On‐line monitoring of radiotherapy beams: Experimental results with proton beams

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Abstract
Proton radiotherapy is a powerful tool in the local control of cancer. The advantages of proton radiotherapy over gamma‐ray therapy arise from the phenomenon 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 designed and tested to monitor the positron‐emitting activity created by the beam as a means of verifying the beam's range, monitoring dose, and determining tissue composition. The prototype imaging system consists of 12 pairs of cylindrical BGO detectors shielded in lead. Each crystal was 1.9 cm in diameter, 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 tissue 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 decay of short‐lived isotopes,andwas studied using 1 min irradiations with a continuous beam. These isotopes provide a significant signal on short time scales, making on‐line imaging possible. Macro‐pulsed beams, having a period of 10 s, were used to study on‐line imaging and the production and decay of long‐lived isotopes, andDecay data were acquired and on‐line images were obtained between beam pulses and indicate that 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 delivered 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 fromwhile the remainder is primarily with traces ofandRadioisotopic imaging may also be performed along the beam path by fitting decay data collected after the treatment is complete. Using this technique, it is shown that variations in elemental composition in inhomogenous treatment volumes may be identified and used to locate anatomic landmarks. Radioisotopic imaging also reveals thatis created well beyond the Bragg peak, apparently by secondary neutrons.