced; these cause dose enhancement in the immediate vicinity of the particles. It has been claimed that upon exposure to naturally occurring background gamma-radiation, particles of DU in the human body would produce dose enhancement by a factor of 500–1000, thereby contributing a significant radiation dose in addition to the dose received from the inherent radioactivity of the DU. In this study, we used the Monte Carlo code EGSnrc to accurately estimate the likely maximum dose enhancement arising from the presence of micrometre-sized uranium particles in the body. We found that although the dose enhancement is significant, of the order of 1–10, it is considerably smaller than that suggested previously.