No abstract

LET-painting was suggested as a method to overcome tumour hypoxia. In vitro experiments have demonstrated a wellestablished relationship between the oxygen enhancement ratio (OER) and linear energy transfer (LET), where OER approaches unity for high-LET values. However, high-LET radiation also increases the risk for side effects in normal tissue. LET-painting attempts to restrict high-LET radiation to compartments that are found to be hypoxic, while applying lower LET radiation to normoxic tissues. Methods. Carbon-12 and oxygen-16 ion treatment plans with four fi elds and with homogeneous dose in the target volume, are applied on an oropharyngeal cancer case with an identifi ed hypoxic entity within the tumour. The target dose is optimised to achieve a tumour control probability (TCP) of 95% when assuming a fully normoxic tissue. Using the same primary particle energy fl uence needed for this plan, TCP is recalculated for three cases assuming hypoxia: fi rst, redistributing LET to match the hypoxic structure (LET-painting). Second, plans are recalculated for varying hypoxic tumour volume in order to investigate the threshold volume where TCP can be established. Finally, a slight dose boost (5-20%) is additionally allowed in the hypoxic subvolume to assess its impact on TCP. Results. LET-painting with carbon-12 ions can only achieve tumour control for hypoxic subvolumes smaller than 0.5 cm 3. Using oxygen-16 ions, tumour control can be achieved for tumours with hypoxic subvolumes of up to 1 or 2 cm 3. Tumour control can be achieved for tumours with even larger hypoxic subvolumes, if a slight dose boost is allowed in combination with LET-painting. Conclusion. Our fi ndings clearly indicate that a substantial increase in tumour control can be achieved when applying the LET-painting concept using oxygen-16 ions on hypoxic tumours, ideally with a slight dose boost.

Background. In vitro RBE values for various high LET radiation types have been determined for many different cell types. Occasionally it is criticized that RBE for a given endpoint cannot be single-value dependent on LET alone, but also on particle species, due to the different dose deposition profi les on microscopic scale. Hence LET is not suffi cient as a predictor of RBE, and this is one of the motivations for development of radiobiological models which explicitly depend on the detailed particle energy spectrum of the applied radiation fi eld. The aim of the present study is to summarize the available data in the literature regarding the dependency of RBE on LET for different particles. Method. As RBE is highly dependent on cell type and endpoint, we discriminated the RBE-LET relationship for the three investigated cell lines and at the same endpoint (10% survival in colony formation). Data points were collected from 20, four and four publications for V79, CHO and T1, respectively, in total covering 228 RBE values from a broad range of particle species. Results and discussion. All RBE-LET data points demonstrate surprising agreement within the general error band formed by the numerous data points, and display the expected RBE peak at around 100 -200 keV/ μ m. For all three cell lines, the infl uence of varying the particle type on the RBE was far from obvious, compared to the general experimental noise. Therefore, a dependence of particle type cannot be concluded, and LET alone in fact does seem to be an adequate parameter for describing RBE at 10% survival. High linear energy transfer (LET) radiation is characterized by a higher biological effectiveness compared to photons of low LET. Because high LET radiation is densely ionizing, the correlated damages of the DNA structure within one cell occur more often so that it becomes more diffi cult for the cell to repair the damage, leading to a markedly increased effi ciency of cell killing [1]. The concept of relative biological effectiveness (RBE) has been introduced to account for this increased effi ciency. RBE is defi ned as the ratio of a dose of photons to a dose of any other particle to produce the same biological effect. High LET beams may have RBEs ranging from 1.5 to 3 [2].RBE values for various high LET radiation types have been determined for many different cell types, both in vitro and in vivo. It has been demonstrated in in vitro studies that RBE is highly dependent on both cell type and the studied endpoint [3], but also on particle species, due to the different dose deposition profi les on microscopic scale [4,5].Hence LET is not suffi cient as a predictor of RBE. This is one of the motivations for the development of radiobiological models which explicitly depend on the detailed particle energy spectrum of the applied radiation fi eld. Several models exist which aim to predict the biological response of cells irradiated with high-LET radiation. The most prominent models in radiotherapy context are based on the amorphous track formalism es...