Clinical application ofin vivotreatment delivery verification based on PET/CT imaging of positron activity induced at high energy photon therapy

Abstract

The purpose of this study was to investigate in vivo verification of radiation treatment with high energy photon beams using PET/CT to image the induced positron activity. The measurements of the positron activation induced in a preoperative rectal cancer patient and a prostate cancer patient following 50 MV photon treatments are presented. A total dose of 5 and 8 Gy, respectively, were delivered to the tumors. Imaging was performed with a 64-slice PET/CT scanner for 30 min, starting 7 min after the end of the treatment. The CT volume from the PET/CT and the treatment planning CT were coregistered by matching anatomical reference points in the patient. The treatment delivery was imaged in vivo based on the distribution of the induced positron emitters produced by photonuclear reactions in tissue mapped on to the associated dose distribution of the treatment plan. The results showed that spatial distribution of induced activity in both patients agreed well with the delivered beam portals of the treatment plans in the entrance subcutaneous fat regions but less so in blood and oxygen rich soft tissues. For the preoperative rectal cancer patient however, a 2 ± (0.5) cm misalignment was observed in the cranial-caudal direction of the patient between the induced activity distribution and treatment plan, indicating a beam patient setup error. No misalignment of this kind was seen in the prostate cancer patient. However, due to a fast patient setup error in the PET/CT scanner a slight mis-position of the patient in the PET/CT was observed in all three planes, resulting in a deformed activity distribution compared to the treatment plan. The present study indicates that the induced positron emitters by high energy photon beams can be measured quite accurately using PET imaging of subcutaneous fat to allow portal verification of the delivered treatment beams. Measurement of the induced activity in the patient 7 min after receiving 5 Gy involved count rates which were about 20 times lower than that of a patient undergoing standard (18)F-FDG treatment. When using a combination of short lived nuclides such as (15)O (half-life: 2 min) and (11)C (half-life: 20 min) with low activity it is not optimal to use clinical reconstruction protocols. Thus, it might be desirable to further optimize reconstruction parameters as well as to address hardware improvements in realizing in vivo treatment verification with PET/CT in the future. A significant improvement with regard to (15)O imaging could also be expected by having the PET/CT unit located close to the radiation treatment room.

The physics of Cerenkov light production during proton therapy

“…A positron travels a short distance in the medium (less than 1 mm in living tissue (Levin and Hoffman 1999)) before it annihilates with an electron and produces two or more gamma ray photons of 0.511 MeV (high enough to produce Cerenkov-producing electrons). This provides an additional mechanism for detection of dose deposition using a positron emission tomography system (Parodi et al 2007, Strååt et al 2013. In addition, positrons with energy higher than 0.262 MeV will emit Cerenkov light (Jelley 1958).…”

New Radiation Oncology Optimization Principles Based On In-Vivo Predictive Assay and Recent Developments in Molecular Radiation Biology

“…Interestingly, the patient was a bit too rapidly positioned on the diagnostic couch to not lose too many positron counts, and the left side of the body was partly located outside the couch top, causing a substantial rotation (≈20°) of all the surrounding fatty soft tissues with peak PET activity (no auto setup on the PET-CT!). Thus, it appears that the whole fourfield box technique was performed with severely oblique beams [53]. Even if larger than normal set up errors it indicate a set up problem that for example may broaden penumbras in regular treatment procedures.…”

“…Both these data sets, when used together, will allow a high degree of therapy optimization where practically all major sources of treatment error can be picked up as long as they influence tumor cell survival during the first week or two of therapy, and can thus be corrected for using biologically optimized adaptive treatment techniques. The lower left in vivo dose delivery image (Figure 10) was made using 50 MV photon beams in a fourfield box technique with the help of a 64 slice PET-CT camera with ≈ 4 mm resolution [53]. Interestingly, the patient was a bit too rapidly positioned on the diagnostic couch to not lose too many positron counts, and the left side of the body was partly located outside the couch top, causing a substantial rotation (≈20°) of all the surrounding fatty soft tissues with peak PET activity (no auto setup on the PET-CT!).…”

“…For true biological optimization, we really want to know how to find the dose distribution that maximizes the probability of curing the patient without unacceptable or severe normal tissue side effects and minimal late sequelae [5,53]. To this end, we need to quantitate the probability of eradicating the tumor for a given dose delivery, while at the same time, we need to know the risk for severe normal tissue damage, as discussed in detail elsewhere [5,54].…”

“…By taking the ratio of the tumor FDG uptake before therapy and after a week of treatment and 18 Gy of tumor dose, in this case, it is possible to quantify the change in tumor uptake and the effective radiation resistance, D0 or D0,eff. From D0,eff and the tumor cell density, it is possible to estimate the optimal dose level necessary for tumor eradication (lower right panel, [51]) without considering the risk for normal tissue damage, which is generally is fairly low for light ion and photon IMRT treatments [53].…”