Our aim in the present study was to investigate the effects of initial electron beam characteristics on Monte Carlo calculated absorbed dose distribution for a linac 6 MV photon beam. Moreover, the range of values of these parameters was derived, so that the resulted differences between measured and calculated doses were less than 1%. Mean energy, radial intensity distribution and energy spread of the initial electron beam, were studied. The method is based on absorbed dose comparisons of measured and calculated depth-dose and dose-profile curves. All comparisons were performed at 10.0 cm depth, in the umbral region for dose-profile and for depths past maximum for depth-dose curves. Depth-dose and dose-profile curves were considerably affected by the mean energy of electron beam, with dose profiles to be more sensitive on that parameter. The depth-dose curves were unaffected by the radial intensity of electron beam. In contrast, dose-profile curves were affected by the radial intensity of initial electron beam for a large field size. No influence was observed in dose-profile or depth-dose curves with respect to energy spread variations of electron beam. Conclusively, simulating the radiation source of a photon beam, two of the examined parameters (mean energy and radial intensity) of the electron beam should be tuned accurately, so that the resulting absorbed doses are within acceptable precision. The suggested method of evaluating these crucial but often poorly specified parameters may be of value in the Monte Carlo simulation of linear accelerator photon beams.
The aims of this study were: (a) to determine conceptus dose resulting from brain radiotherapy; (b) to investigate the necessity of using shielding devices over patient's abdomen during treatment; and (c) to estimate the components of conceptus dose. Radiation doses received by conceptus were measured using anthropomorphic phantoms simulating pregnancy at 4, 12 and 24 weeks gestation and thermoluminescent dosemeters. All irradiations were performed with two lateral and opposed fields approximating the minimum, medium and maximum field size used during treatment of brain malignancies. For a treatment course delivering 65 Gy to tumour without using shielding equipment, conceptus dose never exceeded 100 mGy. Appropriate positioning of 5.1 cm of lead over the phantom's abdomen provided reduction of conceptus dose from 26% to 71%, depending upon gestational age, field size and distance from the field isocentre. The contribution of scatter arising from within the phantom to the conceptus dose was small compared with that from head leakage and collimator scatter. Our dosimetric results indicate that the construction of special shielding equipment is not a prerequisite for treating brain malignancies during pregnancy. However, based on the concept that exposures in women of childbearing age should be kept as low as reasonably achievable, we suggest that shielding devices should be used whenever possible.
The purpose of this study was to evaluate the performance of a region growing technique for segmenting prostate, bladder and rectum in CT images of prostate cancer patients. Prostate, bladder and rectum were segmented in all CT images of 10 patients using the region growing technique and manual tracing. Volumes of the above organs computed with the region growing technique were compared with those from manually traced images on a slice-by-slice basis. Measurement reproducibility of both segmentation techniques was evaluated using the data obtained from four independent observers. The region growing technique was 1.5 times faster than manual tracing. There was no statistical difference between the slice volumes of prostate, bladder and rectum obtained by the two segmentation techniques (p > 0.05, paired Student's t-test). Correlation between slice volumes of all organs of interest provided both by region growing and by manual tracing was very good (prostate r2 = 0.84; bladder r2 = 0.93; rectum r2 = 0.85). An overall reasonable agreement was found between the two segmentation techniques. The intraobserver and interobserver variations for prostate, bladder and rectum volume segmentation were found to be lower with the region growing technique than with manual tracing. The suggested semi-automatic technique allows the possibility of generating accurate and reproducible segmentation of prostate, bladder and rectum from CT data with great saving in labour.
Three methods of indirect effective dose estimation were reviewed and compared to a direct effective dose determination method. An anthropomorphic phantom and thermoluminescence dosimetry were used to obtain dosimetric data associated with anterior-posterior (AP) abdominal radiography, posterior-anterior (PA) chest radiography, PA head radiography, and AP heart fluoroscopy. Effective dose was determined using: (i) organ specific dose values directly determined by thermoluminescence dosimeters, (ii) data published by National Radiological Protection Board (NRPB) and entrance surface dose (ESD), (iii) NRPB data and dose area product (DAP), (iv) energy imparted derived from DAP. The effective dose values estimated from the Rando phantom measurements were 161, 32.3, and 8.4 microSv/projection for the abdomen, chest, and head radiographs, respectively. Cardiac fluoroscopy yielded an effective dose value of 111 microSv/min. The effective dose values obtained indirectly using NRPB data and DAP were in good agreement with directly assessed values in all simulated exposures (difference <8%). The effective doses using NRPB data and ESD values differed from directly assessed values by less than 15% for the radiographic exposures and 60% for heart fluoroscopy. The energy imparted method yielded 136, 31, and 6.6 microSv/projection for the abdomen, chest, and head radiographs, respectively, and 111 microSv/min for heart fluoroscopy. Indirect patient effective dose determination using the NRPB dosimetric data and the measured value of incident radiation allows for reliable patient effective dose estimates. The use of DAP rather than ESD is recommended because it yields accurate results even for complex radiologic exposures involving fluoroscopy. The value of energy imparted may be used for the accurate determination of patient effective dose, especially when specific organ dose values are not of interest. The calculation of energy imparted with the use of EAP provides a reliable starting point for estimation of effective dose from radiologic examinations for which dosimetric data are not provided by NRPB.
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