Calculated microdosimetric characteristics of 125I and 103Pd brachytherapy seeds at different depths in water

Patient-specific microdosimetry: a proof of concept

Microscopic energy deposition distributions from ionizing radiation are used to predict the biological effects of an irradiation and vary depending on biological target size. Ionizing radiation is thought to kill cells or inhibit cell cycling mainly by damaging DNA in the cell nucleus. The size of cells and nuclei depends on tissue type, cell cycle, and malignancy, all of which vary between patients. The aim of this study was to develop methods to perform patient-specific microdosimetry, that being, determining microdosimetric quantities in volumes that correspond to the sizes of cells and nuclei observed in a patient's tissue. A histopathological sample extracted from a stage I lung adenocarcinoma patient was analyzed. A pouring simulation was used to generate a three-dimensional tissue model from cell and nucleus size information determined from the histopathological sample. Microdosimetric distributions including f(y) and d(y) were determined for Co-60,Ir-192,Yb-169 and I-125 in a patient-specific model containing a distribution of cell and nucleus sizes. Fixed radius models and a summation method (where f(y) from many fixed radii models are summed) were compared to the full patient-specific model to evaluate their suitability for fast determination of patient-specific microdosimetric parameters. Fixed radius models do not provide a close approximation of the full patient-specific model y ̅_f or y ̅_d for the lower energy sources investigated, Yb-169 and I-125. The higher energy sources investigated, Co-60 and Ir-192 are less sensitive to target size variation than Yb-169 and I-125. A summation method yields the most accurate approximation of the full model d(y) for all radioisotopes investigated. A summation method allows for the computation of patient-specific microdosimetric distributions with the computing power of a personal computer. With appropriate biological inputs the microdosimetric distributions computed using these methods can yield a patient-specific relative biological effectiveness as part of a multiscale treatment planning approach.

MONTE CARLO-BASED INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTION OF HIGH ENERGY BRACHYTHERAPY SOURCES

FLUKA-based Monte Carlo calculations were carried out to study microdosimetric distributions in air and in water for encapsulated high energy brachytherapy sources (60Co, 137Cs, 192Ir and 169Yb) by simulating a Tissue Equivalent Proportional Counter (Model LET1/2) having sensitive diameter of 1. 27 cm for a site size of 1 μm. The study also included microdosimetric distributions of bare sources. When the sources are in air, for a given source, the source geometry does not affect the y¯F and y¯D values significantly. When the encapsulated 192Ir, 137Cs and 60Co sources are in water, y¯F and y¯D values increase with distance in water which is due to degradation in the energy of photons. Using the calculated values of y¯D, relative biological effectiveness (RBE) was obtained for the investigated sources. When 60Co, 137Cs and 192Ir sources are in water, RBE increases from 1.03 ± 0.01 to 1.17 ± 0.01, 1.24 ± 0.01 to 1.46 ± 0.02 and 1.50 ± 0.01 to 1.75 ± 0.03, respectively, when the distance was increased from 3-15 cm, whereas for 169Yb, RBE is about 2, independent of distance in water.

Microdosimetric characteristics of 50 kV X rays at different depths for breast intraoperative radiotherapy

An intraoperative radiation therapy (IORT) device with 50 kV X rays was designed to deliver a single dose to the tumour bed after local excision of breast cancer. The quality of a radiation can be determined by the microscopic distribution of energy transfers along and across the charged particle tracks. The lineal energy, y, serves as an accurate measure of local energy concentration. The dose mean lineal energy, yD, is an indicator of radiation quality. For low linear energy transfer radiation, the ratio of its dose mean lineal energy to that of (60)Co gamma rays can serve as a good indicator of the relative biological effectiveness (RBE) at low doses. In this study, microdosimetric simulations are performed for soft tissue irradiated by 50 kV X rays generated from the IORT device, with a 4-cm breast applicator attached. All energy transfers are recorded with the location coordinates in the tissue. Microdosimetric single events in a sphere of 1 µm in diameter are scored as a function of radial distances from the applicator surface. Single-event spectra are then constructed. From those single-event spectra, dose mean lineal energy is calculated. Compared with dose mean lineal energy of (60)Co gamma rays, the estimated RBEs at low doses are given for the X rays at different depths in the tissue. The RBEs at clinically relevant doses, as a function of depth, are also presented.