This work focused on the potential use of PET for measuring the in vivo-induced tissue activity due to radiation treatment with high-energy photons, which is 50 MV in this case. Since 1972, attempts were already made at our institution using a gamma camera to depict the distribution of the positron-emitting radionuclides produced in patients being irradiated with 42-MV photons from a betatron. The acquired images could clearly verify that the radionuclide distribution coincided with the irradiated regions. However, as the sensitivity of the camera was very low and the output only analogous, no further such attempts were made (L Johansson, personal communication). Other early studies have shown the potential use of tissue activation for the analysis of tumor blood flow in animal studies [14–16]. More recently, by the study of biological washout processes of ion-beam-induced positron emitters, half-lives of various washout components could be measured in animals [10, 11] and in patients [12, 17]. Furthermore, dose and treatment beam verification during as well as immediately after treatment with ion-beams [18, 19] and protons [17, 20, 21] in patients and for high-energy photons in the animal tissue  have been reported.
In this study, measurements of biological washout processes in the patient were not relevant due to the fact that the therapy machine and the PET/CT were not co-located, and most of the radioactivity had been eliminated before the PET examination. Studying half-lives of various washout components, particularly in blood-rich organs such as the lungs and the liver, generally needs activity measurements to be performed during or at least in direct connection to the irradiation . The urine of the bladder, on the other hand, represents an enclosed compartment with a negligible exchange with other tissues. From the CT scan, it was estimated that the urine volume of the bladder at the beginning of PET acquisition was approximately 200 cm3. Assuming a urine excretion of about 0.5 ml/min, the dilution of activity in this case is almost negligible. Also in fat, the dynamic portion constitutes only of a very small fraction, and most of the induced PET activity originates from radionuclides that are stationary within the tissue.
VOI analysis of the subcutaneous fat gave a composition that did not completely agree with ICRU-tabulated values for the adipose tissue , although the fitted half-lives were found to agree well for both 11C and 15O. The reason for the restricted agreement in composition may be the variations in the actual composition of the subcutaneous fat of the current patient. According to , the adipose tissue is the most variable tissue in the body regarding elemental composition. Water content may vary from 10.9% to 21.0%, and lipid content can range from 62% to 91% . From the analysis of urinary bladder contents (i.e., urine), a high level of oxygen content was found, which is expected as it mainly is composed of water. The calculated half-lives and composition were found to correlate well with tabulated ICRU values . In bone tissues, a high content of 40Ca and 31P will produce positron-emitting radionuclides when they are irradiated with high-energy photons. However, as the half-life of 39Ca is only 0.86 s, this activity will probably never be measurable, and 30P that has a half-life of 2.5 min will most likely not be distinguishable from 15O.
Upon arrival at the Nuclear Medicine department, the setup and positioning of the patient on the PET/CT couch were done as fast as possible in order to avoid further loss of induced activity. The main focus was to ensure that the radiation treatment volume was covered by the axial field of view of the PET camera. Subsequent reconstructions showed that the patient had become slightly mis-positioned in all three planes. In addition, the regular curved PET/CT couch did not match the flat couch used during the radiation treatment and the CT planning, resulting in a deformed activity distribution toward the outer edges of the patient as seen on the treatment plan overlaid on the PET/CT image. However, the deformable registration compensated for the different couches in the main part of the abdomen where the beams intersect.
Reconstruction with FBP [24–26], which is based on the inverse of the radon transform , is fast, robust, linear, and known to yield quantitative results. However, in the low count data such as in this study, the results showed a poor visual image quality, disturbing streak artifacts, and high noise. In order to better visualize the activated tissue and, thereby, the positioning of the VOIs, the iterative reconstruction method, 3D-OSEM , was preferred. The images reconstructed by 3D-OSEM lack the streak artifacts and contain no data outside the object. However, further studies are required to assess the quantitative accuracy of the two methods. It might be that the VOIs preferably are outlined using the 3D-OSEM algorithm, while the data from the VOIs preferably should be extracted from the FBP reconstructed data.