Written by: Lawrence Pang
One of the most effective ways of combating cancer is radiation therapy. Traditionally, radiation therapy uses photons in the form of high-energy x-ray radiation, which ionizes (i.e. removes or adds an electron) atoms in the DNA chains of cancerous cells. This changes the chemical properties of the atom, damaging the DNA and preventing the cancerous cell from multiplying.
One significant issue with radiotherapy is that regular cells in the vicinity of cancerous cells are also impacted by the radiation. Photons are not charged so they are not attracted or repulsed by any atoms. They can only interact with matter via absorption. Whether absorption actually occurs is purely reliant on chance. Given a certain amount of tissue, the proportion of photons that are absorbed by the tissue at each depth is constant, so the total photon dose delivered to the tissue increases slowly. This is demonstrated in the Bragg curve, which indicates the energy loss of various types of radiation as they travel through matter. As can be seen in the graph below, the Bragg curve of the photon beam (pink) decreases slowly as only relatively few photons lose their energy (i.e. are absorbed) at each depth. Furthermore, photons are also sometimes re-emitted by atoms at different angles, so some of the dose will also be scattered into surrounding tissue. Therefore, many healthy cells outside of the tumour will also be affected.
Source: Miller, A. Bragg Peak. https://commons.wikimedia.org/wiki/File:BraggPeak.png (accessed March 23, 2016). Copyright 2005 by A. Miller. Reprinted with permission.
This side effect can be resolved with proton therapy, which is simply radiating protons and not photons. As can be seen in the graph on the left, the proton beam (blue and red) has a sharp peak in the Bragg curve. This indicates that a vast majority of the proton dose is delivered to the specific peak area, and almost none to surrounding regions. The location of this peak can be controlled by radiating protons of different energies. Furthermore, because protons are also relatively massive, there is a negligible scattering effect. Therefore, only cancerous cells can be targeted, and no damage is done to the DNA of surrounding healthy cells.
Proton therapy, while exciting, has its own unique disadvantages. The large mass of protons is beneficial when it comes to reducing scatter but is a barrier when it comes to delivering cost-effective therapy. Protons must be accelerated to very high speeds as part of the therapy, which requires expensive equipment in the form of cyclotrons or synchrotrons (i.e. particle accelerators). Few proton therapy centers have been established due to the discouraging capital cost.There is only one in Canada: the TRIUMF center in Vancouver. In addition, the relative cost of proton therapy is more than twice that of photon therapy (Goitein and Jermann, 2003). However, more modern proton beams can reduce the cost dramatically, and as a result the cost of proton therapy is no longer unrealistic for patients (Lievens and Van den Bogaert, 2005).
Ultimately, proton therapy is a solid prospective technology, especially for tumours in sensitive regions such as the eyes. The jury is still out on its effectiveness in general; the consensus seems to be that proton therapy has significant theoretical advantages but not clinical benefits (St. Clair et al, 2004). Currently, a five-year study of proton therapy's effectiveness against prostate cancer is underway at Massachussetts General Hospital. We hope that its results will lead to yet another powerful weapon in the fight against cancer.
ReferencesGreco, C.; Wolden, S. Current status of radiotherapy with proton and light ion beams. Cancer. 2007, 109, 1227-38.
Goitein, M.; Jermann, M. The Relative Costs of Proton and X-ray Radiation Therapy. Clinical Oncology. 2003, 15, 37-50.
Lievens, Y.; Van den Bogaert, W. Proton beam therapy: Too expensive to become true? Radiotherapy and Oncology. 2005, 75, 131-3.
St. Clair, W. H. Advantage of protons compared to conventional X-ray or IMRT in the treatment of a pediatric patient with medulloblastoma. International Journal of Radiation, Oncology, Biology, Physics. 2004, 58, 727-34.
Terasawa, T.; Dvorak, T.; Ip, S. Systematic Review: Charged-Particle Radiation Therapy for Cancer. Annals of Internal Medicine. 2009, 151, 556-65.