Study on accurateness of percentage depth dose with Monte Carlo simulation algorithm / 中华放射肿瘤学杂志

Objective To study the percentage depth dose difference (PDD) between Monte Carlo method and the measurement method.Methods Based on the detail treatment head structure designing by the manufacture,the BEAMnrc Monte Carlo code was used to simulate the 6 MV photon beams of a Siemens Primus linear accelerator.DOSXYZnrc program generated 6 MV X-ray parameters such as PDD,and it was measured by MP3 three-dimensional water of PTW corporation phantom.The deviation of PDD was calculated using formulas ((measured value-simulation algorithm value)/measured value × 100％).Results Distance from the surface of the water was less than 1.2 cm,the deviation of PDD was ＞ 2％.Distance from the surface greater than 1.2 cm,the deviation of PDD was ＜ 2％.Conclusion In the built-up areas,PDD obtained by the Monte Carlo simulation algorithm is more accurate.

GafChromic EBT in measurement for percent depth dose of high-energy electron beams / 中华放射肿瘤学杂志

Objective To evaluate the dosimetry of high-energy electron beams by using GafChromic EBT film. Methods The percent depth doses of electron beams of 4 MeV,6 MeV,8 MeV,10MeV, 12 MeV and 15 MeV were measured with EBT. The results were then compared with the measurements with diode detector in RFA and parallel plate chamber in water tank. Results The percent depth dose curves using EBT film had a good agreement with those using the other two detectors. Furthermore, no differences were found between up-right and tilt setup methods. When film upper edge is higher than water surface, a sharp drop of measurement results using EBT film was observed in comparison with those from diode detector in build-up region. Conclusions EBT film can be applied to measure percent depth doses of high-energy electron beams. During the EBT film measurements, the film should be tilted at the angle of 5degrees to the central axis of the field. When choosing up-right setup method, the edge of the film should be parallel to the water surface.

Compare the Clinical Tissue Dose Distributions to the Derived from the Energy Spectrum of 15 MV X Rays Linear Accelerator by Using the Transmitted Dose of Lead Filter / 의학물리

Recent radiotherapy dose planning system (RTPS) generally adapted the kernel beam using the convolution method for computation of tissue dose. To get a depth and profile dose in a given depth concerened a given photon beam, the energy spectrum was reconstructed from the attenuation dose of transmission of filter through iterative numerical analysis. The experiments were performed with 15 MV X rays (Oncor, Siemens) and ionization chamber (0.125 cc, PTW) for measurements of filter transmitted dose. The energy spectrum of 15 MV X-rays was determined from attenuated dose of lead filter transmission from 0.51 cm to 8.04 cm with energy interval 0.25 MeV. In the results, the peak flux revealed at 3.75 MeV and mean energy of 15 MV X rays was 4.639 MeV in this experiments. The results of transmitted dose of lead filter showed within 0.6% in average but maximum 2.5% discrepancy in a 5 cm thickness of lead filter. Since the tissue dose is highly depend on the its energy, the lateral dose are delivered from the lateral spread of energy fluence through flattening filter shape as tangent 0.075 and 0.125 which showed 4.211 MeV and 3.906 MeV. In this experiments, analyzed the energy spectrum has applied to obtain the percent depth dose of RTPS (XiO, Version 4.3.1, CMS). The generated percent depth dose from 6x6 cm2 of field to 30x30 cm2 showed very close to that of experimental measurement within 1% discrepancy in average. The computed dose profile were within 1% discrepancy to measurement in field size 10x10 cm, however, the large field sizes were obtained within 2% uncertainty. The resulting algorithm produced x-ray spectrum that match both quality and quantity with small discrepancy in this experiments.

Influence of field size on electron beam central axis dose on a radiotherapy linear accelerator / 中华放射肿瘤学杂志

Objective Clinical application of electrons often involves some beam in which the field size varies with the applicators. The work was done to understand the electron beam characteristics in different field sizes. Methods Percent depth dose and the dose output factor were measured for square and rectangular fields at 100?cm source to surface distance ( SSD ) . Central axis percent depth dose (PDD) measurements were made using the RFA 300 three dimensional radiation field analyzer with a shielded p type diode detector . The dose output factors were measured with the RFA 300 three dimensional radiation field analyzer with a PTW 0.1?cm 3 chamber and a Farmer 2570/1 dosimeter with a 2571 ion chamber in a water phantom. Results The measurements showed that the depth dose curves and the output factors were sometimes dependent on how the field sizes were formed. The change in depth dose with field size was more pronounced in the high energy beams than the low energy ones. However, the output factor did not show any systematic energy dependence because each applicator had it's own X ray jaw setting at each energy. Conclusions When using small inserted apertures to treat small lesions, we should verify the conformation of depth dose and output factors. In this case, we should use applicator dependent output factors at each energy to calculate the monitor units for irradiation.

Treatment Planning and Dosimetry of Small Radiation Fields for Stereotactic Radiosurgery / 대한치료방사선과학회지

The treatment planning and dosimetry of small fields for stereotactic radiosurgery with 10 MV x-ray isocentrically mounted linear accelerator is presented. Special consideration in this study was given to the variation of absorbed dose with field size, the central axis percent depth doses and the combined moving beam dose distributon. The collimator scatter correction factors of small fields (1x1~3x3 cm2) were measured with ion chamber at a target chamber distance of 300cm where the projected fields were larger than the polystyrene buildup caps and it was calibrated with the tissue equivalent solid state detectors of small size (TLD, PLD, ESR and semiconductors). The central axis percent depth doses for 1x1 and 3x3 cm2 fields could be derived with the same acuracy by interpolating between measured values for larger fields and calcu1ated zero area data, and it was also calibrated with semiconductor detectors. The agreement between experimental and calculated data was found to be under +/-2% within the fields. The three dimensional dose planning of stereotactic focusing irradiation on small size tumor regions was performed with dose planning computer system (Therac 2300) and was verified with film dosimetry. The more the number of strips and the wider the angle of arc rotation, the larger were the dose delivered on tumor and the less the dose to surrounding the normal tissues. The circular cone, we designed, improves the alignment, minimizes the penumbra of the beam and formats ball shape of treatment area without stellate patterns. These dosimetric techniques can provide adequate physics background for stereotactic radiosurgery with small radiation fields and 10 MV x-ray beam.