Masters Degrees (Medical Physics)

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Browsing Masters Degrees (Medical Physics) by Author "Mahuvava, Courage"

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    Monte Carlo evaluation of the dose perturbation effect of various hip prostheses during pelvic megavoltage photon radiotherapy
    (University of the Free State, 2016-11) Mahuvava, Courage; Du Plessis, F. C. P.
    English: Introduction: Hip prostheses (HPs) are routinely used in hip augmentation surgery to replace painful or dysfunctional hip joints, especially in the elderly population. A number of patients with HPs are undergoing pelvic radiotherapy (RT) for localised prostate cancer. However, radiographic discrepancies between high-density and high-atomic-number (Z) inserts and surrounding tissue may cause considerable dose perturbations within the target volume and in regions where tissues interface with the prosthetic device. Furthermore, conventional treatment planning systems (TPSs) do not accurately predict dose effects incurred around metallic implants. Therefore, concerns regarding dose inhomogeneities near the prosthesis always arise, especially in patients with bilateral hip prostheses (bHPs) who require teletherapy of prostate cancer, where the tumour typically lies between the prostheses. The aim of this study was to evaluate the dosimetric effect of various HPs during 3D conformal prostate RT using Monte Carlo (MC) simulations. Materials and methods: The MC radiation transport simulation user-code BEAMnrc was used to simulate an Elekta Precise linear accelerator (linac) head, based on the manufacturer’s specifications. The MC linac model was validated by comparing dosimetric features including depth dose and dose profile data simulated in a cubic water tank (WT) with measured values. DOSXYZnrc was used to calculate 3D absorbed dose distributions in a CT based phantom (patient model) with and without HPs. Simulations were performed for 6, 10, 15 and 20 MV conformal photon beams using different beam arrangements. Three treatment plans were generated by XiO TPS and incorporated into MC simulations: a four–field (4F) box plan, a five–field (5F) plan and a six–field (6F) plan. The planning target volume (PTV) was generated by a 1 cm expansion of the prostate alone. The HP materials used were stainless steel (SS316L), titanium (Ti6Al4V) and ultra-high-molecular-weight-polyethylene (UHMWPE). These prosthetic models were manually drawn into the CT dataset from actual CT images of the patient pelvis using MCSHOW graphical user interface (GUI). The prosthesis was made part of the patient using a locally-developed Interactive Data Language (IDL) code that converts the density of the drawn volume into the desired HP material density. Both unilateral and bilateral models were considered in the simulations and dose perturbation factors (DPFs) were calculated on the proximal and distal interfaces of the implant. The dose reduction in the PTV as well as the dose to critical organs was also evaluated. Results: Results indicated that the central axis depth dose within and beyond the inhomogeneity drops significantly due to beam attenuation. For patients with bHPs, the dose contribution from lateral ports at 6 MV was attenuated by up to 23% and 17% for SS316L and Ti6Al4V, respectively. For a unilateral HP (uHP), the respective dose attenuations were 19% and 12%. The dose perturbation was always < 1% for a patient fitted with UHMWPE. Up to 38% dose increase was found at the proximal bone–HP interface due to backscattered electrons from the metal implant. There was a weak dependence of dose distribution on beam energy at the target isocenter, with the maximum dose reduction ranging only from 22.8 to 16.9% from 6 to 20 MV in a patient with bilateral steel HPs. However, interface effects were more pronounced at higher beam energies. However, increasing the number of treatment beams improved the plan quality. The greatest PTV dose perturbation was observed in a 4F box and lowest in a 6F plan. Production of scatter radiation was found to be larger for backscatter compared to forward scatter in this study. Conclusions: The dose perturbation effect of metallic HPs is significant and must be taken into account during treatment planning. UHMWPE poses no significant dose perturbation in the shadow of the implant and on the interface with tissue or bone. The use of MC–based TPSs is recommended for treatments using beam portals passing through HPs. MCSHOW allows the addition of HP contours in the virtual phantom from CT dataset of a patient without a HP. This allows one to carry out MC calculations for several implant models without metal artefacts. Results also highlight the significant influence of the implant’s composition and the beam position relative to the HP as well as beam energy on the dose distribution. Increasing the beam energy may help overcome the attenuation effects of metallic HPs and to improve target coverage. Therefore this study recommends plans with a larger number of beams that would allow avoiding the hip inhomogeneity in order to effectively compensate for dose attenuated in fields passing through HPs. 1It is also evident from the results that the shadowing effect is density-dependent, and its maximum value is for the SS316L HP. A more sophisticated, non-coplanar beam orientation may be necessary to avoid the HPs whilst sparing organs at risk (OARs) and giving sufficient target coverage.