Photon activation—¹⁵O decay studies of tumor blood flow

Abstract

A direct, noninvasive method for measuring absolute values of specific capillary blood flow in living tissue is described. The method is based on the photon activation, in situ, of tissue elements and the measurement of the subsequent decay of the positron activity induced, employing coincidence detection of the photon pairs produced in positron annihilation. Analysis of the time-dependent coincidence spectrum reveals the contribution to the total signal from the decay of 15O, from which the specific capillary blood flow in the imaged, activated volume is ultimately determined. By virtue of its introduction of the radioisotope of interest (15O) directly and uniformly into the tissue volume under investigation, the method described permits both the nonperfused and well perfused fractions of an activated volume to be estimated and hence, the average specific blood flow within imaged tumor volumes to be computed. The model employed to describe and analyze the data is discussed in detail. Results of application of the technique to measurement of specific blood flow in rhabdomyosarcoma tumors grown in WAG/Rij rats are presented and discussed. The method is shown to be reliable and well suited to studies designed to determined the effects of various agents, such as heat, radiation and drugs, on tumor blood flow.

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

“…11 was fit and found to contain primarily 15 O, 13 N, 11 C, and some 14 O. 10 C is not present at the 95% confidence level in the summed decay data. Again, while 14 O is present at the 99% confidence level in the total decay data, it was not found at this level in any of the individual positions.…”

“…40,43,44 The presence of activity beyond the proton-beam range suggests that nuclear reactions induced by secondary neutrons and other energetic secondary charged particles may produce a significant amount of 14 O in the treatment volume. However, since 10 C, 13 N, and 14 O are only produced in small quantities, the uncertainties in their cross sections are of little consequence to on-line range verification. Most tissues are primarily composed of oxygen, carbon, and hydrogen for which the cross sections for producing positron-emitting radioisotopes are reasonably well known.…”

“…In this situation, the decay of 15 O may be roughly modeled by two exponential terms. 10 The first term models the nonmobile fraction of the 15 O activity which disappears solely through radioactive decay. The second term models the mobile fraction of the 15 O activity which disappears through radioactive decay and in vivo biological washout.…”

“…Tissue activation was first discussed in detail by Tobias in 1949 7 and was first reported in gamma-ray experiments by Mayneord et al later that year. 8 Since then the radioactivity induced in tissues by photon, [8][9][10] proton, 11,12 neutron, 13 pion, [14][15][16] and heavy-ion radiotherapy beams [17][18][19] has been investigated. This induced activity has been studied as a source of patient dose, 11,20,21 as a means of determining elemental tissue composition, 10,11,13 and for determining dose distributions.…”

“…8 Since then the radioactivity induced in tissues by photon, [8][9][10] proton, 11,12 neutron, 13 pion, [14][15][16] and heavy-ion radiotherapy beams [17][18][19] has been investigated. This induced activity has been studied as a source of patient dose, 11,20,21 as a means of determining elemental tissue composition, 10,11,13 and for determining dose distributions. 14,15,17,[22][23][24][25][26][27][28][29][30][31] In vivo dose localization by imaging positron-emitting nuclei is considered essential for light-ion, and radioactive-ion beam therapy.…”