Irradiation with Carbon Ions Effectively Counteracts Hypoxia-related Radioresistance in a Rat Prostate Carcinoma.

PURPOSE: Hypoxia in tumors is associated with increased malignancy and resistance to conventional photon radiation therapy. This study investigated the potential of particle therapy to counteract radioresistance in syngeneic rat prostate carcinoma. METHODS AND MATERIALS: Subcutaneously transplanted R3327-HI tumors were irradiated with photons or carbon ions under acute hypoxic conditions, induced by clamping the tumor-supplying artery 10 min before and during irradiation. Dose-response curves were determined for the endpoint "local tumor control within 300 days" and compared with previously published data acquired under oxic conditions. Doses at 50% tumor control probability (TCD50) were used to quantify hypoxia-induced radioresistance relative to that under oxic conditions and also to quantify the increased effectiveness of carbon ions under oxic and hypoxic conditions relative to photons. RESULTS: Compared with those under oxic conditions, TCD50 values under hypoxic conditions increased by a factor of 1.53 ± 0.08 for photons and by a factor of 1.28 ± 0.06 for carbon ions (oxygen enhancement ratio). Compared with those for photons, TCD50 values for carbon ions decreased by a factor of 2.08 ± 0.13 under oxic conditions and by a factor of 2.49 ± 0.08 under hypoxic conditions (relative biological effectiveness). While the slope of the photon dose-response curves increased when changing from oxic to hypoxic conditions, the slopes were steeper and remained unchanged for carbon ions. CONCLUSIONS: The reduced oxygen enhancement ratio of carbon ions indicated that the required dose increase in hypoxic tumors was 17% lower for carbon ions than for photons. Additionally, carbon ions reduced the effect of intertumor heterogeneity on the radiation response. Therefore, carbon ions may confer a significant advantage for the treatment of hypoxic tumors that are highly resistant to conventional photon radiation therapy.

Relative biological effectiveness of oxygen ion beams in the rat spinal cord: Dependence on linear energy transfer and dose and comparison with model predictions.

Background and purpose: Ion beams exhibit an increased relative biological effectiveness (RBE) with respect to photons. This study determined the RBE of oxygen ion beams as a function of linear energy transfer (LET) and dose in the rat spinal cord. Materials and methods: The spinal cord of rats was irradiated at four different positions of a 6 cm spread-out Bragg-peak (LET: 26, 66, 98 and 141 keV/µm) using increasing levels of single and split oxygen ion doses. Dose-response curves were established for the endpoint paresis grade II and based on ED50 (dose at 50 % effect probability), the RBE was determined and compared to model predictions. Results: When LET increased from 26 to 98 keV/µm, ED50 decreased from 17.2 ± 0.3 Gy to 13.5 ± 0.4 Gy for single and from 21.7 ± 0.4 Gy to 15.5 ± 0.5 Gy for split doses, however, at 141 keV/µm, ED50 rose again to 15.8 ± 0.4 Gy and 17.2 ± 0.4 Gy, respectively. As a result, the RBE increased from 1.43 ± 0.05 to 1.82 ± 0.08 (single dose) and from 1.58 ± 0.04 to 2.21 ± 0.08 (split dose), respectively, before declining again to 1.56 ± 0.06 for single and 1.99 ± 0.06 for split doses at the highest LET. Deviations from RBE-predictions were model-dependent. Conclusion: This study established first RBE data for the late reacting central nervous system after single and split doses of oxygen ions. The data was used to validate the RBE-dependence on LET and dose of three RBE-models. This study extends the existing data base for protons, helium and carbon ions and provides important information for future patient treatments with oxygen ions.

Relative biological effectiveness of single and split helium ion doses in the rat spinal cord increases strongly with linear energy transfer.

BACKGROUND AND PURPOSE: Determination of the relative biological effectiveness (RBE) of helium ions as a function of linear energy transfer (LET) for single and split doses using the rat cervical spinal cord as model system for late-responding normal tissue. MATERIAL AND METHODS: The rat cervical spinal cord was irradiated at four different positions within a 6 cm spread-out Bragg-peak (SOBP) (LET 2.9, 9.4, 14.4 and 20.7 keV/µm) using increasing levels of single or split doses of helium ions. Dose-response curves were determined and based on TD50-values (dose at 50% effect probability using paresis II as endpoint), RBE-values were derived for the endpoint of radiation-induced myelopathy. RESULTS: With increasing LET, RBE-values increased from 1.13 ± 0.04 to 1.42 ± 0.05 (single dose) and 1.12 ± 0.03 to 1.50 ± 0.04 (split doses) as TD50-values decreased from 21.7 ± 0.3 Gy to 17.3 ± 0.3 Gy (single dose) and 30.6 ± 0.3 Gy to 22.9 ± 0.3 Gy (split doses), respectively. RBE-models (LEM I and IV, mMKM) deviated differently for single and split doses but described the RBE variation in the high-LET region sufficiently accurate. CONCLUSION: This study established the LET-dependence of the RBE for late effects in the central nervous system after single and split doses of helium ions. The results extend the existing database for protons and carbon ions and allow systematic testing of RBE-models. While the RBE-values of helium were generally lower than for carbon ions, the increase at the distal edge of the Bragg-peak was larger than for protons, making detailed RBE-modeling necessary.

DCE-MRI detected vascular permeability changes in the rat spinal cord do not explain shorter latency times for paresis after carbon ions relative to photons.

BACKGROUND AND PURPOSE: Radiation-induced myelopathy, an irreversible complication occurring after a long symptom-free latency time, is preceded by a fixed sequence of magnetic resonance- (MR-) visible morphological alterations. Vascular degradation is assumed the main reason for radiation-induced myelopathy. We used dynamic contrast-enhanced (DCE-) MRI to identify different vascular changes after photon and carbon ion irradiation, which precede or coincide with morphological changes. MATERIALS AND METHODS: The cervical spinal cord of rats was irradiated with iso-effective photon or carbon (12C-)ion doses. Afterwards, animals underwent frequent DCE-MR imaging until they developed symptomatic radiation-induced myelopathy (paresis II). Measurements were performed at certain time points: 1 month, 2 months, 3 months, 4 months, and 6 months after irradiation, and when animals showed morphological (such as edema/syrinx/contrast agent (CA) accumulation) or neurological alterations (such as, paresis I, and paresis II). DCE-MRI data was analyzed using the extended Toft's model. RESULTS: Fit quality improved with gradual disintegration of the blood spinal cord barrier (BSCB) towards paresis II. Vascular permeability increased three months after photon irradiation, and rapidly escalated after animals showed MR-visible morphological changes until paresis II. After 12C-ion irradiation, vascular permeability increased when animals showed morphological alterations and increased further until animals had paresis II. The volume transfer constant and the plasma volume showed no significant changes. CONCLUSION: Only after photon irradiation, DCE-MRI provides a temporal advantage in detecting early physiological signs in radiation-induced myelopathy compared to morphological MRI. As a generally lower level of vascular permeability after 12C-ions led to an earlier development of paresis as compared to photons, we conclude that other mechanisms dominate the development of paresis II.

Longitudinal MRI study after carbon ion and photon irradiation: shorter latency time for myelopathy is not associated with differential morphological changes.

BACKGROUND: Radiation-induced myelopathy is a severe and irreversible complication that occurs after a long symptom-free latency time if the spinal cord was exposed to a significant irradiation dose during tumor treatment. As carbon ions are increasingly investigated for tumor treatment in clinical trials, their effect on normal tissue needs further investigation to assure safety of patient treatments. Magnetic resonance imaging (MRI)-visible morphological alterations could serve as predictive markers for medicinal interventions to avoid severe side effects. Thus, MRI-visible morphological alterations in the rat spinal cord after high dose photon and carbon ion irradiation and their latency times were investigated. METHODS: Rats whose spinal cords were irradiated with iso-effective high photon (n = 8) or carbon ion (n = 8) doses as well as sham-treated control animals (n = 6) underwent frequent MRI measurements until they developed radiation-induced myelopathy (paresis II). MR images were analyzed for morphological alterations and animals were regularly tested for neurological deficits. In addition, histological analysis was performed of animals suffering from paresis II compared to controls. RESULTS: For both beam modalities, first morphological alterations occurred outside the spinal cord (bone marrow conversion, contrast agent accumulation in the musculature ventral and dorsal to the spinal cord) followed by morphological alterations inside the spinal cord (edema, syrinx, contrast agent accumulation) and eventually neurological alterations (paresis I and II). Latency times were significantly shorter after carbon ions as compared to photon irradiation. CONCLUSIONS: Irradiation of the rat spinal cord with photon or carbon ion doses that lead to 100% myelopathy induced a comparable fixed sequence of MRI-visible morphological alterations and neurological distortions. However, at least in the animal model used in this study, the observed MRI-visible morphological alterations in the spinal cord are not suited as predictive markers to identify animals that will develop myelopathy as the time between MRI-visible alterations and the occurrence of myelopathy is too short to intervene with protective or mitigative drugs.

Oxygen-Sensitive MRI: A Predictive Imaging Biomarker for Tumor Radiation Response?

PURPOSE: To develop a noninvasive prognostic imaging biomarker related to hypoxia to predict SABR tumor control. METHODS AND MATERIALS: A total of 145 subcutaneous syngeneic Dunning prostate R3327-AT1 rat tumors were focally irradiated once using cone beam computed tomography guidance on a small animal irradiator at 225 kV. Various doses in the range of 0 to 100 Gy were administered, while rats breathed air or oxygen, and tumor control was assessed up to 200 days. Oxygen-sensitive magnetic resonance imaging (MRI) (T1-weighted, ΔR1, ΔR2*) was applied to 79 of these tumors at 4.7 T to assess response to an oxygen gas breathing challenge on the day before irradiation as a probe of tumor hypoxia. RESULTS: Increasing radiation dose in the range of 0 to 90 Gy enhanced tumor control of air-breathing rats with a TCD50 estimated at 59.6 ± 1.5 Gy. Control was significantly improved at some doses when rats breathed oxygen during irradiation (eg, 40 Gy; P < .05), and overall there was a modest left shift in the control curve: TCD50(oxygen) = 53.1 ± 3.1 Gy (P < .05 vs air). Oxygen-sensitive MRI showed variable response to oxygen gas breathing challenge; the magnitude of T1-weighted signal response (%ΔSI) allowed stratification of tumors in terms of local control at 40 Gy. Tumors showing %ΔSI >0.922 with O2-gas breathing challenge showed significantly better control at 40 Gy during irradiation while breathing oxygen (75% vs 0%, P < .01). In addition, increased radiation dose (50 Gy) substantially overcame resistance, with 50% control for poorly oxygenated tumors. Stratification of dose-response curves based on %ΔSI >0.922 revealed different survival curves, with TCD50 = 36.2 ± 3.2 Gy for tumors responsive to oxygen gas breathing challenge; this was significantly less than the 54.7 ± 2.4 Gy for unresponsive tumors (P < .005), irrespective of the gas inhaled during tumor irradiation. CONCLUSIONS: Oxygen-sensitive MRI allowed stratification of tumors in terms of local control at 40 Gy, indicating its use as a potential predictive imaging biomarker. Increasing dose to 50 Gy overcame radiation resistance attributable to hypoxia in 50% of tumors.

The RBE in ion beam radiotherapy: In vivo studies and clinical application.

Ion beams used for radiotherapy exhibit an increased relative biological effectiveness (RBE), which depends on several physical treatment parameters as well as on biological factors of the irradiated tissues. While the RBE is an experimentally well-defined quantity, translation to patients is complex and requires radiobiological studies, dedicated models to calculate the RBE in treatment planning as well as strategies for dose prescription. Preclinical in vivo studies and analysis of clinical outcome are important to validate and refine RBE-models. This review describes the concept of the experimental and clinical RBE and explains the fundamental dependencies of the RBE based on in vitro experiments. The available preclinical in vivo studies on normal tissue and tumor RBE for ions heavier than protons are reviewed in the context of the historical and present development of ion beam radiotherapy. In addition, the role of in vivo RBE-values in the development and benchmarking of RBE-models as well as the transition of these models to clinical application are described. Finally, limitations in the translation of experimental RBE-values into clinical application and the direction of future research are discussed.

Effectiveness of fractionated carbon ion treatments in three rat prostate tumors differing in growth rate, differentiation and hypoxia.

PURPOSE: To quantify the fractionation dependence of carbon (12C) ions and photons in three rat prostate carcinomas differing in growth rate, differentiation and hypoxia. MATERIAL AND METHODS: Three sublines (AT1, HI, H) of syngeneic rat prostate tumors (R3327) were treated with six fractions of either 12C-ions or 6 MV photons. Dose-response curves were determined for the endpoint local tumor control within 300 days. The doses at 50% control probability (TCD50) and the relative biological effectiveness (RBE) of 12C-ions were calculated and compared with the values from single and split dose studies. RESULTS: Experimental findings for the three tumor sublines revealed (i) a comparably increased RBE (2.47-2.67), (ii) a much smaller variation of the radiation response for 12C-ions (TCD50: 35.8-43.7 Gy) than for photons (TCD50: 91.3-116.6 Gy), (iii) similarly steep (AT1) or steeper (HI, H) dose-response curves for 12C-ions than for photons, (iv) a larger fractionation effect for photons than for 12C-ions, and (v) a steeper increase of the RBE with decreasing fractional dose for the well-differentiated H- than for the less-differentiated HI- and AT1-tumors, reflected by (vi) the smallest α/ß-value for H-tumors after photon irradiation. CONCLUSION: 12C-ions reduce the radiation response heterogeneity between the three tumor sublines as well as within each subline relative to photon treatments, independently of fractionation. The dose dependence of the RBE varies between tumors of different histology. The results support the use of hypofractionated carbon ion treatments in radioresistant tumors.

High Doses of Photons and Carbon Ions Comparably Increase Vascular Permeability in R3327-HI Prostate Tumors: A Dynamic Contrast-Enhanced MRI Study.

Carbon- (12C-) ion radiotherapy exhibits enhanced biological effectiveness compared to photon radiotherapy, however, the contribution of its interaction with the vasculature remains debatable. The effect of high-dose 12C-ion and photon irradiation on vascular permeability in moderately differentiated rat prostate tumors was compared using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). Syngeneic R3327-HI rat prostate tumors were irradiated with a single dose of either 18 or 37 Gy 12C ions, or 37 or 75 Gy 6-MV photons (sub-curative and curative dose levels, respectively). DCE-MRI was performed one day prior to and 3, 7, 14 and 21 days postirradiation. Voxel-based tumor concentration-time curves were clustered based on their curve shape and treatment response was assessed as the longitudinal changes in the relative abundance per cluster. Radiation-induced vascular damage and increased permeability occurred at day 7 postirradiation for all treatment groups except for the 75 Gy photon-irradiated group, where the onset of vascular damage was delayed until day 14. No differences between irradiation modalities were found. Therefore, early vascular damage cannot explain the higher effectiveness of 12C ions relative to photons in terms of local tumor control for this moderately differentiated prostate tumor and the applied single high doses.

Ramipril reduces incidence and prolongates latency time of radiation-induced rat myelopathy after photon and carbon ion irradiation.

To test the hypothesis that the use of an angiotensin-converting enzyme inhibitor (ACEi) during radiotherapy may be ameliorative for treatment-related normal tissue damage, a pilot study was conducted with the clinically approved (ACE) inhibitor ramipril on the outcome of radiation-induced myelopathy in the rat cervical spinal cord model. Female Sprague Dawley rats were irradiated with single doses of either carbon ions (LET 45 keV/µm) at the center of a 6 cm spread-out Bragg peak (SOBP) or 6 MeV photons. The rats were randomly distributed into 4 experimental arms: (i) photons; (ii) photons + ramipril; (iii) carbon ions and (iv) carbon ions + ramipril. Ramipril administration (2 mg/kg/day) started directly after irradiation and was maintained during the entire follow-up. Complete dose-response curves were generated for the biological endpoint radiation-induced myelopathy (paresis grade II) within an observation time of 300 days. Administration of ramipril reduced the rate of paralysis at high dose levels for photons and for the first time a similar finding for high-LET particles was demonstrated, which indicates that the effect of ramipril is independent from radiation quality. The reduced rate of myelopathy is accompanied by a general prolongation of latency time for photons and for carbon ions. Although the already clinical approved drug ramipril can be considered as a mitigator of radiation-induced normal tissue toxicity in the central nervous system, further examinations of the underlying pathological mechanisms leading to radiation-induced myelopathy are necessary to increase and sustain its mitigative effectiveness.

Fractionated carbon ion irradiations of the rat spinal cord: comparison of the relative biological effectiveness with predictions of the local effect model.

BACKGROUND: To determine the relative biological effectiveness (RBE) and α/ß-values after fractionated carbon ion irradiations of the rat spinal cord with varying linear energy transfer (LET) to benchmark RBE-model calculations. MATERIAL AND METHODS: The rat spinal cord was irradiated with 6 fractions of carbon ions at 6 positions within a 6 cm spread-out Bragg-peak (SOBP, LET: 16-99 keV/µm). TD50-values (dose at 50% complication probability) were determined from dose-response curves for the endpoint radiation induced myelopathy (paresis grade II) within 300 days after irradiation. Based on TD50-values of 15 MV photons, RBE-values were calculated and adding previously published data, the LET and fractional dose-dependence of the RBE was used to benchmark the local effect model (LEM I and IV). RESULTS: At six fractions, TD50-values decreased from 39.1 ± 0.4 Gy at 16 keV/µm to 17.5 ± 0.3 Gy at 99 keV/µm and the RBE increased accordingly from 1.46 ± 0.05 to 3.26 ± 0.13. Experimental α/ß-ratios ranged from 6.9 ± 1.1 Gy to 44.3 ± 7.2 Gy and increased strongly with LET. Including all available data, comparison with model-predictions revealed that (i) LEM IV agrees better in the SOBP, while LEM I fits better in the entrance region, (ii) LEM IV describes the slope of the RBE within the SOBP better than LEM I, and (iii) in contrast to the strong LET-dependence, the RBE-deviations depend only weakly on fractionation within the measured range. CONCLUSIONS: This study extends the available RBE data base to significantly lower fractional doses and performes detailed tests of the RBE-models LEM I and IV. In this comparison, LEM IV agrees better with the experimental data in the SOBP than LEM I. While this could support a model replacement in treatment planning, careful dosimetric analysis is required for the individual patient to evaluate potential clinical consequences.

Impact of Single Dose Photons and Carbon Ions on Perfusion and Vascular Permeability: A Dynamic Contrast-Enhanced MRI Pilot Study in the Anaplastic Rat Prostate Tumor R3327-AT1.

We collected initial quantitative information on the effects of high-dose carbon (12C) ions compared to photons on vascular damage in anaplastic rat prostate tumors, with the goal of elucidating differences in response to high-LET radiation, using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). Syngeneic R3327-AT1 rat prostate tumors received a single dose of either 16 or 37 Gy 12C ions or 37 or 85 Gy 6 MV photons (iso-absorbed and iso-effective doses, respectively). The animals underwent DCE-MRI prior to, and on days 3, 7, 14 and 21 postirradiation. The extended Tofts model was used for pharmacokinetic analysis. At day 21, tumors were dissected and histologically examined. The results of this work showed the following: 1. 12C ions led to stronger vascular changes compared to photons, independent of dose; 2. Tumor growth was comparable for all radiation doses and modalities until day 21; 3. Nonirradiated, rapidly growing control tumors showed a decrease in all pharmacokinetic parameters (area under the curve, Ktrans, ve, vp) over time; 4. 12C-ion-irradiated tumors showed an earlier increase in area under the curve and Ktrans than photon-irradiated tumors; 5. 12C-ion irradiation resulted in more homogeneous parameter maps and histology compared to photons; and 6. 12C-ion irradiation led to an increased microvascular density and decreased proliferation activity in a largely dose-independent manner compared to photons. Postirradiation changes related to 12C ions and photons were detected using DCE-MRI, and correlated with histological parameters in an anaplastic experimental prostate tumor. In summary, this pilot study demonstrated that exposure to 12C ions increased the perfusion and/or permeability faster and led to larger changes in DCE-MRI parameters resulting in increased vessel density and presumably less hypoxia at the end of the observation period when compared to photons. Within this study no differences were found between curative and sub-curative doses in either modality.

Examining correlations of oxygen sensitive MRI (BOLD/TOLD) with [18F]FMISO PET in rat prostate tumors.

Hypoxia is regarded as a potential prognostic biomarker for tumor aggressiveness, progression, and response to therapy. The radiotracer 18F-fluoromisonidazole ([18F]FMISO) has been used with positron emission tomography (PET) to reveal tumor hypoxia. Meanwhile, blood oxygen level dependent (BOLD) MRI and tissue oxygen level dependent (TOLD) MRI offer insight into oxygenation based on endogenous signals without the need for radiolabels. Here, we compared BOLD and TOLD MRI with [18F]FMISO uptake using Dunning prostate R3327-AT1 tumor bearing rats. BOLD and TOLD MRI were acquired with respect to an oxygen gas breathing challenge. The following day, dynamic PET was performed up to 90 minutes following IV injection of [18F]FMISO. Tumors showed distinct heterogeneity based on each technique. Correlations were observed between magnitude of mean BOLD or TOLD MRI signal responses to oxygen-breathing challenge and initial distribution of [18F]FMISO. Correlations were observed for whole tumor as well on a regional basis with stronger correlations in the well perfused tumor periphery indicating the strong influence of perfused vasculature. After 90 minutes most correlations with signal intensity became quite weak, but correlations were observed between hypoxic fraction based on FMISO and fractions of tumor showing BOLD or TOLD response in a subset of tumors. This emphasizes the importance of considering regional heterogeneity and responsive fractions, as opposed to simple magnitudes of responses. Although the data represent a small cohort of tumors they present direct correlations between oxygen sensitive MRI and PET hypoxia reporter agents in the same tumors, indicating the potential utility of further investigations.

Intrinsic and extrinsic tumor characteristics are of minor relevance for the efficacy of split-dose carbon ion irradiation in three experimental prostate tumors.

OBJECTIVE: To quantify the impact of tumor-associated resistance factors on local tumor control after split doses of carbon (12C-) ions or photons in an experimental prostate tumor model. MATERIAL AND METHODS: Three sublines (AT1, H, HI) of syngeneic rat prostate tumors (R3327) differing in growth rate, differentiation and hypoxic status were irradiated with split doses of either 12C-ions or 6MV photons. Dose-response curves were determined for the endpoint local tumor control within 300 days. The relative biological effectiveness (RBE) of 12C-ions was calculated from the TCD50-values (dose at 50% control probability) of photons and 12C-ions. RESULTS: Experimental findings demonstrated: (i) The RBE was highest for the least differentiated AT1-tumor (2.39 ±â€¯0.16 (AT1) vs 2.06 ±â€¯0.11 (H) and 2.03 ±â€¯0.17 (HI)). (ii) TCD50-values between the three tumor sublines differed much less for 12C-ions (26.0-37.9 Gy) than for photons (53.7-90.6 Gy). (iii) While the slope of the dose-response curves for photons and 12C-ions were very similar for the AT1- and H-tumors, the histologically heterogeneous HI-tumor showed a shallow dose-response curve for photons, which is transformed into a steep dose-response curve after 12C-ion irradiation. CONCLUSION: The response to carbon ion irradiations is much less dependent on biological differences between and within the tumor-sublines. Tumors showing a high resistance against photon treatments, also exhibit the largest RBE for carbon ions. Carbon ions could therefore be of clinical advantage for the treatment of tumors with known resistance factors against photons.

Determination of the proton RBE in the rat spinal cord: Is there an increase towards the end of the spread-out Bragg peak?

BACKGROUND AND PURPOSE: To determine the relative biological effectiveness (RBE) of protons in the rat spinal cord as a function of linear energy transfer (LET) and dose. MATERIALS AND METHODS: The rat cervical spinal cord was irradiated with single or two equal fractions (split doses) of protons at four positions (LET 1.4-5.5 keV/µm) along a 6 cm spread-out Bragg peak (SOBP). From dose-response analysis, TD50- (dose at 50% effect probability) and RBE-values were derived using the endpoint of radiation-induced myelopathy. RESULTS: Along the SOBP, the TD50-values decreased from 21.7 ±â€¯0.3 Gy to 19.5 ±â€¯0.5 Gy for single and from 32.3 ±â€¯0.3 Gy to 27.9 ±â€¯0.5 Gy for split doses. The corresponding RBE-values increased from 1.13 ±â€¯0.04 to 1.26 ±â€¯0.05 (single doses) and from 1.06 ±â€¯0.02 to 1.23 ±â€¯0.03 (split doses). CONCLUSIONS: For the relative high fractional doses, the experimental RBE at the distal edge of the proton SOBP is moderately increased. The conventionally applied RBE of 1.1 appears to be valid for the mid-SOBP region, but the higher values occurring more distally could be of clinical significance, especially if critical structures are located in this area. Further in vivo studies at lower fractional doses are urgently required.

Late normal tissue response in the rat spinal cord after carbon ion irradiation.

BACKGROUND: The present work summarizes the research activities on radiation-induced late effects in the rat spinal cord carried out within the "clinical research group ion beam therapy" funded by the German Research Foundation (DFG, KFO 214). METHODS AND MATERIALS: Dose-response curves for the endpoint radiation-induced myelopathy were determined at 6 different positions (LET 16-99 keV/µm) within a 6 cm spread-out Bragg peak using either 1, 2 or 6 fractions of carbon ions. Based on the tolerance dose TD50 of carbon ions and photons, the relative biological effectiveness (RBE) was determined and compared with predictions of the local effect model (LEM I and IV). Within a longitudinal magnetic resonance imaging (MRI)-based study the temporal development of radiation-induced changes in the spinal cord was characterized. To test the protective potential of the ACE (angiotensin converting enzyme)-inhibitor ramipril™, an additional dose-response experiment was performed. RESULTS: The RBE-values increased with LET and the increase was found to be larger for smaller fractional doses. Benchmarking the RBE-values as predicted by LEM I and LEM IV with the measured data revealed that LEM IV is more accurate in the high-LET, while LEM I is more accurate in the low-LET region. Characterization of the temporal development of radiation-induced changes with MRI demonstrated a shorter latency time for carbon ions, reflected on the histological level by an increased vessel perforation after carbon ion as compared to photon irradiations. For the ACE-inhibitor ramipril™, a mitigative rather than protective effect was found. CONCLUSIONS: This comprehensive study established a large and consistent RBE data base for late effects in the rat spinal cord after carbon ion irradiation which will be further extended in ongoing studies. Using MRI, an extensive characterization of the temporal development of radiation-induced alterations was obtained. The reduced latency time for carbon ions is expected to originate from a dynamic interaction of various complex pathological processes. A dominant observation after carbon ion irradiation was an increase in vessel perforation preferentially in the white matter. To enable a targeted pharmacological intervention more details of the molecular pathways, responsible for the development of radiation-induced myelopathy are required.

Carbon ion radiotherapy: impact of tumor differentiation on local control in experimental prostate carcinomas.

BACKGROUND: To summarize the research activities of the "clinical research group heavy ion therapy", funded by the German Research Foundation (DFG, KFO 214), on the impact of intrinsic tumor characteristics (grading, hypoxia) on local tumor control after carbon (12C-) ion- and photon irradiations. METHODS: Three sublines of syngeneic rat prostate tumors (R3327) with various differentiation levels (highly (-H), moderately (-HI) or anaplastic (-AT1), (diameter 10 mm) were irradiated with 1, 2 and 6 fractions of either 12C-ions or 6 MV photons using increasing dose levels. Primary endpoint was local tumor control at 300 days. The relative biological effectiveness (RBE) of 12C-ions was calculated from TCD50-values (dose at 50% tumor control probability) of photons and 12C-ions and correlated with intrinsic tumor parameters. For the HI-subline, larger tumors (diameter 18 mm) were irradiated with either carbon ions, oxygen ions or photons under ambient as well as hypoxic conditions to determine the variability of the RBE under different oxygenation levels. In addition, imaging, histology and molecular analyses were performed to decipher the underlying mechanisms. RESULTS: Experimental results revealed (i) a smaller variation of the TCD50-values between the three tumor sublines for 12C-ions (23.6 - 32.9 Gy) than for photons (38.2 - 75.7 Gy), (ii) steeper dose-response curves for 12C-ions, and (iii) an RBE that increased with tumor grading (1.62 ± 0.11 (H) vs 2.08 ± 0.13 (HI) vs 2.30 ± 0.08 (AT1)). Large HI-tumors resulted in a marked increase of TCD50, which was increased further by 15% under hypoxic relative to oxic conditions. Noninvasive imaging, histology and molecular analyses identified hypoxia as an important radioresistance factor in photon therapy. CONCLUSIONS: The dose-response studies revealed a higher efficacy of 12C-ions relative to photon therapy in the investigated syngeneic tumor model. Hypoxia turned out to be at least one important radioresistance factor, which can be partly overridden by high-LET ion beams. This might be used to increase treatment effectiveness also in patients. The results of this project served as a starting point for several ongoing research projects.

RBE and related modeling in carbon-ion therapy.

Carbon ion therapy is a promising evolving modality in radiotherapy to treat tumors that are radioresistant against photon treatments. As carbon ions are more effective in normal and tumor tissue, the relative biological effectiveness (RBE) has to be calculated by bio-mathematical models and has to be considered in the dose prescription. This review (i) introduces the concept of the RBE and its most important determinants, (ii) describes the physical and biological causes of the increased RBE for carbon ions, (iii) summarizes available RBE measurements in vitro and in vivo, and (iv) describes the concepts of the clinically applied RBE models (mixed beam model, local effect model, and microdosimetric-kinetic model), and (v) the way they are introduced into clinical application as well as (vi) their status of experimental and clinical validation, and finally (vii) summarizes the current status of the use of the RBE concept in carbon ion therapy and points out clinically relevant conclusions as well as open questions. The RBE concept has proven to be a valuable concept for dose prescription in carbon ion radiotherapy, however, different centers use different RBE models and therefore care has to be taken when transferring results from one center to another. Experimental studies significantly improve the understanding of the dependencies and limitations of RBE models in clinical application. For the future, further studies investigating quantitatively the differential effects between normal tissues and tumors are needed accompanied by clinical studies on effectiveness and toxicity.

Effects of CTGF Blockade on Attenuation and Reversal of Radiation-Induced Pulmonary Fibrosis.

Background: Radiotherapy is a mainstay for the treatment of lung cancer that can induce pneumonitis or pulmonary fibrosis. The matricellular protein connective tissue growth factor (CTGF) is a central mediator of tissue remodeling. Methods: A radiation-induced mouse model of pulmonary fibrosis was used to determine if transient administration of a human antibody to CTGF (FG-3019) started at different times before or after 20 Gy thoracic irradiation reduced acute and chronic radiation toxicity. Mice (25 mice/group; 10 mice/group in a confirmation study) were examined by computed tomography, histology, gene expression changes, and for survival. In vitro experiments were performed to directly study the interaction of CTGF blockade and radiation. All statistical tests were two-sided. Results: Administration of FG-3019 prevented (∼50%-80%) or reversed (∼50%) lung remodeling, improved lung function, improved mouse health, and rescued mice from lethal irradiation ( P < .01). Importantly, when antibody treatment was initiated at 16 weeks after thoracic irradiation, FG-3019 reversed established lung remodeling and restored lung function. CTGF blockade abrogated M2 polarized macrophage influx, normalized radiation-induced gene expression changes, and reduced myofibroblast abundance and Osteopontin expression. Conclusion: These results indicate that blocking CTGF attenuates radiation-induced pulmonary remodeling and can reverse the process after initiation. CTGF has a central role in radiation-induced fibrogenesis, and FG-3019 may benefit patients with radiation-induced pulmonary fibrosis or patients with other forms or origin of chronic fibrotic diseases.