Kliniken & Institute … Kliniken Radiologische Klinik … RadioOnkologie und… Research Research Sektion Medizinphysik …

Sektion Medizinphysik

Der Erfolg der Strahlentherapie beruht auf über 100 Jahren Erfahrung. Die zugrundeliegenden Prozesse, welche die Wirkung von Strahlung auf Normalgewebe bestimmen, sind jedoch noch immer größtenteils unverstanden. Wir versuchen, Strahlenbiologie von der Nanometer-Skala der Ionisierung bis zur Größe des gesamten Organs zu verstehen. Dazu nutzen wir Werkzeuge aus nichtlinearer Dynamik, Nichtgleichgewichts-Thermodynamik, und Netzwerktheorie in Verbindung mit Statistischen Methoden, mit welchen wir biologische Daten aus internationalen Kollaborationen mit Biologen modellieren und analysieren. Eine enge Partnerschaft mit den Experten für Monte Carlo Dosisberechnung und mit Ärzten am HIT ermöglicht uns eine Verbindung präklinischer und klinischer Ergebnisse und gewährleistet die Translation von mikroskopischen Beobachtungen in die Klinik.

The success of radiation therapy is based on over 100 years of experience. However, the underlying processes that determine the effect of radiation on normal tissue are still largely misunderstood. We try to understand radiation biology from the nanometer scale of ionization to the size of the entire organ. We use tools from nonlinear dynamics, non-equilibrium thermodynamics, and network theory in connection with statistical methods with which we model and analyze biological data from international collaborations with biologists. A close partnership with the experts for Monte Carlo dose calculation and with doctors at the HIT enables us to combine preclinical and clinical results and ensures the translation of microscopic observations into the clinic.

Aktuelle Forschungsprojekte | Current Research Projects

Mechanisms of late radiation-induced lesions in the central nervous system

Dr. Anika Simon, Dr. Emanuel Bahn, Prof. Dr. Markus Alber

Therapeutic irradiation of brain tissue may lead to delayed effects that range from cognitive impairment to the occurrence of necrosis. With this project we want to address the delayed radiation necrosis experimentally on a molecular level in vitro and in vivo. Inflammation, vascular damage and demyelination belong to the relatively wellcharacterized acute phase that is observed shortly after irradiation. The acute phase is followed by a sub-acute phase, in which progenitor cells start to regenerate the harmed tissue. At last, a late phase is initiated that is characterized by a remodeling of the tissue and the progenitor cells. During this phase, which takes about months or years, the severe side effect delayed radiation necrosis may be observed. Histologically, the late radiation necrosis is characterized by edema, white matter necrosis, inflammation and blood-brain barrier disruption. In contrast, very little is known about the molecular mechanisms behind this side effect, especially during the extended latent phase.

Volume Effects in the Central Nervous System and Sparing in Microbeam/ Minibeam Radiation

Dr. Emanuel Bahn, Prof. Dr. Markus Alber

Experiments with narrow proton beams in the millimeter-range and bath/shower dose distributions have produced evidence of an abnormally high volume effect in rat spinal cord more than a decade ago. Whether these results have pertinence for the effects observed in microbeam irradiation remains a fundamental question. In other words, whether a microbeam irradiation is a kind of bath/shower dose.

We have developed a spinal cord dose response model that predicts relative frequency and localization of late radiation effects after irradiation with arbitrary dose distributions, over scales from few micrometers to the entire organ. The dynamic repair model (DRM) can predict the posited sparing effect in micro-/ minibeam radiation and thereby help to optimize microbeam patterns. The DRM views a complication as a result of a failed repair/homeostasis process, caused by deletion of critical tissue information via radiation. From general thermodynamic principles, we derive a closed form that relies on only three parameters. In this framework, the volume effect emerges as a result of a finite communication distance in a network-like information storage system.

Fig. 1 shows 50 % response doses D50 as a function of irradiated length (or effective length in the case of inhomogeneous irradiation). The DRM provides a very good prediction of all small volume and bath/shower datasets. For the rat spinal cord, we determine a critical information communication distance of (4.0±0.3) mm. The microbeam experiment compares a 1.35 mm wide homogeneous field with an 11 mm wide comb of 35μm wide microbeams. The homogeneous field response is well predicted by the DRM. The microbeam D50 is underestimated (predicted: 180±20 Gy measured: 370±90 Gy).

In micro-/minibeam radiation, dose distributions are highly inhomogeneous on very small scales. The abnormally high volume effect on small scales might provide an explanation, but this requires a model that permits comparison of effects of inhomogeneous doses. The observation that the same complication arises for vastly different dose distributions (only the dose scale changes) implies that the underlying mechanism does not change with volume. The DRM allows a prediction of the dose response for arbitrary dose distributions and it particularly explains the effect of low dose baths adjacent to small high dose regions through a deleterious effect on information critical for repair. Microbeam experiments with peak doses in excess of 200 Gy may lie outside the valid range of the DRM because effects that arise from killing every cell in the high dose region are not modeled (overkill). Microbeam dose response data are scarce and thus definitive conclusions are not justified, but comparison of model prediction with a published dataset indicates an additional dose tolerance beside the volume effect, possibly caused by overkill.

Volume Effects and the Intestinal Stem Cell Niche

Dr. Emanuel Bahn, Prof. Dr. Markus Alber In cooperation with Dr. William Shaw (UFS, Bloemfontein, South Africa)

Radiation toxicity of normal tissue depends on the irradiated volume on a macroscopic level, which forms the basis of conformal radiotherapy. On a microscopic level, differential tissue response is hypothesized to be driven by the radio-sensitivity of stem cell niches, which are maintained by a resilient reserve stem cell population. A mechanism of the volume effect that links both length scales remains elusive. We recently observed a volume effect of the intestinal stem cell niche and posit a connection to pathological checkpoint recovery as underlying mechanism.

Regeneration of jejunal crypts in mice was studied following irradiation of small jejunal sections with single doses of 6 to 24 Gy of 6 MV X-rays using an ex vivo technique. Field sizes of 10 mm, 7 mm and 5 mm applied to externalized jejunum were investigated in comparison to total body irradiation (TBI). We devised a biomathematical model that delivers a full statistical dynamic description of epithelial radiation injury and subsequent regeneration. We validated the model against cellular and crypt distribution statistics.

We observed a differential response of the intestinal stem cell niche, but only for field sizes of 5 and 7 mm. Crypt survival was enhanced by up to an order of magnitude compared to 10 mm field and TBI. Model-based data evaluation relates this behavior to two effects: (1) Time to checkpoint recovery becomes less dosedependent for smaller fields. (2) Millimeter-scale injuries exhibit accelerated proliferation and/or reduced radiation sensitivity. The small-scale volume effect appears to be an adaptation of niche regenerative paths to the size of the lesion. A full dynamic description of the evolution of stem cell niche population statistics is obtained.

The regeneration process of the stem cell niche is localized in the intestine and does not rely on radiation-sensitive resources of the organism. Yet, the niche interacts with intact tissue in few millimeters distance. Our findings generate several hypotheses about the driving mechanism behind the volume effect in radiotherapy. Premature checkpoint recovery in the stem cell niche, which may lead to delayed mitotic cell death and hence render tissue more sensitive to radiation, seems to be suppressed in small injuries, which seem to favor apoptosis.

A second (third, fourth...) look at the In Vitro Clonogenic Assay

Robin Koch, Christoph Harmel, Dr. Emanuel Bahn In cooperation with Dr. Ivana Dokic (DKFZ)

The in vitro clonogenic assay (IVCA) presents the standard in vitro experimental method in radiobiology: the cell survival curve lays the foundation for most biological models in radiotherapy. Since its introduction in 1956, the IVCA has remained basically unchanged: irradiated cells are incubated to form colonies, which are subsequently scored as either doomed or vital, based on their size. Here, we suggest to record the colony growth and show by temporally resolved statistical analysis of colony formation that the underlying basic assumptions of the in IVCA are often not fulfilled, which may lead to systematic errors in cell radiation response parameters α and β.

We analyzed the effects of time-of-scoring and of scoring threshold on cell survival curves and on extracted parameters. These thresholds should have no influence on the outcome within a reasonable range. However, we made contrary observations: cell survival curves undergo an apparent shift when scored at subsequent time points, with (logarithmic) slope decreasing and curvature increasing over time (Fig. 1a). As a result, cell survival parameters and shift considerably when a linear/quadratic formula is fitted to the cell survival curves, the derived /ratio varies over a large range (Fig. 1b). Analysis of colony growth curves suggests that this effect is connected to a dose-dependent proliferation rate.

The clonogenic assay rests on the working hypothesis that irradiated cells will either perish after few mitoses, or proliferate constantly to form vital colonies. We observe that, since this condition is not generally fulfilled, systematic errors arise. We propose a temporally and statistically resolved analysis of colony size distributions. This removes the scoring bias by separating cell survival from the effects of colony growth. Additionally, a biomathematical model can be employed for in-depth analysis of the underlying colony formation.

Development of a High Precision Irradiation System for In Vivo RBE Measurements with Ion Beams

Judith Besuglow, Dr. Emanuel Bahn
In cooperation with Dr. Andrea Mariani (HIT), Gernot Echner (DKFZ)

Research platforms for experimental small animal irradiation can foster to the acute need of robust in vivo data for current challenges in radiotherapy, such as the determination of the RBE of high-LET radiation in the brain. Due to the strong dependence of dose response on the volume of the small irradiation fields employed in these experiments, highly identical dose distributions are required for inter-comparison of different ion types, which presents a major challenge for experimental design.

We devised a multi-component system that meets the requirements of a precise comparative RBE measurement within the pristine Bragg peak of ion beams. Via a passive collimation system, we obtain range adjustment to sub-millimeter precision and sharp lateral beam collimation, combined with precise and rapid animal positioning. The model requires irradiation of the frontal lobe of a mouse brain with a pristine Bragg peak while sparing sensitive adjacent regions. The dose volume is a 3.5 mm x 7 mm wide cuboid of 4 mm depth for proton, helium, carbon and oxygen ion irradiation. This is achieved via passive collimation with a 3D printed bolus that can be accurately adjusted relative to the fixated animal head. The collimator was optimized by Monte Carlo (FLUKA) based beam simulation in water and on CT scans.

In this study we could show that conformity of dose distributions between highly different ion types is achievable. The proposed setup allows a detailed examination of biological effects at the distal end of the Bragg peak, thus providing valuable in-vivo data for high RBE irradiation.

Fig. 1: Dose distribution obtained by Monte Carlo simulation in water. a) Projected dose distribution for He Ions. b) Depth-dose curves (solid lines) and depth-LET curves (dotted lines) for the four studied ion types.