On-line monitoring and PET imaging of the positron-emitting activity created in tissue by proton radiotherapy beams.

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 beams range and determining tissue composition. Design studies of the detection system are presented, the data acquisition system used is described and the results of on-line experiments with proton beams are presented. The depth-distribution of positron-emitting activity created by proton radiotherapy beams was imaged on-line using this system. Decay data was acquired and imaged between beam pulses and after the irradiation. Over 80% of the initial positron-emitting activity is from $\sp{15}$O which has a half-life of 122 seconds. The residual range of the treatment beam below the energy threshold for producing $\sp{15}$O is 0.3 cm. Consequently, the end of the activity distribution and the location of the Bragg peak are well correlated in homogenous tissue. The results show that the range of a 150 MeV proton radiotherapy beam may be verified after a single beam pulse to within a detectors width of the imaging system. The integrated total dose delivered to the patient may also be monitored by observing the increase in the number of coincidence events detected between successive beam pulses. It is also shown that in some situations the width of the plateau region of a Spread-Out Bragg Peak (SOBP) may be inferred from the fall of activity at the distal end of the distribution. Radioisotopic imaging may be performed along the beam path if decay data is collected after the treatment is completed. It is shown that using this technique, variations in elemental composition in inhomogenous treatment volumes may be identified and used to locate anatomical landmarks. Radioisotopic imaging also reveals that $\sp{14}$O is created well beyond the Bragg peak, apparently by secondary neutrons.