Biomedical subjects

E E Klein

Publications and source records attributed to E E Klein.

At least 37 records · Page 2Linked to original sources

PubMed1996

Design and dosimetric characteristics of a high dose rate remotely afterloaded endocavitary applicator system.

PURPOSE: An applicator is described for endocavitary treatment of rectal cancers using a high dose rate (HDR) remote afterloading system with a single high-intensity 192Ir source as an alternative to the 50 kVp x-ray therapy contact unit most frequently used in this application. METHODS AND MATERIALS: The applicator consists of a tungsten-alloy collimator with a 45 degree beveled end, placed in a protoscope with an elliptical cross-section. The resultant 3 cm diameter circular treatment aperture, located in the beveled face of the proctoscope, is irradiated by circular array of dwell positions located about 6.5 mm from the applicator surface. This beveled end allows patients with posterior wall tumors to be treated in the dorsal lithotomy position. The dose-rate distributions about the applicator were determined using a combination of thermoluminescent dosimetry (TLD-100 detectors) and radiochromic film dose measurement techniques along with Monte Carlo dosimetry calculations. TLD-100 (3 x 3 x 0.9 mm3 chips) measurements were used to measure the distribution of dose over the proctoscope surface as well as the central axis dose-rate distribution. Relative radiochromic film measurements were used to measure off-axis ratios (flatness and penumbra width) within the treatment aperture. These data were combined with Monte Carlo simulation results to obtain the final dose distribution. RESULTS: The tungsten collimator successfully limits the dose to the tissue in contact with the proctoscope walls to less than 12% of the prescribed dose. These results indicate that the HDR applicator system has slightly more penetrating depth-dose characteristics than the most widely used contact therapy x-ray machine. Flatness characteristics of the two treatment delivery systems are comparable, although the HDR endocavitary applicator has a significantly wider penumbra. Finally, the HDR applicator has a lower surface dose rate (1.5-4 Gy/min of dwell time) compared to 9-10 Gy/min for the x-ray unit. CONCLUSIONS: An applicator system has been developed for endocavitary treatment of early stage rectal carcinoma that uses a single-stepping source HDR remote afterloading system as a radiation source. The advantages of the HDR-based system over x-ray therapy contact units currently used in this clinical application are (a) enhanced flexibility in applicator design and (b) widespread availability of single-stepping source HDR remote afterloading systems.

Brachytherapy↗

PubMed1996

Commissioning and periodic quality assurance of a clinical electronic portal imaging device.

PURPOSE: An electronic portal imaging device (EPID) was recently installed on our dual-energy linear accelerator. Commissioning and quality assurance techniques were developed for the EPID. METHODS AND MATERIALS: A commissioning procedure was developed consisting of five parts: (a) physical operation and safety; (b) image acquisition, resolution, and sensitivity calibration; (c) image storage, analysis, and handling; (d) reference image acquisition; and (e) clinical operations. RESULTS: The physical operation and safety tests relate to the motions of the unit, stability of the unit supports, safety interlocks, and interlock overrides. Imager contrast and spatial resolutions are monitored by imaging a contrast-detail phantom. The imager calibration procedure consists of a no-radiation image to compensate for signal offsets, as well as a "flat-field image." The flat-field image is taken with 5.0 cm of homogeneous phantom material placed at isocenter to provide some photon scatter and to approximate the presence of a patient. Daily quality assurance procedures consists of safety tests and the acquisition and inspection of images of the contrast-detail phantom. After 1 year, the frequency of the daily procedure was reduced to weekly. Quarterly QA procedures are conducted by the physicist and consist of the same procedures conducted in the weekly test. The annual QA procedure consists of a duplication of the commissioning procedure. CONCLUSION: The procedures discussed in this article were applied to an ionization-chamber device. They have been useful in identifying difficulties with the EPID operation, including the need for recalibrating and monitoring the accelerator output stability.

Calibration↗

PubMed1996

A quality assurance program for ancillary high technology devices on a dual-energy accelerator.

Our facility has added high-technology ancillary devices to our dual-energy linear accelerator. After commissioning and acceptance testing of dual asymmetric jaws, dynamic wedge, portal imaging, and multileaf collimation (MLC), quality assurance programs were instituted. The programs were designed to be both periodic and patient specific when required. In addition, when dosimetric aspects were affected by these technologies, additional quality assurance checks were added. Positional accuracy checks (light and radiation) are done for both asymmetric jaws and MLC. Each patient MLC field is checked against the original simulation or digitally reconstructed radiographs. Off-axis factors and output checks are performed for asymmetric fields. Dynamic wedge transmission factors and profiles are checked periodically, and a patient diode check is performed for every new dynamic wedge portal. On-line imaging checks encompass safety checks along with periodic measurement of contrast and spatial resolution. The most important quality assurance activity is the annual review of proper operation and procedures for each device. Our programs have been successful in avoiding patient-related errors or device malfunctions. The programs are a team effort involving physicists, maintenance engineers, and therapists.

Engineering↗

PubMed1996

Feasibility study of multileaf collimated electrons with a scattering foil based accelerator.

BACKGROUND AND PURPOSE: There is an ever evolving process to improve the technical aspects of electron beam delivery. Both the foil/applicator and scanning electron beam systems have gone through recent upheavals. Concomitantly, multileaf collimators are now a staple method for collimating photons. We undertook a study of multileaf collimated electron beam (MLCEB) using a dual scattering foil system. MATERIALS AND METHODS: We compared MLCEB with applicator collimated electron beams (AEB) by examining the dosimetric aspects of the two systems using 70 and 80 cm SSDs for the MLCEB, the minimum practical SSDs achievable. Percent depth dose, isodose profiles, output factors, leakage, surface dose, bremstrahlung, effective SSDs, etc. were measured using film and/or ion chamber. Clinical fields, such as posterior neck node (PNN), were compared. We also investigated the use of MLCEB for arc therapy using segments. RESULTS: In all cases, the MLCEB performed inferior, as judged by isodoses, uniformity index (UI) and penumbra analysis. The 80 cm SSD (minimum for PNN), low energy, small fields, was the worst case. For a 6 MeV beam, the UI/penumbra was 0.823/10 mm for the AEB, and 0.561/29 mm for the MLCEB at 80 cm SSD. The PNN multileaf fields exhibited narrow 90% and 80% isodose lines, and wide 20% and 10% lines. CONCLUSIONS: We conclude that multileaf for PNN fields could not be matched to adjacent off-cord photon fields. The "stair-stepping' effect associated with MLC photons was absent for electrons.

Electrons↗

PubMed1996

Verification of milled two-dimensional photon compensating filters using an electronic portal imaging device.

A computer-driven milling machine is being installed at the Mallinckrodt Institute of Radiology to fabricate photon compensating filters for conventional compensation and beam-intensity modulation. Commissioning and quality assurance procedures are being developed for the design, fabrication, and delivery systems prior to using the milled filters in the clinic. The portion of the quality assurance procedures governing the filter fabrication using a computer-driven milling machine includes, in part, a comparison of designed and fabricated filters. Test filters include geometrically regular filters, such as flat surfaces and steps. The verification of these shapes is accomplished using spatial measurements. However, to test the mill's ability to generate complex curved surfaces, filters with more complicated surfaces are generated. These filters do not lend themselves to precise verification using physical measurement of the thickness distribution. A method has been developed to verify the thickness distributions of these complex filters by irradiating the filters with high-energy x rays and comparing the scattered and transmitted fluence to the fluence calculated using the intended filter shape. The fluence measurement is made using a calibrated commercial liquid ionization chamber electronic portal imaging device. The calculated fluence is separated into transmitted primary and scattered fluences and is determined using a convolution of a distributed radiation source kernel with an exponential filter attenuation function. The attenuation coefficient is measured for the filter material (Lipowitz metal) and fit to a second-order polynomial in filter thickness and off-axis distance. The distributed source kernel is measured using a split-field technique and fit to a sum of three two-dimensional Gaussian distributions. The scattered radiation is modeled by the Klein-Nishina cross section. The algorithm is tested by comparing calculated fluences with measured fluences for a series of machined filters: an open field, flat filters of 9.6-, 15.8-, and 31.6-mm thickness, split-field filters, and a pyramid-shaped filter. In each case, the algorithm calculates the fluence to within 3% of the measured values over the entire irradiated field size to within 1.5 cm of the collimated field edges.

Algorithms↗

PubMed1995

Clinical implementation of a commercial multileaf collimator: dosimetry, networking, simulation, and quality assurance.

PURPOSE: Clinical implementation of multileaf collimation (MLC) includes commissioning (including leaf calibration), dosimetric measurements (penumbra, transmission, calculation parameters), shaping methods, networking for file transfer, verification simulation, and development of a quality assurance (QA) program. Differences of MLC and alloy shaping in terms of penumbra and stair-step effects must be analyzed. METHODS AND MATERIALS: Leaf positions are calibrated to light field. The resultant decrement line, penumbras, leaf transmission data, and isodoses in various planes were measured with film. Penumbra was measured for straight edges and corners, in various media. Ion chambers were used to measure effects of MLC on output, scatter, and depth dose. We maintain midleaf intersection criteria. MLC fields are set 7 mm beyond planning target volumes. After shaping by vendor software or by our three-dimensional planning system, files are transferred to the MLC workstation by means of sharing software, interface cards, and cabling. A MLC emulator was constructed for simulation. Our QA program includes file checks, monthly checks (leaf position accuracy and interlock tests), and annual review. RESULTS: We found the MLC leaf position (light field) corresponds to decrement lines ranging from 50 to 59%. Transmission through MLC (1.5-2.5%) is less than alloy (3.5%). Multileaf penumbra is slightly wider than for alloy. Relative penumbra did not increase in the lung, and composite field dosimetry exhibited negligible differences compared with alloy. Verification simulations provide diagnostic image quality hard copies of the MLC fields. Monitor unit parameters used for alloy held for MLC. DISCUSSION: Clinical implementation for MLC as a block replacement was conducted on a site-by-site basis. Time studies indicate significant (25%) in-room time reductions. Through imaging and dosimetric analysis, the accuracy of field delivery has increased with MLC. The most significant impact of MLC is the ability to increase the number of daily treatment fields, thereby reducing normal tissue dosing, which is vital for dose escalation.

Calibration↗

PubMed1995

Changes in electron beam dosimetry with a new scattering foil-applicator system on a CL2100C.

PURPOSE: The optimization of clinical electron beams is a challenge to accelerator manufacturers. There are numerous variations and reports of scattering-foil and applicator configurations. The accelerator at our facility was recently updated with new foils and applicators. We conducted many dosimetric tests to critically evaluate dosimetric changes and their clinical effects. METHODS AND MATERIALS: The new dual foil systems are thicker and have shaped disks seated on the lower foils. The 12 MeV beam no longer shares a common foil used for 6 and 9 MeV. The applicators now have denser collimating plates, and Fiberglas no longer connects the plates. The new applicator set includes a rectangular 10 x 6 cm applicator that uses one photon jaw setting for all energies. After the electron beam energies were tuned to previous specifications (energy according to ionization depths, symmetry to +/- 2%, and flatness to +/- 6%), recommissioning took place. Electron beam output checks at various source-to-skin distances (SSD) were conducted for all energies and applicators. Computer-driven water scanning provided percent depth dose, profile, isodose, and Bremsstrahlung data. Surface doses, in-air electron dispersion, effective SSDs, and leakage were also measured. All results compared the previous and updated systems. RESULTS: We found little change in relative percent depth doses for 100 cm SSD between the two systems. The differences in PDD due to increasing SSD, however, decreased with the updated system. Surface doses decreased in most cases, while Bremsstrahlung increased in all cases (typically by a factor of two). Beam uniformity indices increased significantly, while penumbra widths decreased. Diagonal profiles are now quite flat for large fields. For a 20 MeV beam, the 90% width along the diagonal axis for a 25 x 25 applicator at dmax depth has increased from 25 to 32 cm. There was little or no change in 'effective SSD' or in-air dispersion. Leakage outside the applicators was reduced by a factor of two to three. The flatness characteristics of the 10 x 6 cm applicator were poor in comparison to the improved flatness of the new square applicators. CONCLUSIONS: The updated scattering foil-applicator electron beam system has yielded many dosimetric changes. Major improvements have been made in beam flatness and leakage. These positive changes have not been accompanied by any clinically significant dosimetric deficiencies.

Calibration↗

PubMed1995

Simultaneous delivery of electron beam therapy and ultrasound hyperthermia using scanning reflectors: a feasibility study.

PURPOSE: The feasibility of simultaneously delivering external electron beam radiation and superficial hyperthermia using a scanning ultrasound reflector-array system (SURAS) was experimentally investigated and demonstrated. METHODS AND MATERIALS: A new system uses a scanning reflector to distribute the acoustic energy from a planar ultrasound array over the surface of the target volume. External photon/electron beams can be concurrently delivered with hyperthermia by irradiating through the scanning reflectors. That is, this system enables the acoustic waves and the radiation beams to enter the target volume from the same direction. Reflectors were constructed of air-equivalent materials for maximum acoustic reflection and minimum radiation attenuation. Acoustically, the air reflectors were compared to brass reflectors (assumed ideal) for reflectivity and specular quality using several single transducers ranging in frequency from 0.68 to 4.8 MHz. The relative reflectivity was determined from acoustic power measurements using a force-balance technique. The specular quality was assessed by comparing the acoustic pressure fields reflected by air reflectors with those reflected by brass reflectors. Also, acoustic pressure fields generated by a SURAS prototype for two different arrays (2.24 and 4.5 MHz) were measured to investigate field distribution variations as a function of the distance separating the array and the scanning reflector. All pressure fields were measured with a hydrophone in a degassed water tank. Finally, to determine the effect of the air reflectors on electron dose distributions, these were measured using film in a water-equivalent solid phantom after passage of a 20 MeV electron beam through the SURAS. These measurements were performed with the reflector scanning continuously across the electron beam and at rest within the electron beam. RESULTS: The measurements performed using single ultrasound transducers showed that the air reflectors had power reflectivities of 87-96% that of brass, and that for smooth surfaces the reflections from air reflectors were as specular as those from brass reflectors. Acoustic pressure fields measurements of the SURAS for two different arrays showed that the 50% pressure amplitude contours were well-distributed across the projected surface area of the array for different distances separating the array and the reflector. Finally, film dosimetry showed that the electron dose distribution was not affected by the air reflector of the SURAS either for the scanning case or the stationary case. This indicates that the reflectors as made are basically water-equivalent in terms of high energy ionizing radiation. The measured isodoses also indicate that the constructed SURAS prototype would allow the delivery of adequate radiation (90% isodose) to a depth of 2.0 cm. CONCLUSIONS: The results presented show that the SURAS design has the potential to deliver hyperthermia to large superficial tumors, while allowing simultaneous irradiation with 20 MeV electron beams without adverse effects on the radiation dose delivery.

Acoustics↗

PubMed1995

Dosimetry and clinical implementation of dynamic wedge.

PURPOSE: Wedge-shaped isodoses are desired in a number of clinical situations. Physical wedge filters have provided nominal angled isodoses with dosimetric consequences of beam hardening, increased peripheral dosing, nonidealized gradients at deep depths, along with the practical consequences of filter handling and placement problems. Dynamic wedging uses a combination of a moving jaw and changing dose rate to achieve angled isodoses. The clinical implementation of dynamic wedge and an accompanying quality assurance program are discussed in detail. METHODS AND MATERIALS: The accelerator at our facility has two photon energies (6 MV and 18 MV), currently with dynamic wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees. The segmented treatment tables (STT) that drive the jaw in concert with a changing dose rate are unique for field sizes ranging from 4.0 cm to 20.0 cm in 0.5 cm steps, resulting in 256 STTs. Transmission wedge factors were measured for each STT with an ion chamber. Isodose profiles were accumulated with film after dose conversion. For treatment-planning purposes, dmax orthogonal dose profiles were measured for open and dynamic fields. Physical filters were assigned empirically via the ratio of open and wedge profiles. RESULTS: A nonlinear relationship with wedge factor and field size was found. The factors were found to be independent of the stationary field setting or second order blocking. Dynamic wedging provided more consistent gradients across the field compared with physical filters. Percent depth doses were found to be closer to open field. The created physical filters provided planned isodoses that closely resembled measured isodoses. Comparative isodose plans show improvement with dynamic wedging. CONCLUSIONS: Dynamic wedging has practical and dosimetric advantages over physical filters. Table collisions with physical filters are alleviated. Treatment planning has been solved with an empirical solution. Dynamic wedge is a positive replacement for physical filters, and a first step for commercial introduction of dynamic conformal therapy.

Humans↗

PubMed1995

Use of a lucite beam spoiler for high-energy breast irradiation.

Treatment planning for breast cancer is a challenge for bridge separations greater than 24 cm. protocols require dose homogeneity to the breast parenchyma to 10% while maintaining good cosmesis (skin sparing). Low-energy beams (6 MV or lower) may not achieve protocol specifications because of lateral or medial hot spots. The buildup region for high-energy beams (i.e., 18 MV) may lead to underdosing in the superficial regions. Adding bolus will pull the isodoses to surface, surrendering skin sparing. A spoiler will increase the contribution of scattered medium energy electrons and photons, thereby increasing superficial dosing and maintaining skin sparing due to displacement of the spoiler from the skin. We chose Lucite as the spoiler material for our 18-MV beam. Buildup measurements were performed with a parallel-plate ionization chamber to find the optimal spoiler thickness of 18 mm at a spoiler-to-skin distance of 22 cm. A breast phantom was designed allowing thermoluminescent dosimeter measurements. The spoiler has increased the percent depth dose from the depths of 0.5 cm to 3.0 cm to mimic those delivered by a 6 MV beam, while the relative skin dose was held to 50%. Our treatment schema is typically 6-MV photons with customized compensation for half or more of the fractions and uncompensated 18-MV photons with the spoiler for the remaining fractions. The spoiler may be used in conjunction with thin bolus (1.0 cm) for therapeutic dosing to the skin (scars, inflammatory disease, etc.). The treatment plans have maintained dose homogeneity for large patients, without consequence of skin reaction.

Breast Neoplasms↗

PubMed1995

Clinical introduction of a commercial treatment chair to facilitate thorax irradiation.

There are occasions when patients can benefit significantly from being treated upright. This technique is beneficial in the treatment of Hodgkin's disease when the mediastinum is involved. The main benefit is the lung volume spared resulting from narrowing of a bulky mediastinum. We purchased a commercial treatment chair to facilitate upright treatment. Clinical implementation required rigorous testing, documentation of procedures, and acknowledgment of and corresponding solutions to limitations. The purchased chair is designed to couple with the spine bar of the treatment couch, allowing patient positioning via pendant functions. The chair has no obstruction as immobilization is afforded by arm, shoulder, and knee supports, which allows isocentric treatments. An additional hydraulic mount is used for simulation. We have treated five patients thus far, including Hodgkin's disease patients with and without mediastinal involvement, and one lung cancer patient with a large lower lobe lesion. All patients have benefited by having a greater volume of lung spared (as much as 60% additional blocking) when upright simulation films are compared with supine and prone films. Immobilization has been adequate as confirmed by daily light field projections and portal films. We are routinely evaluating Hodgkin's disease patients for upright treatment. Patients treated in the chair have benefited in terms of lung volume spared without loss of immobilization. A patient with a large lower lobe lung lesion has also benefited as the tumor "dropped," thus affording lung sparing via blocking.

Equipment Design↗

PubMed1995

Stereotactic external beam irradiation using a linear accelerator: the Washington University experience.

From February 1989 to December 1993, 139 patients with a variety of brain lesions were treated with stereotactic external beam irradiation using a 6MV linear accelerator. The largest group consisted of patients with recurrent brain metastases (n = 46). Twenty seven patients had malignant gliomas, most of which were recurrent. Several benign conditions were treated, including arteriovenous malformations (n = 27), acoustic neuromas (n = 9), meningiomas (n = 7), and pituitary adenomas (n = 2). Durable responses were seen in the majority of patients with transient, mild, side effects. This experience suggests that stereotactic external beam irradiation is a safe, reliable, and effective method for non-invasive treatment of selected patients with small, localized brain lesions.

Academic Medical Centers↗

PubMed1994

Assurance of high quality linac-based stereotactic radiosurgery.

PURPOSE: Stereotactic radiosurgery is generally a single, high-dose radiation treatment for the brain requiring targeting accuracy on the order of a millimeter. From the initial implementation of radiosurgery, therefore, quality assurance is an ongoing process of paramount importance. In this paper, we outline the basic elements of a quality assurance program for our linear accelerator that has been in use at Washington University Medical Center over the past 2 years. METHODS AND MATERIALS: Various devices and procedures have been developed to verify the accuracy and safety of the stereotactic radiosurgery regimen. Specifically, we present methods for assessing the attainment of spatially correct patient images, the reliability of the computerized treatment planning system, achieving physical safety for the patient, as well as the proper operation of the radiation treatment device. RESULTS: Our procedures have allowed us to assure quality patient treatments and, additionally, has permitted monitoring our performance for continual improvement. For example, a plot of targeting accuracy with the number of patients shows an asymptotic approach to a value within 0.6 mm of that ideally expected. CONCLUSION: To maintain high-quality patient care, one must review critical aspects of the treatment regimen on a periodic basis. Providing for the appropriate level of staff training, periodic reviews of procedures and maintenance of forms are also very important.

Humans↗

PubMed1994

A mono isocentric technique for breast and regional nodal therapy using dual asymmetric jaws.

PURPOSE: Definitive radiation therapy for breast cancer with regional nodal involvement often introduces treatment of adjacent abutted regions. Many methods describe techniques to achieve an effective transverse plane match. Our facility recently adopted a matching technique using asymmetric jaws to beam-split all portals along the central axis plane. Our technique uses one isocenter to treat the opposed tangential breast fields, the supraclavicular port and the posterior axillary field. METHODS AND MATERIALS: Our linear accelerator has four collimator jaws capable of being set independently. The longitudinal (Y) jaws beam-split all the portals at the match plane, namely the upper border of the tangential beams and the bottom border of the nodal fields. The transverse (X) jaws define the lateral borders of the nodal fields, and in a near beam-split fashion in conjunction with customized Cerrobend, block the lung for the tangential beams. The unique isocenter is chosen along the mid-bridge through the tangential match plane. Dosimetric qualities and calculational techniques of the asymmetric beams were analyzed with ionimetric water scans, ion chamber studies, and film. The match-line is clinically confirmed with composite port films. RESULTS: Our dosimetric studies show asymmetric jaws provide nearly equivalent field edge definition and superior absorption in comparison with Cerrobend blocks. The use of one isocenter results in a reduction of in-room treatment time by a factor of two. The burden of lifting heavy Cerrobend blocks has been removed. A composite port film, which includes the medial tangential and supraclavicular ports, shows a perfect match-line in all cases. Similar composite port films taken with our previous technique of geometric matching with collimator and table angulation exhibit slight overlap or underdose regions in many cases. CONCLUSION: Our treatment technique takes full advantage of dual asymmetric jaws to achieve a perfect match-line, necessitates only one isocenter and set-up point, and supplies more absorption in reference to lung and contralateral breast dose. The pure match-line is accompanied by the fact that the patient does not have to move in any direction.

Breast Neoplasms↗

PubMed1994

Quality assurance and dosimetric evaluation for an endocavitary unit.

The use of endocavitary contact therapy for selected rectal carcinomas continues to be an effective treatment option. Very small volumes are treated with an extremely high dose rate associated with rapid fall-off in depth and an overall high dose. The clinical benefits of the high dose rate leads to dosimetric and quality-assurance challenges. In addition, the operating room environment creates concerns in terms of dosimetry and radiation safety due to varying room line voltages and uncontrolled environments. The unit at our facility delivers 50 kVp X-rays using an SSD of 35 mm with lead-lined procotscopes of 24 and 29 mm. The dose rate is approximately 10 Gy per minute. Establishment of daily, monthly, and annual quality assurance reviews have been made and assessed. Parallel-plate ionization chambers measure outputs and relative depth dose with different phantoms. Silver bromide film is used to evaluate beam profiles. An electron diode system was chosen for day-of-treatment output checks. An ion chamber survey meter measures scatter and leakage exposure rates. Day-of-treatment output checks have assured output stability in various operating rooms. Trends in output have been confirmed by monthly ion chamber checks. Percent depth dose measurements carried out in liquid water compared well with accepted published data as did corrected polystyrene measurements. Radiation survey measurements detected acceptable exposure rate levels. The established comprehensive quality assurance program incorporates cross checking with multiple dosimetry systems. The confidence level of dose delivery has increased with the introduction of a day-of-treatment output checks.(ABSTRACT TRUNCATED AT 250 WORDS)

Brachytherapy↗

PubMed1993

Pre-installation empirical testing of room shielding for high dose rate remote afterloaders.

PURPOSE: Many facilities are acquiring high dose rate remote afterloading units. It is economical that these units be placed in existing shielded teletherapy rooms. Scatter-radiation barriers marginally protect uncontrolled areas from a high dose rate source especially in a room that houses a non-dynamic Cobalt-60 unit. In addition the exact thickness and material composition of the barriers are unknown and therefore, a calculation technique may give misleading results. Also, it would be impossible to evaluate an entire wall barrier by taking isolated core samples in order to assist in the calculations. A quick and inexpensive measurement of dose equivalent using a rented high activity 192Ir source evaluates the barriers and locates shielding deficiencies. METHODS AND MATERIALS: We performed transmission calculations for primary and scattered radiation based on National Council on Radiation Protection and Measurements Reports 49 and 51, respectively. We then rented a high activity 21.7 Ci (8.03 x 10(11) Bq) Ir-192 source to assess our existing teletherapy room shielding for adequacy and voids. This source was placed at the proposed location for clinical high dose rate treatment and measurements were performed. RESULTS: No deficiencies were found in controlled areas surrounding the room, but large differences were found between the calculated and measured values. Our survey located a region in the uncontrolled area above the room requiring augmented shielding which was not predicted by the calculations. A canopy shield was designed to potentially augment the shielding in the ceiling direction. CONCLUSION: Pre-installation testing by measurement is an invaluable method for locating shielding deficiencies and avoiding unnecessary enhancement of shielding particularly when there is lack of information of the inherent shielding.

Brachytherapy↗

PubMed1993

Independent collimator dosimetry for a dual photon energy linear accelerator.

PURPOSE: The independent collimator feature in medical linear accelerators can define radiation fields that are asymmetric with respect to the flattening filter and oblique to the incident surface. Prior to clinical implementation, it is necessary to evaluate the dosimetry of this non-standard treatment delivery technique. An investigation of the independent collimator dosimetry for 6 MV and 18 MV x-ray beams has been undertaken. METHODS AND MATERIALS: Dose to tissue in free space, percent depth dose and dose distribution were measured and compared to that for symmetric field collimation. RESULTS: The dosimetry results were consistent for both photon modes. Dose in free space with asymmetric collimation can be calculated from the corresponding symmetric field dose in free space to within 1.2 +/- 0.7% by applying an appropriate off-axis factor. Asymmetric field percent depth dose differs from symmetric field percent depth dose on average by 1.1 +/- 0.7% for 6 MV and by 0.7 +/- 0.5% for 18 MV for field sizes ranging from 5 x 5 to 20 x 20, centered 3 cm and 10 cm off-axis. The measured isodose curves demonstrate divergence effects and reduced doses (less than 3%) adjacent to the field edge closest to the flattening filter center. This dose asymmetry result is identical to that from secondary collimation. CONCLUSION: The methodology for clinical implementation of the independent collimator feature is straightforward. However, accurate representation of the isodose distributions by commercial radiotherapy treatment planning systems requires special dose calculation algorithms.

Particle Accelerators↗

PubMed1993

The influence of air cavities on interface doses for photon beams.

PURPOSE: As the quantification of dose in homogeneous media is now better understood, it is necessary to further quantify effects from heterogeneous media. The most extreme case is related to air cavities. Although dose corrections at large distances beyond a cavity are accountable by attenuation differences, perturbations at air-tissue interfaces are complex to measure or calculate. These measurements helps understand the physical processes that govern these perturbations. METHODS AND MATERIALS: A thin window parallel-plate chamber and a special diode were used for measurements with various air cavity geometries (layer, channel, cubic cavity, triangle) in x-ray beams of 4 and 15 MV. RESULTS: Underdosing effects occur at both the distal and proximal air cavity interfaces. The magnitude depends on geometry, energy, and field sizes. As the cavity thickness increases, the central axis dose at the distal interface decreases. Increasing field size remedied the underdosing, as did the introduction of lateral walls. Following a 2.0 cm wide air channel for a 4 MV, 4 x 4 cm2 field there was an 11% underdose at the distal interface, while a 2.0 cm cubic cavity yielded only a 3% loss. Measurements at the proximal interface showed losses of 5% to 8%. For a 4 MV parallel opposed beam irradiation the losses at the interfaces were 10% for a channel cavity (in comparison with the homogeneous case) and 1% for a cube. The losses were slightly larger for the 15 MV beam. Underdosage at the lateral interface was 4% and 8% for the 4 MV and 15 MV beams, respectively. CONCLUSION: Although reports suggest better clinical results using lower photon energies with the presence of air cavities, there is no reliable dose calculation algorithm to predict interface doses accurately. The measurements reported here can be used to guide the development of new calculation models under nonequilibrium conditions. This situation is of clinical concern when lesions such as larynx carcinoma beyond air cavities are irradiated.

Absorption↗