Future development of biologically relevant dosimetry

Abstract

Proton and ion beams are radiotherapy modalities of increasing importance and interest. Because of the different biological dose response of these radiations as compared with high-energy photon beams, the current approach of treatment prescription is based on the product of the absorbed dose to water and a biological weighting factor, but this is found to be insufficient for providing a generic method to quantify the biological outcome of radiation. It is therefore suggested to define new dosimetric quantities that allow a transparent separation of the physical processes from the biological ones. Given the complexity of the initiation and occurrence of biological processes on various time and length scales, and given that neither microdosimetry nor nanodosimetry on their own can fully describe the biological effects as a function of the distribution of energy deposition or ionization, a multiscale approach is needed to lay the foundation for the aforementioned new physical quantities relating track structure to relative biological effectiveness in proton and ion beam therapy. This article reviews the state-of-the-art microdosimetry, nanodosimetry, track structure simulations, quantification of reactive species, reference radiobiological data, cross-section data and multiscale models of biological response in the context of realizing the new quantities. It also introduces the European metrology project, Biologically Weighted Quantities in Radiotherapy, which aims to investigate the feasibility of establishing a multiscale model as the basis of the new quantities. A tentative generic expression of how the weighting of physical quantities at different length scales could be carried out is presented.

What will the medical physics of proton therapy look like 10 yr from now? A personal view

“…In this context, if used as a parameter during the planning process, LET can be considered to be just another “agent” to be manipulated and therefore measured as a commissioning, QA or plan specific parameter. And advanced devices are being developed that will allow us to directly measure LET (Palmans et al . and references therein) at the micro‐ and nano‐meter scales.…”

“…In this context, if used as a parameter during the planning process, LET can be considered to be just another "agent" to be manipulated and therefore measured as a commissioning, QA or plan specific parameter. And advanced devices are being developed that will allow us to directly measure LET (Palmans et al 46 and references therein) at the micro-and nano-meter scales. These could enable us to more fully understand the biological consequences of proton interactions, and perhaps lead to well defined, and clinically derived, proton specific dose and LET constraints to tumors and critical organs, making the RBE concept, with all its uncertainties, more or less redundant.…”

Proton and light ion RBE for the induction of direct DNA double strand breaks

“…Proton and light ion beams are rapidly gaining popularity as a radiation therapy treatment option due to their ability to conform dose to the target while sparing surrounding healthy tissues. 1 These beams have different clinical effects for equivalent physical doses as compared to conventional x-rays. Therefore, the relative biological effectiveness (RBE) is a useful measure in comparing their effectiveness.…”

Energy dependent track structure parametrisations for protons and carbon ions based on nanometric simulations

“…However it is still a current topic of interest to find a reliable way to estimate the biological effectiveness of ion beams. Benefiting from a multi-scale approach, the BioQuaRT 1 project [2,3] aims at creating a new dosimetric quantity to define the local radiation quality, which strongly influences the biological response. In the long run, this quantity should be integrated into clinical treatment planning systems which simulate and optimise the treatment for each individual patient.…”

“…We therefore present here three parametrisations which can be used to extract track structure properties for each desired energy in a clinical energy range without the need to perform a full nanometric Monte Carlo simulation for each patient. These parametrisations may be useful for treatment planning, where the three-dimensional patient geometry is represented by voxels with a size of ≈1 mm 3 . These voxels are exposed to a mixed radiation field where several particle types with different energies hit a voxel from different directions.…”