PubMed · 6949991
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M L Howard, H S Stocks. Dental exposure normalization technique (DENT)--summary.. https://pubmed.ncbi.nlm.nih.gov/6949991/
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In intensity modulated radiation therapy (IMRT) the aim of an accurate conformal dose distribution is obtained through a complex process. This ranges from the calculation of the optimal distribution of fluence by the treatment planning system (TPS), to the dose delivery through a multilamellar collimator (MLC), with several segments per beam in the step and shoot approach. The above-mentioned consideration makes mandatory an accurate dosimetric verification of the IM beams. A high resolution and integrating dosimeter, like the radiographic film, permits one to simultaneously measure the dose in a matrix of points, providing a good means of obtaining dose distributions. The intrinsic limitation of film dosimetry is the sensitivity dependence on the field size and on the measurement depth. However, the introduction of a scattered radiation filter permits the use of a single calibration curve for all field sizes and measurement depths. In this paper the quality control procedure developed for dosimetric verification of IMRT technique is reported. In particular a system of film dosimetry for the verification of a 6 MV photon beam has been implemented, with the introduction of the scattered radiation filter in the clinical practice that permits one to achieve an absolute dose determination with a global uncertainty within 3.4% (1 s.d.). The film has been calibrated to be used both in perpendicular and parallel configurations. The work also includes the characterization of the Elekta MLC. Ionimetric independent detectors have been used to check single point doses. The film dosimetry procedure has been applied to compare the measured absolute dose distributions with the ones calculated by the TPS, both for test and clinical plans. The agreement, quantified by the gamma index that seldom reaches the 1.5 value, is satisfying considering that the comparison is performed between absolute doses.
OBJECTIVE: To determine accurately the relative output factors, defined as the ratio of the nominal dose rate for a given collimator to that of the 60-mm collimator. This is particularly important for radiosurgical treatment of functional disorders, such as trigeminal neuralgia, in which a single large radiation dose is delivered to the target with a small collimator, such as the 5-mm collimator for CyberKnife radiosurgery. Numerous studies on the output factors have been reported for the Leksell gamma knife unit but none for the CyberKnife system. METHODS: Measurements of the relative output factors for all 12 collimators were performed by three different methods: silicon diode, radiographic film, and thermoluminescent dosimetry microcubes. The silicon diode is designed for measurements in small (1-50 MV) photon beams performed in water or air. Film and thermoluminescent dosimetry measurements were performed in a plastic phantom. RESULTS: The measured relative output factors for the three methods were very similar except for the three smallest collimators (5, 7.5, and 10 mm). The measured difference between the above methods was approximately 2%. The mean value of the output factor for the 5-mm collimator was 0.686 +/- 0.024. The uncertainties of the output factors are expected to increase with the decrease of collimator diameter. They range from approximately 1 to 4% of the relative output factor. CONCLUSION: The relative output factor can be measured with an acceptable accuracy even for the smallest (5-mm) CyberKnife collimators. This requires the selection of appropriate dosimetric detectors and measuring procedures. The results obtained with the diode are considered more accurate than with the other two methods.
BACKGROUND: Orthopaedic surgeons use intraoperative portable fluoroscopy and roentgenography. The present study was an attempt to find out if there is a difference between the occupational radiation exposure to the orthopaedic surgeon and assistant surgeon in the operating room while using intraoperative fluoroscopy or radiographic control and to measure it. METHODS: During a 3-month period, 107 consecutive operations were monitored for radiation exposure. At monthly intervals, the radiation doses were measured in millirem and recorded. The distance of the orthopaedic surgeon and the assistant surgeon from the X-ray source were noted in every fluoroscopic check. The orthopaedic surgeon was always at a safe distance (more than 90 cm), but the assistant surgeon always stood nearby (10 cm) the X-ray source for positioning of the patient. RESULTS: The radiation exposure according to the badge on the shoulder was consecutively 3, 4, 3 mrem for the orthopaedic surgeon and 20, 19, 22 mrem for the assistant surgeon. The radiation exposure according to the badges on the anaesthetic machine, in the room and under the apron of the orthopaedic surgeon were all zero, whereas the readings of the badge under the apron of the assistant surgeon were 7, 6, 5 mrem consecutively. CONCLUSION: Our findings show that although the radiation exposure during orthopaedic operations is below the recommendations of the European Committee on Radiation Protection, there is a higher risk of exposure for the assistant surgeon. It has to be kept in mind that there could be morphological and functional damage in cells exposed to radiation. Therefore, we should continue to use appropriate shielding precautions in view of the unknown long-term risks.