Skip to ContentSkip to Navigation
KVI - Center for Advanced Radiation TechnologyResearch and educationMedical Physics


Radiotherapy is an important modality in the treatment of tumors: about half of the cured cancer patients has benefited from radiotherapy. However, in most cases not only the tumor is irradiated but also the surrounding healthy tissues and organs. The co-irradiation of these healthy tissues may cause long-term complications that significantly influence the quality of life of the patients. The severity of these complications depends on the function of the co-irradiated organ, their radiation sensitivity and the radiation dose they incur. In specific cases this may be the limiting factor of the radiation dose to the tumor and thereby compromise the chance of curing the patient

The development of radiotherapy with photons, initially from radioactive sources and nowadays produced with small linear electron accelerators, has resulted in a very significant reduction of the radiation-induced complications, but they still remain a very serious issue that calls for further improvement of the irradiation techniques.

One possible route towards a further reduction of the radiation dose to the surrounding healthy tissues, and thus of the complications, is the use of heavy charged particles (protons and other ions) instead of photons. This type of irradiation generally results in a significantly lower radiation dose in the co-irradiated healthy tissues due to its superior dose deposition properties: ions have a well-defined, finite penetration depth in matter and a high dose deposition close to the end of their trajectory. This is illustrated in the figure below which shows a schematic dose distribution for protons and high-energy photons.

In a typical treatment plan for proton therapy, the Spread Out Bragg Peak (SOBP, dashed blue line), is the therapeutic radiation distribution. The SOBP is the sum of several individual Bragg peaks (thin blue lines) at staggered depths. The depth-dose plot of an x-ray beam (red line) is provided for comparison. The pink area represents the additional dose delivered by x-ray radiotherapy which can be the source of damage to normal tissues and of secondary cancers, especially of the skin. Source of image: Wikipedia, adapted from "Proton beam therapy" W P Levin, H Kooy, J S Loeffler and T F DeLaney British Journal of Cancer (2005) 93, 849–854.

These advantages were pointed out already in 1947, in a paper by Robert R. Wilson, who later on became a prominent particle physicist. Only with the advent of advanced 3-dimensional imaging techniques such as X-ray Computed Tomography (CT) and magnetic resonance imaging (MRI) and the revolution in computer technology in the last two decades, has it become possible to fully exploit the advantages of protons (and other ions) over photons in radiotherapy. This has resulted in a rapid expansion of the clinical use of protons in radiotherapy and research to further improve this treatment modality. In 2012 worldwide about 10 000 patients were treated with protons and carbon ions. The annual growth of this number is around 25 %.

The finite penetration depth of ions and the high dose deposit at the end of their path, which make it possible to substantially reduce the radiation dose to the surrounding healthy tissue, are, unfortunately, not only a benefit. They also cause the dose distribution to be rather sensitive to small errors in the modelling of the tissue composition and density, based on X-ray imaging, used to predict the energy loss of the protons. Furthermore, small changes in the patient anatomy can cause clinically significant differences between the intended and actually delivered dose distribution. These effects result in an underdose in the tumor and/or an overdose in the surrounding healthy tissue, compromising treatment outcome in terms of both tumor control and normal tissue complications.

In the Netherlands, proton therapy is part of the basic healthcare package for specific indications since 2012. Currently, three clinical proton therapy facilities are being constructed, including one at the University Medical Center Groningen (UMCG). It is expected that the first patient treatments in the Netherlands will take place in 2017.

In conjunction with these plans, the KVI-Center for Advanced Radiation Technology (KVI-CART) and UMCG, together with several other parties, have established the PARticle Therapy REsearch Center (PARTREC), a research collaboration with a broad research program aiming to further improve the quality of proton therapy treatment. In the framework of PARTREC, KVI-CART has a physics research program that focuses on the following topics:

  • Combining advanced X-ray imaging techniques with proton imaging to reduce the uncertainty in the stopping power predictions to below 1%
  • Methods for in-vivo verification of the irradiation by imaging the very weak secondary radiation produced by the interaction of the protons and helium ions with tissue
  • A standard for dosimetry in proton therapy that will ensure that treatments in all treatment centers can be compared

In addition, we facilitate the research programme of the UMCG Radiation Biology group on biological damage in healthy tissues induced by irradiations with protons and carbon ions.

Last modified:28 June 2017 10.40 a.m.