On-line monitoring of radiotherapy beams: Experimental results with protonbeams

Proton radiotherapy is a powerful tool in the local control of cancer. The advantages of proton radiotherapy over gamma-ray therapy arise from the phe nomenon known as the Bragg peak. This phenomenon enables large doses to be delivered to well-defined volumes while sparing surrounding healthy tissue. To fully realize the potential of this technique the location of the high- dose volume must be controlled very accurately. An imaging system was desig ned and tested to monitor the positron-emitting activity created by the bea m as a means of verifying the beam's range, monitoring dose, and determinin g tissue composition. The prototype imaging system consists of 12 pairs of cylindrical EGO detectors shielded in lead. Each crystal was 1.9 cm in diam eter, 5.0 cm long, and separated by 0.5 cm from other detectors in the row. These are arranged in two rows, 60 cm apart, with the proton beam and tiss ue phantoms half-way between and parallel to the detector rows. Experiments were conducted with 150 MeV continuous and macro-pulsed proton beams which had beam currents ranging from 0.14 nA to 1.75 nA. The production and deca y of short-lived isotopes, O-15 and O-14, was studied using 1 min irradiati ons with a continuous beam. These isotopes provide a significant signal on short time scales, making on-line imaging possible. Macro-pulsed beams, hav ing a period of 10 s, were used to study on-line imaging and the production and decay of long-lived isotopes, N-13, C-11 and F-18. Decay data were acq uired and on-line images were obtained between beam pulses and indicate tha t range verification is possible, for a 150 MeV beam, after one beam pulse, to within the 1.2 cm resolution limit of the imaging system. The dose deli vered to the patient may also be monitored by observing the increase in the number of coincidence events detected between successive beam pulses. Over 80% of the initial positron-emitting activity is From O-15 while the remai nder is primarily C-11, N-13, O-14 With traces of F-18, and C-10. Radioisot opic imagingmay also be performed along the beam path by fitting decay data collected after the treatment is complete. Using this technique, it is sho wn that variations in elemental composition in inhomogenous treatment volum es may be identified and used to locate anatomic landmarks. Radioisotopic i maging also reveals that O-14 is created well beyond the Bragg peak, appare ntly by secondary neutrons. (C) 1999 American Association of Physicists in Medicine.