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The uranium collimator of the Varian 4-MeV accelerator produces a dose rate of 100 mR/h near the collimator opening. It can be decreased to 4% by a 6-mm Lucite shield at the level of the collimator opening. A better solution is a lead glass-mylar (EM) shield which decreases the uranium radiation to less than 3% and also provides better skin sparing during treatment than the open collimator.
Angular distributions of photon depth dose produced by 25-MeV electrons incident on several fully stopping single-element targets (C, Al, Cu, Mo, Ta, Pb) and two composite layered targets (Ni-Al, W-Al) were studied. Depth-dose curves measured using TLD-700 (thermoluminescent dosimeter) chips embedded in lucite phantoms. Several useful therapy electron accelerator design curves were determined, including relative flattener thickness as a function of target atomic number, "effective" bremsstrahlung endpoint energy or beam "hardness" as a function of target atomic number and photon emission angle, and estimates of shielding thickness as a function of angle required to reduce the radiation outside the treatment cone to required levels.
Monte Carlo calculations have been made to determine the energies delivered by photons and neutrons to the human body irradiated by collimated photon beams. The beams were monoenergetic and ranged from 100 keV to 40 MeV. The energy deposition in the body was sorted into two regions: inside and outside the irradiated volume. Most of the results obtained were for a beam size of 100 cm2 although some calculations were also made to 600 cm2 beams. The effect of beam size on energy deposition in the two regions was investigated for 60Co gamma rays. Graphs are presented which give the integral doses delivered by neutrons and photons to the two regions for therapy beams of various energies. These graphs can be used to calculate the integral doses which are delivered inside and outside the treatment volume for photon spectra from most medical accelerators. Calculations of energy deposition were also made for the spectra from two particular accelerators. These were done using Monte Carlo as well as by simply "folding" the spectra into the results for monoenergetic photons. The results obtained by both methods were in good agreement and indicated that the integral doses deposited outside the treatment volume by neutrons are more than two orders of magnitude smaller than those deposited by scattered photons.
Instrument AB Scanditronix offers a 6-MV therapeutic photon beam as an option with the 22-MeV medical microtron (MM 22). The method of acceleration, target assembly, and use of two flattening filters produce beams having several characteristics (e.g., field flatness, penetration, and intensity) that are superior to many 6-MV Linac beams. Characteristics of the microtron's 6-MV photon beams including quality, surface dose, depth of maximum dose, peak dose rates, peak scatter factors, central axis percentage depth doses, tissue maximum ratios, and beam profiles are described.
The Therac-25 is a new 25-MeV linear accelerator manufactured by Atomic Energy of Canada, Ltd. The first two units have recently been installed in Toronto, Ontario and Halifax, Nova Scotia. Calculations and measurements of primary and secondary radiation levels were made. Neutron dose-equivalent rates were measured inside and outside the room. The maximum leakage rate at 1 m from the accelerator target was 0.4% Sv per peak photon Gy. The tenth value layer for neutrons from the Therac-25, at the entrance to a one-legged maze was found to be 5.5 cm of polyethylene. Measurements were done to estimate daily technologist exposure due to induced activity in the treatment room.
The epidemic form of Kaposi's sarcoma is the most frequent tumor in sieropositive patients. Every part of the body including oral cavity is affected by these lesions. According to modern acknowledgement in treating oropharynge carcinoma, radiotherapy is used for management of oral Kaposi's sarcoma. This paper reports a study of 10 patients suffering from Kaposi's sarcoma correlated to AIDS (EKS) treated with radiotherapy and chemiotherapy, achieving good results, at the Istituto Nazionale per lo Studio e la Cura dei Tumori of Milan (Divisone di Radioterapia C) from 1988 to 1992. Treatment has been performed using linear accelerator (6 Mev) or Co 60 unity in order to reach the deepest layer of mucosa lesions. Radiotherapy schedule consisted of 150-200 cGy daily fractions given 5 times/week (w) for 4-5 w in split-course.
PURPOSE: A mutation in the p53 gene is believed to play an important role in the radioresistance of many cancer cell lines. We studied cytotoxic effects of high linear energy transfer (LET) carbon beams on glioma cell lines with either mutant or wild-type p53. METHODS AND MATERIALS: Cell lines U-87 and U-138 expressing wild-type p53 and U-251 and U-373 expressing mutant p53 were used. These cells were irradiated with 290 MeV/u carbon beams generated by the Heavy Ion Medical Accelerator in the National Institute of Radiologic Science or X-rays. A standard colony-forming assay and flow cytometric detection of apoptosis were performed. Cell cycle progression and the expression of p53, p21, and bax proteins were examined. RESULTS: High LET carbon radiation was more cytotoxic than low LET X-ray treatment against glioma cells. The effects of the carbon beams were not dependent on the p53 gene status but were reduced by G(1) arrest, which was independent of p21 expression. The expression of bax remained unchanged in all four cell lines. CONCLUSION: These results indicate that high LET charged particle radiation can induce cell death in glioma cells more effectively than X-rays and that cell death other than p53-dependent apoptosis may participate in the cytotoxicity of heavy charged particles. Thus, it might prove to be an effective alternative radiotherapy for patients with gliomas harboring mutated p53 gene.
A stereotactic radiosurgery technique is described which allows stereotactic radiation therapy to be easily fractionated on a daily or weekly basis. This permits adequate and safe radiation therapy to lesions larger than 2.5 cm, such as large arteriovenous malformations, and possibly safer radiation to smaller lesions near crucial intracranial structures. The technique utilizes external scalp landmarks and avoids the need for standard stereotactic head devices.
A new method for external stereotactic focal irradiation of three-dimensional irregular target volumes is proposed. In this method the target is irradiated by a linear accelerator set in various angular positions around the isocenter. During the irradiation the target is translated in a direction perpendicular to the beam. By controlling the velocity of the translation it is possible to modify the configuration of therapeutic isodoses so as to make them follow the borders of the target.
An empirical technique for calculation of the capture gamma ray dose in the maze of high-energy medical accelerators is described. The total photon dose was measured along the maze centre line and by use of a graphical technique the capture gamma dose and the low-energy photon dose component were determined. Tenth-value lengths for both photon dose components are reported for four medical accelerators operating with megavoltages in the range 16.8-22.0 MV. The capture gamma dose and the low-energy photon dose in the maze were related to the following parameters: the room surface area, the distance from the isocentre to the inner maze entrance, and the neutron head leakage at a fixed distance from the target.
Knowledge of the angular distribution of an electron beam at the applicator face is a necessary parameter in defining a beam when the Hogstrom pencil beam method of dose calculation is used. The angular spread can be found experimentally using penumbra widths measured at various distances from the applicator face. Using knowledge of the geometry and composition of the scattering foils of the linear accelerator, the angular standard deviation was calculated theoretically using Fermi-Eyges theory. The obtained angular spread values agree with experimentally derived values to within experimental error for electron energies from 6 to 21 MeV. The Fermi-Eyges calculation is fast, and can be used as a quick check to validate experimental angular spread values.
The purpose of this work was to establish procedures for the implementation of the Varian Enhanced Dynamic Wedge into a treatment planning system (TPS), based as much as possible on simple theoretical considerations and already available data. A method is presented for the calculation (rather than measurement) of off-axis relative wedge transmission curves that are required by the TPS for relative dose calculations. We also present a method for absolute dose (monitor unit) calculations, based on the calculation of an effective wedge factor on the prescription point. A simple formula has been derived for the calculation of the effective wedge factor for the most general case, i.e. an arbitrary effective wedge angle, field size and prescription point. Relative dose calculations have been verified by measurements performed on a Varian Clinac 2300C/D linear accelerator, for 6 MV and 20 MV photon energies. Monitor unit calculations have also been verified experimentally for several cases such as symmetric and asymmetric fields with prescription on the collimator axis or on the geometrical centre of the asymmetric field. The presented technique provides results within 2% for both relative and absolute dose calculations for clinically relevant cases.
High-energy linear accelerators are replacing lower energy units in radiation therapy centers. Radiation protection requirements necessitate expensive reconstruction of existing treatment rooms to accommodate these new machines. We describe two shielding materials: one made by embedding small pieces of scrap steel in cement, and the other made with cast iron in cement. Both materials produce high-density barriers at low cost using standard construction methods.
A device for shaping electron fields from a Therac-20 accelerator is described. The considerable advantage of continuously variable field sizes is enhanced when the shaping device is placed on the lower trimmer bars of the shorter set of electron trimmers, which remain fully adjustable. Cerrobend blocks of 1-cm thickness are sufficient for a 5% attenuation level with 20-MeV electrons and large field sizes.
Radiosurgical techniques are becoming increasingly popular for the selective destruction of brain lesions. To ensure precision in the procedure, set-up and treatment of lesions in this manner, we have adopted standard stereotactic methods to allow one to calculate accurately the absorbed dose and also to preserve accuracy in locating the target site in three dimensions. At McGill University, radiosurgery is performed using the dynamic technique, which utilizes the concurrent rotation of both the 10-MV photon beam linear accelerator (from 30 to 330 degrees) and the patient couch (from 75 to -75 degrees) about a common point centered on the target within the lesion. A three-dimensional treatment planning system for the calculation of dose distributions implemented in conjunction with CT, MRI and DSA stereotactic image analysis systems is presented.
Since 1954, 39 cases of glomus jugulare or vagale tumor have been treated by the authors with radiotherapy. The initial results with orthovoltage (280 kV) therapy were unpredictable, but 30 patients with 32 tumors treated with megavoltage radiotherapy (Cobalt-60 [60Co] or linear accelerator) have shown either no evidence of recurrence or complications, with one exception, or died of causes unrelated to their glomus tumor. Megavoltage radiotherapy is recommended for primary treatment of all glomus tumors that present with evidence of bone invasion or nerve involvement.
This report describes now a set of applicators, convering fields with dimensions of 4 to 20 cm, for the 6 to 20 MeV electron beams of a MEL SL75-20 linear accelerator was developed. The electron scatter contribution of the applicator walls to the treatment field was investigated, varying the applicator entrance opening and the scattering foil, with the aim of optimizing the resulting field flatness, with a minimum loss of depth dose. Experiments with field defining end frames and additional perspex scatterers for large field sizes are also reported.
It was shown that RBE coefficients of protons (9 GeV) and accelerated helium ions (4 GeV/nucleon) are within the range from 1.0 to 11.6 and 1.0 to 7.2, respectively, depending on the object under study, the criterium of estimation, the registration time, and the dose value.