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

Development of 18F-Fluoromisonidazole Hypoxia PET/CT Diagnostic Interpretation Criteria and Validation of Interreader Reliability, Reproducibility, and Performance

Tumor hypoxia, an integral biomarker to guide radiotherapy, can be imaged with 18F-fluoromisonidazole (18F-FMISO) hypoxia PET. One major obstacle to its broader application is the lack of standardized interpretation criteria. We sought to develop and validate practical interpretation criteria and a dedicated training protocol for nuclear medicine physicians to interpret 18F-FMISO hypoxia PET. Methods: We randomly selected 123 patients with human papillomavirus-positive oropharyngeal cancer enrolled in a phase II trial who underwent 123 18F-FDG PET/CT and 134 18F-FMISO PET/CT scans. Four independent nuclear medicine physicians with no 18F-FMISO experience read the scans. Interpretation by a fifth nuclear medicine physician with over 2 decades of 18F-FMISO experience was the reference standard. Performance was evaluated after initial instruction and subsequent dedicated training. Scans were considered positive for hypoxia by visual assessment if 18F-FMISO uptake was greater than floor-of-mouth uptake. Additionally, SUVmax was determined to evaluate whether quantitative assessment using tumor-to-background ratios could be helpful to define hypoxia positivity. Results: Visual assessment produced a mean sensitivity and specificity of 77.3% and 80.9%, with fair interreader agreement (κ = 0.34), after initial instruction. After dedicated training, mean sensitivity and specificity improved to 97.6% and 86.9%, with almost perfect agreement (κ = 0.86). Quantitative assessment with an estimated best SUVmax ratio threshold of more than 1.2 to define hypoxia positivity produced a mean sensitivity and specificity of 56.8% and 95.9%, respectively, with substantial interreader agreement (κ = 0.66), after initial instruction. After dedicated training, mean sensitivity improved to 89.6% whereas mean specificity remained high at 95.3%, with near-perfect interreader agreement (κ = 0.86). Conclusion: Nuclear medicine physicians without 18F-FMISO hypoxia PET reading experience demonstrate much improved interreader agreement with dedicated training using specific interpretation criteria.

Combination of proton- or X-irradiation with anti-PDL1 immunotherapy in two murine oral cancers

Combining radiation therapy with immunotherapy is a strategy to improve both treatments. The purpose of this study was to compare responses for two syngeneic head and neck cancer (HNC) tumor models in mice following X-ray or proton irradiation with or without immune checkpoint inhibition (ICI). MOC1 (immunogenic) and MOC2 (less immunogenic) tumors were inoculated in the right hind leg of each mouse (C57BL/6J, n = 398). Mice were injected with anti-PDL1 (10 mg/kg, twice weekly for 2 weeks), and tumors were treated with single-dose irradiation (5-30 Gy) with X-rays or protons. MOC2 tumors grew faster and were more radioresistant than MOC1 tumors, and all mice with MOC2 tumors developed metastases. Irradiation reduced the tumor volume in a dose-dependent manner. ICI alone reduced the tumor volume for MOC1 with 20% compared to controls, while no reduction was seen for MOC2. For MOC1, there was a clear treatment synergy when combining irradiation with ICI for radiation doses above 5 Gy and there was a tendency for X-rays being slightly more biologically effective compared to protons. For MOC2, there was a tendency of protons being more effective than X-rays, but both radiation types showed a small synergy when combined with ICI. Although the responses and magnitudes of the therapeutic effect varied, the optimal radiation dose for maximal synergy appeared to be in the order of 10-15 Gy, regardless of tumor model.

High-LET charged particles: radiobiology and application for new approaches in radiotherapy

The number of patients treated with charged-particle radiotherapy as well as the number of treatment centers is increasing worldwide, particularly regarding protons. However, high-linear energy transfer (LET) particles, mainly carbon ions, are of special interest for application in radiotherapy, as their special physical features result in high precision and hence lower toxicity, and at the same time in increased efficiency in cell inactivation in the target region, i.e., the tumor. The radiobiology of high-LET particles differs with respect to DNA damage repair, cytogenetic damage, and cell death type, and their increased LET can tackle cells' resistance to hypoxia. Recent developments and perspectives, e.g., the return of high-LET particle therapy to the US with a center planned at Mayo clinics, the application of carbon ion radiotherapy using cost-reducing cyclotrons and the application of helium is foreseen to increase the interest in this type of radiotherapy. However, further preclinical research is needed to better understand the differential radiobiological mechanisms as opposed to photon radiotherapy, which will help to guide future clinical studies for optimal exploitation of high-LET particle therapy, in particular related to new concepts and innovative approaches. Herein, we summarize the basics and recent progress in high-LET particle radiobiology with a focus on carbon ions and discuss the implications of current knowledge for charged-particle radiotherapy. We emphasize the potential of high-LET particles with respect to immunogenicity and especially their combination with immunotherapy.

Carbon Ions for Hypoxic Tumors: Are We Making the Most of Them?

Hypoxia, which is associated with abnormal vessel growth, is a characteristic feature of many solid tumors that increases their metastatic potential and resistance to radiotherapy. Carbon-ion radiation therapy, either alone or in combination with other treatments, is one of the most promising treatments for hypoxic tumors because the oxygen enhancement ratio decreases with increasing particle LET. Nevertheless, current clinical practice does not yet fully benefit from the use of carbon ions to tackle hypoxia. Here, we provide an overview of the existing experimental and clinical evidence supporting the efficacy of C-ion radiotherapy in overcoming hypoxia-induced radioresistance, followed by a discussion of the strategies proposed to enhance it, including different approaches to maximize LET in the tumors.

Ultrasound-assisted carbon ion dosimetry and range measurement using injectable polymer-shelled phase-change nanodroplets: in vitro study

Methods allowing for in situ dosimetry and range verification are essential in radiotherapy to reduce the safety margins required to account for uncertainties introduced in the entire treatment workflow. This study suggests a non-invasive dosimetry concept for carbon ion radiotherapy based on phase-change ultrasound contrast agents. Injectable nanodroplets made of a metastable perfluorobutane (PFB) liquid core, stabilized with a crosslinked poly(vinylalcohol) shell, are vaporized at physiological temperature when exposed to carbon ion radiation (C-ions), converting them into echogenic microbubbles. Nanodroplets, embedded in tissue-mimicking phantoms, are exposed at 37 °C to a 312 MeV/u clinical C-ions beam at different doses between 0.1 and 4 Gy. The evaluation of the contrast enhancement from ultrasound imaging of the phantoms, pre- and post-irradiation, reveals a significant radiation-triggered nanodroplets vaporization occurring at the C-ions Bragg peak with sub-millimeter shift reproducibility and dose dependency. The specific response of the nanodroplets to C-ions is further confirmed by varying the phantom position, the beam range, and by performing spread-out Bragg peak irradiation. The nanodroplets' response to C-ions is influenced by their concentration and is dose rate independent. These early findings show the ground-breaking potential of polymer-shelled PFB nanodroplets to enable in vivo carbon ion dosimetry and range verification.

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/β-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.

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.

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.

Inhibition of Wnt signalling pathway by XAV939 enhances radiosensitivity in human cervical cancer HeLa cells

Cervical cancer is the second most common malignant tumour threatening women's health. In recent years, heavy-ion beam therapy is becoming a newly emerging therapeutic mean of cancer; however, radio-resistance and radiation-induced damage constitute the main obstacles for curative treatment of cervical cancer. Therefore, to identify the radiosensitizers is essential. Here, we investigated the effects of Wnt signalling pathway on the response of ¹²C⁶⁺ radiation in HeLa cells. XAV939, an inhibitor of Wnt signalling pathway, was added two hours before ¹²C⁶⁺ radiation.¹²C⁶⁺ radiation inhibited the viability of HeLa cells in a time-dependent manner, and inhibiting Wnt signalling using XAV939 significantly intensified this stress. Meanwhile, ¹²C⁶⁺ radiation induced a significant increased cell apoptosis, G2/M phase arrest, and the number of γ-H2AX foci. Supplementation with XAV939 significantly increased the effects induced by ¹²C⁶⁺ radiation alone. Combining XAV939 with ¹²C⁶⁺ irradiation, the expression of apoptotic genes (p53, Bax, Bcl-2) was significantly increased, while the expression of Wnt-related genes (Wnt3a, Wnt5a, β-catenin, cyclin D1 and c-Myc) was significantly decreased. Overall, these findings suggested that blockage of the Wnt/β-catenin pathway effectively sensitizes HeLa cells to ¹²C⁶⁺ irradiation, and it may be a potential therapeutic approach in terms of increasing the clinical efficacy of ¹²C⁶⁺ beams.

Carbon Ion Radiobiology

Radiotherapy using accelerated charged particles is rapidly growing worldwide. About 85% of the cancer patients receiving particle therapy are irradiated with protons, which have physical advantages compared to X-rays but a similar biological response. In addition to the ballistic advantages, heavy ions present specific radiobiological features that can make them attractive for treating radioresistant, hypoxic tumors. An ideal heavy ion should have lower toxicity in the entrance channel (normal tissue) and be exquisitely effective in the target region (tumor). Carbon ions have been chosen because they represent the best combination in this direction. Normal tissue toxicities and second cancer risk are similar to those observed in conventional radiotherapy. In the target region, they have increased relative biological effectiveness and a reduced oxygen enhancement ratio compared to X-rays. Some radiobiological properties of densely ionizing carbon ions are so distinct from X-rays and protons that they can be considered as a different "drug" in oncology, and may elicit favorable responses such as an increased immune response and reduced angiogenesis and metastatic potential. The radiobiological properties of carbon ions should guide patient selection and treatment protocols to achieve optimal clinical results.

Combination Therapy With Charged Particles and Molecular Targeting: A Promising Avenue to Overcome Radioresistance

Radiotherapy plays a central role in the treatment of cancer patients. Over the past decades, remarkable technological progress has been made in the field of conventional radiotherapy. In addition, the use of charged particles (e.g., protons and carbon ions) makes it possible to further improve dose deposition to the tumor, while sparing the surrounding healthy tissues. Despite these improvements, radioresistance and tumor recurrence are still observed. Although the mechanisms underlying resistance to conventional radiotherapy are well-studied, scientific evidence on the impact of charged particle therapy on cancer cell radioresistance is restricted. The purpose of this review is to discuss the potential role that charged particles could play to overcome radioresistance. This review will focus on hypoxia, cancer stem cells, and specific signaling pathways of EGFR, NFκB, and Hedgehog as well as DNA damage signaling involving PARP, as mechanisms of radioresistance for which pharmacological targets have been identified. Finally, new lines of future research will be proposed, with a focus on novel molecular inhibitors that could be used in combination with charged particle therapy as a novel treatment option for radioresistant tumors.

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, K_trans, v_e, v_p) over time; 4. ¹²C-ion-irradiated tumors showed an earlier increase in area under the curve and K_trans 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.

Bolus arrival time estimation in dynamic contrast-enhanced magnetic resonance imaging of small animals based on spline models

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is used to quantify perfusion and vascular permeability. In most cases a bolus arrival time (BAT) delay exists between the arterial input function (AIF) and the contrast agent arrival in the tissue of interest which needs to be estimated. Existing methods for BAT estimation are tailored to tissue concentration curves, which have a fast upslope to the peak as frequently observed in patient data. However, they may give poor results for curves that do not have this characteristic shape such as tissue concentration curves of small animals. In this paper, we propose a method for BAT estimation of signals that do not have a fast upslope to their peak. The model is based on splines which are able to adapt to a large variety of concentration curves. Furthermore, the method estimates BATs on a continuous time scale. All relevant model parameters are automatically determined by generalized cross validation. We use simulated concentration curves of small animal and patient settings to assess the accuracy and robustness of our approach. The proposed method outperforms a state-of-the-art method for small animal data and it gives competitive results for patient data. Finally, it is tested on in vivo acquired rat data where accuracy of BAT estimation was also improved upon the state-of-the-art method. The results indicate that the proposed method is suitable for accurate BAT estimation of DCE-MRI data, especially for small animals.

DNA damage response of clinical carbon ion versus photon radiation in human glioblastoma cells

BACKGROUND AND PURPOSE

Carbon ion radiotherapy is a promising therapeutic option for glioblastoma patients due to its high physical dose conformity and greater biological effectiveness than photons. However, the biological effects of carbon ion radiation are still incompletely understood. Here, we systematically compared the biological effects of clinically used carbon ion radiation to photon radiation with emphasis on DNA repair.

MATERIALS AND METHODS

Two human glioblastoma cell lines (U87 and LN229) were irradiated with carbon ions or photons and DNA damage response was systematically analyzed, including clonogenic survival, induction and repair of DNA double-strand breaks (DSBs), cell cycle arrest and apoptosis or autophagy. γH2AX foci were analyzed by flow cytometry, conventional light microscopy and 3D superresolution microscopy.

RESULTS

DSBs were repaired delayed and with slower kinetics after carbon ions versus photons. Carbon ions caused stronger and longer-lasting cell cycle delays, predominantly in G2 phase, and a higher rate of apoptosis. Compared to photons, the effectiveness of carbon ions was less cell cycle-dependent. Homologous recombination (HR) appeared to be more important for DSB repair after carbon ions versus photons in phosphatase and tensin homolog (PTEN)-deficient U87 cells, as opposed to PTEN-proficient LN229 cells.

CONCLUSION

Carbon ions induced more severe DSB damage than photons, which was repaired less efficiently in both cell lines. Thus, carbon ion radiotherapy may help to overcome resistance mechanisms of glioblastoma associated with DNA repair for example in combination with repair pathway-specific drugs in the context of personalized radiotherapy.

A comparison of mechanism-inspired models for particle relative biological effectiveness (RBE)

BACKGROUND AND SIGNIFICANCE

The application of heavy ion beams in cancer therapy must account for the increasing relative biological effectiveness (RBE) with increasing penetration depth when determining dose prescriptions and organ at risk (OAR) constraints in treatment planning. Because RBE depends in a complex manner on factors such as the ion type, energy, cell and tissue radiosensitivity, physical dose, biological endpoint, and position within and outside treatment fields, biophysical models reflecting these dependencies are required for the personalization and optimization of treatment plans.

AIM

To review and compare three mechanism-inspired models which predict the complexities of particle RBE for various ion types, energies, linear energy transfer (LET) values and tissue radiation sensitivities.

METHODS

The review of models and mechanisms focuses on the Local Effect Model (LEM), the Microdosimetric-Kinetic (MK) model, and the Repair-Misrepair-Fixation (RMF) model in combination with the Monte Carlo Damage Simulation (MCDS). These models relate the induction of potentially lethal double strand breaks (DSBs) to the subsequent interactions and biological processing of DSB into more lethal forms of damage. A key element to explain the increased biological effectiveness of high LET ions compared to MV x rays is the characterization of the number and local complexity (clustering) of the initial DSB produced within a cell. For high LET ions, the spatial density of DSB induction along an ion's trajectory is much greater than along the path of a low LET electron, such as the secondary electrons produced by the megavoltage (MV) x rays used in conventional radiation therapy. The main aspects of the three models are introduced and the conceptual similarities and differences are critiqued and highlighted. Model predictions are compared in terms of the RBE for DSB induction and for reproductive cell survival.

RESULTS AND CONCLUSIONS

Comparisons of the RBE for DSB induction and for cell survival are presented for proton (¹ H), helium (⁴ He), and carbon (¹² C) ions for the therapeutically most relevant range of ion beam energies. The reviewed models embody mechanisms of action acting over the spatial scales underlying the biological processing of potentially lethal DSB into more lethal forms of damage. Differences among the number and types of input parameters, relevant biological targets, and the computational approaches among the LEM, MK and RMF models are summarized and critiqued. Potential experiments to test some of the seemingly contradictory aspects of the models are discussed.

Clinical trials involving carbon-ion radiation therapy and the path forward

To describe the international landscape of clinical trials in carbon-ion radiotherapy (CIRT), the authors reviewed the current status of 63 ongoing clinical trials (median, 47 participants) involving CIRT identified from the US clinicaltrials.gov trial registry and the World Health Organization International Clinical Trials Platform Registry. The objectives were to evaluate the potential for these trials to define the role of this modality in the treatment of specific cancer types and identify the major challenges and opportunities to advance this technology. A significant body of literature suggested the potential for advantageous dose distributions and, in preclinical biologic studies, the enhanced effectiveness for CIRT compared with photons and protons. In addition, clinical evidence from phase I/II trials, although limited, indicated the potential for CIRT to improve cancer outcomes. However, current high-level phase III randomized clinical trial evidence does not exist. Although there has been an increase in the number of trials investigating CIRT since 2010, and the number of countries and sites offering CIRT is slowly growing, this progress has excluded other countries. Several recommendations are proposed to study this modality to accelerate progress in the field, including: 1) increasing the number of multinational randomized clinical trials, 2) leveraging the existing CIRT facilities to launch larger multinational trials directed at common cancers combined with high-level quality assurance; and 3) developing more compact and less expensive next-generation treatment systems integrated with radiobiologic research and preclinical testing.