Positron emission tomography for quality assurance in proton therapy

Abstract

Introduction Cancer is one of the leading causes of death in The Netherlands. In 2017, all types of cancer combined caused 47,000 of 150,000 recorded causes of death (Centraal Bureau voor de Statistiek, 2017). There are several ways to treat cancer. The most common treatments include radiotherapy, surgery, chemotherapy, targeted therapy, hormonal therapy and immunotherapy. Often, different treatment modalities are combined to maximize their efficacy. For example, patients might receive radiotherapy after surgery to remove any traces of cancer cells that were left. Radiotherapy uses ionizing radiation to kill tumor cells by damaging their DNA. This radiation can be applied internally (brachytherapy) or externally. For brachytherapy, radioactive sources are implanted in and around the tumor, which deliver dose directly at the right location. However, for this method, the tumor needs to be in a relatively easily accessible location. For some patients, radioactive substances that accumulate in the tumor are injected. This radiation then delivers most dose at the site where it accumulates. More often, the radiation is applied using a source outside of the body. In the past, radioactive sources such as 60 Co were used to supply MeV gamma rays. Nowadays, a linear accelerator is used in most radiotherapy facilities to produce MeV electron beams. These electrons are stopped in a tungsten absorber to generate MeV X-rays, which penetrate deeply into the body. Other particles can also be used, such as protons or even heavier nuclides. Accelerating these particles to clinically useful energies requires large particle accelerators. Already in 1946, Robert R. Wilson wrote about how protons with an energy in the order of 100 MeV are very interesting for radio-3. Beam-on imaging of short-lived positron emitters during proton therapy proton bunches, such as prompt gamma rays, were removed from the data via an anti-coincidence filter with the cyclotron RF. The resulting energy spectrum allowed good identification of the 511 keV PET counts during beam-on. A method was developed to subtract the long-lived background from the 12 N image by introducing a beam-off period into the cyclotron beam time structure. We measured 2D images and 1D profiles of the 12 N distribution. A range shift of 5 mm was measured as 6 ± 3 mm using the 12 N profile. A larger, more efficient, PET system with a higher data throughput capability will allow beam-on 12 N PET imaging of single spots in the distal layer of an irradiation with an increased signal-to-background ratio and thus better accuracy. A simulation shows that a large dual panel scanner, which images a single spot directly after it is delivered, can measure a 5 mm range shift with millimeter accuracy: 5.5 ± 1.1 mm for 1.64 × 10 8 protons and 5.2 ± 0.5 mm for 8.2 × 10 8 protons. This makes fast and accurate feedback on the dose delivery during treatment possible.