ESTRO 38 Abstract book

S23 ESTRO 38

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.

pertinence for the effects observed in microbeam radiation (whether a microbeam radiation is a kind of bath/shower dose) remains a fundamental question. We present 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 model can predict the posited sparing effect in micro-/minibeam radiation and thereby help to optimize microbeam patterns. Material and Methods The dynamic repair model (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. We tested the DRM on six well-known published datasets of volume effects and bath/shower effects in rat spinal cord, as well as one dataset of synchrotron-based microbeam radiation. Model parameters were obtained from regression to the volume-effect data. Model validation was subsequently performed on the bath/shower and microbeam data. Results The DRM provides a very good prediction of all small volume and bath/shower datasets. Fig. 1 shows 50 % response doses D50 as a function of irradiated length (or effective length in the case of inhomogeneous irradiation). 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).

Proffered Papers: CL 1: Proffered papers : Lung

OC-0059 Stereotactic radiotherapy for oligoprogressive NSCLC: clinical scenarios affecting survival S. Kroeze 1 , C. Fritz 1 , D. Kaul 2 , O. Blanck 3 , K.H. Kahl 4 , F. Roeder 5 , S. Siva 6 , J. Verhoeff 7 , A. Grosu 8 , M. Schymalla 9 , M. Glatzer 10 , M. Szücs 11 , M. Geier 12 , S. Mose 13 , I. Sackerer 14 , F. Lohaus 15 , F. Eckert 16 , M. Guckenberger 1 1 University Hospital Zürich, Radiation Oncology, Zurich, Switzerland; 2 Charité‐University Hospital Berlin, Radiation Oncology, Berlin, Germany ; 3 Universitätsklinikum Schleswig‐Holstein, Radiation Oncologyz, Kiel, Germany ; 4 Klinikum Augsburg, Radiation Oncology, Augsburg, Germany; 5 University Hospital Munich, Radiation Oncology, Munich, Germany ; 6 Peter MacCallum Cancer Center, Radiation Oncology, Melbourne, Australia; 7 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands; 8 University Medical Center Freiburg, Radiation Oncology, Freiburg, Germany; 9 Universitätsklinikum Gießen und Marburg, Radiation Oncology, Marburg, Germany; 10 Kantonsspital St. Gallen, Radiation Oncology, St. Gallen, Switzerland; 11 University Hospital Rostock, Radiation Oncology, Rostock, Germany; 12 Ordensklinikum Linz‐ Barmherzige Schwestern, Radiation Oncology, Linz, Austria; 13 Schwarzwald‐Baar Klinikum, Radiation Oncology, Villingen‐Schwenningen, Germany; 14 Strahlentherapie Freising und Dachau, Radiation Oncology, Freising, Germany; 15 University Hospital Dresden, Radiation Oncology, Dresden, Germany; 16 University Hospital Tübingen, Radiation Oncology, Tübingen, Germany Purpose or Objective Developments of local and systemic therapies have improved the prognosis of metastatic NSCLC. Oligometastatic patients may especially benefit from a more aggressive treatment approach. However, the concept of “oligometastasis” is complex: e.g. limited progression or resistance of disease to systemic treatment. This study aimed to evaluate the outcome of stereotactic radiotherapy (SRT) to oligoprogressive or oligoresistant NSCLC in patients receiving concurrent immuno- or targeted therapy. Material and Methods The retrospective international multicenter register study (TOaSTT) collected data on metastatic NSCLC patients receiving SRT concurrent (≤30d) to immuno- or targeted therapy. Patients were grouped in: patients treated for ≤5 metastases without additional metastases (oligoprogression), treated for ≤5 progressive metastases with controlled disease of all other metastases (oligopersistent), and patients with mixed response/ uncontrolled disease. SRT was performed to ≤5 extracranial or cranial lesions. Overall survival (OS) was analyzed using Kaplan-Meier survival curves and log rank testing. Secondary outcomes were progression free survival (PFS), local control (LC), time to switch of systemic therapy after SRT and toxicity as defined by the CTCAE v4 criteria. Results SRT of 192 lesions in 108 patients was performed between 7/2009 and 5/2018 in 16 clinics. Median age was 63y (range 33-80). 75% had synchronous metastatic disease, driver mutations were: EGFR 41%, ALK 14%, other 21%, unknown/no 24%. Median follow-up was 18.7 (range 1-102)

Conclusion 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 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 modelled. Microbeam