Wing-Chee Lam

An on-line electronic portal imaging system for external beam radiotherapy

A digital imaging system has been constructed to obtain the treatment portal image of a patient during external beam radiotherapy. A DEC LSI-11/23 microcomputer controls a stepper motor, which moves a linear array of 256 silicon diodes to scan the radiation transmitted through the patient. The computer also processes the collected data to generate an image for displaying on a video monitor. The quality of the digital image is comparable to that of the conventional verification radiographs. Major advantages of the system include the speed and simplicity in data storage and retrieval and in its capability of direct comparison with simulation radiographs.

On-line measurement of field placement errors in external beam radiotherapy

An on-line electronic portal imaging system was used in a quantitative measurement of field placement errors in external beam radiotherapy. Data from 30 patients, giving a total of 150 portals with 66% in the thoracic region and 34% in the abdominal region, were analysed. Displacements from the prescription delineated by physicians were measured. The displacements included errors to block malposition, field malposition, field malrotation and patient malposition, and these were classified in two categories of displacements, those associated with the portal boundary and those with anatomic structures. Except for one rotational error, only translational errors were found. Displacements greater than 1 cm amounted to 17% with respect to the portal boundary and between 10% and 46% with respect to anatomic landmarks within the portal. From the results of the study, a margin of 1 cm around the tumour is barely sufficient if a 5% accuracy in dose delivery is desired.

Total body irradiation in bone marrow transplantation: The influence of fractionation and delay of marrow infusion

Abstract Bone marrow transplantation (BMT) after total body irradiation (TBI) and cyclophosphamide is being employed increasingly in the therapy of end stage leukemia. Interstitial pneumonitis (IP) represents a major acute toxicity after allogeneic transplantation. A more rapid reconstitution of lymphoid organs and bone marrow post transplant may result in increased immune competence and hence fewer opportunistic pulmonary infections and IP. By delaying the infusion of marrow to 72 hr after TBI (1250 rod at 7.5 rad/min) instead of the customary 24 hr, we can demonstrate an increase in initial repopulation of thymus, spleen and marrow, with syngeneic transplants in Lewis rats. Interstitial pneumonitis may also be caused, in part, by the pulmonary toxicity of large single exposures of TBI. Clinical and laboratory data suggest that fractionated TBI may be less toxic to the lung. When fractionated TBI (625 rad × 2, 7.5 rad/min) is compared to single dose TBI (1250 rad, 7.5 rad/min), an increased initial repopulation of lymphoid organs is observed when fractionated therapy is employed. Delay in marrow infusion and fractionation of TBI exposure may have clinical advantages in patients who receive BMT.

The dosimetry of 60Co total body irradiation

Abstract The dosimetry characteristics of a single source 60 Co total body irradiation (TBI) facility have been studied. The dose distribution for AP-PA irradiation in an adult size and an infant size phantom was measured in detail using thermoluminescent dosimeters. A 10% homogeneity relative to the midline dose was found over most of the body. An increase of 10% relative to midline dose was noted in the thoracic region because of lower density lung tissue. A relative decrease of more than 10% was found in the head region because of the reduced beam intensity away from the central axis and reduced scatter volume. Dose in the abdomen was homogeneous to ± 5%.

Electron arc therapy: Beam data requirements and treatment planning

In electron arc therapy a long narrow beam defined by a secondary collimator is used to sweep across the treatment area of the patient. Central axis percentage depth doses and off-axis ratios measured with such a collimator show variations with the source-to-surface distance (SSD). Errors may be introduced into isodose distributions calculated with the effective SSD method when beam data measured at a fixed SSD are used. A treatment planning algorithm using beam data measured at two SSDs to allow an interpolation and extrapolation procedure to account for SSD variations has been implemented. For calculations in off-axis planes, off-axis ratios normal to the plane of rotation are also required. Results of the calculations were checked with film dosimetry measurements and found to be in good agreement.

An on-line data acquisition system for radiation therapy: External photon and electron beams

An on-line data acquisition system for external beam treatment planning has been developed. The hardware consists of a Tektronix 4054 Computer, an SHM Nuclear 3-D scanning water phantom, interfaces for 16 channels of ADC and 32 channels of digital I/O, and two detector probes for measuring dose distribution and monitoring fluctuations in machine output. The program to control the movement of the scanning probe and to collect data is written in BASIC. Results of the measurements are plotted in real time on the Tektronix screen. The beam data format consists of a set of percentage depth doses along the central axis and five sets of off-axis ratios at five different depths. In the electron mode, when ionization chambers are used, data points are averages of two measurements, one with positive bias and the other with negative bias. The beam data are stored on disk which can be accessed by the Capintec treatment planning system.

Errors in off‐axis treatment planning for a 4‐MeV machine

Computer treatment planning systems allow dose computation in planes parallel to the central one (off-axis plans). The beam data may consist of, e.g., percentage depth doses along the central axis plus off-axis ratios (OAR) at several depths. In some systems, the calculation of an off-axis plan is based on the assumption that the OAR can be represented by a separable function: OAR(x,y) = f(x).g(y), where x and y are the symmetry axes perpendicular to the beam axis and the functions f and g are equal for a square open field. The errors of this assumption for a 4-MeV machine were measured for open fields and wedged fields at five different depths. The measured dose was compared with that predicted by the above equation for 50%, 75%, and 88% of the half field width from the beam axis. Maximum deviation of more than 10% was observed with the probable error of the measurement being 1%.

Clinical use of on-line portal imaging for daily patient treatment

PURPOSE To determine the ease of use by clinical staff and reliability of an electronic portal imaging system and evaluate the potential to utilize on-line imaging to assess accuracy of daily patient treatment positioning in radiation therapy. METHODS AND MATERIALS A computer controlled fluorescent screen-mirror imaging system was used to acquire on-line portal images. A physician panel assessed on-line image quality relative to standard portal film. Clinical use of the imager was implemented through a protocol where images were obtained during the first six monitor units of external beam. The images were visually compared to a reference portal and patient setup was adjusted for errors exceeding 5 mm. Subsequent off-line analysis was utilized to give insight into the magnitude of clinical setup error in the visually accepted images. RESULTS Physician evaluation of on-line image quality with an initial 211 images found that 70% were comparable or superior to standard film portal images. Eighty percent of treatment fields fit completely within the on-line imaging area. Eight percent of on-line images were rejected due to poor image quality. Twelve percent of the daily treatment setups imaged required adjustment overall, but specific field types predictably required more frequent adjustment (pelvic and mantle fields). Off-line analysis of accepted images demonstrates that 18% of the final images had setup errors exceeding 5 mm. CONCLUSION On-line imaging facilitated daily portal alignment and verification. Ease of use, almost instantaneous viewing and consistent ability to identify and locate anatomical landmarks imply the potential for on-line imaging to replace film based approaches. Retrospective analysis of daily images reveals that visual assessment of setup is not sufficient for eliminating localization errors. Further improvement is required with respect to detecting localization error and fully encompassing larger field sizes.

Tissue compensator fabrication using a simple photographic technique

A simple method of fabricating photon beam tissue compensators is described. A plaster cast of the patients surface contours is partially immersed in a mixture of India ink and water to obtain a topographic map relative to a reference plane which is perpendicular to the photon beam. The contours are photographed, and successive contours corresponding to different missing tissue thicknesses are used to fabricate a compensator from lead sheets cut to the same shape with the proper magnification.