The study shows that the doses absorbed by organs far from the treatment volume are substantially smaller in proton therapy than in external beam photon therapy, even when taking into account the imaging dose.
For cancer patients receiving radiotherapy, reducing radiation exposure of normal tissue can prevent adverse effects, such as the development of second cancers later in life. This is especially relevant for children, given their longer life expectancy.
But understanding – and mitigating – the risks associated with radiotherapy needs a better estimation of total radiation doses absorbed by the patient. This means integrating doses received by the target (in-field) and the normal (out-of-field) tissues, not only during treatment but also during imaging procedures conducted before and during the treatment.
Integrating doses for assessing the risk
In this study, HARMONIC researchers applied a validated set of tools to evaluate the complete (whole body) patient exposure during paediatric brain cancer treatment and imaging procedures. “To the best of our knowledge this is the first time in which an out-of-field dose comparison is done taking into account the overall dose received by the patient during treatment and imaging procedures,” says Lorenzo Brualla, researcher at the University of Essen’s Proton Therapy Centre and task leader in the HARMONIC work package (WP4) dedicated to dosimetry.
The research team used a 5-year-old anthropomorphic phantom to estimate organ doses for treatment of a brain tumour (glioma) with different photon (3D-CRT, IMRT, VMAT) or proton (PBS) radiotherapy techniques, and included x-ray doses received during imaging procedures.
To estimate doses, they used a series of tools based on the Monte Carlo method that simulates how radiation interacts with tissues in the body. They then used a previously published model to estimate the risk of cancer according to the doses received by the tissue.
Normal tissues are less affected with proton therapy
The results show that out-of-field organ doses are lower for proton than for photon therapy. For example, the thyroid received 120 mSV with proton therapy versus 575 mSv for photon therapy. By allowing to reduce the doses received by healthy tissues, proton therapy significantly reduces the risk that these tissues will give rise to cancer cells later in life.
The analysis also shows that, given the lower doses from proton therapy, the imaging dose accounts for a considerable proportion of the dose compared to photon treatments, where the contribution is much lower.
“The published results represent the summit of the efforts conducted within the dosimetry tasks of WP4, and shows the importance of the collaborations established with other partners from the HARMONIC consortium,” says Brualla.
De Saint-Hubert M, Boissonnat G, Schneider U et al. Complete patient exposure during paediatric brain cancer treatment for photon and proton therapy techniques including imaging procedures. Front. Oncol., 19 Sept 2023. doi: 10.3389/fonc.2023.1222800