Normal Tissue Response Research Articles

The Metformin Immunoregulatory Actions in Tumor Suppression and Normal Tissues Protection.

The immune system is the key player in a wide range of responses in normal tissues and tumors to anticancer therapy. Inflammatory and fibrotic responses in normal tissues are the main limitations of chemotherapy, radiotherapy, and also some newer anticancer drugs such as immune checkpoint inhibitors (ICIs). Immune system responses within solid tumors including anti-tumor and tumor-promoting responses can suppress or help tumor growth. Thus, modulation of immune cells and their secretions such as cytokines, growth factors and epigenetic modulators, pro-apoptosis molecules, and some other molecules can be suggested to alleviate side effects in normal tissues and drug-resistance mechanisms in the tumor. Metformin as an anti-diabetes drug has shown intriguing properties such as anti-inflammation, anti-fibrosis, and anticancer effects. Some investigations have uncovered that metformin can ameliorate radiation/chemotherapy toxicity in normal cells and tissues through the modulation of several targets in cells and tissues. These effects of metformin may ameliorate severe inflammatory responses and fibrosis after exposure to ionizing radiation or following treatment with highly toxic chemotherapy drugs. Metformin can suppress the activity of immunosuppressive cells in the tumor through the phosphorylation of AMP-activated protein kinase (AMPK). In addition, metformin may stimulate antigen presentation and maturation of anticancer immune cells, which lead to the induction of anticancer immunity in the tumor. This review aims to explain the detailed mechanisms of normal tissue sparing and tumor suppression during cancer therapy using adjuvant metformin with an emphasis on immune system responses.

Long Noncoding VIM-AS1: Biomarker of Breast Fibrosis Susceptibility After Radiation Therapy and Promoter of Transforming Growth Factor Beta1–Driven Fibrosis

Fibrosis is a common late complication of radiation therapy. Molecular dysregulations leading to fibrosis have been characterized for the coding part of the genome, notably those involving the TGFB1 gene network. However, because a large part of the human genome encodes RNA transcripts that are not translated into proteins, exploring the involvement of the noncoding part of the genome in fibrosis susceptibility and development was the aim of this work. Breast cancer patients having or not having developed severe breast fibrosis after radiation therapy were retrospectively selected from the COPERNIC collection. Exome sequencing and RNA-seq transcriptomic profiling were performed on 19 primary dermal fibroblast strains isolated from the patients' nonirradiated skin. Functional experiments were based on fibrogenic induction by transforming growth factor-Beta1 (TGFB1) and gene knockdown in healthy donor fibroblasts. Coding and noncoding transcriptomes discriminated fibrosis from nonfibrosis conditions, and a signature of breast fibrosis susceptibility comprising 15 long noncoding RNAs (lncRNAs) was identified. A hazard ratio validation showed that the lncRNA vimentin antisense long noncoding RNA 1 (VIM-AS1) was the best biomarker associated with fibrosis risk. This lncRNA has not been previously associated with any fibrotic disorder, but we found it upregulated in data sets from cardiac fibrosis and scleroderma, suggesting a general role in tissue fibrosis. Functional experiments demonstrated a profibrotic action of VIM-AS1 because its knockdown reduced myofibroblast activation, collagen matrix production, and dermal organoid contraction. RNA-seq data analysis after VIM-AS1 silencing also pointed out the regulation of replication, cell cycle, and DNA repair. Mechanistically, because VIM-AS1 was found coregulated with the vimentin gene, these data support a profibrotic function of the TGFB1/VIM-AS1/vimentin axis, targeting the dynamics of fibroblast-myofibroblast transition. Noncoding RNA analysis can provide specific biomarkers relevant to the prediction of normal tissue responses after radiation therapy, which opens perspectives of next-generation approaches for treatment, in the frame of the recent developments of RNA-based technologies.

On the significance of the different geometrical and dosimetric parameters in microbeam and minibeam radiation therapy a retrospective evaluation.

Spatially Fractionated Radiation Therapy (SFRT) is an unconventional therapeutic approach with the potential to disrupt the classical paradigms of conventional radiation therapy. The high spatial dose modulation in SFRT is believed to activate distinct radiobiological mechanisms which lead to remarkable increases in normal tissue tolerance. To make optimal use of SFRT and its benefits, a deeper understanding of the biological response and its relationship with the complex dosimetric and geometric components of SFRT is essential. A retrospective evaluation of preclinical studies was conducted to gain insight into the dosimetric and geometric parameters that are most correlated with normal tissue response. Current literature evaluates the response of tissue to MBRT and MRT according to various end points, e.g. the level of desquamation, degree of necrosis, or the amount of malcalcification. A set of metrics was developed to allow a quantitative comparison of these results. The strongest correlations were observed with the doses in both the peaks and valleys as well as the ratio of the area covered by the peak over the total area. This emphasises the geometry of the beam. MBRT challenged previous uniform dose-distribution paradigms by highlighting the critical role of Peak Dose alongside Valley Dose in tissue sparing whereas MRT underscores the significant influence of geometric beam parameters on tissue preservation. The data exhibits variability in the results obtained using different animal models and endpoints and additional research is warranted to explore the trends observed in this study under controlled conditions.

Proton- compared to X-irradiation leads to more acinar atrophy and greater hyposalivation accompanied by a differential cytokine response.

Proton therapy gives less dose to healthy tissue compared to conventional X-ray therapy, but systematic comparisons of normal tissue responses are lacking. The aim of this study was to investigate late tissue responses in the salivary glands following proton- or X-irradiation of the head and neck in mice. Moreover, we aimed at investigating molecular responses by monitoring the cytokine levels in serum and saliva. Female C57BL/6J mice underwent local fractionated irradiation with protons or X-rays to the maximally tolerated acute level. Saliva and serum were collected before and at different time points after irradiation to assess salivary gland function and cytokine expression. To study late responses in the major salivary glands, histological analyses were performed on tissues collected at day 105 after onset of irradiation. Saliva volume after proton and X-irradiation was significantly lower than for controls and remained reduced at all time points after irradiation. Protons caused reduced saliva production and fewer acinar cells in the submandibular glands compared to X-rays at day 105. X-rays induced a stronger inflammatory cytokine response in saliva compared to protons. This work supports previous preclinical findings and indicate that the relative biological effectiveness of protons in normal tissue might be higher than the commonly used value of 1.1.

Artificial intelligence for response prediction and personalisation in radiation oncology.

Artificial intelligence (AI) systems may personalise radiotherapy by assessing complex and multifaceted patient data and predicting tumour and normal tissue responses to radiotherapy. Here we describe three distinct generations of AI systems, namely personalised radiotherapy based on pretreatment data, response-driven radiotherapy and dynamically optimised radiotherapy. Finally, we discuss the main challenges in clinical translation of AI systems for radiotherapy personalisation.

Radiation induces acute and subacute vascular regression in a three-dimensional microvasculature model.

Radiation treatment is one of the most frequently used therapies in patients with cancer, employed in approximately half of all patients. However, the use of radiation therapy is limited by acute or chronic adverse effects and the failure to consider the tumor microenvironment. Blood vessels substantially contribute to radiation responses in both normal and tumor tissues. The present study employed a three-dimensional (3D) microvasculature-on-a-chip that mimics physiological blood vessels to determine the effect of radiation on blood vessels. This model represents radiation-induced pathophysiological effects on blood vessels in terms of cellular damage and structural and functional changes. DNA double-strand breaks (DSBs), apoptosis, and cell viability indicate cellular damage. Radiation-induced damage leads to a reduction in vascular structures, such as vascular area, branch length, branch number, junction number, and branch diameter; this phenomenon occurs in the mature vascular network and during neovascularization. Additionally, vasculature regression was demonstrated by staining the basement membrane and microfilaments. Radiation exposure could increase the blockage and permeability of the vascular network, indicating that radiation alters the function of blood vessels. Radiation suppressed blood vessel recovery and induced a loss of angiogenic ability, resulting in a network of irradiated vessels that failed to recover, deteriorating gradually. These findings demonstrate that this model is valuable for assessing radiation-induced vascular dysfunction and acute and chronic effects and can potentially improve radiotherapy efficiency.

Vascular effects of continuous hyperbaric oxygen exposure: experimental outlook.

Hyperbaric oxygen (HBO) treatment aims to restore tissue oxygenation by inhaling 100% oxygen in pressure rooms. Although beneficial effects have been reported with regard to re-oxygenated ischemic tissues, conflicting findings have been presented concerning the paradoxical tissue response following reperfusion and/or the different responses of non-ischemic normal tissues to increased oxygen exposure. The present study sought to experimentally investigate the impact of continuous HBO treatments on normal aortic tissue. New Zealand rabbits were placed in pressure rooms for 90 minutes per day under 2.5 atmospheric pressure and exposed to HBO for 28 days. Normal structural histology was obtained in the control group. Foam cells were detected in the aortic intimae, thickening and undulation were visualized in the endothelium, and localized separations were observed in the tunica media in the study group compared with the control group. Moreover, salient vasa vasorum was detected in the study group via histopathology. These findings suggest that continuous HBO exposures disrupt the normal vascular structure of a healthy aorta.

Phosphorylated Ataxia Telangiectasia Mutated (pATM) Enzyme-Linked Immunosorbent Assay (ELISA) for Predicting Radiation Induces Normal Tissue Toxicity in Radiotherapy Patients: A Systematic Review

Background: An adverse normal tissue response, such as normal tissue toxicity (NTT), is present in radiotherapy (RT) patients and can limit the effectiveness of the RT treatment. Identifying patients with adverse tissue responses before RT had clinical benefits and individual radiosensitivity (IRS) is considered an important factor in NTT incidences. Therefore, this systematic study aimed to determine the possibility of using phosphorylated Ataxia Telangiectasia Mutated (pATM) Enzyme-Linked Immunosorbent Assay (ELISA) to predict NTT in RT patients.Methods: A comprehensive data search was conducted in three electronic databases, namely PUBMED CENTRAL, ScienceDirect, and SCOPUS. The quality of relevant publications was independently evaluated using the PICO (participants, intervention/exposure, comparison, and outcome) approach.Results: : A total of 47 articles were retrieved, 41 of which were assessed based on the titles and abstracts. Furthermore, 39 articles were excluded, and 2 were included in this study.Conclusions: The phosphorylated ATM ELISA on lymphocytes showed promising results for IRS prediction in RT patients. However, these assumptions should be validated on a larger RT patient cohort.

Acute normal tissue responses in a murine model following fractionated irradiation of the head and neck with protons or X-rays

Background The purpose of this study was to investigate acute normal tissue responses in the head and neck region following proton- or X-irradiation of a murine model. Materials and methods Female C57BL/6J mice were irradiated with protons (25 or 60 MeV) or X-rays (100 kV). The radiation field covered the oral cavity and the major salivary glands. For protons, two different treatment plans were used, either with the Bragg Peak in the middle of the mouse (BP) or outside the mouse (transmission mode; TM). Delivered physical doses were 41, 45, and 65 Gy given in 6, 7, and 10 fractions for BP, TM, and X-rays, respectively. Alanine dosimetry was used to assess delivered doses. Oral mucositis and dermatitis were scored using CTC v.2.0-based tables. Saliva was collected at baseline, right after end of irradiation, and at day 35. Results The measured dose distribution for protons (TM) and X-rays was very similar. Oral mucositis appeared earlier, had a higher score and was found in a higher percentage of mice after proton irradiation compared to X-irradiation. Dermatitis, on the other hand, had a similar appearance after protons and X-rays. Compared to controls, saliva production was lower right after termination of proton- and X-irradiation. The BP group demonstrated saliva recovery compared to the TM and X-ray group at day 35. Conclusion With lower delivered doses, proton irradiation resulted in similar skin reactions and increased oral mucositis compared to X-irradiation. This indicates that the relative biological effectiveness of protons for acute tissue responses in the mouse head and neck is greater than the clinical standard of 1.1. Thus, there is a need for further investigations of the biological effect of protons in normal tissues.

Redefining precision radiotherapy through liquid biopsy.

Precision radiotherapy refers to the ability to deliver radiation doses with sub-millimetre accuracy. It does not however consider individual variation in tumour or normal tissue response, failing to maximise tumour control and minimise toxicity. Combining precise delivery with personalised dosing, through analysis of cell-free DNA, would redefine precision in radiotherapy.

Flash Radiotherapy: Innovative Cancer Treatment

Flash radiotherapy (Flash-RT) is an innovative technique used in radiotherapy for cancer treatment because it delivers an extremely high dose of radiation (>40 Gy/s) to the tumour in a very short period of time, typically within a fraction of a second. This ultra-fast delivery of radiation distinguishes Flash-RT from conventional radiotherapy, which typically involves the delivery of radiation over a longer time period, often several minutes. Studies conducted in cell and preclinical models suggested that Flash-RT may spare normal tissues from radiation-related side effects, such as skin toxicity, gastrointestinal complications, and damage to organs-at-risk. This is believed to be due to the unique normal tissue response to the ultra-high dose rate. Nevertheless, while Flash-RT shows promising results in preclinical and early clinical studies, one should note that the technique is still in the early stages of development. This entry provides a comprehensive exploration of the immense potentials of Flash-RT, covering its background, mechanisms, radiation sources, recent experimental findings based on cell and preclinical models, and future prospects. It aims to provide valuable insights into this innovative radiotherapy technology for anyone interested in the subject.

Tumor Microenvironment-Responsive Hydrogel for Direct Extracellular ATP Imaging-Guided Surgical Resection with Clear Boundaries.

Most solid tumors are clinically treated using surgical resection, and the presence of residual tumor tissues at the surgical margins often determines tumor survival and recurrence. Herein, a hydrogel (AHB Gel) is developed for fluorescence-guided surgical resection. AHB Gel is constructed by tethering a polyacrylamide hydrogel and ATP-responsive aptamers together. It exhibits strong fluorescence under high ATP concentrations corresponding to the TME (100-500μM) but shows little fluorescence at low ATP concentrations (10-100nM) such as those in normal tissues. AHB Gel can rapidly (within 3min) emit fluorescence after exposure to ATP, and the fluorescence signal only occurs at sites exposed to high ATP, resulting in a clear boundary between the ATP-high and ATP-low regions. In vivo, AHB Gel exhibits specific tumor-targeting capacity with no fluorescence response in normal tissue, providing clear tumor boundaries. In addition, AHB Gel has good storage stability, which is conducive to its future clinical application. In summary, AHB Gel is a novel tumor microenvironment-targeted DNA-hybrid hydrogel for ATP-based fluorescence imaging. It could enable the precise imaging of tumor tissues, showing promising application in fluorescence-guided surgeries in the future. This article is protected by copyright. All rights reserved.

Biological sex differences in renin angiotensin system enzymes ACE and ACE2 regulate normal tissue response to radiation injury.

Introduction: In experimental animal models, biological sex-differences in the manifestation and severity of normal tissue radiation injury have been well-documented. Previously we demonstrated male and female rats have differential and highly reproducible responses to high-dose partial body irradiation (PBI) with male rats having greater susceptibility to both gastrointestinal acute radiation syndrome (GI-ARS) and radiation pneumonitis than female rats. Methods: In the current study, we have investigated whether differential expression of the renin-angiotensin system (RAS) enzymes angiotensin converting enzyme (ACE) and ACE2 contribute to the observed sex-related differences in radiation response. Results: During the period of symptomatic pneumonitis, the relative ratio of ACE to ACE2 (ACE/ACE2) protein in the whole lung was significantly increased by radiation in male rats alone. Systemic treatment with small molecule ACE2 agonist diminazene aceturate (DIZE) increased lung ACE2 activity and reduced morbidity during radiation pneumonitis in both sexes. Notably DIZE treatment also abrogated morbidity in male rats during GI-ARS. We then evaluated the contribution of the irradiated bone marrow (BM) compartment on lung immune cell infiltration and ACE imbalance during pneumonitis. Transplantation of bone marrow from irradiated donors increased both ACE-expressing myeloid cell infiltration and immune ACE activity in the lung during pneumonitis compared to non-irradiated donors. Discussion: Together, these data demonstrate radiation induces a sex-dependent imbalance in the renin-angiotensin system enzymes ACE and ACE2. Additionally, these data suggest a role for ACE-expressing myeloid cells in the pathogenesis of radiation pneumonitis. Finally, the observed sex-differences underscore the need for consideration of sex as a biological variable in the development of medical countermeasures for radiation exposure.

SP-0523 How much can radiotherapy benefit from biomarkers of normal tissue response to radiation

SP-0523 How much can radiotherapy benefit from biomarkers of normal tissue response to radiation

PO-2091 A method to explore dose and LET for normal tissue response studies in large proton therapy cohorts

PO-2091 A method to explore dose and LET for normal tissue response studies in large proton therapy cohorts

Abstract 3498: GTP-dependent signaling links metabolism, non-homologous end joining and treatment responses in cancer and normal tissues

Abstract Introduction/Objectives: Purines regulate DNA repair in brain tumors through uncertain mechanisms. We sought to define the mechanisms by which purines regulate DNA repair and therapy response. Methods: Crispr/Cas9, siRNA, and CDNA overexpression and immunoblot were used to modulate and confirm protein levels. γ-H2AX and Rad51 foci were enumerated using immunofluorescence. Fluorescent reporter assays were performed to detect DNA repair activity. Phosphoproteomics was used to identify GTP-dependent (de)phosphorylation events after RT and antibodies generated against novel sites. Celltiter-glo or clonogenic assay were performed to evaluate drug/RT responses. IHC was used to detect protein expression. Animal models of glioblastoma and normal tissue (irradiation of GI system) were used to assess DNA repair and treatment responses in vivo. Results: Pharmacogenomic inhibition of GTP (but not ATP) synthesis sensitized GBM cells to RT, slowed the repair of RT-induced DSBs and inhibited the activity of non-homologous end joining (NHEJ), but not homologous recombination (HR). These effects could be rescued by GTP (but not ATP) supplementation. We found a GTP-dependent RT-induced dephosphorylation event on Abl interactor 1 (Abi-1) serine 323 (S323) using phosphoproteomics. We generated a new antibody for p-Abi-1 (S323), validated its specificity, and confirmed that RT causes a GTP-dependent dephosphorylation of Abi-1 (S323). Knockout of Abi-1 slowed RT-induced DSB repair and this was rescued by re-expression of dephosphomimetic Abi-1 (S323A) but not phosphomimetic Abi-1 (S323D). Abi-1 canonically binds to the small GTP-activated protein Rac1. Expression of constitutively active Rac1 promoted the dephosphorylation of Abi-1 (S323) and DSB repair while these effects were blocked by dominant negative Rac1. Pharmacogenomic inhibition of protein phosphatase 5 reversed the GTP- and Rac1-mediated dephosphorylation of Abi-1 and DSB repair. These findings have therapeutic relevance. In GBM PDX samples, levels of p-Abi-1 (S323) negatively correlated with Rac1 activity and predicted favorable efficacy of genotoxic treatments. In orthotopic GBM mouse models, inhibition of Rac1 enhanced RT responses and suppressed Abi-1 S323 dephosphorylation. Abi-1 knockout enhanced efficacy of numerous genotoxic treatments and could be rescued by Abi-1 S323A (but not Abi-1 S323D) re-expression. This regulation is generalizable beyond brain cancer, as GTP supplementation promoted DNA repair and p-Abi1 (S323) dephosphorylation in normal astrocytes and enteroids and protected mice from RT-mediated gastrointestinal injury. Conclusions: GTP promotes DNA repair by activating Rac1 and PP5 to dephosphorylate Abi-1 and stimulate NHEJ. Disrupting this regulation could improve brain tumor treatment responses while augmenting it could help protect normal tissues from genotoxic injury. Citation Format: Weihua Zhou, Angelica Lin, Zitong Zhao, John Yang, Kari Wilder-Romans, Jie Xu, Sumeet Solanki, Jing Li, Annabel Yang, Andrew Scott, Ayesha Kothari, Erik Peterson, Navyateja Korimerla, Jessica Liang, Janna Jacobson, Sravya Palavalasa, Alexandra Obrien, Sean Ferris, Shuang Zhao, Jann N. Sarkaria, Meredith Morgan, Theodore S. Lawrence, Costas A. Lyssiotis, Yatrik Shah, Daniel R. Wahl. GTP-dependent signaling links metabolism, non-homologous end joining and treatment responses in cancer and normal tissues [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3498.

Predicting tumour and normal tissue response during radiotherapy using radiomic analysis from on-board imaging: a systematic review

Predicting tumour and normal tissue response during radiotherapy using radiomic analysis from on-board imaging: a systematic review

Immunoregulation by resveratrol; implications for normal tissue protection and tumour suppression.

Immune reactions are involved in both tumour and normal tissue in response to therapy. Elevated secretion of certain chemokines, exosomes and cytokines triggers inflammation, pain, fibrosis and ulceration among other normal tissue side effects. On the other hand, secretion of tumour-promoting molecules suppresses activity of anticancer immune cells and facilitates the proliferation of malignant cells. Novel anticancer drugs such as immune checkpoint inhibitors (ICIs) boost anticancer immunity via inducing the proliferation of anticancer cells such as natural killer (NK) cells and CD8+ T lymphocytes. Certain chemotherapy drugs and radiotherapy may induce anticancer immunity in the tumour, however, both have severe side effects for normal tissues through stimulation of several immune responses. Thus, administration of natural products with low side effects may be a promising approach to modulate the immune system in both tumour and normal organs. Resveratrol is a well-known phenol with diverse effects on normal tissues and tumours. To date, a large number of experiments have confirmed the potential of resveratrol as an anticancer adjuvant. This review focuses on ensuing stimulation or suppression of immune responses in both tumour and normal tissue after radiotherapy or anticancer drugs. Later on, the immunoregulatory effects of resveratrol in both tumour and normal tissue following exposure to anticancer agents will be discussed.

T cell‐dependent bispecific antibodies alter organ‐specific endothelial cell–T cell interaction

Preclinical and clinical studies demonstrate that T cell‐dependent bispecific antibodies (TDBs) induce systemic changes in addition to tumor killing, leading to adverse events. Here, we report an in‐depth characterization of acute responses to TDBs in tumor‐bearing mice. Contrary to modest changes in tumors, rapid and substantial lymphocyte accumulation and endothelial cell (EC) activation occur around large blood vessels in normal organs including the liver. We hypothesize that organ‐specific ECs may account for the differential responses in normal tissues and tumors, and we identify a list of genes selectively upregulated by TDB in large liver vessels. Using one of the genes as an example, we demonstrate that CD9 facilitates ICAM‐1 to support T cell–EC interaction in response to soluble factors released from a TDB‐mediated cytotoxic reaction. Our results suggest that multiple factors may cooperatively promote T cell infiltration into normal organs as a secondary response to TDB‐mediated tumor killing. These data shed light on how different vascular beds respond to cancer immunotherapy and may help improve their safety and efficacy.

Dosimetric quantities and cell survival for spatially fractionated radiation therapy

Purpose: Spatially Fractionated Radiation Therapy (SFRT) is characterized by large differences in peak and valley doses. Preclinical and clinical studies suggest that differences in biological mechanisms lead to differential normal tissue and tumor response compared to uniform irradiation. We hypothesize that to evaluate clinical effectiveness and understand fundamental biological mechanisms, radiobiological rather than physical dose quantities should be utilized for comparisons. The aim of this work is to determine whether Equivalent Uniform Dose (EUD) is a superior predictor of cell survival than absorbed dose.Methods: Absorbed dose parameters were compared to the Equivalent Uniform Dose to assess their predictive value for the relative effectiveness of uniform and SFRT with X-rays. A Bayesian bootstrap technique was utilized to model uncertainties in the biological fit parameters for a human fibroblast (MRC5) and two human tumor cell lines (LN18 and A549). Dose uncertainties were evaluated through measurements and error modeling of SFRT profiles. A dimensionless Relative Effectiveness Factor (REF) is proposed to quantify differences between uniform and SFRT irradiation.Results: For all cell lines, cell survival after SFRT matched uniform irradiation within the estimated uncertainties at equal values of the EUD. Average and peak dose showed poor correlation with in vitro cell survival. The proposed REF factor is dose dependent and suggests enhanced cell killing for both tumor cell lines (1.14 ± .08 for LN18, 1.32 ± .13 for A549 at 8 Gy EUD) for SFRT. Normal human fibroblasts showed reduced cell killing relative to uniform irradiation (.58 ± .06 for MRC5). Synthetically generated SFRT dose profiles revealed that EUD uncertainties are dominated by valley dose uncertainties, especially at high doses.Discussion: EUD is more predictive of cell survival than average or valley dose. Valley dose is close to equal to the EUD for values >10 Gy and has the advantage of being independent of uncertainties in biological parameters. The REF is a novel and useful metric to compare quantitative differences in SFRT and uniform irradiation.Conclusion: EUD is recommended for comparisons of SFRT and uniform irradiation. The results suggest an increase in survival of normal-human fibroblast cells and reduced survival for both tumor cell lines after SFRT relative to uniform irradiation.