The controversial abscopal effect

Exploiting sensitization windows of opportunity in hyper and hypo-fractionated radiation therapy

In contrast to the conventional radiotherapy/chemoradiotherapy paradigms used in the treatment of majority of cancer types, this review will describe two areas of radiobiology, hyperfractionated and hypofractionated radiation therapy, for cancer treatment focusing on application of novel concepts underlying these treatment modalities. The initial part of the review discusses the phenomenon of hyper-radiation sensitivity (HRS) at lower doses (0.1 to 0.6 Gy), describing the underlying mechanisms and how this could enhance the effects of chemotherapy, particularly, in hyperfractionated settings. The second part examines the radiobiological/physiological mechanisms underlying the effects of high-dose hypofractionated radiation therapy that can be exploited for tumor cure. These include abscopal/bystander effects, activation of immune system, endothelial cell death and effect of hypoxia with re-oxygenation. These biological properties along with targeted dose delivery and distribution to reduce normal tissue toxicity may make high-dose hypofractionation more effective than conventional radiation therapy for treatment of advanced cancers. The novel radiation physics based methods that take into consideration the tumor volume to be irradiated and normal tissue avoidance/tolerance can further improve treatment outcome and post-treatment quality of life. In conclusion, there is enough evidence to further explore novel avenues to exploit biological mechanisms from hyper-fractionation by enhancing the efficacy of chemotherapy and hypo-fractionated radiation therapy that could enhance tumor control and use imaging and technological advances to reduce toxicity.

Indirect application of near infrared light induces neuroprotection in a mouse model of parkinsonism - an abscopal neuroprotective effect

We have previously shown near infrared light (NIr), directed transcranially, mitigates the loss of dopaminergic cells in MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-treated mice, a model of parkinsonism. These findings complement others suggesting NIr treatment protects against damage from various insults. However one puzzling feature of NIr treatment is that unilateral exposure can lead to a bilateral healing response, suggesting NIr may have 'indirect' protective effects. We investigated whether remote NIr treatment is neuroprotective by administering different MPTP doses (50-, 75-, 100-mg/kg) to mice and treating with 670-nm light directed specifically at either the head or body. Our results show that, despite no direct irradiation of the damaged tissue, remote NIr treatment produces a significant rescue of tyrosine hydroxylase-positive cells in the substantia nigra pars compacta at the milder MPTP dose of 50-mg/kg (∼30% increase vs sham-treated MPTP mice, p<0.05). However this protection did not appear as robust as that achieved by direct irradiation of the head (∼50% increase vs sham-treated MPTP mice, p<0.001). There was no quantifiable protective effect of NIr at higher MPTP doses, irrespective of the delivery mode. Astrocyte and microglia cell numbers in substantia nigra pars compacta were not influenced by either mode of NIr treatment. In summary, the findings suggest that treatment of a remote tissue with NIr is sufficient to induce protection of the brain, reminiscent of the 'abscopal effect' sometimes observed in radiation treatment of metastatic cancer. This discovery has implications for the clinical translation of light-based therapies, providing an improved mode of delivery over transcranial irradiation.

Radiation for Awakening the Dormant Immune System, a Promising Challenge to be Explored

Recent advances that have been made in our understanding of cancer biology and immunology show that infiltrated immune cells and cytokines in the tumor microenvironment may play different functions that appear tightly related to clinical outcomes. Strategies aimed at interfering with the cross-talk between microenvironment tumor cells and their cellular partners have been considered for the development of new immunotherapies. These novel therapies target different cell components of the tumor microenvironment and importantly, they may be coupled and boosted with classical treatments, such as radiotherapy. In this work, we try to summarize recent data on the microenvironment impact of radiation therapy, from pre-clinical research to the clinic, while taking into account that this new knowledge will probably translate into indication and objective of radiation therapy changes in the next future.

Radiation for Awakening the Dormant Immune System, a Promising Challenge to be Explored

Recent advances that have been made in our understanding of cancer biology and immunology show that infiltrated immune cells and cytokines in the tumor microenvironment may play different functions that appear tightly related to clinical outcomes. Strategies aimed at interfering with the cross-talk between microenvironment tumor cells and their cellular partners have been considered for the development of new immunotherapies. These novel therapies target different cell components of the tumor microenvironment and importantly, they may be coupled and boosted with classical treatments, such as radiotherapy. In this work, we try to summarize recent data on the microenvironment impact of radiation therapy, from pre-clinical research to the clinic, while taking into account that this new knowledge will probably translate into indication and objective of radiation therapy changes in the next future.

Responses to ionizing radiation mediated by inflammatory mechanisms

Since the early years of the twentieth century, the biological consequences of exposure to ionizing radiation have been attributed solely to mutational DNA damage or cell death induced in irradiated cells at the time of exposure. However, numerous observations have been at variance with this dogma. In the 1950s, attention was drawn to abscopal effects in areas of the body not directly irradiated. In the 1960s reports began appearing that plasma factors induced by irradiation could affect unirradiated cells, and since 1990 a growing literature has documented an increased rate of DNA damage in the progeny of irradiated cells many cell generations after the initial exposure (radiation-induced genomic instability) and responses in non-irradiated cells neighbouring irradiated cells (radiation-induced bystander effects). All these studies have in common the induction of effects not in directly irradiated cells but in unirradiated cells as a consequence of intercellular signalling. Recently, it has become clear that all the various effects demonstrated in vivo may reflect an ongoing inflammatory response to the initial radiation-induced injury that, in a genotype-dependent manner, has the potential to contribute primary and/or ongoing damage displaced in time and/or space from the initial insult. Importantly, there is direct evidence that non-steroidal anti-inflammatory drug treatment reduces such damage in vivo. These new findings highlight the importance of tissue responses and indicate additional mechanisms of radiation action, including the likelihood that radiation effects are not restricted to the initiation stage of neoplastic diseases, but may also contribute to tumour promotion and progression. The various developments in understanding the responses to radiation exposures have implications not only for radiation pathology but also for therapeutic interventions.

Abscopal effect of radiation therapy: Interplay between radiation dose and p53 status

PURPOSE

This study investigates whether the abscopal effect induced by radiation-therapy (RT) is able to sterilize non-irradiated tumour cells through bystander signals.

MATERIAL AND METHODS

Wild-type (wt)-p53 or p53-null HCT116 human colon cancer cells were xenografted into both flanks of athymic female nude mice. When tumours reached a volume of 0.2 cm(3), irradiation was performed, under strict dose monitoring, with a dedicated mobile accelerator designed for intra-Operative-RT (IORT). A dose of 10 or 20 Gy (IR groups), delivered by a 10 MeV electron beam, was delivered to a tumour established in one side flank, leaving the other non-irradiated (NIR groups). A subset of mice were sacrificed early on to carry out short-term molecular analyses.

RESULTS

All directly-irradiated tumours, showed a dose-dependent delayed and reduced regrowth, independent of the p53 status. Importantly, a significant effect on tumour-growth inhibition was also demonstrated in NIR wt-p53 tumours in the 20 Gy-irradiation group, with a moderate effect also evident after 10 Gy-irradiation. In contrast, no significant difference was observed in the NIR p53-null tumours, independent of the dose delivered. Molecular analyses indicate that p53-dependent signals might be responsible for the abscopal effect in our model system, via a pro-apoptotic pathway.

CONCLUSIONS

We suggest that the interplay between delivered dose and p53 status might help to sterilize out-of-field tumour cells.

ER stress, autophagy and immunogenic cell death in photodynamic therapy-induced anti-cancer immune responses

Tumours are a form of pseudo-organs with their own microenvironment where the cancer cells nurture a dysfunctional immune environment incapable of inciting anti-tumour immunity. It had been proposed that the only way to counteract such an immune system dysfunction in tumours is by eliciting, therapeutically, a cancer cell death pathway that is accompanied by high immunogenicity and possibly inhibits or reduces the influence of the pro-tumourigenic cytokine signalling. Subsequently, a small and a large-scale screening study as well as several targeted studies found that few, selected anticancer therapeutic regimens are able to induce a promising kind of cancer cell demise called immunogenic cell death (ICD), which can activate the immune system owing to the spatiotemporally defined emission of danger signals. Recently, photodynamic therapy (PDT) utilizing the photosensitiser, hypericin (Hyp), became the first PDT paradigm characterized to be capable of inducing bona fide ICD. In the present perspective, we discuss the various technical, conceptual, and molecular advancements and unprecedented results revealed by Hyp-PDT that have influenced the fields of ICD, ER stress biology, cancer cell death, anti-cancer immune responses, photoimmunology and PDT.

Transmission of signals from rats receiving high doses of microbeam radiation to cage mates: an inter-mammal bystander effect

Inter-animal signaling from irradiated to non-irradiated organisms has been demonstrated for whole body irradiated mice and also for fish. The aim of the current study was to look at radiotherapy style limited exposure to part of the body using doses relevant in preclinical therapy. High dose homogenous field irradiation and the use of irradiation in the microbeam radiation therapy mode at the European Synchrotron Radiation Facility (ESRF) at Grenoble was tested by giving high doses to the right brain hemisphere of the rat. The right and left cerebral hemispheres and the urinary bladder were later removed to determine whether abscopal effects could be produced in the animals and also whether effects occurred in cage mates housed with them. The results show strong bystander signal production in the contra-lateral brain hemisphere and weaker effects in the distant bladder of the irradiated rats. Signal strength was similar or greater in each tissue in the cage mates housed for 48hrs with the irradiated rats. Our results support the hypothesis that proximity to an irradiated animal induces signalling changes in an unirradiated partner. If similar signaling occurs between humans, the results could have implications for caregivers and hospital staff treating radiotherapy patients.

Bystander effects as manifestation of intercellular communication of DNA damage and of the cellular oxidative status

It is becoming increasingly clear that cells exposed to ionizing radiation (IR) and other genotoxic agents (targeted cells) can communicate their DNA damage response (DDR) status to cells that have not been directly irradiated (bystander cells). The term radiation-induced bystander effects (RIBE) describes facets of this phenomenon, but its molecular underpinnings are incompletely characterized. Consequences of DDR in bystander cells have been extensively studied and include transformation and mutation induction; micronuclei, chromosome aberration and sister chromatid exchange formation; as well as modulations in gene expression, proliferation and differentiation patterns. A fundamental question arising from such observations is why targeted cells induce DNA damage in non-targeted, bystander cells threatening thus their genomic stability and risking the induction of cancer. Here, we review and synthesize available literature to gather support for a model according to which targeted cells modulate as part of DDR their redox status and use it as a source to generate signals for neighboring cells. Such signals can be either small molecules transported to adjacent non-targeted cells via gap-junction intercellular communication (GJIC), or secreted factors that can reach remote, non-targeted cells by diffusion or through the circulation. We review evidence that such signals can induce in the recipient cell modulations of redox status similar to those seen in the originating targeted cell - occasionally though self-amplifying feedback loops. The resulting increase of oxidative stress in bystander cells induces, often in conjunction with DNA replication, the observed DDR-like responses that are at times strong enough to cause apoptosis. We reason that RIBE reflect the function of intercellular communication mechanisms designed to spread within tissues, or the entire organism, information about DNA damage inflicted to individual, constituent cells. Such responses are thought to protect the organism by enhancing repair in a community of cells and by eliminating severely damaged cells.

High dose bystander effects in spatially fractionated radiation therapy

Traditional radiotherapy of bulky tumors has certain limitations. Spatially fractionated radiation therapy (GRID) and intensity modulated radiotherapy (IMRT) are examples of advanced modulated beam therapies that help in significant reductions in normal tissue damage. GRID refers to the delivery of a single high dose of radiation to a large treatment area that is divided into several smaller fields, while IMRT allows improved dose conformity to the tumor target compared to conventional three-dimensional conformal radiotherapy. In this review, we consider spatially fractionated radiotherapy approaches focusing on GRID and IMRT, and present complementary evidence from different studies which support the role of radiation induced signaling effects in the overall radiobiological rationale for these treatments.

Embryos of the zebrafish Danio rerio in studies of non-targeted effects of ionizing radiation

The use of embryos of the zebrafish Danio rerio as an in vivo tumor model for studying non-targeted effects of ionizing radiation was reviewed. The zebrafish embryo is an animal model, which enables convenient studies on non-targeted effects of both high-linear-energy-transfer (LET) and low-LET radiation by making use of both broad-beam and microbeam radiation. Zebrafish is also a convenient embryo model for studying radiobiological effects of ionizing radiation on tumors. The embryonic origin of tumors has been gaining ground in the past decades, and efforts to fight cancer from the perspective of developmental biology are underway. Evidence for the involvement of radiation-induced genomic instability (RIGI) and the radiation-induced bystander effect (RIBE) in zebrafish embryos were subsequently given. The results of RIGI were obtained for the irradiation of all two-cell stage cells, as well as 1.5 hpf zebrafish embryos by microbeam protons and broad-beam alpha particles, respectively. In contrast, the RIBE was observed through the radioadaptive response (RAR), which was developed against a subsequent challenging dose that was applied at 10 hpf when <0.2% and <0.3% of the cells of 5 hpf zebrafish embryos were exposed to a priming dose, which was provided by microbeam protons and broad-beam alpha particles, respectively. Finally, a perspective on the field, the need for future studies and the significance of such studies were discussed.

Abscopal effects of radiation therapy: a clinical review for the radiobiologist

An "abscopal" effect occurs when localized irradiation perturbs the organism as a whole, with consequences that can be either beneficial or detrimental. Mechanistic explanations of this effect are challenging. From the oncologist's perspective, the term refers to distant tumor regression after localized irradiation. On the other hand, from a biologist's point of view, abscopal effects include induction of genomic instability, cell death, and oncogenic transformation in normal tissues. This conceptual dichotomy is explored in this review, with a focus on clinically documented cases of anti-tumor abscopal effects and abscopal effects in normal tissues. This review also outlines several suggested mechanisms for abscopal effects.

Spontaneous regression of metastatic melanoma - clinical evidence of the abscopal effect

INTRODUCTION

Metastatic melanoma is poorly understood. Regression of primary lesions has been associated with poor prognosis, but spontaneous regression of all metastatic disease is clearly beneficial. A patient's own immune responses occasionally appear to stimulate spontaneous regression of metastatic disease in melanoma.

PATIENTS AND METHODS

We present six interesting cases of complete or nearly complete spontaneous regression of metastatic melanoma, suggest possible causes and review the literature.

RESULTS AND CONCLUSIONS

These cases show clear radiological, pathological or clinical evidence of spontaneous regression of metastatic melanoma. This remains a poorly understood phenomena warranting further investigation and may prove useful in the development of immune mediated solutions.

Head and neck irradiation modulates pharmacokinetics of 5-fluorouracil and cisplatin

BACKGROUND

5-fluorouracil (5-FU) and cisplatin (CDDP) are used to enhance radiotherapy (RT) effect for head and neck (HN) cancers. However, the effect of local RT on systemic chemotherapeutics remains unclear. Here, we evaluated the influence of HN irradiation on the pharmacokinetics (PK) of 5-FU and CDDP in rats as experimental model.

METHODS

The radiation dose distributions of HN cancer patients were determined for the low dose areas, which are generously deposited around the target volume. Two Gy and 0.5 Gy RT were selected. Single-fraction radiation was delivered to the HN of Sprague-Dawley rats. 5-FU at 100 mg/kg or CDDP at 5 mg/kg was intravenously infused 24 hours after radiation.

RESULTS

Radiation at 2 Gy reduced the area under the plasma concentration vs. time curve (AUC) of 5-FU and CDDP by 16% and 29% compared to non-irradiated controls, respectively. This was accompanied by incremental total plasma clearance values. Intriguingly, low dose radiation at 0.5 Gy resulted in a similar pharmacokinetic profile, with a 17% and 33% reduction in the AUC of 5-FU and CDDP, respectively. The changes in AUC of bile, which increases with RT, were opposite to AUC of plasma for both drugs.

CONCLUSIONS

The local HN RT could modulate systemic PK of 5-FU and CDDP in rats. This unexpected RT-PK phenomena may provide a reference for adjustment of drug administration and is worthy of further investigation.

TRIAL REGISTRATION

ClinicalTrials.gov ID NCT01755585 and NCT01609114.

Oxidized extracellular DNA as a stress signal that may modify response to anticancer therapy

An increase in the levels of oxidation is a universal feature of genomic DNA of irradiated or aged or even malignant cells. In case of apoptotic death of stressed cells, oxidized DNA can be released in circulation (cfDNA). According to the results of the studies performed in vitro by our group and other researchers, the oxidized cfDNA serves as a biomarker for a stress and a stress signal that is transmitted from the "stressed" area i.e. irradiated cells or cells with deficient anti-oxidant defenses to distant (bystander) cells. In recipient cells, oxidized DNA stimulates biosynthesis of ROS that is followed up by an increase in the number of single strand and double strand breaks (SSBs and DSBs), and activation of DNA Damage Response (DDR) pathway. Effects of oxidized DNA are considered similar to that of irradiation. It seems that downstream effects of irradiation, in part, depend on the release of oxidized DNA fragments that mediate the effects in distant cells. The responses of normal and tumor cell to oxidized DNA may differ. It seems that tumor cells are more sensitive to oxidized DNA-dependent DNA damage, while developing pronounced adaptive response. This may suggest that in chemotherapy or irradiation-treated human body, the release of oxidized DNA from dying cancer cells may give a boost to remaining malignant cells by augmenting their survival and stress resistance. Further studies of the effects of oxidized DNA in both in vitro and in vivo systems are warranted.

Oxidative DNA damage caused by inflammation may link to stress-induced non-targeted effects

A spectrum of radiation-induced non-targeted effects has been reported during the last two decades since Nagasawa and Little first described a phenomenon in cultured cells that was later called the "bystander effect". These non-targeted effects include radiotherapy-related abscopal effects, where changes in organs or tissues occur distant from the irradiated region. The spectrum of non-targeted effects continue to broaden over time and now embrace many types of exogenous and endogenous stressors that induce a systemic genotoxic response including a widely studied tumor microenvironment. Here we discuss processes and factors leading to DNA damage induction in non-targeted cells and tissues and highlight similarities in the regulation of systemic effects caused by different stressors.

The peripheral myeloid expansion driven by murine cancer progression is reversed by radiation therapy of the tumor

Expansion of myeloid-lineage leukocytes in tumor-bearing mice has been proposed as a cause of systemic immunosuppression. We demonstrate that radiation therapy of tumors leads to a decline in myeloid cell numbers in the blood and a decrease in spleen size. The frequency of myeloid cells does not decline to the level seen in tumor-free mice: we demonstrate that metastatic disease can prevent myeloid cell numbers from returning to baseline, and that tumor recurrence from residual disease correlates with re-expansion of myeloid lineage cells. Radiation therapy results in increased proliferation of T cells in the spleen and while T cell responses to foreign antigens are not altered by tumor burden or myeloid cell expansion, responses to tumor-associated antigens are increased after radiation therapy. These data demonstrate that myeloid cell numbers are directly linked to primary tumor burden, that this population contracts following radiation therapy, and that radiation therapy may open a therapeutic window for immunotherapy of residual disease.

Bystander signalling: exploring clinical relevance through new approaches and new models

Classical radiation biology research has centred on nuclear DNA as the main target of radiation-induced damage. Over the past two decades, this has been challenged by a significant amount of scientific evidence clearly showing radiation-induced cell signalling effects to have important roles in mediating overall radiobiological response. These effects, generally termed radiation-induced bystander effects (RIBEs) have challenged the traditional DNA targeted theory in radiation biology and highlighted an important role for cells not directly traversed by radiation. The multiplicity of experimental systems and exposure conditions in which RIBEs have been observed has hindered precise definitions of these effects. However, RIBEs have recently been classified for different relevant human radiation exposure scenarios in an attempt to clarify their role in vivo. Despite significant research efforts in this area, there is little direct evidence for their role in clinically relevant exposure scenarios. In this review, we explore the clinical relevance of RIBEs from classical experimental approaches through to novel models that have been used to further determine their potential implications in the clinic.

Charged particle therapy--optimization, challenges and future directions

The use of charged particle therapy to control tumours non-invasively offers advantages over conventional radiotherapy. Protons and heavy ions deposit energy far more selectively than X-rays, allowing a higher local control of the tumour, a lower probability of damage to healthy tissue, low risk of complications and the chance for a rapid recovery after therapy. Charged particles are also useful for treating tumours located in areas that surround tissues that are radiosensitive and in anatomical sites where surgical access is limited. Current trial outcomes indicate that accelerated ions can potentially replace surgery for radical cancer treatments, which might be beneficial as the success of surgical cancer treatments are largely dependent on the expertise and experience of the surgeon and the location of the tumour. However, to date, only a small number of controlled randomized clinical trials have made comparisons between particle therapy and X-rays. Therefore, although the potential advantages are clear and supported by data, the cost:benefit ratio remains controversial. Research in medical physics and radiobiology is focusing on reducing the costs and increasing the benefits of this treatment.

Non-targeted effects of ionising radiation--implications for low dose risk

Non-DNA targeted effects of ionising radiation, which include genomic instability, and a variety of bystander effects including abscopal effects and bystander mediated adaptive response, have raised concerns about the magnitude of low-dose radiation risk. Genomic instability, bystander effects and adaptive responses are powered by fundamental, but not clearly understood systems that maintain tissue homeostasis. Despite excellent research in this field by various groups, there are still gaps in our understanding of the likely mechanisms associated with non-DNA targeted effects, particularly with respect to systemic (human health) consequences at low and intermediate doses of ionising radiation. Other outstanding questions include links between the different non-targeted responses and the variations in response observed between individuals and cell lines, possibly a function of genetic background. Furthermore, it is still not known what the initial target and early interactions in cells are that give rise to non-targeted responses in neighbouring or descendant cells. This paper provides a commentary on the current state of the field as a result of the non-targeted effects of ionising radiation (NOTE) Integrated Project funded by the European Union. Here we critically examine the evidence for non-targeted effects, discuss apparently contradictory results and consider implications for low-dose radiation health effects.

Role of intrahepatic tumor control in the prognosis of patients with hepatocellular carcinoma and extrahepatic metastases

BACKGROUND AND AIM

There has been little information about the long-term outcome and prognostic factors in patients with hepatocellular carcinoma (HCC) and extrahepatic metastases. The purpose of this study was to investigate the clinical factors affecting survival after extrahepatic metastasis and to determine the survival benefit of controlling intrahepatic HCC.

METHODS

Between 2004 and 2009, a total of 240 consecutive patients with HCC and extrahepatic metastasis were recruited. Based on tumor extent, performance, and hepatic function, the patients underwent locoregional and/or systemic treatments. The treatment response of the intrahepatic tumor after extrahepatic metastasis and other prognostic parameters were analyzed retrospectively.

RESULTS

During the mean follow up of 276 days, 222 patients died; the median survival time was 146 days. Multivariate analysis revealed that Child-Pugh class A, smaller hepatic tumor size, absence of portal venous invasion, single metastatic organ involvement, and objective treatment response of the intrahepatic tumor were the favorable prognostic factors for survival. Of the 183 evaluable patients, 24 achieved complete or partial response for intrahepatic tumors after treatment. The overall survival for the 24 responders was significantly improved, with a median of 521 days, as compared to 170 days for the remaining 159 patients without objective tumor response. The leading cause of death was progressive intrahepatic tumor.

CONCLUSIONS

Intrahepatic tumor status and hepatic reserve are among the significant predictors of survival in patients with HCC and extrahepatic metastases. This study indicates that even in patients with metastases from advanced HCC, therapeutic approaches to control intrahepatic tumors are important in improving patient survival.

Abscopal regression of antigen disparate tumors by antigen cascade after systemic tumor vaccination in combination with local tumor radiation

Radiation is a primary modality in cancer treatment. Radiation can also reduce tumor growth outside the treatment field, often referred to as the abscopal effect. The mechanisms and therapeutic potential of the abscopal effect have not been fully elucidated. We evaluated the role of vaccination directed against a tumor-associated antigen (TAA) in the induction and amplification of radiation induced abscopal effects. Active-specific immunotherapy with a TAA-specific vaccine regimen was used to induce and potentiate T-cell responses against carcinoembryonic antigen (CEA) in combination with local irradiation of subcutaneous tumors. We examined the potential synergy of a poxvirus-based CEA vaccine regimen in CEA-transgenic (Tg) mice in combination with either external beam radiation or brachytherapy of local tumors. The induction of CD8(+) T cells specific for multiple TAAs not encoded by the vaccine was observed after the combination therapy. In two tumor models, the antigen cascade responses induced by vaccine and local irradiation mediated the regression of antigen negative metastases at distal subcutaneous or pulmonary sites. Clinically, local control of the primary tumor is necessary and can sometimes prevent metastases; however, irradiation generally fails to control preexisting metastases. These studies suggest that by coupling tumor irradiation with immunotherapy, the abscopal effect can transcend from anecdotal observation to a defined mechanism that can be exploited for the treatment of systemic disease.

Biological complexities in radiation carcinogenesis and cancer radiotherapy: impact of new biological paradigms

Although radiation carcinogenesis has been shown both experimentally and epidemiologically, the use of ionizing radiation is also one of the major modalities in cancer treatment. Various known cellular and molecular events are involved in carcinogenesis. Apart from the known phenomena, there could be implications for carcinogenesis and cancer prevention due to other biological processes such as the bystander effect, the abscopal effect, intrinsic radiosensitivity and radioadaptation. Bystander effects have consequences for mutation initiated cancer paradigms of radiation carcinogenesis, which provide the mechanistic justification for low-dose risk estimates. The abscopal effect is potentially important for tumor control and is mediated through cytokines and/or the immune system (mainly cell-mediated immunity). It results from loss of growth and stimulatory and/or immunosuppressive factors from the tumor. Intrinsic radiosensitivity is a feature of some cancer prone chromosomal breakage syndromes such as ataxia telangectiasia. Radiosensitivity is manifested as higher chromosomal aberrations and DNA repair impairment is now known as a good biomarker for breast cancer screening and prediction of prognosis. However, it is not yet known whether this effect is good or bad for those receiving radiation or radiomimetic agents for treatment. Radiation hormesis is another major concern for carcinogenesis. This process which protects cells from higher doses of radiation or radio mimic chemicals, may lead to the escape of cells from mitotic death or apoptosis and put cells with a lower amount of damage into the process of cancer induction. Therefore, any of these biological phenomena could have impact on another process giving rise to genome instability of cells which are not in the field of radiation but still receiving a lower amount of radiation. For prevention of radiation induced carcinogenesis or risk assessment as well as for successful radiation therapy, all these phenomena should be taken into account.

Changing paradigms in radiobiology

The last 25 years have seen a major shift in emphasis in the field of radiobiology from a DNA-centric view of how radiation damage occurs to a much more biological view that appreciates the importance of macro-and micro-environments, hierarchical organization, underlying genetics, evolution, adaptation and signaling at all levels from atoms to ecosystems. The new view incorporates concepts of hormesis, nonlinear systems, bioenergy field theory, uncertainty and homeodynamics. While the mechanisms underlying these effects and responses are still far from clear, it is very apparent that their implications are much wider than the field of radiobiology. This reflection discusses the changing views and considers how they are influencing thought in environmental and medical science and systems biology.

Future of radiation therapy for malignant melanoma in an era of newer, more effective biological agents

The incidence of melanoma is rising. The primary initial treatment for melanoma continues to be wide local excision of the primary tumor and affected lymph nodes. Exceptions to wide local excision include cases where surgical excision may be cosmetically disfiguring or associated with increased morbidity and mortality. The role of definitive or adjuvant radiotherapy has largely been relegated to palliative measures because melanoma has been viewed as a prototypical radiotherapy-resistant cancer. However, the emerging clinical and radiobiological data summarized here suggests that many types of effective radiation therapy, such as radiosurgery for melanoma brain metastases, plaque brachytherapy for uveal melanoma, intensity modulated radiotherapy for melanoma of the head and neck, and adjuvant radiotherapy for selected high-risk, node-positive patients can improve outcomes. Similarly, although certain chemotherapeutic agents and biologics have shown limited responses, long-term control for unresectable tumors or disseminated metastatic disease has been rather disappointing. Recently, several powerful new biologics and treatment combinations have yielded new hope for this patient group. The recent identification of several clinically linked melanoma gene mutations involved in mitogen-activated protein kinase (MAPK) pathway such as BRAF, NRAS, and cKIT has breathed new life into the drive to develop more effective therapies. Some of these new therapeutic approaches relate to DNA damage repair inhibitors, cellular immune system activation, and pharmacological cell cycle checkpoint manipulation. Others relate to the investigation of more effective targeting and dosing schedules for underutilized therapeutics, such as radiotherapy. This paper summarizes some of these new findings and attempts to give some context to the renaissance in melanoma therapeutics and the potential role for multimodality regimens, which include certain types of radiotherapy as aids to locoregional control in sensitive tissues.

Matrix metalloproteinase-8 mediates the unfavorable systemic impact of local irradiation on pharmacokinetics of anti-cancer drug 5-Fluorouracil

Concurrent chemoradiation with 5-fluorouracil (5-FU) is widely accepted for cancer treatment. However, the interactions between radiation and 5-FU remain unclear. Here, we evaluated the influence of local irradiation on the pharmacokinetics of 5-FU in rats. The single-fraction radiation was delivered to the whole pelvic fields of Sprague-Dawley rats after computerized tomography-based planning. 5-FU at 100 mg/kg was prescribed 24 hours after radiation. A high-performance liquid chromatography system was used to measure 5-FU in the blood. Matrix metalloproteinase-8 (MMP-8) inhibitor I was administered to examine whether or not RT modulation of 5-FU pharmacokinetic parameters could be blocked. Compared with sham-irradiated controls, whole pelvic irradiation reduced the area under the concentration versus time curve (AUC) of 5-FU in plasma and, in contrast, increased in bile with a radiation dose-dependent manner. Based on protein array analysis, the amount of plasma MMP-8 was increased by whole pelvic irradiation (2.8-fold by 0.5 Gy and 5.3-fold by 2 Gy) in comparison with controls. Pretreatment with MMP-8 inhibitor reversed the effect of irradiation on AUC of 5-FU in plasma. Our findings first indicate that local irradiation modulate the systemic pharmacokinetics of 5-FU through stimulating the release of MMP-8. The pharmacokinetics of 5-FU during concurrent chemoradiaiton therapy should be rechecked and the optimal 5-FU dose should be reevaluated, and adjusted if necessary, during CCRT.

Radiation-induced bystander effects: what are they, and how relevant are they to human radiation exposures?

The term radiation-induced bystander effect is used to describe radiation-induced biological changes that manifest in unirradiated cells remaining within an irradiated cell population. Despite their failure to fit into the framework of classical radiobiology, radiation-induced bystander effects have entered the mainstream and have become established in the radiobiology vocabulary as a bona fide radiation response. However, there is still no consensus on a precise definition of radiation-induced bystander effects, which currently encompasses a number of distinct signal-mediated effects. These effects are classified here into three classes: bystander effects, abscopal effects and cohort effects. In this review, the data have been evaluated to define, where possible, various features specific to radiation-induced bystander effects, including their timing, range, potency and dependence on dose, dose rate, radiation quality and cell type. The weight of evidence supporting these defining features is discussed in the context of bystander experimental systems that closely replicate realistic human exposure scenarios. Whether the manifestation of bystander effects in vivo is intrinsically limited to particular radiation exposure scenarios is considered. The conditions under which radiation-induced bystander effects are induced in vivo will ultimately determine their impact on radiation-induced carcinogenic risk.

Systemic DNA damage related to cancer

The importance of bystander effects is becoming more appreciated, as studies show they may affect the course of cancer and other chronic diseases. The term "bystander effects" refers to changes in naïve cells sharing the same milieu with cells that have been damaged. Bystander cells may be in contact with, or distant from, damaged cells. In addition, it has been shown in culture that not only physically damaged cells, but also cells that have become abnormal (i.e., cancerous or senescent) may induce bystander effects. Recently, we have shown a similar effect in animals. Mice harboring subcutaneous tumors exhibited elevated levels of DNA damage in distant organs. In contrast to cell culture, immune cells seemed to be involved in tumor-induced bystander effects in animals because CCL2-null tumor-bearing mice did not exhibit increased distant DNA damage. Here, we discuss some of the implications of these observations.

The evolving role of radiation therapy in the management of malignant melanoma

The incidence of melanoma is rising in the United States, leading to an estimated 68,720 new diagnoses and 8,650 deaths annually. The natural history involves metastases to lymph nodes, lung, liver, brain, and often to other sites. Primary treatment for melanoma is surgical excision of the primary tumor and affected lymph nodes. The role of adjuvant or definitive radiation therapy in the treatment of melanoma remains controversial, because melanoma has traditionally been viewed as a prototypical radioresistant cancer. However, recent studies suggest that under certain clinical circumstances, there may be a significant role for radiation therapy in melanoma treatment. Stereotactic radiosurgery for brain metastases has shown effective local control. High dose per fraction radiation therapy has been associated with a lower rate of locoregional recurrence of sinonasal melanoma. Plaque brachytherapy has evolved into a promising alternative to enucleation at the expense of moderate reduction in visual acuity. Adjuvant radiation therapy following lymphadenectomy in node-positive melanoma prevents local and regional recurrence. The newer clinical data along with emerging radiobiological data indicate that radiotherapy is likely to play a greater role in melanoma management and should be considered as a treatment option.

Influence of low dose irradiation on differentiation, maturation and T-cell activation of human dendritic cells

Ionizing irradiation could act directly on immune cells and may induce bystander effects mediated by soluble factors that are released by the irradiated cells. This is the first study analyzing both the direct effect of low dose ionizing radiation (LDIR) on the maturation and cytokine release of human dendritic cells (DCs) and the functional consequences for co-cultured T-cells. We showed that irradiation of DC-precursors in vitro does not influence surface marker expression or cytokine profile of immature DCs nor of mature DCs after LPS treatment. There was no difference of single dose irradiation versus fractionated irradiation protocols on the behavior of the mature DCs. Further, the low dose irradiation did not change the capacity of the DCs to stimulate T-cell proliferation. But the irradiation of the co-culture of DCs and T-cells revealed significantly lower proliferation of T-cells with higher doses. Summarizing the data from approx. 50 DC preparations there is no significant effect of low dose ionizing irradiation on the cytokine profile, surface marker expression and maturation of DCs in vitro although functional consequences cannot be excluded.

Combination approaches to immunotherapy: the radiotherapy example

The marquee cell of the immune response, the T lymphocyte, is exquisitely sensitive to ionizing radiation. Moreover, the majority of cells of the hematopoietic lineage, with their high turnover and radiation sensitivity, are effectively ablated by relatively low radiation doses, such that total-body irradiation is an effective immunosuppressive therapy. Despite and sometimes because of these facts, the immune system may underlie some portion of the therapeutic effects of radiation. In this review, we will discuss these processes, and go on to discuss current therapies combining radiation therapy with immunotherapy for the treatment of cancer.

Time-series clustering of gene expression in irradiated and bystander fibroblasts: an application of FBPA clustering

Background

The radiation bystander effect is an important component of the overall biological response of tissues and organisms to ionizing radiation, but the signaling mechanisms between irradiated and non-irradiated bystander cells are not fully understood. In this study, we measured a time-series of gene expression after α-particle irradiation and applied the Feature Based Partitioning around medoids Algorithm (FBPA), a new clustering method suitable for sparse time series, to identify signaling modules that act in concert in the response to direct irradiation and bystander signaling. We compared our results with those of an alternate clustering method, Short Time series Expression Miner (STEM).

Results

While computational evaluations of both clustering results were similar, FBPA provided more biological insight. After irradiation, gene clusters were enriched for signal transduction, cell cycle/cell death and inflammation/immunity processes; but only FBPA separated clusters by function. In bystanders, gene clusters were enriched for cell communication/motility, signal transduction and inflammation processes; but biological functions did not separate as clearly with either clustering method as they did in irradiated samples. Network analysis confirmed p53 and NF-κB transcription factor-regulated gene clusters in irradiated and bystander cells and suggested novel regulators, such as KDM5B/JARID1B (lysine (K)-specific demethylase 5B) and HDACs (histone deacetylases), which could epigenetically coordinate gene expression after irradiation.

Conclusions

In this study, we have shown that a new time series clustering method, FBPA, can provide new leads to the mechanisms regulating the dynamic cellular response to radiation. The findings implicate epigenetic control of gene expression in addition to transcription factor networks.

Tumors induce complex DNA damage in distant proliferative tissues in vivo

That tumors cause changes in surrounding tissues is well documented, but whether they also affect distant tissues is uncertain. Such knowledge may be important in understanding the relationship between cancer and overall patient health. To address this question, we examined tissues distant to sites of implanted tumors for genomic damage using cohorts of C57BL/6 and BALB/c mice with early-stage subcutaneous syngeneic grafts, specifically, B16 melanoma, MO5076 sarcoma, and COLON26 carcinoma. Here we report that levels of two serious types of DNA damage, double-strand breaks (DSBs) measured by γ-H2AX focus formation and oxidatively induced non-DSB clustered DNA lesions (OCDLs), were elevated in tissues distant from the tumor site in tumor-bearing mice compared with their age- and sex-matched controls. Most affected were crypts in the gastrointestinal tract organs and skin, both highly proliferative tissues. Further investigation revealed that, compared with controls, tumor-bearing mice contained elevated amounts of activated macrophages in the distant gastrointestinal tissues, as well as elevated serum levels of several cytokines. One of these cytokines, CCL2/MCP-1, has been linked to several inflammation-related conditions and macrophage recruitment, and strikingly, CCL2-deficient mice lacked increased levels of DSBs and OCDLs in tissues distant from implanted tumors. Thus, this study is unique in being a direct demonstration that the presence of a tumor may induce a chronic inflammatory response in vivo, leading to increased systemic levels of DNA damage. Importantly, these findings suggest that tumors may have more profound effects on their hosts than heretofore expected.

Harnessing the immune response to treat cancer

It is well established that the immune system has the capacity to attack malignant cells. During malignant transformation cells acquire numerous molecular and biochemical changes that render them potentially vulnerable to immune cells. Yet it is self-evident that a growing tumour has managed to evade these host defence mechanisms. The exact ways in which the immune system interacts with tumour cells and how cancers are able to escape immunological eradication have only recently started to be fully elucidated. Understanding the relationship between the tumour and the anti-tumour immune response and how this can be altered with conventional treatments and immune-targeted therapies is crucial to developing new treatments for patients with cancer. In this review, focusing on the anti-tumour T-cell response, we summarize our understanding of how tumours, cancer treatments and the immune system interact, how tumours evade the immune response and how this process could be manipulated for the benefit of patients with cancer.

Genomic instability after targeted irradiation of human lymphocytes: evidence for inter-individual differences under bystander conditions

Environmental (222)radon exposure is a human health concern, and many studies demonstrate that very low doses of high LET alpha-particle irradiation initiate deleterious genetic consequences in both irradiated and non-irradiated bystander cells. One consequence, radiation-induced genomic instability (RIGI), is a hallmark of tumorigenesis and is often assessed by measuring delayed chromosomal aberrations. We utilised a technique that facilitates transient immobilization of primary lymphocytes for targeted microbeam irradiation and have reported that environmentally relevant doses, e.g. a single (3)He(2+) particle traversal to a single cell, are sufficient to induce RIGI. Herein we sought to determine differences in radiation response in lymphocytes isolated from five healthy male donors. Primary lymphocytes were irradiated with a single particle per cell nucleus. We found evidence for inter-individual variation in radiation response (RIGI, measured as delayed chromosome aberrations). Although this was not highly significant, it was possibly masked by high levels of intra-individual variation. While there are many studies showing a link between genetic predisposition and RIGI, there are few studies linking genetic background with bystander effects in normal human lymphocytes. In an attempt to investigate inter-individual variation in the induction of bystander effects, primary lymphocytes were irradiated with a single particle under conditions where fractions of the population were traversed. We showed a marked genotype-dependent bystander response in one donor after exposure to 15% of the population. The findings may also be regarded as a radiation-induced genotype-dependent bystander effect triggering an instability phenotype.

Manifestations and mechanisms of non-targeted effects of ionizing radiation

A well-established radiobiological paradigm is that the biological effects of ionizing radiation occur in irradiated cells as a consequence of the DNA damage they incur. However, many observations of, so-called, non-targeted effects indicate that genetic alterations are not restricted to directly irradiated cells. Non-targeted effects are responses exhibited by non-irradiated cells that are the descendants of irradiated cells (radiation-induced genomic instability) or by cells that have communicated with irradiated cells (radiation-induced bystander effects). Radiation-induced genomic instability is characterized by chromosomal abnormalities, gene mutations and cell death. Similar effects, as well as responses that may be regarded as protective, have been attributed to bystander mechanisms. The majority of studies to date have used in vitro systems but some non-targeted effects have been demonstrated in vivo and there is also evidence for radiation-induced instability in the mammalian germ line. However, there may be situations where radiation-induced genomic instability in vivo may not necessarily identify genomically unstable somatic cells but the manifestation of responses to ongoing production of damaging signals generated by genotype-dependent mechanisms having properties in common with inflammatory processes. Non-targeted mechanisms have significant implications for understanding mechanisms of radiation action but the current state of knowledge does not permit definitive statements about whether these phenomena have implications for assessing radiation risk.

Toxicity risk of non-target organs at risk receiving low-dose radiation: case report

The spine is the most common site for bone metastases. Radiation therapy is a common treatment for palliation of pain and for prevention or treatment of spinal cord compression. Helical tomotherapy (HT), a new image-guided intensity modulated radiotherapy (IMRT), delivers highly conformal dose distributions and provides an impressive ability to spare adjacent organs at risk, thus increasing the local control of spinal column metastases and decreasing the potential risk of critical organs under treatment. However, there are a lot of non-target organs at risk (OARs) occupied by low dose with underestimate in this modern rotational IMRT treatment. Herein, we report a case of a pathologic compression fracture of the T9 vertebra in a 55-year-old patient with cholangiocarcinoma. The patient underwent HT at a dose of 30 Gy/10 fractions delivered to T8-T10 for symptom relief. Two weeks after the radiotherapy had been completed, the first course of chemotherapy comprising gemcitabine, fluorouracil, and leucovorin was administered. After two weeks of chemotherapy, however, the patient developed progressive dyspnea. A computed tomography scan of the chest revealed an interstitial pattern with traction bronchiectasis, diffuse ground-glass opacities, and cystic change with fibrosis. Acute radiation pneumonitis was diagnosed. Oncologists should be alert to the potential risk of radiation toxicities caused by low dose off-targets and abscopal effects even with highly conformal radiotherapy.

Does scientific evidence support a change from the LNT model for low-dose radiation risk extrapolation?

The linear no-threshold (LNT) model has been widely used to establish international rules and standards in radiation protection. It is based on the notion that the physical energy deposition of ionizing radiation (IR) increases carcinogenic risk linearly with increasing dose (i.e., the carcinogenic effectiveness remains constant irrespective of dose) and, within a factor of two, also with dose-rate. However, recent findings have strongly put into question the LNT concept and its scientific validity, especially for very low doses and dose-rates. Low-dose effects are more difficult to ascertain than high-dose effects. Epidemiological studies usually lack sufficient statistical power to determine health risks from very low-dose exposures. In this situation, studies of the fundamental mechanisms involved help to understand and assess short- and long-term effects of low-dose IR and to evaluate low-dose radiation risks. Several lines of evidence demonstrate that low-dose and low dose-rate effects are generally lower than expected from high-dose exposures. DNA damage signaling, cell cycle checkpoint activation, DNA repair, gene and protein expression, apoptosis, and cell transformation differ qualitatively and quantitatively at high- and low-dose IR exposures, and most animal and epidemiological data support this conclusion. Thus, LNT appears to be scientifically invalid in the low-dose range.

Ionizing radiation-induced bystander effects, potential targets for modulation of radiotherapy

Cells exposed to ionizing radiation show DNA damage, apoptosis, chromosomal aberrations or increased mutation frequency and for a long time it was generally accepted that these effects resulted from ionization of cell structures and the action of reactive oxygen species formed by water radiolysis. In the last few years, however, it has appeared that cells exposed to ionizing radiation and other genotoxic agents can release signals that induce very similar effects in non-targeted neighboring cells, phenomena known as bystander effects. These signals are transmitted to the neighboring non-hit cells by intercellular gap-junction communication or are released outside the cell, in the case of cultured cells into the medium. The signaling is mutual, and irradiated cells can also receive signals from non-irradiated neighbors. Most experiments show a decrease in survival of unirradiated bystander cells, but some studies of the influence of unirradiated or low dose-irradiated cells on those irradiated with higher doses show that intercellular bystander signaling can also increase the survival of irradiated cell populations. In the last few years, communication between irradiated and non-irradiated cells has attracted interest in many studies as a possible target for modulation of radiotherapy. Understanding the mechanisms underlying bystander effects is important for radiation risk assessment and for evaluation of protocols for cancer radiotherapy. In this review we describe different aspects of ionizing radiation-induced bystander effects: experimental examples, types of DNA damage, situations in vivo, and their possible role in adaptive response to irradiation, and we discuss their possible significance for anticancer therapy.

Systemic effects of local radiotherapy

Radiotherapy is generally used to treat a localised target that includes cancer. Increasingly, evidence indicates that radiotherapy recruits biological effectors outside the treatment field and has systemic effects. We discuss the implications of such effects and the role of the immune system in standard cytotoxic treatments. Because the effects of chemotherapy and radiotherapy are sensed by the immune system, their combination with immunotherapy presents a new therapeutic opportunity. Radiotherapy directly interferes with the primary tumour and possibly reverses some immunosuppressive barriers within the tumour microenvironment-ideally, recovering the role of the primary tumour as an immunogenic hub. Local radiation also triggers systemic effects that can be used in combination with immunotherapy to induce responses outside the radiation field.

Radiation-induced bystander signalling in cancer therapy

Our understanding of how radiation kills normal and tumour cells has been based on an intimate knowledge of the direct induction of DNA damage and its cellular consequences. What has become clear is that, as well as responses to direct DNA damage, cell-cell signalling -- known as the bystander effect -- mediated through gap junctions and inflammatory responses may have an important role in the response of cells and tissues to radiation exposure and also chemotherapy agents. This Review outlines the key aspects of radiation-induced intercellular signalling and assesses its relevance for existing and future radiation-based therapies.