To fractionate or not to fractionate? That is the question for the radiosurgery of hypoxic tumors

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.

Illuminating and Radiosensitizing Tumors with 2DG-Bound Gold-Based Nanomedicine for Targeted CT Imaging and Therapy

Although radiotherapy is one of the most important curative treatments for cancer, its clinical application is associated with undesired therapeutic effects on normal or healthy tissues. The use of targeted agents that can simultaneously achieve therapeutic and imaging functions could constitute a potential solution. Herein, we developed 2-deoxy-d-glucose (2DG)-labeled poly(ethylene glycol) (PEG) gold nanodots (2DG-PEG-AuD) as a tumor-targeted computed tomography (CT) contrast agent and radiosensitizer. The key advantages of the design are its biocompatibility and targeted AuD with excellent sensitivity in tumor detection via avid glucose metabolism. As a consequence, CT imaging with enhanced sensitivity and remarkable radiotherapeutic efficacy could be attained. Our synthesized AuD displayed linear enhancement of CT contrast as a function of its concentration. In addition, 2DG-PEG-AuD successfully demonstrated significant augmentation of CT contrast in both in vitro cell studies and in vivo tumor-bearing mouse models. In tumor-bearing mice, 2DG-PEG-AuD showed excellent radiosensitizing functions after intravenous injection. Results from this work indicate that 2DG-PEG-AuD could greatly potentiate theranostic capabilities by providing high-resolution anatomical and functional images in a single CT scan and therapeutic capability.

Physics and biomedical challenges of cancer therapy with accelerated heavy ions

Radiotherapy should have low toxicity in the entrance channel (normal tissue) and be very effective in cell killing in the target region (tumour). In this regard, ions heavier than protons have both physical and radiobiological advantages over conventional X-rays. Carbon ions represent an excellent combination of physical and biological advantages. There are a dozen carbon-ion clinical centres in Europe and Asia, and more under construction or at the planning stage, including the first in the USA. Clinical results from Japan and Germany are promising, but a heated debate on the cost-effectiveness is ongoing in the clinical community, owing to the larger footprint and greater expense of heavy ion facilities compared with proton therapy centres. We review here the physical basis and the clinical data with carbon ions and the use of different ions, such as helium and oxygen. Research towards smaller and cheaper machines with more effective beam delivery is necessary to make particle therapy affordable. The potential of heavy ions has not been fully exploited in clinics and, rather than there being a single 'silver bullet', different particles and their combination can provide a breakthrough in radiotherapy treatments in specific cases.

Genomic and Transcriptomic Profiling of Brain Metastases

Simple Summary

Brain metastases (BM) are the most common brain tumors in adults and are the main cause of cancer-associated death. Omics analysis of BM will allow for a better understanding of metastatic progression, prognosis and therapeutic targeting. In this study, BM samples underwent comprehensive molecular profiling with genomics and transcriptomics. Mutational signatures suggested that most mutations were gained prior to metastasis. A novel copy number event centered around the MCL1 gene was found in 75% of all samples. Transcriptomics revealed that melanoma BM formed a distinct cluster in comparison to other subtypes. Poor survival correlated to self-identified black race and absence of radiation treatment but not molecular profiles. These data identify potential new drivers of brain metastatic progression, implicate that melanoma BM are distinctive and likely responsive to unique therapies, and further investigation of sociodemographic and clinical features are needed in BM cohorts.

Abstract

Brain metastases (BM) are the most common brain tumors in adults occurring in up to 40% of all cancer patients. Multi-omics approaches allow for understanding molecular mechanisms and identification of markers with prognostic significance. In this study, we profile 130 BM using genomics and transcriptomics and correlate molecular characteristics to clinical parameters. The most common tumor origins for BM were lung (40%) followed by melanoma (21%) and breast (15%). Melanoma and lung BMs contained more deleterious mutations than other subtypes (p < 0.001). Mutational signatures suggested that the bulk of the mutations were gained before metastasis. A novel copy number event centered around the MCL1 gene was found in 75% of all samples, suggesting a broader role in promoting metastasis. Unsupervised hierarchical cluster analysis of transcriptional signatures available in 65 samples based on the hallmarks of cancer revealed four distinct clusters. Melanoma samples formed a distinctive cluster in comparison to other BM subtypes. Characteristics of molecular profiles did not correlate with survival. However, patients with self-identified black race or those who did not receive radiation correlated with poor survival. These data identify potential new drivers of brain metastatic progression. Our data also suggest further investigation of sociodemographic and clinical features is needed in BM cohorts.

Possible Overcoming of Tumor Hypoxia with Adaptive Hypofractionated Radiosurgery of Large Brain Metastases: A Biological Modeling Study

OBJECTIVE

The present biological modeling study evaluated possible application of adaptive hypofractionated stereotactic radiosurgery (HSRS), which involves escalation of the prescription dose according to the gradual decrease in the tumor volume between treatment sessions separated by 2- to 3-week intervals, in the management of large brain metastases.

METHODS

To investigate the effects of dose escalation during three-stage adaptive HSRS, a generalized biologically effective dose (gBED) model was applied. Accounting for both a nonuniform dose distribution inside the target and tumor hypoxia was implemented, and normal brain radiation dose distributions were assessed.

RESULTS

In comparison with conventional three-stage HSRS (with an identical prescription dose of 10 Gy at each treatment session), adaptive HSRS resulted in a 30-40% increase in gBED. This effect was especially prominent in late-responding targets (with α/β ratios from 3 to 10 Gy) and in neoplasms containing a high percentage of hypoxic cells. Despite dose escalation in the target, irradiation of the adjacent normal brain tissue was kept within safe limits at a level similar to that applied in conventional three-stage HSRS.

CONCLUSION

Adaptive HSRS theoretically results in significant enhancement of gBED in the target and may possibly overcome resistance to irradiation, which is caused by tumor hypoxia. These advantages may translate into higher treatment efficacy in cases of large brain metastases.

Current Evidence for Stereotactic Body Radiotherapy in Lung Metastases

Lung metastases are the second most common malignant neoplasms of the lung. It is estimated that 20–54% of cancer patients have lung metastases at some point during their disease course, and at least 50% of cancer-related deaths occur at this stage. Lung metastases are widely accepted to be oligometastatic when five lesions or less occur separately in up to three organs. Stereotactic body radiation therapy (SBRT) is a noninvasive, safe, and effective treatment for metastatic lung disease in carefully selected patients. There is no current consensus on the ideal dose and fractionation for SBRT in lung metastases, and it is the subject of study in ongoing clinical trials, which examines different locations in the lung (central and peripheral). This review discusses current indications, fractionations, challenges, and technical requirements for lung SBRT.

Carbon Ion Radiobiology

Simple Summary

Radiotherapy with carbon ions has been used for over 20 years in Asia and Europe and is now planned in the USA. The physics advantages of carbon ions compared to X-rays are similar to those of protons, but their radiobiological features are quite distinct and may lead to a breakthrough in the treatment of some cancers characterized by high mortality.

Abstract

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.

FSRT vs. SRS in Brain Metastases-Differences in Local Control and Radiation Necrosis-A Volumetric Study

Background: While the role of stereotactic radiotherapy for brain metastases is increasing, evidence on the comparative efficacy and safety of fractionated stereotactic radiotherapy (FSRT) and single-session radiosurgery (SRS) is scarce.

Methods: Longitudinal volumetric analysis was performed in a consecutive cohort of 120 patients and 190 brain metastases (>0.065 cm³ in volume / > ~5 mm in diameter) treated exclusively with FSRT (n = 98) and SRS (n = 92), respectively. A total of 972 tumor segmentations was used, averaging 5.1 time points per metastasis. Progression was defined using a volumetric extension of the RANO-BM criteria. Local control and radionecrosis were compared for lesions treated with FSRT and SRS, respectively.

Results: Metastases treated with FSRT were significantly larger at baseline (mean, 4.66 vs. 0.40 cm³, p < 0.001). Biologically effective dose (BED) for metastases (α/β = 12, linear-quadratic-cubic model) was significantly associated with local control, whereas BED for normal brain (α/β = 2, linear-quadratic model) was significantly associated with radionecrosis. Median time to local progression was 22.9 months in the FSRT group compared to 14.5 months in the SRS group (p = 0.022). Overall radionecrosis rate at 12 months was 3.4% for FSRT and 14.8% for SRS (p = 0.010). Radionecrosis °IV requiring resection with histologic proof of radiation necrosis also was significantly reduced in the FSRT group (FSRT 0.0% vs. SRS 3.9%, p = 0.041). In multivariate analysis, FSRT was associated with reduced risk of progression (HR 0.47, p = 0.015) and reduced risk of radionecrosis (HR 0.18, p = 0.045).

Conclusions: This volumetric study provides initial evidence that the improvements in therapeutic ratio expected for FSRT in larger brain metastases, might equally extend into the domain of smaller metastases, traditionally less considered for fractionated treatment. FSRT might constitute an important tool to further increase local control and reduce radionecrosis risk in stereotactic radiotherapy for brain metastases, that should be assessed in randomized intervention trials.

Annealing-modulated nanoscintillators for nonconventional X-ray activation of comprehensive photodynamic effects in deep cancer theranostics

Photodynamic therapy (PDT), which involves the generation of reactive oxygen species (ROS) through interactions of a photosensitizer (PS) with light and oxygen, has been applied in oncology. Over the years, PDT techniques have been developed for the treatment of deep-seated cancers. However, (1) the tissue penetration limitation of excitation photon, (2) suppressed efficiency of PS due to multiple energy transfers, and (3) insufficient oxygen source in hypoxic tumor microenvironment still constitute major challenges facing the clinical application of PDT for achieving effective treatment. We present herein a PS-independent, ionizing radiation-induced PDT agent composed of yttrium oxide nanoscintillators core and silica shell (Y₂O₃:Eu@SiO₂) with an annealing process. Our results revealed that annealed Y₂O₃:Eu@SiO₂ could directly induce comprehensive photodynamic effects under X-ray irradiation without the presence of PS molecules. The crystallinity of Y₂O₃:Eu@SiO₂ was demonstrated to enable the generation of electron-hole (e^--h⁺) pairs in Y₂O₃ under ionizing irradiation, giving rise to the formation of ROS including superoxide, hydroxyl radical and singlet oxygen. In particular, combining Y₂O₃:Eu@SiO₂ with fractionated radiation therapy increased radio-resistant tumor cell damage. Furthermore, photoacoustic imaging of tumors showed re-distribution of oxygen saturation (_SO₂) and reoxygenation of the hypoxia region. The results of this study support applicability of the integration of fractionated radiation therapy with Y₂O₃:Eu@SiO₂, achieving synchronously in-depth and oxygen-insensitive X-ray PDT. Furthermore, we demonstrate Y₂O₃:Eu@SiO₂ exhibited radioluminescence (RL) under X-ray irradiation and observed the virtually linear correlation between X-ray-induced radioluminescence (X-RL) and the Y₂O₃:Eu@SiO₂ concentration in vivo. With the pronounced X-RL for in-vivo imaging and dosimetry, it possesses significant potential for utilization as a precision theranostics producing highly efficient X-ray PDT for deep-seated tumors.

Approaches to combat hypoxia in cancer therapy and the potential for in silico models in their evaluation

AIM

The negative impact of tumour hypoxia on cancer treatment outcome has been long-known, yet there has been little success combating it. This paper investigates the potential role of in silico modelling to help test emerging hypoxia-targeting treatments in cancer therapy.

METHODS

A Medline search was undertaken on the current landscape of in silico models that simulate cancer therapy and evaluate their ability to test hypoxia-targeting treatments. Techniques and treatments to combat tumour hypoxia and their current challenges are also presented.

RESULTS

Hypoxia-targeting treatments include tumour reoxygenation, hypoxic cell radiosensitization with nitroimidazoles, hypoxia-activated prodrugs and molecular targeting. Their main challenges are toxicity and not achieving adequate delivery to hypoxic regions of the tumour. There is promising research toward combining two or more of these techniques. Different types of in silico therapy models have been developed ranging from temporal to spatial and from stochastic to deterministic models. Numerous models have compared the effectiveness of different radiotherapy fractionation schedules for controlling hypoxic tumours. Similarly, models could help identify and optimize new treatments for overcoming hypoxia that utilize novel hypoxia-targeting technology.

CONCLUSION

Current therapy models should attempt to incorporate more sophisticated modelling of tumour angiogenesis/vasculature and vessel perfusion in order to become more useful for testing hypoxia-targeting treatments, which typically rely upon the tumour vasculature for delivery of additional oxygen, (pro)drugs and nanoparticles.

Impact of SBRT fractionation in hypoxia dose painting - Accounting for heterogeneous and dynamic tumor oxygenation

PURPOSE

Tumor hypoxia, often found in nonsmall cell lung cancer (NSCLC), implies an increased resistance to radiotherapy. Pretreatment assessment of tumor oxygenation is, therefore, warranted in these patients, as functional imaging of hypoxia could be used as a basis for dose painting. This study aimed at investigating the feasibility of using a method for calculating the dose required in hypoxic subvolumes segmented on ¹⁸ F-HX4 positron emission tomography (PET) imaging of NSCLC.

METHODS

Positron emission tomography imaging data based on the hypoxia tracer ¹⁸ F-HX4 of 19 NSCLC patients were included in the study. Normalized tracer uptake was converted to oxygen partial pressure (pO₂ ) and hypoxic target volumes (HTVs) were segmented using a threshold of 10 mmHg. Uniform doses required to overcome the hypoxic resistance in the target volumes were calculated based on a previously proposed method taking into account the effect of interfraction reoxygenation, for fractionation schedules ranging from extremely hypofractionated stereotactic body radiotherapy (SBRT) to conventionally fractionated radiotherapy.

RESULTS

Gross target volumes ranged between 6.2 and 859.6 cm³ , and the hypoxic fraction < 10 mmHg between 1.2% and 72.4%. The calculated doses for overcoming the resistance of cells in the HTVs were comparable to those currently prescribed in clinical practice as well as those previously tested in feasibility studies on dose escalation in NSCLC. Depending on the size of the HTV and the distribution of pO₂ , HTV doses were calculated as 43.6-48.4 Gy for a three-fraction schedule, 51.7-57.6 Gy for five fractions, and 59.5-66.4 Gy for eight fractions. For patients in whom the HTV pO₂ distribution was more favorable, a lower dose was required despite a bigger volume. Tumor control probability was lower for single-fraction schedules, while higher levels of tumor control probability were found for schedules employing several fractions.

CONCLUSIONS

The method to account for heterogeneous and dynamic hypoxia in target volume segmentation and dose prescription based on ¹⁸ F-HX4-PET imaging appears feasible in NSCLC patients. The distribution of oxygen partial pressure within HTV could impact the required prescribed dose more than the size of the volume.

Postoperative local fractionated radiotherapy for resected single brain metastases

PURPOSE

Evaluation of postoperative fractionated local 3D-conformal radiotherapy (3DRT) of the resection cavity in brain metastases.

PATIENTS AND METHODS

Between 2011 and 2016, 57 patients underwent resection of a single, previously untreated (37/57, 65%) or recurrent (20/57, 35%) brain metastasis (median maximal diameter 3.5 cm [1.1-6.5 cm]) followed by 3DRT. For definition of the gross tumor volume (GTV), the resection cavity was used and for the clinical target volume (CTV), margins of 1.0-1.5 cm were added. Median dose was 48.0 Gy (30.0-50.4 Gy) in 25 (10-28) fractions; most patients had 36.0-42.0 Gy in 3.0 Gy fractions (n = 16, EQD2_10Gy 39.0-45.5 Gy) or 40.0-50.4 Gy in 1.8-2.0 Gy fractions (n = 37, EQD2_10Gy 39.3-50.0 Gy).

RESULTS

Median follow-up was 18 months. Local control rates were 83% at 1 year and 78% at 2 years and were significantly influenced by histology (breast cancer 100%, non-small lung cancer 87%, melanoma 80%, colorectal cancer 26% at 2 years, p = 0.006) and resection status (p < 0.0001), but not by EQD2_10Gy or size of the planning target volume (median 96.7 ml [16.7-282.8 ml]). At 1 and 2 years, 74% and 52% of the patients were free from distant brain metastases. Salvage procedures were applied in 25/27 (93%) of recurrent patients. Survival was 68% at 1 year and 41% at 2 years and was significantly improved in younger patients (p = 0.006) with higher Karnofsky performance score (p < 0.0001) and without prior radiotherapy (54% vs. 9% at 2 years, p = 0.006). No cases of radiographic or symptomatic radionecrosis were observed.

CONCLUSION

Adjuvant fractionated local 3DRT is highly effective in radiosensitive, completely resected metastases and should be considered for treating large resection cavities as an alternative to postoperative stereotactic single dose or hypofractionated radiosurgery.

Improved effectiveness of stereotactic radiosurgery in large brain metastases by individualized isotoxic dose prescription: an in silico study

INTRODUCTION

In large brain metastases (BM) with a diameter of more than 2 cm there is an increased risk of radionecrosis (RN) with standard stereotactic radiosurgery (SRS) dose prescription, while the normal tissue constraint is exceeded. The tumor control probability (TCP) with a single dose of 15 Gy is only 42%. This in silico study tests the hypothesis that isotoxic dose prescription (IDP) can increase the therapeutic ratio (TCP/Risk of RN) of SRS in large BM.

MATERIALS AND METHODS

A treatment-planning study with 8 perfectly spherical and 46 clinically realistic gross tumor volumes (GTV) was conducted. The effects of GTV size (0.5-4 cm diameter), set-up margins (0, 1, and 2 mm), and beam arrangements (coplanar vs non-coplanar) on the predicted TCP using IDP were assessed. For single-, three-, and five-fraction IDP dose-volume constraints of V_12Gy = 10 cm³, V_19.2 Gy = 10 cm³, and a V_20Gy = 20 cm³, respectively, were used to maintain a low risk of radionecrosis.

RESULTS

In BM of 4 cm in diameter, the maximum achievable single-fraction IDP dose was 14 Gy compared to 15 Gy for standard SRS dose prescription, with respective TCPs of 32 and 42%. Fractionated SRS with IDP was needed to improve the TCP. For three- and five-fraction IDP, a maximum predicted TCP of 55 and 68% was achieved respectively (non-coplanar beams and a 1 mm GTV-PTV margin).

CONCLUSIONS

Using three-fraction or five-fraction IDP the predicted TCP can be increased safely to 55 and 68%, respectively, in large BM with a diameter of 4 cm with a low risk of RN. Using IDP, the therapeutic ratio of SRS in large BM can be increased compared to current SRS dose prescription.

Challenges of Using High-Dose Fractionation Radiotherapy in Combination Therapy

Radiotherapy is crucial and substantially contributes to multimodal cancer treatment. The combination of conventional fractionation radiotherapy (CFRT) and systemic therapy has been established as the standard treatment for many cancer types. With advances in linear accelerators and image-guided techniques, high-dose fractionation radiotherapy (HFRT) is increasingly introduced in cancer centers. Clinicians are currently integrating HFRT into multimodality treatment. The shift from CFRT to HFRT reveals different effects on the tumor microenvironment and responses, particularly the immune response. Furthermore, the combination of HFRT and drugs yields different results in different types of tumors or using different treatment schemes. We have reviewed clinical trials and preclinical evidence on the combination of HFRT with drugs, such as chemotherapy, targeted therapy, and immune therapy. Notably, HFRT apparently enhances tumor cell killing and antigen presentation, thus providing opportunities and challenges in treating cancer.