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Session Item

Monday
November 30
16:45 - 17:45
Clinical Stream 1
Proffered papers 34: Particle therapy in CNS tumours
Proffered Papers
Clinical
16:55 - 17:05
RBE-dependence on LET and fractionation in the rat cervical spinal cord after oxygen ion irradiation
OC-0687

Abstract

RBE-dependence on LET and fractionation in the rat cervical spinal cord after oxygen ion irradiation
Authors: Hintz|, Lisa(1,2,3)*[l.hintz@dkfz-heidelberg.de];Saager|, Maria(1,3);Peschke|, Peter(1,3);Brons|, Stephan(3,4);Debus|, Jürgen(3,5,6);Glowa|, Christin(1,3,5);Karger|, Christian Peter(1,3);
(1)German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology E040, Heidelberg, Germany;(2)Heidelberg University, Faculty of Biosciences, Heidelberg, Germany;(3)National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany;(4)Heidelber Ion Beam Therapy Center HIT, Heidelberg, Germany;(5)University Hospital Heidelberg, Radiation Oncology and Radiotherapy, Heidelberg, Germany;(6)German Cancer Research Center DKFZ, Clinical Cooperation Unit Radiotherapy, Heidelberg, Germany;
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Purpose or Objective

The cervical spinal cord (CSC) is one of the dose-limiting organs when treating head and neck cancer (HNC) patients with irradiation since an overdose in the spinal cord may lead to irradiation-induced myelopathy, an irreversible and severe side effect. Thus HNC is one of the most relevant indications for radiation treatments with ions. The local effect model (LEM) – actually used for treatment planning in carbon (12C) ion therapy in Germany – predicts the increased relative biological effectiveness (RBE) needed for RBE-weighted dose calculations. Accordingly, uncertainties in the RBE can lead to under- or over-dosages, making it mandatory to validate the predictions by experimental data. As the further increased RBE of oxygen (16O) ions could have a great benefit for treating radioresistant hypoxic tumors and are therefore planned to be implemented at the Heidelberg Ion Beam Therapy Center (HIT), the RBE-dependence on linear energy transfer (LET) and fractionation has to be investigated.

Material and Methods

The segments C1-C6 of the CSC of female Sprague Dawley (SD) rats were irradiated with increasing dose levels of 16O-ions. Experiments were performed with either 1 or 2 fractions (fx) and at 4 different positions within a 6 cm Spread-out Bragg-peak (SOBP) covering the plateau-region (LET of 26 keV/µm) and different positions within the SOBP (LETs of 68, 100 and 140 keV/µm). Dose-response curves were measured for the endpoint paresis grade II (PII – palsy of the forelimbs similar to irradiation-induced myelopathy) within 300 days after irradiation. RBEs were calculated based on the TD50-values (dose at 50% complication probability) and using previously measured values for photons [Radiat Res. 2003 Nov;160(5):536-42].

Results

For all positions RBEs rose from 1 to 2 fx. Also RBEs increased with increasing LETs up to 100 keV/µm and this increase was seen to be stronger for irradiations with 2 fx than with 1 fx. At 140 keV/µm, however, both the 1 and 2 fx studies showed RBEs lower than that at 100 keV/µm.

Conclusion

In accordance with our previous studies with 12C-ions [Radiat Oncol. 2018 Jan 11;13(1):5] we found a clear fractionation- and LET-dependence of the RBE also for 16O-ions. Excluding the data point at 140 keV/µm, comparison with the previously measured 12C-ion RBEs at the same SOBP-positions in addition revealed a higher effectiveness of 16O-ions. Interestingly, the RBE of 16O-ions decreased beyond 100 keV/µm for both fractionation schedules, which might be interpreted as an “overkill-effect". This study provides the first systematic RBE-data for late effects in the CSC, which can be used to benchmark RBE-models.