Fluoroscopy as a surrogate for lung tumour motion.

OBJECTIVES: The aim of this article was to test a simple approach of using pixel density values from fluoroscopy images to enable gated radiotherapy. METHODS: Anterior and lateral (LAT) from images were acquired from 18 patients referred for radical radiotherapy for non-small cell lung cancer for a period of 30-45 s. The amplitude of movement and the number of breathing cycles were determined in the right-left (RL) and superoinferior (SI) directions on the anterior images and the anteroposterior (AP) and SI directions on the lateral images. The breathing pattern was created by analysing the variation in a summation of pixel values within a defined area. The greatest and lowest 30% of pixel values were set as the duty cycle to represent inhale and exhale amplitude-based gating. RESULTS: A median of eight breathing cycles was captured for each patient with a duration of 2.2-11.8 s per cycle. The mean (range) motion was 4.7 mm (2.4-5.8 mm), 7.2 mm (2.3-17.6 mm), 6.2 mm (1.9-13.8 mm) and 4.8 mm (2.4-11.3 mm) in the RL, SI (AP), SI (LAT) and AP directions, respectively. A total of 10/14 anterior videos and 7/11 LAT videos had correlations between motion and breathing of >0.6. Margins of 5.5 mm, 6.8 mm and 6.6 mm in the RL, SI and AP directions, respectively, were determined to gate in exhale. The benefit of gating was greater when motion was >5 mm. CONCLUSION: The simple approach of using pixel density values from fluoroscopy images to distinguish inhale from exhale and enable gating was successfully applied in all patients. This technique may potentially provide an accurate surrogate for tumour position.

Accuracy and precision of an external-marker tracking-system for radiotherapy treatments.

The purpose of this work was to determine the accuracy and precision of a real-time motion-tracking system (Osiris+) for the monitoring of external markers used on patients receiving radiotherapy treatments. Random and systematic errors in the system were evaluated for linear (1D), circular (2D) and elliptical (3D) continuous motions, and for a set of static positions offset from an origin. A Wellhofer beam data measurement system and a computer controlled platform (which could be programmed to give motion in 3D) were used to move a hemi-spherical test object. The test object had four markers of the type used on patients. Three markers were aligned in the central plane and a fourth was positioned out of plane. Errors were expressed as deviations from the planned positions at the sampled time points. The marked points on the test object were tracked for the linear motion case with a variation from the true position of less than +/-1 mm, except for two extreme situations. The variation was within +/-2 mm when the lights were dimmed and when the amplitude of the movement was +/-5.0 cm. The 2D circular motion was tracked with a standard deviation of 1 mm or less over four cycles. The sampling rates of the system were found to be 0.3-0.4 s when it was monitoring actively and 1.5-1.6 s otherwise. The recorded Osiris+ measurements of known static positions were within +/-1 mm of the value from the computer controlled platform moving the test object. The elliptical motions in 3D were tracked to +/-1 mm in two directions (Y,Z), and generally to within +/-2 mm for the third direction (X); however, specific marked points could display an error of up to 5 mm at certain positions in X. The overall displacement error for the 3D motion was +/-1 mm with a standard deviation of 2.5 mm. The system performance is satisfactory for use in tracking external marker motion during radiotherapy treatments.

Characterization of a transmission ionization beam-imager for radiotherapy verification.

A system for radiotherapy treatment verification is proposed, using an air-ionization chamber with 1600 simultaneously readable 1 cm pixels. An image of the entire beam may be used to calibrate a portal image, to verify the position of the multi-leaf collimator with respect to delivered dose (either before or during treatment) and to check beam flatness and symmetry. This study characterizes the physical behaviour of such a system. A test chamber has been constructed and its temporal and spatial resolution and noise characteristics are evaluated. Several parameters of the design are varied, and their effects assessed. Temporal resolution is adequate to allow readout between each linear accelerator pulse at 400 pulses per second. Application of low atomic-number build-up and reduction of plate separation were the most effective methods to improve spatial resolution. The full width at half maximum of the line-spread function is shown to be 4.5 mm using a pre-sampling technique. The peak pixel-signal to x-ray quantum noise ratio exceeds 100. Prototype electronics have been tested, demonstrating that electronic noise could be reduced to a level below the x-ray quantum noise. The results of the study allow the simulation of any possible application to evaluate the proposed verification system.