Search

The Low-Energy Photon-Scattering (LSCAT) expansion to the EGS4 Monte Carlo code has been applied to the "scatter sphere" EGS4 code scasph.mortran to compute energy deposition kernels at photon energies below 100 keV. The LSCAT routines were used to replace the default atomic form factor and cross-section data used by EGS4 to model coherent scatter with measured molecular form factor and cross-section data. Energy deposition kernels were calculated with: (1) coherent scatter effects neglected, (2) coherent scatter effects included using the default EGS4 atomic form factor, and (3) coherent scatter effects included using the molecular form factor. These energy deposition kernels were utilized in a convolution/superposition code developed for mega-voltage therapeutic X-rays modified to compute dose at low photon energy. Depth dose calculations were made for 30 keV photons, where the coherent scatter contribution is maximum, using kernels which model coherent scatter in each of the three ways described. These calculations show a difference between the kernel that neglects coherent scatter compared to the kernels that include coherent scatter, but show no distinction between the atomic and molecular form factor kernels. A separation of the kernels into their primary and scatter components reveals that the depth dose calculation is dominated by the primary component. An examination of the kernels separated into their primary and scatter components suggest that the "scatter sphere" EGS4 code includes the coherent scatter contribution in the primary component of the kernel.

The goal of this project is to investigate the use of multi-strength and multi-specie radioactive sources in permanent prostate implant brachytherapy. In order to fulfill the requirement for an optimal dose distribution, the prescribed dose should be delivered to the target in a nearly uniform dose distribution while simultaneously sparing sensitive structures. The treatment plan should use a small number of needles and sources while satisfying the treatment requirements. The hypothesis for the use of multi-strength and/or multi-specie sources is that a better treatment plan using fewer sources and needles could be obtained than by treatment plans using single-strength sources could reduce the overall number of sources used for treatment. We employ a recently developed greedy algorithm based on the adjoint concept as the optimization search engine. The algorithm utilizes and ''adjoint ratio'', which provides a means of ranking source positions, as the pseudo-objective function. It ha s been shown that the greedy algorithm can solve the optimization problem efficiently and arrives at a clinically acceptable solution in less than 10 seconds. Our study was inclusive, that is there was no combination of sources that clearly stood out from the others and could therefore be considered the preferred set of sources for treatment planning. Source strengths of 0.2 mCi (low), 0.4 mCi (medium), and 0.6 mCi (high) of sup 1 sup 2 sup 5 I in four different combinations were used for the multi-strength source study. The combination of high- and medium-strength sources achieved a more uniform target dose distribution due to few source implants whereas the combination of low-and medium-strength sources achieved better sparing of sensitive tissues including that of the single-strength 0.4 mCi base case. sup 1 sup 2 sup 5 I at 0.4 mCi and sup 1 sup 9 sup 2 Ir at 0.12 mCi and 0.25 mCi source strengths were used for the multi-specie source study. This study also proved inconclusiv supposing a combination of two 0.12 mCi sup 1 sup 9 sup 2 Ir sources and sup 1 sup 2 sup 5 I sources or a combination of two 0.25 mCi sup 1 sup 9 sup 2 Ir sources and sup 1 sup 2 sup 5 I sources did not have better target and sensitive structures DVHs than that of the single specie sup 1 sup 2 sup 5 I 0.4 mCi base case. However, because of the high dose delivered by a sup 1 sup 9 sup 2 Ir source, the multi-specie treatment plan required fewer sources and needles and hence is less invasive than the treatment plan for the 0/4 mCi sup 1 sup 2 sup 5 I base case.

The goal of this project is to investigate the use of the adjoint method, commonly used in the reactor physics community, for the optimization of radiation therapy patient treatment plans. Two different types of radiation therapy are being examined, interstitial brachytherapy and radiotherapy. In brachytherapy radioactive sources are surgically implanted within the diseased organ such as the prostate to treat the cancerous tissue. With radiotherapy, the x-ray source is usually located at a distance of about 1-metere from the patient and focused on the treatment area. For brachytherapy the optimization phase of the treatment plan consists of determining the optimal placement of the radioactive sources, which delivers the prescribed dose to the disease tissue while simultaneously sparing (reducing) the dose to sensitive tissue and organs. For external beam radiation therapy the optimization phase of the treatment plan consists of determining the optimal direction and intensity of beam, which provides complete coverage of the tumor region with the prescribed dose while simultaneously avoiding sensitive tissue areas. For both therapy methods, the optimal treatment plan is one in which the diseased tissue has been treated with the prescribed dose and dose to the sensitive tissue and organs has been kept to amore » minimum.« less