Film In Phantom Research Articles

Adaptive neighborhood contrast enhancement in mammographic phantom images

With a view of quantitatively evaluating image quality in mammographic systems from an automated computerized analysis of digitized mammographic phantom films, an image enhancement method for such images is presented. In this framework an adaptive neighborhood image processing technique with a contrast-enhancement function is used. The method consists of computing a local contrast around each pixel using a variable neighborhood whose size and shape depend on the statistical properties around the given pixel. The obtained image is then transformed into a new contrast image using several contrast-enhancement functions. Finally, an inverse contrast transform is applied to the previous image. To compare contrast-enhancement functions, images simulating objects similar to those observed in the phantom image, with various relative contrasts and signal-to-noise ratio (SNR) levels, are generated. Several parameters including the output-to-input SNR ratio and the mean squared error are used as comparison criteria. Results show that this process enhances features in the image with little enhancement of noise and that a trigonometric contrast-enhancement function performs best for studying phantom images.

The use of rectal balloon during the delivery of intensity modulated radiotherapy (IMRT) for prostate cancer: more than just a prostate gland immobilization device?

The purpose of this study was to investigate the role of a rectal balloon for prostate immobilization and rectal toxicity reduction in patients receiving dose-escalated intensity-modulated radiotherapy for prostate cancer. Patients with localized prostate cancer who were undergoing intensity-modulated radiotherapy were treated in a prone position, immobilized with a customized Vac-Lok bag (MED-TEC, Orange City, IA). A rectal balloon with 100 cc of air was used to immobilize the prostate. The prostate displacements were measured using computed tomography (CT)-CT fusion on 10 patients who received radioactive seed implant before intensity-modulated radiotherapy. They were scanned twice weekly during 5 weeks of intensity-modulated radiotherapy, and breathing studies were also performed. Rectal toxicity was evaluated by use of Radiation Therapy Oncology Group scoring in 100 patients. They were treated to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). Dose-volume histogram of the rectum was assessed. A film phantom was constructed to simulate the 4-cm diameter air cavity that was created by the rectal balloon. Kodak XV2 films (Rochester NY) were used to measure and compare dose distribution with and without the air cavity. A fraction of 1.25 Gy was delivered to the phantom at isocenter with 15-MV photons by use of the NOMOS Peacock system and the MIMiC treatment delivery system (Sewickley, PA). The anterior-posterior and lateral prostate displacements were minimal, on the order of measurement uncertainty (approximately 1 mm). The standard deviation of superior-inferior displacement was 1.78 mm. Breathing studies showed no organ displacement during normal breathing when the rectal balloon was in place. The rectal toxicity profile was very favorable: 83% (83/100) patients had no rectal complaint, and 11% and 6% had grade 1 and 2 toxicity, respectively. Dose-volume histogram analysis revealed that in all of the patients, no more than 25% of the rectum received 70 Gy or greater. As visualized by film dosimetry, the dose at air-tissue interface was approximately 15% lower than that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, the differential was approximately 8% and 5%, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal, with or without the air cavity. The use of a rectal balloon during intensity-modulated radiotherapy significantly reduces prostate motion. Prostate immobilization thus allows a safer and smaller planning target volume margin. It has also helped spare the anterior rectal wall (by its dosimetric effects) and reduced the rectal volume that received high-dose radiation (by rectal wall distension). All these factors may have further contributed to the decreased rectal toxicity achieved by intensity-modulated radiotherapy, despite dose escalation and higher-than-conventional fraction size.

An automated phantom-film QA procedure for intensity-modulated radiation therapy

To verify that the calculated dose distribution is delivered accurately during intensity-modulated radiation therapy (IMRT), we have implemented an automated plan/film validation protocol. The cubic polystyrene film phantom provided with the Peacock IMRT system and the Radiation Imaging Technology (RIT) film dosimetry system were used to compare planned and delivered dose distributions. The calculated dose matrix from CORVUS was transferred to RIT and analyzed. The analysis included dose-difference histograms, dose comparison in low-gradient areas, distance to agreement in high-gradient areas, dose profiles, and isodose comparisons. Dose differences of up to 5% were commonly observed in the high-dose and low-gradient areas between verification films and treatment plans for prostate patients. The most prominent discrepancies were detected in the high-gradient areas of dose distributions. The automated protocol is an efficient technique that provides information about spatial differences between calculated and delivered doses.

A thin film phantom for blood flow simulation and Doppler test.

The thin film phantom is a new type of ultrasound resolution test object. It consists of a thin planar substrate that is acoustically matched to the surrounding media. Precisely located scatterers on the surface of the substrate generate echo signals. The patterning of scatterers on the substrate allows echogenicity to be controlled as a function of position, which enables the production of a test object with highly reproducible and controllable scattering characteristics. We show that by vibrating the substrate in a suitable manner, an echo signal may be generated that simulates bi-directional flow. We demonstrate that a vibration of low amplitude at frequency f0 produces a Doppler spectral signal at f0 and -f0, within the limits of aliasing. Furthermore, by driving the film with a bandlimited noise signal, we illustrate how a velocity distribution may be simulated. A time-varying flow velocity may be simulated by varying the noise bandwidth with time. Finally, using this technique, we demonstrate a system that simulates an arterial flow pattern, including its characteristic velocity distribution in forward and reverse directions simultaneously.

Quality control in interstitial brachytherapy of the breast using pulsed dose rate: treatment planning and dose delivery with an Ir-192 afterloading system

Background and purpose: In the Radiotherapy Department of Leuven, about 20% of all breast cancer patients treated with breast conserving surgery and external radiotherapy receive an additional boost with pulsed dose rate (PDR) Ir-192 brachytherapy. An investigation was performed to assess the accuracy of the delivered PDR brachytherapy treatment. Secondly, the feasibility of in vivo measurements during PDR dose delivery was investigated. Materials and methods: Two phantoms are manufactured to mimic a breast, one for thermoluminescent dosimetry (TLD) measurements, and one for dosimetry using radiochromic films. The TLD phantom allows measurements at 34 dose points in three planes including the basal dose points. The film phantom is designed in such a way that films can be positioned in a plane parallel and orthogonal to the needles. Results: The dose distributions calculated with the TPS are in good agreement with both TLD and radiochromic film measurements (average deviations of point doses <±5%). However, close to the interface tissue-air the dose is overestimated by the TPS since it neglects the finite size of a breast and the associated lack of backscatter (average deviations of point doses −14%). Conclusion: Most deviations between measured and calculated doses, are in the order of magnitude of the uncertainty associated with the source strength specification, except for the point doses measured close to the skin. In vivo dosimetry during PDR brachytherapy treatment was found to be a valuable procedure to detect large errors, e.g. errors caused by an incorrect data transfer.

Chest wall irradiation with MLC-shaped photon and electron fields

Purpose: To improve the treatment technique for chest wall irradiation, using the multileaf collimator (MLC) of the MM50 Racetrack Microtron to shape both photon and electron beams, and to check the dose delivery in the match-line region of these fields for the routine and improved technique. Methods and Materials: Using diode and film phantom measurements, the optimal number of photon beam segments and their positions relative to the electron beam were determined. On phantoms, and during actual patient treatment using in vivo dosimetry, the dose homogeneity in the match-line region was determined for both the routine and improved techniques. Results: Three photon beam segments (9-mm gap, perfect match, and 9-mm overlap) were used to match the electron beam, resulting in minimum-maximum dose values in the match-line region of 88–109%, compared to 80–115% for the routine technique (2 photon beam segments). During patient treatment, the average minimum and maximum dose values were 95% and 115%, respectively, compared to 78% and 127%, respectively, for the routine technique. The interfraction variation in dose delivery was reduced from 11.0% (1 SD) to 4.6% (1 SD). The actual treatment time was reduced from 10 to 4.5 min. Conclusion: Using the MLC of the MM50 to shape both photon and electron beams, an improved treatment technique for chest wall irradiation was developed, which is less labor intensive, faster, and yields a more homogeneous, and better reproducible dose delivery.

Abutment region dosimetry for serial tomotherapy

Purpose: A commercial intensity modulated radiation therapy system (Corvus, NOMOS Corp.) is presently used in our clinic to generate optimized dose distributions delivered using a proprietary dynamic multileaf collimator (DMLC) (MIMiC) composed of 20 opposed leaf pairs. On our accelerator (Clinac 600C/D, Varian Associates, Inc.) each MIMiC leaf projects to either 1.00 × 0.84 or 1.00 × 1.70 cm 2 (depending on the treatment plan and termed 1 cm or 2 cm mode, respectively). The MIMiC is used to deliver serial (axial) tomotherapy treatment plans, in which the beam is delivered to a nearly cylindrical volume as the DMLC is rotated about the patient. For longer targets, the patient is moved (indexed) between treatments a distance corresponding to the projected leaf width. The treatment relies on precise indexing and a method was developed to measure the precision of indexing devices. A treatment planning study of the dosimetric effects of incorrect patient indexing and concluded that a dose heterogeneity of 10% mm -1 resulted. Because the results may be sensitive to the dose model accuracy, we conducted a measurement-based investigation of the consequences of incorrect indexing using our accelerator. Although the indexing provides an accurate field abutment along the isocenter, due to beam divergence, hot and cold spots will be produced below and above isocenter, respectively, when less than 300° arcs were used. A preliminary study recently determined that for a 290° rotation in 1 cm mode, 15% cold and 7% hot spots were delivered to 7 cm above and below isocenter, respectively. This study completes the earlier work by investigating the dose heterogeneity as a function of position relative to the axis of rotation, arc length, and leaf width. The influence of random daily patient positioning errors is also investigated. Methods and Materials: Treatment plans were generated using 8.0 cm diameter cylindrical target volumes within a homogeneous rectilinear film phantom. The plans included both 1 and 2 cm mode, optimized for 300°, 240°, and 180° gantry rotations. Coronal-oriented films were irradiated throughout the target volumes and scanned using a laser film digitizer. The central target irradiated in 1 cm mode was also used to investigate the effects of incorrect couch indexing. Results: The dose error as a function of couch index error was 25% mm -1, significantly greater than previously reported. The clinically provided indexing system yielded 0.10 mm indexing precision. The intrinsic dose distributions indicated that more heterogeneous dose distributions resulted from the use of smaller gantry angle ranges and larger leaf projections. Using 300° gantry angle and 1 cm mode yielded 7% hot and 15% cold spots 7 cm below and above isocenter, respectively. When a 180° gantry angle was used, the values changed to 22% hot and 27% cold spots for the same locations. The heterogeneities for the 2 cm mode were 70% greater than the corresponding 1 cm values. Conclusions: While serial tomotherapy is used to deliver highly conformal dose distributions, significant dosimetric factors must be considered before treatment. The patient must be immobilized during treatment to avoid dose heterogeneities caused by incorrect indexing due to patient movement. Even under ideal conditions, beam divergence can cause significant abutment-region dose heterogeneities. The use of larger gantry angle ranges, smaller leaf widths, and appropriate locations of the gantry rotation axis can minimize these effects.

Phantoms for IMRT Dose Distribution Measurement and Treatment Verification

Background: The verification of intensity-modulated radiation therapy (IMRT) patient treatment dose distributions is currently based on custom-built or modified dose measurement phantoms. The only commercially available IMRT treatment planning and delivery system (Peacock, NOMOS Corp.) is supplied with a film phantom that allows accurate spatial localization of the dose distribution using radiographic film. However, measurements using other dosimeters are necessary for the thorough verification of IMRT. Methods: We have developed a phantom to enable dose measurements using a cylindrical ionization chamber and the localization of prescription isodose curves using a matrix of thermoluminescent dosimetry (TLD) chips. The external phantom cross-section is identical to that of the commercial phantom, to allow direct comparisons of measurements. A supplementary phantom has been fabricated to verify the IMRT dose distributions for pelvis treatments. Results: To date, this phantom has been used for the verification of IMRT dose distributions for head and neck and prostate cancer treatments. Designs are also presented for a phantom insert to be used with polymerizing gels (e.g., BANG-2) to obtain volumetric dose distribution measurements. Conclusion: The phantoms have proven useful in the quantitative evaluation of IMRT treatments.

Review of the first 50 cases completed by the RACR mammography QA programme: Phantom image quality, processor control and dose considerations

The Mammography Quality Assurance Programme, recently established by the Royal Australasian College of Radiologists, has processed the first 50 applications. This programme, which closely follows the programme of the American College of Radiology (ACR), utilizes phantom film images, thermoluminescent dosimetry measurement of mean glandular dose, processor control charts, clinical images, equipment reports and required survey information to establish that a centre conforms to a minimum standard in mammography. The present paper describes the initial results of the first phantom images, dose measurements, processor control and survey information. A film review panel of six members has been trained in phantom film reading. Their evaluation of phantom films was compared with film readings by members of the ACR and was found to be in close agreement. Fifty films have been evaluated up to the present time with a failure rate of 26%. The major causes of failure were unacceptable film artefacts and poor contrast (as indicated by reduced fibre and mass visibility). A surprising result was the high failure in processing, where 23% of units reviewed had significant problems, including failure to keep the processor within required control limits. Only one centre recorded a mean glandular dose above 2 mGy with no centre over the 3 mGy limit. A review of the frequency of the quality control testing shows that the acceptance of quality assurance in mammography, while greater than in the initial stages of the ACR programme, is less than current US practice. These initial results for the accreditation process probably reflect an initial period of adjustment, as seen by the high pass rate achieved by centres that have resubmitted material to gain accreditation.

Vertebral morphometry derived from digital images.

We describe a method for capturing measurement data directly from digitized images using specialized software and high-resolution workstations. We have evaluated the reliability, accuracy, and reproducibility of this method in an international clinical trial involving vertebral morphometry. Accuracy was determined using clinical radiographs measured with vernier calipers and a film phantom. Intra- and interobserver variabilities were assessed, and longitudinal reproducibility was evaluated. As part of the trial, spinal radiographs were collected from more than 200 international health care facilities and digitized at four screening centers. Digitized images were stored and sent to our central facility for morphometry and archiving. Timeliness and variability of the process were tracked. Relative accuracy was nearly 100%. Correlation with clinical measurements was high (r = .96; p < .05). The mean coefficient of variation for interobserver variability was 2%. Intraobserver variation was 3-5%. The coefficient of variation for longitudinal reproducibility ranged from 4% to 6%. After 9 months of operation, our trial included 9494 patients. Of approximately 36,000 radiographs, 98% passed quality review. Only 1% of vertebral levels were not measurable. Hardware and software problems were minimal. The use of digitized images for morphometry is accurate, reproducible, and convenient. When applied to a large-scale clinical trial, it offers unique advantages that may justify the cost and complexity that exceed those of conventional radiographs.

Improved field edge definition in electron arc therapy with dynamic collimation techniques

Purpose : The diffuse shape of the electron beam used for arc therapy requires collimation on the patient's surface to sharply define treatment field edges. The electron beam arcs 10–15° past field edges defined by a custom fitted and manufactured cast which acts as a tertiary collimator. This allows the entire beam profile to be integrated at the field edge. The tertiary collimator is heavy and bulky, requiring two therapists to lift and position the cast. An alternative technique for field edge definition would require the electron arc collimators to dynamically close to zero, while maintaining the projection of the leading edge of the field coincident with the geometric edge of the treatment field. This would allow integration of the entire electron are profile and maintain a sharply defined treatment edge at the medial and lateral margins of the are. The present customized cast could be replaced by generic lead strips at only the superior and inferior treatment field borders. This study investigates the dosimetry of dynamically collimated electron arc treatment volumes at field margins and its potential for eliminating the need for tertiary collimation at the arc field margins. Methods and Materials : Electron arc isodose distributions were calculated using a pencil beam algorithm for treatment volumes defined by tertiary collimation at the surface of a cylindrical phantom and compared to distributions generated by simulating dynamic collimation to define the same field edges. Phantom measurements were performed using film densitometry to verify computer predictions. Results : Penumbra width is one measure of the sharpness of dose fall off at a treatment field edge. We define it as the distance between the 90% and 20% isodose lines at the field edge measured orthogonal to the incident electron beam. Calculations and phantom film densitometry measurements were performed for electron energies from 6–20 MeV. Dynamic and tertiary collimation both reduce penumbra width by approximately 50% compared to no collimation. There is a small advantage in minimizing penumbra width at low electron energy with tertiary collimation. This shifts to a small advantage with dynamic collimation at high electron energy. Conclusion : Dynamic collimation produces a field edge isodose distribution equivalent to tertiary collimation for clinical purposes. These results suggest that tertiary collimation at medial and lateral electron arc treatment field margins can be eliminated with dynamic collimation. This should result in greater clinical acceptance of breast electron arc therapy as the capacity for dynamic collimation is added to the next generation of linear accelerators.

Subjective evaluations of mammographic accreditation phantom images by three observer groups.

Mammography providers are under increasing pressure to become certified by the American College of Radiology (ACR). Successful accreditation is contingent on passing a phantom image quality test. This study was undertaken to evaluate viewer performance with the phantom image evaluation process and to determine any observer group differences. A series of standard phantom images were viewed by 30 medical physicists, 30 diagnostic radiologists, and 30 inexperienced observers. From the responses, object detection rates and passing rates according to the ACR criteria were established. These responses were analyzed with standard nonparametric tests to assess the degree of variability, correlation, and agreement among different observer groups. Median passing scores were similar for the radiologists and untrained readers, but the medical physicists appear to differ from the other two groups. There was not sufficient evidence to indicate that special training among physicists made a significant difference in median passing scores or mass detection rates. However, such training appeared to significantly affect the detection of microcalcification and fibril test patterns among the physicists' subgroups. Agreement among observer groups was high for all groups, but tended to be lower for overall passing rates than for any of the individual test objects. Agreement among physicists was affected by their subspecialty, presumably caused by their levels of specific training for these visual tasks. The authors conclude that choosing medical physicists to evaluate mammographic phantom films appears to be a good choice among potential observer groups, and that special training for reading these images affects their detection abilities and consistency. However, because passing rates did not appear to be affected by special training and given the current rapid degree of change in this area, more testing of medical physicists is desirable to examine these effects over time, and to study the effect of developing standards for training.

Methods of Scoring Mammographic Phantom Films

Films were taken of five different mammographic phantoms under a wide variety of conditions. The phantoms were two from Leeds, one from Newcastle, the ‘Bars’ and the Ackerman (Du Pont). Two sets of films were obtained with each phantom, one set at four different kV values, and another set using three different focus sizes and three degrees of magnification. All films were Fuji MA, and the two sets of films were obtained using the same models of X ray set, processor and cassette. Background density of all films was matched to approximately 1.4. Three films were obtained for each combination of conditions and scored independently by each author. Scoring systems varied between phantoms and between types of detail, but all involved grading according to the ease with which details could be seen. Merit order of films was found to be insensitive to scoring system, but some systems appeared quicker or easier to use than others. Merits of all scoring systems are discussed for each phantom.

Methods of Scoring Mammographic Phantom Films

Films were taken of five different mammographic phantoms under a wide variety of conditions. The phantoms were two from Leeds, one from Newcastle, the ‘Bars’ and the Ackerman (Du Pont). Two sets of films were obtained with each phantom, one set at four different kV values, and another set using three different focus sizes and three degrees of magnification. All films were Fuji MA, and the two sets of films were obtained using the same models of X ray set, processor and cassette. Background density of all films was matched to approximately 1.4. Three films were obtained for each combination of conditions and scored independently by each author. Scoring systems varied between phantoms and between types of detail, but all involved grading according to the ease with which details could be seen. Merit order of films was found to be insensitive to scoring system, but some systems appeared quicker or easier to use than others. Merits of all scoring systems are discussed for each phantom.

Scoring of image quality phantom films in mammography

We were very interested to read the paper by J. Law “A new phantom for mammography” (1991). We have been advocates of this particular phantom for some time and have used it regularly on a total of 29 mammography units (nine different makes/types in all) over the past 2 years. One could of course use different methods of scoring the image quality perceived on test films and the degree of sensitivity to changes of kilovoltage, film processing etc. will clearly depend on the scoring system chosen.

A new phantom for mammography

In reply to Dr Moores, the account he gives of my paper in paragraphs 1–6 of his letter in broadly correct, but I do not think it affects the conclusions reached. The Du Pont scores may be divided by three (or the relevant vertical axes of the graphs relabelled) to obtain a mean single-film result if desired. For the Du Pont phantom films, scores for contrast details and for resolution have been separated in Tables and Figures exactly as Dr Moores advocates in his seventh paragraph but appears to criticize in paragraph six.

Scoring of image quality phantom films in mammography

I would like to comment briefly on the interesting letter from Mr A. Robinson and Dr A. C. Underwood. The “0–3 scale of visiblity” referred to in the fourth paragraph of the letter from Robinson and Underwood was the basis of the “common scoring system” referred to in my reply to Dr Moores. Since learning of it from them, I have made some use of it with the Du Pont phantom and elsewhere, and found it helpful.

A Film Phantom for Routine Film Dosimetry in the Clinical Environment

This paper describes a new film phantom we have designed that eliminates many of the problems found in phantoms used in film dosimetry. This phantom employs two sheets of solid water attached with nylon hinges and aligned with pins. It is made light-impermeable through the use of nylon hook-and-loop fasteners. This new design shows good agreement with ion-chamber measurements over the full depth of electron penetration.

External Beam Profiles Measured with Film or Ionization Chamber

Radiation therapy beam profiles measured with an ionization chamber in a scanning water phantom are compared with profiles measured with a standard film dosimetry technique. An 18 cm field was filmed with 4 MV photons in a polystyrene phantom at depths of 1, 7, and 13 cm. The film was scanned with a scanning densitometer and the resulting profiles were converted to dose profiles with a sensitometric curve. To determine the effect of backscatter on the film a special phantom was constructed to eliminate the backscatter from the film measurement. This provided a comparison with the solid film phantom where full backscattering was present. The beam profiles measured with the film differed by less than 3% from the ionization chamber profiles and there was less than 1% difference between the film techniques.

A comparative heating‐pattern study of direct‐contact applicators in microwave diathermy

Heating patterns, induced in planar and limb phantoms of simulated fat and muscle tissue by large and by small, direct‐contact microwave diathermy applicators operating at 2.45 GHz, were studied. Teflon slab‐loaded rectangular waveguides, circularly polarized horns, and other large radiators were investigated as well as the only two low‐power direct‐contact emitters presently in noninvasive clinical use. The integrity of temperature measurements by a thermographic camera was established and probe thermometry was used to compare polyethelene with a silk screen as the midplane phantom film. The width and depth of penetration of microwave heating were determined to evaluate energy deposition inside simulated muscle tissue. The results of temperature profiles show that a Teflon slab‐loaded rectangular waveguide and a circularly polarized horn provide uniform heating in the center of the phantom's midplane and that the focusing effects of limb phantoms enhance depth of penetration. Measurements of the electric field demonstrated that direct‐contact applicators can be designed to maintain leakage levels below 5 mW/cm2 per 100 watts of forward power for large applicators, and below 5 mW/cm2 per 10 watts of forward power for small applicators at a distance of 5 cm. Leakage of a circular aperture horn can be minimized with an output flange choke.