Portal film quality: a multiple institutional study

Radiation oncology research in the cancer and leukemia group B

Radiation oncology initiatives have been an integral component in the evolution of multidisciplinary research in the Cancer and Leukemia Group B. Although early studies in the Group primarily focused on chemotherapy for hematologic and pediatric malignancies, the Radiation Oncology Committee was established in 1972, reflecting the broadening scope of clinical investigation with an increased emphasis on solid tumor research. A major early contribution of the Radiation Oncology Committee was the recognition of the importance of formalized radiation quality review, which led to the development of the Quality Assurance Review Center. The committee has been instrumental in designing trials, in conjunction with our medical oncology and surgical oncology colleagues, to assess multimodality therapy. The results of many of these studies have had important implications for clinical practice. Recent efforts have explored our major research theme of treatment intensification via radiotherapy dose modulation and novel combinations of radiotherapy with sensitizing agents, with an emphasis on safely implementing advanced technologies in the cooperative group setting.

An evaluation of the relationship between the quality of prophylactic cranial radiotherapy in childhood acute leukemia and institutional experience: a Quality Assurance Review Center-Pediatric Oncology Group study

PURPOSE

The Pediatric Oncology Group Protocol 9404 was a prospective clinical trial of two forms of chemotherapy in childhood T-cell acute lymphoblastic leukemia and advanced stage T-cell lymphoblastic non-Hodgkin's lymphoma. The protocol called for prophylactic C1 whole brain external beam irradiation, 18 Gy in 2 Gy/fraction for 9 fractions. We hypothesized that a correlation would be found between the number of children irradiated on protocol by an institution and the compliance rate of that institution with radiotherapy quality assurance (QA) guidelines. We also hypothesized that QA compliance would improve as the study progressed.

METHODS AND MATERIALS

We scored the radiation dose as a minor deviation from protocol guidelines if the dose to the prescription point differed from the protocol by 6-10%, and a major deviation if it differed from protocol by > 10%. Treatment volumes were scored as a minor deviation if the margins were less than specified or the fields were excessively large. A major deviation was defined as the transection of a potential leukemia-bearing volume such as would be caused by blocking the cribriform plate, optic nerve, or temporal lobe. When the treating physician submitted a treatment plan and simulator film at the initiation of therapy to the Quality Assurance Review Center (QARC), a rapid turn-around review of the plan and suggestions for improvement was provided. At the end of therapy, all simulator and port films were reviewed at the QARC.

RESULTS

We reviewed the data from 353 patients treated at 73 institutions in the United States, Canada, and Europe. Of these patients, 2% (n = 7) were not assessable for QA because of incomplete information. Minor quality deviations were found in 27.7% of patients (n = 98) and major deviations in 7.9% (n = 28). The frequency of major deviations for institutions placing 1-4 patients on study was 11% vs. 5.5% for institutions placing > or =5 patients (p < 0.09). The frequency of minor deviations was 28.6% for institutions placing 1-4 patients on study vs. 27.1% for institutions placing >5 patients (p not significant). The frequency of major deviations fell over time (1996-1997, 15.5% vs. 1998-2001, 4.7%, p < 0.001). The frequency of minor deviations did not (1996-1997, 29.9% vs. 1998-2001, 26.9%, p not significant).

CONCLUSION

For a relatively simple radiotherapy field, with clearly written protocol guidelines and rapid turn-around corrections from QARC, the rate of minor deviations was no different between institutions placing 1-4 patients on study and those placing > or =5. A trend (p < 0.09) was noted, however, for major deviations to decrease as a function of institutional experience, as well as over time (p < 0.001), supporting the validity of the hypothesis that pediatric clinical experience matters in QA for C1 whole brain leukemia radiotherapy.

Evaluation of several Dupont portal film systems

The sensitometric curves (also known as the characteristic curves, or more commonly as the H-D curves after Hurler and Driffield who first used such curves in 1890 to describe the response of photographic film to light) of three DuPont portal films (CRONEX 10T, 10TL and Ultra-Vision C) in combination with two DuPont cassettes (CRONEX Radiation Therapy and Radiation Therapy Verification) were produced utilizing a 60Co beam and a 18 MV beam. The results are compared with that of a Kodak portal film system (T-Mat G/RA film and X-Onratic V cassette). The sensitometric curves of the DuPont films in the Kodak cassette were also measured. The results show that the Kodak system is superior to the DuPont systems with respect to the imaging quality and the feasibility of film techniques, and that if the DuPont films are used, significant improvements can be made by combining the DuPont films with the Kodak cassette.

Objective decision-making following a portal film: the results of a pilot study

PURPOSE

To discriminate between random and systematic treatment setup errors using portal films.

METHODS AND MATERIALS

A bi-dimensional analytic techniques using multiple analyses of variance based on Hotelling's T2 statistics to derive numerical and graphical measures of daily portal film accuracy and precision has been trialed using 88 daily portal films from seven patients' treatment.

RESULTS

A demonstration is provided of how a reasonable approximation of random variation from the intended (Simulator) field center, and systematic displacement of the mean position of the portal film centers may be derived from a minimum number of portal films. If a random error as great as 10 mm exists, at least six or seven portal films are considered necessary to reliably detect and quantify the size of a systematic error.

CONCLUSION

Our results suggest that a modest systematic error could go undetected until the end of a 5 or 6 week course of treatment if only one portal film is obtained each week. A greater number of portal films should be performed during the first week of treatment to reduce the frequency of such errors. Efforts to separate and quantify both random and systematic errors in setup are worthwhile and will lead to improvements in outcome at the individual patient level and at a departmental level in the development of quality assurance programs.

Portal film enhancement: technique and clinical utility

We report on the results a 3-year project which had as its goal the development of methods to enhance radiation portal films to improve their readability. We had previously reported on a portal film enhancement technique, contrast limited adaptive histogram equalization, which could enhance low contrast detail, but degraded sharply contrasted edges. A new method, unsharp masking followed by contrast limited adaptive histogram equalization, now appears to overcome this problem. A clinical trial to test whether enhanced portal films could be read more accurately than standard ones was undertaken. The trial involved 12 readers from two institutions doing 276 readings. In this trial the enhanced films were judged to be of higher quality than the non-enhanced films (p < .001) and were read more accurately (p = .026). The usefulness and difficulties of routinely performing portal film enhancement in a busy radiation therapy department are discussed.

Surface topography for patient repositioning

For many years, this group has been interested in the measurement of patient surface contours in order to prepare radiotherapy compensators. Recently, these surface measurements have been made using optical ranging techniques (Medical Physics 16:425; 1989). A laser line is projected onto the patient surface and the video image of the line is digitized and used to calculate the coordinates of each point on the line. This surface information can be used not only for the calculation of compensators but can also be used to verify patient position from simulation to treatment and from one treatment to another. Because the calculation of surface coordinates takes approximately 30 seconds, the technique has been modified to allow the comparison of setup position in real time. In the new system, a series of narrow fan beams is projected to intersect the patient surface. At simulation, the reference video image of the lines is digitized and stored. At treatment setup, repeat video images are digitized and compared with the reference in real time. Operator adjustments to patient position are immediately shown on the composite image, and a congruency value is calculated to help in this process of alignment. Uses and accuracy of the technique will be discussed.

Clinical utility of high-resolution portal films

We have evaluated the film quality and overall clinical utility of a unique high-resolution portal film system which utilizes high contrast graphics art film. A total of 127 unselected patients had their conventional portal films and high-resolution films of the same site, compared and graded subjectively for a variety of parameters by an independent panel of radiotherapists and radiographers. Although consistent improvements in most aspects of image quality were noted, improved overall clinical usefulness of the high-resolution portal film system in comparison to the conventional films, when subjected to multiple regression analysis, was confined to lateral neck views only. High-resolution portal film utility was also found to depend significantly on field size with areas less than 110 cm2 having a markedly worse clinical utility score.

Recent advances in radiotherapy treatment planning

Radiation treatment planning is currently in a state of rapid change. Dissatisfaction with past planning technology stems from the growing realization that: (1) Increases in the local regional tumor control rate will increase the cure rate in many malignancies. (2) Even at the best treatment centers geometric tumor misses are commonplace. (3) Traditional constraints on treatment techniques, originally imposed for simplicity and reproducibility, are no longer necessary, and can result in suboptimal treatment. (4) Treatment plans judged "optimal" in two dimensions may be far from optimal when viewed over the entire treatment volume. (5) Lack of treatment reproducibility is also commonplace, and can be demonstrated to adversely affect treatment outcome. On the positive side, recent developments in computer graphics, image processing, radiation physics, and radiation biology are now making it possible to define, design, and deliver sophisticated 3D radiation treatments. However, because many of these technologies are being developed for other disciplines, their applicability to radiation therapy treatment planning is not widely appreciated. We outline the current status and new developments in radiation therapy treatment planning.

The use of digitized port films in assessing new treatment modalities

An inexpensive computer-based imaging system has been used to compare simulator and portal radiographs. The films were digitized, normalized, and then overlaid for measurement. In a test group of patients, a new treatment protocol for pelvis irradiation was evaluated using this technique. For 90% of the patients, the average error was found to be smaller than 5 mm, and there were no field placement errors greater than 1 cm.

A new system for port films

A novel system for port films is described which incorporates a high-quality fluorescent screen lying behind a flat, heavy-metal screen or filter and the film pressed between them is not an X ray type but a single-emulsion, very high contrast graphics or line-type film. In experimental comparisons using 4 MV, 6 MV, and 25 MV photon beams, the new Peter MacCallum Cancer Institute (PMCI) system produced clearer images, about X2 higher in contrast yet lower in relative noise levels, than did the conventional commercially-available systems which use x-ray film between heavy-metal screens. Direct clinical comparisons, chiefly using 6 MV accelerators, confirmed that anatomical details were correspondingly better visualized and in general, the PMCI port film could be matched against the simulator radiograph with greater confidence. Routine use for greater than 2 yr in this Institute (4000 new patients per yr, 20 cassettes made for use with four 6 MV accelerators) has shown the PMCI system to be an economical and practical device, giving reproducibly high quality images in all the common port film applications.

Evaluation of several film/screen combinations for megavoltage radiation therapy treatment field verification

The introduction of a lower cost alternative to a film commonly used for megavoltage radiation therapy port films led to a comparison of two films (Kodak X-TL and X-OMAT G) in four metal screen cassette configurations. In addition to determination of H-D curves and point spread functions, images of humanoid phantom sections were obtained and evaluated for clinical "usefulness." The X-OMAT G film-screen combinations compared favorably with the X-TL film combinations with respect to the H-D Curves and point spread functions but there was a slight preference for X-TL film. There were no differences noted whether the film was used in the standard "ready pack" or naked in the cassettes.

Technique for verifying treatment fields using portal images with diagnostic quality

The image quality of portal films for megavoltage photon beams, when using the double-exposure technique, is poor compared to diagnostic quality, X ray images. A technique is described to record on a single film a megavoltage portal image superimposed upon a diagnostic X ray image, which provides the radiotherapist with "diagnostic quality" portal images. The technique uses a commercially available X ray tube mounted on the head of a 60Co unit. The alignment procedure, which uses a leveling device to ensure that the X ray focal spot and 60Co source are at the same location for each exposure, is confirmed by registering on film the image of an alignment marker. An evaluation of film-screen combination showed therapy verification film in a rare earth intensifying screen cassette to be best suited for this technique. The relationship between off-axis dose and the penumbral region of the portal image has been evaluated and should be useful in the interpretation of portal verification film relative to the treatment volume.

Automatic digital contrast enhancement of radiotherapy films

The practice of radiotherapy involves the precise geometric localization of both anatomic and non-anatomic structures using radiographs which are typically of very low contrast. Portal and verification films suffer from poor contrast as a result of the dominance of Compton interactions at therapeutic energies, and implant localization films often are degraded by extreme patient thickness (lateral pelvis) or projection of bony structures (head and neck). Automatic contrast enhancement techniques developed and proven for optimization of the display of digitally produced images such as CT have been applied to radiotherapy films to improve contrast and augment readability. This approach has become viable only recently with the advent of high speed, high resolution film digitizers and laser cameras and the evolution of sufficiently powerful computer hardware.

Portal film technique charts

A method to produce consistent radiographic density in portal film images is described. The exposure required to produce the desired film density can be calculated on the basis of tissue depth dose data. The required exposure is tabulated as a function of patient thickness, field size, and air gap between the patient and the cassette. The table, referred to as a "technique chart," helps to maintain consistent image quality by minimizing the frequency of over and under exposed films. Alternatively, treatment planning software may be modified to determine the film exposure required for a specific patient. The calculated exposures are accurate only if the image receptor employs a sufficiently thick front metal screen, a requirement consistent with optimum image contrast.