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Assessment of out-of-field doses in proton therapy with variation of clinical parameters

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Nowadays pencil beam scanning (PBS) is the most promising form of proton therapy (PT) with clear reduction in out-of-field doses compared to older PT techniques and conventional photon radiotherapy [W. Newhauser, Phys Med Biol, 2015]. Nevertheless, PT is unavoidably accompanied by the production of secondary high-energy neutrons in the patient and structural materials of the beamline [Stolarczyk, Phys Med Biol, 2018]. Neutrons are of particular concern as they are capable of travelling large distances to deposit out-of-field doses in organs located far from the primary treatment field and with a relatively high biological effectiveness [Ottolenghi, Radiat Prot Dosim, 2015]. Furthermore, non-elastic nuclear reactions will also produce secondary protons, heavier ions and photons. Especially the secondary protons and heavier ions might have a significant detrimental effect on the surrounding healthy tissue. These out-of-field doses are especially an issue for radiation protection of children, due to their high radiosensitivity and long life-expectancy. Another group of patients deserving special attention are pregnant women, who can benefit from PT during pregnancy if foetal radiation exposure is kept as low as possible, avoiding treatment delays, preterm deliveries and maternal death [Geng, Phys Med Biol, 2016]. Some researchers are cautious that the existing knowledge and understanding of the out-of-field doses and associated risk of inducing Secondary Malignant Neoplasms (SMNs) is not sufficiently mature to justify the use of modern techniques, such as PT, for treating children or pregnant women [Newhauser, et al., Nat Rev Cancer, 2011]. Therefore, a full characterization of the out-of-field doses, particularly at the PT field edge requires special attention for radiation protection and prevention of SMNs [Harisson, EURADOS SRA, Radiat Prot Dos, 2021].

Even though the out-of-field doses can be reduced through optimization of clinical parameters (such as patient alignment, direction and energy of primary beam, choice of beam modifiers, etc.), the doses originating from secondary radiation in PT are generally not calculated by treatment planning systems. This leaves the medical doctors and medical physicists without a tool for out-of-field dose assessment and optimization.

While previous work has been focussing on the characterisation of suitable detectors and assessment of out-of-field doses in specific cases, the main goal of this PhD is to assess the impact of varying clinical parameters on the out-of-field radiation using advanced measurement techniques and building a Monte Carlo framework based on a radiation transport code.

During this PhD a Monte Carlo radiation transport model will be developed to simulate out-of-field doses. This model will be validated by comparison with results from previous measurement campaigns and additional measurements with different detectors such as also the novel Timepix based pixelated silicon detectors and the MicroPlus and mini-TEPC microdosimetric detectors. This validated model for out-of-field doses will then be used to assess out-of-field doses for a large number of clinical plans, with a strong focus to paediatric patients, including brain and craniospinal irradiations, and pregnant women, including brain, lymphoma and breast cancers. An important aspect will be to change systematically specific treatment parameters such as beam angle, proton energies, field size, modulation width, presence of range shifter, apertures etc. Based on these results it will be investigated how sensitive the out-of-field doses are to the clinical plan parameters and how this sensitivity can be parameterized most efficiently. Furthermore, we will explore the translation of out-of-field doses to associated risks by applying current risk models but also using more advanced risk models considering for example dose inhomogeneity, fractionation schemes in radiotherapy [Dasu, Acta Oncol, 2005] and dose deposition at cellular scale [Ottolenghi, Rad Prot Dos, 2015].

Ultimately, these results will allow medical physicists to assess out-of-field doses as a function of the most important treatment parameters enabling optimisation of treatment plans and providing an essential input for epidemiological studies on treatment associated side effects and the risk of secondary cancer of treated children and/or exposed foetus.

The minimum diploma level of the candidate needs to be

  • Master of sciences
  • Master of sciences in engineering

The candidate needs to have a background in

  • Physics

Estimated duration

4 years

Expert group

Research in Dosimetric Applications

SCK CEN Mentor

De Saint-Hubert Marijke
mdsainth [at]
+32 (0)14 33 21 40

SCK CEN Co-mentor

Van Hoey Olivier
ovhoey [at]
+32 (0)14 33 27 09


Depuydt Tom
tom.depuydt [at]