Purpose: Due to the poor quality of megavoltage (MV) radiographs, detection and assessment of discrepancies in radiation field placement are difficult. Furthermore, the high imaging dose required to produce the megavoltage radiograph prohibits frequent image acquisition, particularly for those fields that require the use of an "open-field" exposure. For these small, or conformal, radiation fields, an alternate method of verifying field placement is required if the out-of-field dose is to be minimized. An open-field image acquired with a kilovoltage (kV) source would (a) deliver a very low patient dose, (b) increase the visibility of bony landmarks, and (c) simplify intercomparison of portal and prescription images. This article describes the development of a dual-beam imaging system that produces diagnostic quality "double-exposure" portal images for verifying radiation field placement.
Methods and materials: The dual-beam system consists of a kV x-ray tube mounted on the gantry of a medical linear accelerator. The kV beam shares the same isocenter (+/- 1 mm) as the treatment beam but is at 45 degrees to the central axis. Both the kilovoltage and megavoltage images are collected with a fluoroscopic imaging system that uses a low-noise CCD camera to accumulate the light emitted from a phosphor screen. Two 45 degrees mirrors are used to remove the CCD camera from the x-ray beam. The light integration on the CCD array is controlled by a mechanical shutter, allowing easy synchronization with the radiation exposures. The camera is shielded by a lead housing to reduce the number of x-rays reaching the CCD array. A conventional thickness phosphor screen is used for both the kV and MV exposures. In the dual-beam imaging procedure, an open-field kV radiograph is acquired with the patient in treatment position. Immediately following, a MV image is acquired with the beam-defining blocks in position. Summation of the two images produces an online double-exposure image. The anatomical information in either the kV or MV image can be emphasized by weighting the images appropriately. This system was used to acquire MV and kV images of both a contrast-detail phantom and a Rando head phantom. Dual-beam images were also acquired for a pituitary treatment, demonstrating the feasibility and usefulness of the dual-beam technique.
Results: Analysis of the contrast-detail images produced with the MV and kV beams shows the expected advantage of using the kV x-ray beam. Images of a Rando head phantom confirm these results. A clinical demonstration of the dual-beam system for verifying the delivery of a pituitary field is shown. The quality of the dual-beam image is similar to the prescription (simulation) image, contains a larger anatomical region, and delivers a lower integral dose to the patient. In addition, the kV beam also enhances the visibility of small markers implanted in the prostate.
Conclusions: A dual-beam imaging system has been developed for the radiographic verification of small, conformal fields. This development demonstrates the advantages and feasibility of using a kV x-ray beam in combination with the treatment beam to improve the accuracy of detecting patient setup errors.