![]() Following its introduction over 20 years ago, IMRT has continued to steadily evolve and is now considered both state-of-the-art and standard of care for many malignancies. With IMRT, each of a group of broad beams of photons is subdivided into narrow beamlets of cross-sections of the order of ½ cm x ½ cm and delivered using dynamic multi-leaf collimators. In the mid-1990 s, radiotherapy with photons took a giant leap forward when Intensity modulated photon radiotherapy (IMRT) was introduced. (Adapted from Jones, reproduced with permission). ![]() The curves are normalized in each case to 100 at maximum dose. We assert that, with such research, proton therapy will be a commonly applied radiotherapy modality for most types of solid cancers in the near future.ĭepth-dose curves for a 200 MeV proton beam: both unmodulated and with a 5 cm spread-out Bragg peak (SOBP), compared with a 16 MV x-ray beam (for 10 ×10 cm 2 fields). To increase the resilience of dose distributions in the face of uncertainties and improve our confidence in dose distributions seen on treatment plans, robust optimization techniques are being developed and implemented. However, residual uncertainties will remain in spite of the best efforts. Ongoing research is aimed at better understanding the consequences of the various uncertainties on proton therapy, and reducing the uncertainties image-guidance, adaptive radiotherapy, further study of biological properties of protons, and the development of novel dose computation and optimization methods. ![]() These uncertainties, approximations and current technological limitations of proton therapy may limit the achievement of the true potential of proton therapy. In reality, the RBE is variable and a complex function of energy of protons, dose per fraction, tissue and cell type, end point, etc. Furthermore, the relative biological effectiveness (RBE) of protons is simplistically assumed to have a constant value of 1.1. In addition to anatomy variations, other sources of uncertainty in dose delivered to the patient include the approximations and assumptions of models used for computing dose distributions for planning of treatments. These factors must be considered in designing as well as evaluating treatment plans. The differences in techniques arise from the unique physical properties of protons but are also necessary because of the greater vulnerability of protons to uncertainties, especially from inter- and intra-fractional variations in anatomy. Treatment planning and plan evaluation of PSPT and IMPT requires special considerations compared to the processes used for photon treatment planning. General consensus is that there is a need to conduct randomized trials and/or collect outcomes data in multi-institutional registries to unequivocally demonstrate the advantage of protons. Considering the high cost or establishing and operating proton therapy centers, questions have been raised about their cost effectiveness. Although promising results have been and continue to be reported for many other types of cancers, they are based on small studies. It is generally acknowledged that proton therapy is safe, effective and recommended for many types of pediatric cancers, ocular melanomas, chordomas and chondrosarcomas. The latter technique can be used to treat patients with optimized intensity modulated proton therapy (IMPT), the most powerful proton modality.ĭespite the high potential of proton therapy, the clinical evidence supporting the broad use of protons is mixed. Spreading and shaping can be achieved by electro-mechanical means to treat the patients with “passively-scattered proton therapy (PSPT) or using magnetic scanning of thin “beamlets” of protons of a sequence of initial energies. The initial thin beams of protons are spread laterally and longitudinally and shaped appropriately to deliver treatments. Protons, accelerated to therapeutic energies ranging from 70 to 250 MeV, typically with a cyclotron or a synchrotron, are transported to the treatment room where they enter the treatment head mounted on a rotating gantry. These may, in turn, allow escalation of tumor doses, greater sparing of normal tissues, thus potentially improving local control and survival while at the same time reducing toxicity and improving quality of life. ![]() This is because of the unique depth-dose characteristics of protons, which can be exploited to achieve significant reductions in normal tissue doses proximal and distal to the target volume. In principle, proton therapy offers a substantial clinical advantage over the conventional photon therapy. ![]()
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