Stereotactic ablative radiotherapy should be combined with a hypoxic cell radiosensitizer

Tumor hypoxia and radiotherapy: A major driver of resistance even for novel radiotherapy modalities

Hypoxia in solid tumors is an important predictor of poor clinical outcome to radiotherapy. Both physicochemical and biological processes contribute to a reduced sensitivity of hypoxic tumor cells to ionizing radiation and hypoxia-related treatment resistances. A conventional low-dose fractionated radiotherapy regimen exploits iterative reoxygenation in between the individual fractions, nevertheless tumor hypoxia still remains a major hurdle for successful treatment outcome. The technological advances achieved in image guidance and highly conformal dose delivery make it nowadays possible to prescribe larger doses to the tumor as part of single high-dose or hypofractionated radiotherapy, while keeping an acceptable level of normal tissue complication in the co-irradiated organs at risk. However, we insufficiently understand the impact of tumor hypoxia to single high-doses of RT and hypofractionated RT. So-called FLASH radiotherapy, which delivers ionizing radiation at ultrahigh dose rates (> 40 Gy/sec), has recently emerged as an important breakthrough in the radiotherapy field to reduce normal tissue toxicity compared to irradiation at conventional dose rates (few Gy/min). Not surprisingly, oxygen consumption and tumor hypoxia also seem to play an intriguing role for FLASH radiotherapy. Here we will discuss the role of tumor hypoxia for radiotherapy in general and in the context of novel radiotherapy treatment approaches.

Individualized Stereotactic Ablative Radiotherapy for Lung Tumors: The iSABR Phase 2 Nonrandomized Controlled Trial

Importance

Stereotactic ablative radiotherapy (SABR) is used for treating lung tumors but can cause toxic effects, including life-threatening damage to central structures. Retrospective data suggested that small tumors up to 10 cm3 in volume can be well controlled with a biologically effective dose less than 100 Gy.

Objective

To assess whether individualizing lung SABR dose and fractionation by tumor size, location, and histological characteristics may be associated with local tumor control.

Design, Setting, and Participants

This nonrandomized controlled trial (the iSABR trial, so named for individualized SABR) was a phase 2 multicenter trial enrolling participants from November 15, 2011, to December 5, 2018, at academic medical centers in the US and Japan. Data were analyzed from December 9, 2020, to May 10, 2023. Patients were enrolled in 3 groups according to cancer type: initial diagnosis of non-small cell lung cancer (NSCLC) with an American Joint Committee on Cancer 7th edition T1-3N0M0 tumor (group 1), a T1-3N0M0 new primary NSCLC with a history of prior NSCLC or multiple NSCLCs (group 2), or lung metastases from NSCLC or another solid tumor (group 3).

Intervention

Up to 4 tumors were treated with once-daily SABR. The dose ranged from 25 Gy in 1 fraction for peripheral tumors with a volume of 0 to 10 cm3 to 60 Gy in 8 fractions for central tumors with a volume greater than 30 cm3.

Main outcome

Per-group freedom from local recurrence (same-lobe recurrence) at 1 year, with censoring at time of distant recurrence, death, or loss to follow-up.

Results

In total, 217 unique patients (median [IQR] age, 72 [64-80] years; 129 [59%] male; 150 [69%] current or former smokers) were enrolled (some multiple times). There were 240 treatment courses: 79 in group 1, 82 in group 2, and 79 in group 3. A total of 285 tumors (211 [74%] peripheral and 74 [26%] central) were treated. The most common dose was 25 Gy in 1 fraction (158 tumors). The median (range) follow-up period was 33 (2-109) months, and the median overall survival was 59 (95% CI, 49-82) months. Freedom from local recurrence at 1 year was 97% (90% CI, 91%-99%) for group 1, 94% (90% CI, 87%-97%) for group 2, and 96% (90% CI, 89%-98%) for group 3. Freedom from local recurrence at 5 years ranged from 83% to 93% in the 3 groups. The proportion of patients with grade 3 to 5 toxic effects was low, at 5% (including a single patient [1%] with grade 5 toxic effects).

Conclusions and Relevance

The results of this nonrandomized controlled trial suggest that individualized SABR (iSABR) used to treat lung tumors may allow minimization of treatment dose and is associated with excellent local control. Individualized dosing should be considered for use in future trials.

Trial Registration

ClinicalTrials.gov Identifier: NCT01463423.

[18F]FDG-PET-Based Personalized Radiotherapy Dose Prescription

PET imaging with 2'-deoxy-2'-[18F]fluoro-D-glucose ([18F]FDG) has become one of the pillars in the management of malignant diseases. It has proven value in diagnostic workup, treatment policy, follow-up, and as prognosticator for outcome. [18F]FDG is widely available and standards have been developed for PET acquisition protocols and quantitative analyses. More recently, [18F]FDG-PET is also starting to be appreciated as a decision aid for treatment personalization. This review focuses on the potential of [18F]FDG-PET for individualized radiotherapy dose prescription. This includes dose painting, gradient dose prescription, and [18F]FDG-PET guided response-adapted dose prescription. The current status, progress, and future expectations of these developments for various tumor types are discussed.

Design, Synthesis and Anticancer Evaluation of Nitroimidazole Radiosensitisers

The role of hypoxic tumour cells in resistance to radiotherapy, and in suppression of immune response, continues to endorse tumour hypoxia as a bona fide, yet largely untapped, drug target. Radiotherapy innovations such as stereotactic body radiotherapy herald new opportunities for classical oxygen-mimetic radiosensitisers. Only nimorazole is used clinically as a radiosensitiser, and there is a dearth of new radiosensitisers in development. In this report, we augment previous work to present new nitroimidazole alkylsulfonamides and we document their cytotoxicity and ability to radiosensitise anoxic tumour cells in vitro. We compare radiosensitisation with etanidazole and earlier nitroimidazole sulfonamide analogues and we identify 2-nitroimidazole and 5-nitroimidazole analogues with marked tumour radiosensitisation in ex vivo assays of surviving clonogens and with in vivo tumour growth inhibition.

A simple mathematical model of cyclic hypoxia and its impact on hypofractionated radiotherapy

PURPOSE

There is evidence that the population of cells that experience fluctuating oxygen levels ("acute," or, "cyclic" hypoxia) are more radioresistant than chronically hypoxic ones and hence, this population may determine radiotherapy (RT) response, in particular for hypofractionated RT, where reoxygenation may not be as prominent. A considerable effort has been devoted to examining the impact of hypoxia on hypofractionated RT; however, much less attention has been paid to cyclic hypoxia specifically and the role its kinetics may play in determining the efficacy of these treatments. Here, a simple mathematical model of cyclic hypoxia and fractionation effects was worked out to quantify this.

METHODS

Cancer clonogen survival fraction was estimated using the linear quadratic model, modified to account for oxygen enhancement effects. An analytic approximation for oxygen transport away from a random network of capillaries with fluctuating oxygen levels was used to model inter-fraction tissue oxygen kinetics. The resulting survival fraction formula was used to derive an expression for the iso-survival biologically effective dose (BED), BED_iso-SF . These were computed for some common extra-cranial hypofractionated RT regimens.

RESULTS

Using relevant literature parameter values, inter-fraction fluctuations in oxygenation were found to result in an added 1-2 logs of clonogen survival fraction in going from five fractions to one for the same nominal BED (i.e., excluding the effects of oxygen levels on radiosensitivity). BED_iso-SF 's for most ultra-hypofractionated (five or fewer fractions) regimens in a given tumor site are similar in magnitude, suggesting iso-efficacy for common fractionation schedules.

CONCLUSIONS

Although significant, the loss of cell-killing with increasing hypofractionation is not nearly as large as previous estimates based on the assumption of complete reoxygenation between fractions. Most ultra-hypofractionated regimens currently in place offer sufficiently high doses to counter this loss of cell killing, although care should be taken in implementing single-fraction regimens.

Sensitizing brain metastases to stereotactic radiosurgery using hyperbaric oxygen: A proof-of-principle study

PURPOSE

Increased oxygen levels may enhance the radiosensitivity of brain metastases treated with stereotactic radiosurgery (SRS). This project administered hyperbaric oxygen (HBO) prior to SRS to assess feasibility, safety, and response.

METHODS

38 patients were studied, 19 with 25 brain metastases treated with HBO prior to SRS, and 19 historical controls with 27 metastases, matched for histology, GPA, resection status, and lesion size. Outcomes included time from HBO to SRS, quality-of-life (QOL) measures, local control, distant (brain) metastases, radionecrosis, and overall survival.

RESULTS

The average time from HBO chamber to SRS beam-on was 8.3 ± 1.7 minutes. Solicited adverse events (AEs) were comparable between HBO and control patients; no grade III or IV serious AEs were observed. Radionecrosis-free survival (RNFS), radionecrosis-free survival before whole-brain radiation therapy (WBRT) (RNBWFS), local recurrence-free survival before WBRT (LRBWFS), distant recurrence-free survival before WBRT (DRBWFS), and overall survival (OS) were not significantly different for HBO patients and controls on Kaplan-Meier analysis, though at 1-year estimated survival rates trended in favor of SRS + HBO: RNFS - 83% vs 60%; RNBWFS - 78% vs 60%; LRBWFS - 95% vs 78%; DRBWFS - 61% vs 57%; and OS - 73% vs 56%. Multivariate Cox models indicated no significant association between HBO treatment and hazards of RN, local or distant recurrence, or mortality; however, these did show statistically significant associations (p < 0.05) for: local recurrence with higher volume, radionecrosis with tumor resection, overall survival with resection, and overall survival with higher GPA.

CONCLUSION

Addition of HBO to SRS for brain metastases is feasible without evident decrement in radiation necrosis and other clinical outcomes.

Vascular damage in tumors: a key player in stereotactic radiation therapy?

The use of stereotactic radiation therapy (SRT) for cancer treatment has grown in recent years, showing excellent results for some tumors. The greatly increased doses per fraction in SRT compared to conventional radiotherapy suggest a 'new biology' that determines treatment outcome. Proposed mechanisms include significant damage to tumor blood vessels and enhanced antitumor immune responses, which are also vasculature-dependent. These ideas are mostly based on the results of radiation studies in animal models because direct observations in humans are limited. However, even preclinical findings are somewhat incomplete and result in ambiguous conclusions. Current evidence of vasculature-related mechanisms of SRT is reviewed. Understanding them could result in better optimization of SRT alone or in combination with immune or other cancer therapies.

The Potential of Photoacoustic Imaging in Radiation Oncology

Radiotherapy is recognized globally as a mainstay of treatment in most solid tumors and is essential in both curative and palliative settings. Ionizing radiation is frequently combined with surgery, either preoperatively or postoperatively, and with systemic chemotherapy. Recent advances in imaging have enabled precise targeting of solid lesions yet substantial intratumoral heterogeneity means that treatment planning and monitoring remains a clinical challenge as therapy response can take weeks to manifest on conventional imaging and early indications of progression can be misleading. Photoacoustic imaging (PAI) is an emerging modality for molecular imaging of cancer, enabling non-invasive assessment of endogenous tissue chromophores with optical contrast at unprecedented spatio-temporal resolution. Preclinical studies in mouse models have shown that PAI could be used to assess response to radiotherapy and chemoradiotherapy based on changes in the tumor vascular architecture and blood oxygen saturation, which are closely linked to tumor hypoxia. Given the strong relationship between hypoxia and radio-resistance, PAI assessment of the tumor microenvironment has the potential to be applied longitudinally during radiotherapy to detect resistance at much earlier time-points than currently achieved by size measurements and tailor treatments based on tumor oxygen availability and vascular heterogeneity. Here, we review the current state-of-the-art in PAI in the context of radiotherapy research. Based on these studies, we identify promising applications of PAI in radiation oncology and discuss the future potential and outstanding challenges in the development of translational PAI biomarkers of early response to radiotherapy.

Imaging the Rewired Metabolism in Lung Cancer in Relation to Immune Therapy

Metabolic reprogramming is recognized as one of the hallmarks of cancer. Alterations in the micro-environmental metabolic characteristics are recognized as important tools for cancer cells to interact with the resident and infiltrating T-cells within this tumor microenvironment. Cancer-induced metabolic changes in the micro-environment also affect treatment outcomes. In particular, immune therapy efficacy might be blunted because of somatic mutation-driven metabolic determinants of lung cancer such as acidity and oxygenation status. Based on these observations, new onco-immunological treatment strategies increasingly include drugs that interfere with metabolic pathways that consequently affect the composition of the lung cancer tumor microenvironment (TME). Positron emission tomography (PET) imaging has developed a wide array of tracers targeting metabolic pathways, originally intended to improve cancer detection and staging. Paralleling the developments in understanding metabolic reprogramming in cancer cells, as well as its effects on stromal, immune, and endothelial cells, a wave of studies with additional imaging tracers has been published. These tracers are yet underexploited in the perspective of immune therapy. In this review, we provide an overview of currently available PET tracers for clinical studies and discuss their potential roles in the development of effective immune therapeutic strategies, with a focus on lung cancer. We report on ongoing efforts that include PET/CT to understand the outcomes of interactions between cancer cells and T-cells in the lung cancer microenvironment, and we identify areas of research which are yet unchartered. Thereby, we aim to provide a starting point for molecular imaging driven studies to understand and exploit metabolic features of lung cancer to optimize immune therapy.

Therapeutic targeting of the hypoxic tumour microenvironment

Hypoxia is prevalent in human tumours and contributes to microenvironments that shape cancer evolution and adversely affect therapeutic outcomes. Historically, two different tumour microenvironment (TME) research communities have been discernible. One has focused on physicochemical gradients of oxygen, pH and nutrients in the tumour interstitium, motivated in part by the barrier that hypoxia poses to effective radiotherapy. The other has focused on cellular interactions involving tumour and non-tumour cells within the TME. Over the past decade, strong links have been established between these two themes, providing new insights into fundamental aspects of tumour biology and presenting new strategies for addressing the effects of hypoxia and other microenvironmental features that arise from the inefficient microvascular system in solid tumours. This Review provides a perspective on advances at the interface between these two aspects of the TME, with a focus on translational therapeutic opportunities relating to the elimination and/or exploitation of tumour hypoxia.

A multi-institutional analysis of fractionated versus single-fraction stereotactic body radiotherapy (SBRT) in the treatment of primary lung tumors: a comparison between two antipodal fractionations

OBJECTIVES

Stereotactic body radiotherapy (SBRT) is a consolidate treatment for inoperable early-stage lung tumors, usually delivered in single or multi-fraction regimens. We aimed to compare these two approaches in terms of local effectiveness, safety and survival.

MATERIALS AND METHODS

Patients affected by medically inoperable early-stage lung tumor were treated at two Institutions with two different schedules: 70 Gy in ten fractions (TF) (BED10: 119 Gy) or 30 Gy in single fraction (SF) (BED10: 120 Gy).

RESULTS

73 patients were treated with SBRT delivered with two biological equivalent schedules: SF (44) and TF (29). The median follow-up was 34 months (range 3-81 months). Three-year Overall survival (OS) was 57.9%, 3-year cancer-specific survival (CSS) was 77.2%, with no difference between treatment groups. Three-year progression-free survival (LPFS) was 88.9% and did not differs between SF and TF. Overall, four cases (5.4%) of acute grade ≥ 3 pneumonitis occurred. No differences in acute and late toxicity between the two groups were detected.

CONCLUSION

SF and TF seems to be equally safe and effective in the treatment of primary inoperable lung tumors especially for smaller lesion. The SF may be preferentially offered to reduce patient access to hospital with no negative impact on tumor control and survival.

Potential Clinical Significance of Overall Targeting Accuracy and Motion Management in the Treatment of Tumors That Move With Respiration: Lessons Learnt From a Quarter Century of Stereotactic Body Radiotherapy From Dose Response Models

OBJECTIVE

To determine the long-term normal tissue complication probability with stereotactic body radiation therapy (SBRT) treatments for targets that move with respiration and its relation with the type of respiratory motion management (tracking vs. compression or gating).

METHODS

A PubMed search was performed for identifying literature regarding dose, volume, fractionation, and toxicity (grade 3 or higher) for SBRT treatments for tumors which move with respiration. From the identified papers logistic or probit dose-response models were fitted to the data using the maximum-likelihood technique and confidence intervals were based on the profile-likelihood method in the dose-volume histogram (DVH) Evaluator.

RESULTS

Pooled logistic and probit models for grade 3 or higher toxicity for aorta, chest wall, duodenum, and small bowel suggest a significant difference when live motion tracking was used for targeting tumors with move with respiration which was on the average 10 times lower, in the high dose range.

CONCLUSION

Live respiratory motion management appears to have a better toxicity outcome when treating targets which move with respiration with very steep peripheral dose gradients. This analysis is however limited by sparsity of rigorous data due to poor reporting in the literature.

Local control and toxicity outcomes of stereotactic radiosurgery for spinal metastases of gastrointestinal origin

OBJECTIVE

Colorectal cancer (CRC) and other gastrointestinal (GI) cancers are believed to have greater radioresistance than other histologies. The authors report local control and toxicity outcomes of stereotactic radiosurgery (SRS) to spinal metastases from GI primary cancers.

METHODS

A retrospective single-center review was conducted of patients with spinal metastases from GI primary cancers treated with SRS from 2004 to 2017. Patient demographics and lesion characteristics were summarized using medians, interquartile ranges (IQRs), and proportions. Local failure (LF) was estimated using the cumulative incidence function adjusted for the competing risk of death and compared using Gray's test for equality. Multivariable analyses were conducted using Cox proportional hazard models, adjusting for death as a competing risk, on a per-lesion basis. Patients were stratified in the Cox model to account for repeated measures for clustered outcomes. Median survival was calculated using the Kaplan-Meier method.

RESULTS

A total of 74 patients with 114 spine lesions were included in our analysis. The median age of the cohort was 62 years (IQR 53-70 years). Histologies included CRC (46%), hepatocellular carcinoma (19%), neuroendocrine carcinoma (13%), pancreatic carcinoma (12%), and other (10%). The 1- and 2-year cumulative incidence rates of LF were 24% (95% confidence interval [CI] 16%-33%) and 32% (95% CI 23%-42%), respectively. Univariable analysis revealed that older age (p = 0.015), right-sided primary CRCs (p = 0.038), and single fraction equivalent dose (SFED; α/β = 10) < 20 Gy (p = 0.004) were associated with higher rates of LF. The 1-year cumulative incidence rates of LF for SFED < 20 Gy10 versus SFED ≥ 20 Gy10 were 35% and 7%, respectively. After controlling for gross tumor volume and prior radiation therapy to the lesion, SFED < 20 Gy10 remained independently associated with worse LF (hazard ratio 2.92, 95% CI 1.24-6.89, p = 0.014). Toxicities were minimal, with pain flare observed in 6 patients (8%) and 15 vertebral compression fractures (13%).

CONCLUSIONS

Spinal metastases from GI primary cancers have high rates of LF with SRS at a lower dose. This study found that SRS dose is a significant predictor of failure and that prescribed SFED ≥ 20 Gy10 (biological equivalent dose ≥ 60 Gy10) is associated with superior local control.

Optimization of irradiation interval for fractionated stereotactic radiosurgery by a cellular automata model with reoxygenation effects

The current study aims to determine the optimal irradiation interval of fractionated stereotactic radiosurgery (SRS) by using an improved cellular automata (CA) model. The tumor growth process was simulated by considering the amount of oxygen and the density of blood vessels, which supplied oxygen and nutrient required for cell growth. Cancer cells died by the mitotic death process due to radiation, which was quantified by the LQ-model, or the apoptosis due to the lack of nutrients. The radiation caused increased permeation of plasma protein through the blood vessel or the breakdown of the vasculature. Consequently, these changes lead to a change in radiation sensitivity of cancer cells and tumor growth rate after irradiation. The optimal model parameters were determined with experimental data of the rat tumor volume. The tumor control probability (TCP) was defined as the ratio of the number of histories in which all cancer cells died after the irradiation to the total number of the histories per simulation. The optimal irradiation interval was defined as the irradiation interval that TCP was the maximum. For one fractionation treatment, the ratio of hypoxic cells to the total number of cancer cells kept decreasing until day 16th after irradiation; whereas the number of surviving cancer cells begun increasing immediately after irradiation. This intricate relationship between the hypoxia (or reoxygenation) and the number of cancer cells lead to an optimal irradiation interval for the second irradiation. The optimal irradiation interval for two-fraction SRS was six days. The optimum intervals for the first-second irradiations and the second-third irradiations were five and two days, respectively, for three fraction SRS. For 4 and 5-fraction treatments, the optimum first-interval was five days, which was similar to three fraction treatment. The remaining intervals should be one day. We showed that the improved CA model could be used to optimize the irradiation interval by explicitly including the reoxygenation after irradiation in the model.

Pre-treatment serum bicarbonate predicts for primary tumor control after stereotactic body radiation therapy in patients with localized non-small cell lung cancer

BACKGROUND

Tumor aggressiveness and hypoxia are linked to acidosis in the tumor microenvironment (TME). We hypothesized that low pre-treatment serum bicarbonate, potentially correlating with an acidic and hypoxic TME, predicts for poor outcomes after stereotactic body radiation therapy (SBRT) for non-small cell lung cancer (NSCLC).

METHODS

We included patients with localized NSCLC treated to a biologically effective dose (BED) ≥ 100 Gy, with available pre-treatment bicarbonate values within 3 months of treatment. We used receiver operating characteristic analysis to determine the bicarbonate concentration optimally predicting for primary tumor recurrence, and evaluated its association with recurrence and survival. We validated our findings in an independent cohort of patients from three collaborating institutions.

RESULTS

A total of 110 patients and 114 tumors were included in the training cohort, with median follow-up of 15.0 months. Bicarbonate < 26 mEq/L was associated with primary tumor recurrence on univariate (HR = 5.92; 95% CI 1.69-24.88; p = 0.005) and multivariate analysis (HR = 5.48; 95% CI 1.37-25.19; p = 0.020). The validation cohort consisted of 195 patients and 208 tumors with median follow-up of 27.5 months. In the validation cohort, bicarbonate < 26 mEq/L was again associated with primary tumor recurrence on univariate (HR = 3.38; 95% CI 1.27-9.37; p = 0.015) and multivariate analysis (HR = 3.33; 1.18-10.07; p = 0.023).

CONCLUSIONS

Pre-treatment bicarbonate predicts for primary tumor control in NSCLC treated with SBRT and may be useful for risk stratification. These findings should be confirmed prospectively.

Local Treatment of Breast Cancer Liver Metastasis

Breast cancer represents a leading cause of death worldwide. Despite the advances in systemic therapies, the prognosis for patients with breast cancer liver metastasis (BCLM) remains poor. Especially in case of failure or cessation of systemic treatments, surgical resection for BCLMs has been considered as the treatment standard despite a lack of robust evidence of benefit. However, due to the extent and location of disease and physical condition, the number of patients with BCLM who are eligible for surgery is limited. Palliative locoregional treatments of liver metastases (LM) include transarterial embolization (TAE), transarterial chemoembolization (TACE), and selective internal radiotherapy (SIRT). Percutaneous thermal ablation methods, such as radiofrequency ablation (RFA) and microwave ablation (MWA), are considered potentially curative local treatment options. They are less invasive, less expensive and have fewer contraindications and complication rates than surgery. Because conventional ultrasound- and computed tomography-guided single-probe thermal ablation is limited by tumor size, multi-probe stereotactic radiofrequency ablation (SRFA) with intraoperative image fusion for immediate, reliable judgment has been developed in order to treat large and multiple tumors within one session. This review focuses on the different minimally invasive local and locoregional treatment options for BCLM and attempts to describe their current and future role in the multidisciplinary treatment setting.

Increases in Serial Pretreatment 18F-FDG PET-CT Metrics Predict Survival in Early Stage Non-Small Cell Lung Cancer Treated With Stereotactic Ablative Radiation Therapy

Purpose

Quantitative changes in positron emission tomography with computed tomography imaging metrics over serial scans may be predictive biomarkers. We evaluated the relationship of pretreatment metabolic tumor growth rate (MTGR) and standardized uptake value velocity (SUVV) with disease recurrence or death in patients with early-stage non-small cell lung cancer treated with stereotactic ablative radiation therapy (SABR).

Methods and Materials

Under institutional review board approval, we retrospectively identified patients who underwent positron emission tomography with computed tomography at diagnosis and staging and simulation for SABR. Two cohorts underwent SABR between November 2005 to October 2012 (discovery) and January 2012 to April 2016 (validation). MTGR and SUVV were calculated as the daily change in metabolic tumor volume and maximum standardized uptake value, respectively. Cox proportional hazard models identified predictors of local, regional, and distant recurrence and death for the combined cohort. MTGR and SUVV thresholds dichotomizing risk of death in the discovery cohort were applied to the validation cohort.

Results

A total of 152 lesions were identified in 143 patients (92 lesions in 83 discovery cohort patients). In multivariable models, increasing MTGR trended toward increased hazard of distant recurrence (hazard ratio, 6.98; 95% confidence interval, 0.67-72.61; P = .10). In univariable models, SUVV trended toward risk of death (hazard ratio, 11.8, 95% confidence interval, 0.85-165.1, P = .07). MTGR greater than 0.04 mL/d was prognostic of decreased survival in discovery (P = .048) and validation cohorts (P < .01).

Conclusions

MTGR greater than 0.04 mL/d is prognostic of death in patients with non-small cell lung cancer treated with SABR. Increasing SUVV trends, nonsignificantly, toward increased risk of recurrence and death. MTGR and SUVV may be candidate imaging biomarkers to study in trials evaluating systemic therapy with SABR for patients at high risk of out-of-field recurrence.

Overcoming Radioresistance: Small Molecule Radiosensitisers and Hypoxia-activated Prodrugs

The role of hypoxia in radiation resistance is well established and many approaches to overcome hypoxia in tumours have been explored, with variable success. Two small molecule strategies for targeting hypoxia have dominated preclinical and clinical efforts. One approach has been the use of electron-affinic nitroheterocycles as oxygen-mimetic sensitisers. These agents are best exemplified by the 5-nitroimidazole nimorazole, which has limited use in conjunction with radiotherapy in head and neck squamous cell carcinoma. The second approach seeks to leverage tumour hypoxia as a tumour-specific address for hypoxia-activated prodrugs. These prodrugs are selectively activated by reductases under hypoxia to release cytotoxins, which in some instances may diffuse to kill surrounding oxic tumour tissue. A number of these hypoxia-activated prodrugs have been examined in clinical trial and the merits and shortcomings of recent examples are discussed. There has been an evolution from delivering DNA-interactive cytotoxins to molecularly targeted agents. Efforts to implement these strategies clinically continue today, but success has been elusive. Several issues have been identified that compromised these clinical campaigns. A failure to consider the extravascular transport and the micropharmacokinetic properties of the prodrugs has reduced efficacy. One key element for these 'targeted' approaches is the need to co-develop biomarkers to identify appropriate patients. Hypoxia-activated prodrugs require biomarkers for hypoxia, but also for appropriate activating reductases in tumours, as well as markers of intrinsic sensitivity to the released drug. The field is still evolving and changes in radiation delivery and the impact of immune-oncology will provide fertile ground for future innovation.

Biological Principles of Stereotactic Body Radiation Therapy (SBRT) and Stereotactic Radiation Surgery (SRS): Indirect Cell Death

PURPOSE

To review the radiobiological mechanisms of stereotactic body radiation therapy stereotactic body radiation therapy (SBRT) and stereotactic radiation surgery (SRS).

METHODS AND MATERIALS

We reviewed previous reports and recent observations on the effects of high-dose irradiation on tumor cell survival, tumor vasculature, and antitumor immunity. We then assessed the potential implications of these biological changes associated with SBRT and SRS.

RESULTS

Irradiation with doses higher than approximately 10 Gy/fraction causes significant vascular injury in tumors, leading to secondary tumor cell death. Irradiation of tumors with high doses has also been reported to increase the antitumor immunity, and various approaches are being investigated to further elevate antitumor immunity. The mechanism of normal tissue damage by high-dose irradiation needs to be further investigated.

CONCLUSIONS

In addition to directly killing tumor cells, high-dose irradiation used in SBRT and SRS induces indirect tumor cell death via vascular damage and antitumor immunity. Further studies are warranted to better understand the biological mechanisms underlying the high efficacy of clinical SBRT and SRS and to further improve the efficacy of SBRT and SRS.

Comparison of Three Radiobiological Models in Stereotactic Body Radiotherapy for Non-Small Cell Lung Cancer

Objective: The applicability of the linear quadratic (LQ) model to local control (LC) modeling after hypofractionated radiotherapy to treat lung cancer is highly debated. To date, the differences in predicted outcomes between the LQ model and other radiobiological models, which are characterized by additional dose modification beyond a certain transitional dose (d_T), have not been well established. This study aims to compare the outcomes predicted by the LQ model with those predicted by two other radiobiological models in stereotactic body radiotherapy (SBRT) for non-small cell lung cancer (NSCLC). Methods: Computer tomography (CT) simulation data sets for 20 patients diagnosed with stage Ⅰ primary NSCLC were included in this study. Three radiobiological models, including the LQ, the universal survival curve (USC) and the modified linear quadratic and linear (mLQL) model were employed to predict the tumor control probability (TCP) data. First, the d_T values for the USC and mLQL models were determined. Then, the biologically effective dose (BED) and the predicted TCP values from the LQ model were compared with those calculated from the USC and mLQL models. Results: The d_T values from the USC model were 29.6 Gy, 33.8 Gy and 44.5 Gy, whereas the values were 90.2 Gy, 84.0 Gy and 57.3 Gy for the mLQL model for 1-year, 2-year and 3-year TCP prediction. The remarkable higher d_T values obtained from the mLQL model revealed the same dose-response relationship as the LQ model in the low- and high-dose ranges. We also found that TCP prediction from the LQ and USC models differed by less than 3%, although the BED values for the two models were significantly different. Conclusion: Radiobiological analysis reveals small differences between the models and suggested that the LQ model is applicable for modeling LC using SBRT to treat lung cancer, even when an extremely high fractional dose is used.

Natural radioprotectors and their impact on cancer drug discovery

Cancer is a complex multifaceted illness that affects different patients in discrete ways. For a number of cancers the use of chemotherapy has become standard practice. Chemotherapy is a use of cytostatic drugs to cure cancer. Cytostatic agents not only affect cancer cells but also affect the growth of normal cells; leading to side effects. Because of this, radiotherapy gained importance in treating cancer. Slaughtering of cancerous cells by radiotherapy depends on the radiosensitivity of the tumor cells. Efforts to improve the therapeutic ratio have resulted in the development of compounds that increase the radiosensitivity of tumor cells or protect the normal cells from the effects of radiation. Amifostine is the only chemical radioprotector approved by the US Food and Drug Administration (FDA), but due to its side effect and toxicity, use of this compound was also failed. Hence the use of herbal radioprotectors bearing pharmacological properties is concentrated due to their low toxicity and efficacy. Notably, in silico methods can expedite drug discovery process, to lessen the compounds with unfavorable pharmacological properties at an early stage of drug development. Hence a detailed perspective of these properties, in accordance with their prediction and measurement, are pivotal for a successful identification of radioprotectors by drug discovery process.

A novel index for assessing treatment plan quality in stereotactic radiosurgery

OBJECTIVE

Stereotactic radiosurgery (SRS) is characterized by high levels of conformity and steep dose gradients from the periphery of the target to surrounding tissue. Clinical studies have backed up the importance of these factors through evidence of symptomatic complications. Available data suggest that there are threshold doses above which the risk of symptomatic radionecrosis increases with the volume irradiated. Therefore, radiosurgical treatment plans should be optimized by minimizing dose to the surrounding tissue while maximizing dose to the target volume. Several metrics have been proposed to quantify radiosurgical plan quality, but all present certain weaknesses. To overcome limitations of the currently used metrics, a novel metric is proposed, the efficiency index (η50%), which is based on the principle of calculating integral doses: η50% = integral doseTV/integral dosePIV50%.

METHODS

The value of η50% can be easily calculated by dividing the integral dose (mean dose × volume) to the target volume (TV) by the integral dose to the volume of 50% of the prescription isodose (PIV50%). Alternatively, differential dose-volume histograms (DVHs) of the TV and PIV50% can be used. The resulting η50% value is effectively the proportion of energy within the PIV50% that falls into the target. This value has theoretical limits of 0 and 1, with 1 being perfect. The index combines conformity, gradient, and mean dose to the target into a single value. The value of η50% was retrospectively calculated for 100 clinical SRS plans.

RESULTS

The value of η50% for the 100 clinical SRS plans ranged from 37.7% to 58.0% with a mean value of 49.0%. This study also showed that the same principles used for the calculation of η50% can be adapted to produce an index suitable for multiple-target plans (Gη12Gy). Furthermore, the authors present another adaptation of the index that may play a role in plan optimization by calculating and minimizing the proportion of energy delivered to surrounding organs at risk (OARη50%).

CONCLUSIONS

The proposed efficiency index is a novel approach in quantifying plan quality by combining conformity, gradient, and mean dose into a single value. It quantifies the ratio of the dose “doing good” versus the dose “doing harm,” and its adaptations can be used for multiple-target plan optimization and OAR sparing.

Effects of different doses of X-ray irradiation on cell apoptosis, cell cycle, DNA damage repair and glycolysis in HeLa cells

The present study examined the radiation biological response of cancer cells to different fractional irradiation doses and investigates the optimal fractional irradiation dose with improved biological effects. Radiobiological studies were performed at the molecular and cellular levels to provide insights into DNA damage and repair, and the apoptosis mechanism of cells that were exposed to different doses of X-ray irradiation (0, 2, 4, 6, 8, 10, 12.5, 15 and 20 Gy). Evidence of increased reactive oxygen species (ROS), DNA double strand breaks (DSB), cellular apoptosis, G2/M phase proportion and inhibition of cell proliferation were observed following irradiation. Differences in the ROS amount and apoptotic percentages of cells between the 2 and 4 Gy groups were insignificant. Compared with 0 Gy, the expression of the apoptosis suppression protein B-cell lymphoma-2 was decreased following at increased irradiation doses. However, apoptosis-associated protein Bcl-2-associated X (Bax), caspase-9 and BH3 interacting domain death agonist (Bid) were elevated following irradiation, compared with the control group (0 Gy). Furthermore, the expression levels of Bax in the 6, 8, 10 and 12.5 Gy groups were significantly increased, compared with the other groups. Caspase-9 expression with 2, 4, 6 and 8 Gy were increased compared with other groups, and the Bid levels with 6 and 8 Gy were also increased compared with other groups. G2/M phase arrest was associated with the increase of checkpoint kinase 1 and reduction of cyclin dependent kinase 1. DNA damage repair was associated with the protein Ku70 in the 2, 8, 10, 12.5, 15 and 20 Gy groups were less than other group. Compared with other group, Ku80 levels were reduced in the 6 and 8 Gy groups, and Rad51 levels were reduced in the 2, 8 and 10 Gy groups. The expression of hypoxia inducible factor-1α, c-Myc and glucose transporter 1 (GLUT1) demonstrated an increasing trend following irradiation in a dose-dependent manner, but the expression of pyruvate kinase M2, in the 2–10 Gy irradiation groups, and GLUT1, in the 12.5, 15 and 20 Gy irradiation groups, were reduced, compared with the other groups. Considering the DNA damage repair and apoptosis mechanisms at molecular and cellular levels, it was concluded that 2, 6, 8 and 10 Gy may be the optimal fractional dose that can promote cell apoptosis, and inhibit DNA damage repair and glycolysis.

High Single Doses of Radiation May Induce Elevated Levels of Hypoxia in Early-Stage Non-Small Cell Lung Cancer Tumors

PURPOSE

Tumor hypoxia correlates with treatment failure in patients undergoing conventional radiation therapy. However, no published studies have investigated tumor hypoxia in patients undergoing stereotactic body radiation therapy (SBRT). We aimed to noninvasively quantify the tumor hypoxic volume (HV) in non-small cell lung cancer (NSCLC) tumors to elucidate the potential role of tumor vascular response and reoxygenation at high single doses.

METHODS AND MATERIALS

Six SBRT-eligible patients with NSCLC tumors >1 cm were prospectively enrolled in an institutional review board-approved study. Dynamic positron emission tomography images were acquired at 0 to 120 minutes, 150 to 180 minutes, and 210 to 240 minutes after injection of 18F-fluoromisonidazole. Serial imaging was performed prior to delivery of 18 Gy and at approximately 48 hours and approximately 96 hours after SBRT. Tumor HVs were quantified using the tumor-to-blood ratio (>1.2) and rate of tracer influx (>0.0015 mL·min·cm-3).

RESULTS

An elevated and in some cases persistent level of tumor hypoxia was observed in 3 of 6 patients. Two patients exhibited no detectable baseline tumor hypoxia, and 1 patient with high baseline hypoxia only completed 1 imaging session. On the basis of the tumor-to-blood ratio, in the remaining 3 patients, tumor HVs increased on day 2 after 18 Gy and then showed variable responses on day 4. In the 3 of 6 patients with detectable hypoxia at baseline, baseline tumor HVs ranged between 17% and 24% (mean, 21%), and HVs on days 2 and 4 ranged between 33% and 45% (mean, 40%) and between 18% and 42% (mean, 28%), respectively.

CONCLUSIONS

High single doses of radiation delivered as part of SBRT may induce an elevated and in some cases persistent state of tumor hypoxia in NSCLC tumors. Hypoxia imaging with 18F-fluoromisonidazole positron emission tomography should be used in a larger cohort of NSCLC patients to determine whether elevated tumor hypoxia is predictive of treatment failure in SBRT.

Stereotactic body radiation therapy (SBRT) for early stage non-small cell lung cancer (NSCLC): contemporary insights and advances

The standard-of-care treatment for early-stage non-small cell lung cancer (NSCLC) continues to be surgery in the form of lobectomy or pneumonectomy. Stereotactic body radiation therapy (SBRT) has evolved as a viable alternative to surgery for medically inoperable patients, achieving excellent local control (LC) with relatively minimal toxicity in standard-risk patients. Nevertheless, the maturation of SBRT has fostered debate regarding its use, technique, dose, and fractionation, particularly in the context of patient- and disease-specific characteristics such as tumor size and location. This review will cover the recent trends and future directions of SBRT as it becomes an increasingly individualized modality in the treatment of early-stage NSCLC.

The Impact of Radiation on the Tumor Microenvironment: Effect of Dose and Fractionation Schedules

In addition to inducing lethal DNA damage in tumor and stromal cells, radiation can alter the interactions of tumor cells with their microenvironment. Recent technological advances in planning and delivery of external beam radiotherapy have allowed delivery of larger doses per fraction (hypofractionation) while minimizing dose to normal tissues with higher precision. The effects of radiation on the tumor microenvironment vary with dose and fractionation schedule. In this review, we summarize the effects of conventional and hypofractionated radiation regimens on the immune system and tumor stroma. We discuss how these interactions may provide therapeutic benefit in combination with targeted therapies. Understanding the differential effects of radiation dose and fractionation can have implications for choice of combination therapies.

Next-Generation Hypoxic Cell Radiosensitizers: Nitroimidazole Alkylsulfonamides

Innovations in the field of radiotherapy such as stereotactic body radiotherapy, along with the advent of radio-immuno-oncology, herald new opportunities for classical oxygen-mimetic radiosensitizers. The role of hypoxic tumor cells in resistance to radiotherapy and in suppression of immune response continues to endorse tumor hypoxia as a bona fide, yet largely untapped, drug target. Only nimorazole is used clinically as a radiosensitizer, and there is a dearth of new radiosensitizers in development. Here we present a survey of novel nitroimidazole alkylsulfonamides and document their cytotoxicity and ability to radiosensitize anoxic tumor cells in vitro. We use a phosphate prodrug approach to increase aqueous solubility and to improve tumor drug delivery. A 2-nitroimidazole and a 5-nitroimidazole analogue demonstrated marked tumor radiosensitization in either ex vivo assays of surviving clonogens or tumor regrowth delay.

Effect of primary tumor location and tumor size on the response to radiotherapy for liver metastases from colorectal cancer

Metastatic liver tumors (MLTs) from colorectal cancer (CRC) are often treated with stereotactic body radiation therapy (SBRT). The present study aimed to examine the predictive factors for response of MLTs to SBRT. A total of 39 MLTs from 24 patients with CRC were retrospectively analyzed. Radiotherapy for MLT was typically performed with a prescribed dose equivalent to a biologically effective dose (BED)10 of 100 Gy. The median follow-up period was 16 months (range, 5-64 months). The median prescribed dose and total BED10 were 56 Gy (range, 45-72 Gy) and 97.5 Gy (range, 71.7-115.5 Gy), respectively, in a median of 8 fractions (range, 4-33 fractions). The 1- and 2-year local control rates were 67.2 and 35.9%, respectively. For patients with MLT treated with ablative SBRT (BED10 ≥100 Gy in ≤5 fractions), the 1- and 2-year local control rates were 83.3 and 62.5%, respectively. Univariate analysis showed that primary tumor location (left-sided colon), maximum tumor diameter (≤30 mm) and ablative SBRT (BED10 ≥100 Gy in ≤5 fractions) were significantly associated with improved local control (P=0.0058, P=0.0059 and P=0.0268, respectively). Multivariate analysis showed that tumor diameter was significantly associated with improved local control (P=0.0314). In addition, patients who received ablative SBRT had significantly prolonged overall survival times compared with those treated with non-ablative SBRT (P=0.0261). To conclude, tumors ≤30 mm that can be treated with ablative SBRT are associated with good local control rates. The primary tumor location may affect the radiosensitivity of MLTs.

Modeling the Cellular Response of Lung Cancer to Radiation Therapy for a Broad Range of Fractionation Schedules

Purpose: To demonstrate that a mathematical model can be used to quantitatively understand tumor cellular dynamics during a course of radiotherapy and to predict the likelihood of local control as a function of dose and treatment fractions.Experimental Design: We model outcomes for early-stage, localized non-small cell lung cancer (NSCLC), by fitting a mechanistic, cellular dynamics-based tumor control probability that assumes a constant local supply of oxygen and glucose. In addition to standard radiobiological effects such as repair of sub-lethal damage and the impact of hypoxia, we also accounted for proliferation as well as radiosensitivity variability within the cell cycle. We applied the model to 36 published and two unpublished early-stage patient cohorts, totaling 2,701 patients.Results: Precise likelihood best-fit values were derived for the radiobiological parameters: α [0.305 Gy^-1; 95% confidence interval (CI), 0.120-0.365], the α/β ratio (2.80 Gy; 95% CI, 0.40-4.40), and the oxygen enhancement ratio (OER) value for intermediately hypoxic cells receiving glucose but not oxygen (1.70; 95% CI, 1.55-2.25). All fractionation groups are well fitted by a single dose-response curve with a high χ² P value, indicating consistency with the fitted model. The analysis was further validated with an additional 23 patient cohorts (n = 1,628). The model indicates that hypofractionation regimens overcome hypoxia (and cell-cycle radiosensitivity variations) by the sheer impact of high doses per fraction, whereas lower dose-per-fraction regimens allow for reoxygenation and corresponding sensitization, but lose effectiveness for prolonged treatments due to proliferation.Conclusions: This proposed mechanistic tumor-response model can accurately predict overtreatment or undertreatment for various treatment regimens. Clin Cancer Res; 23(18); 5469-79. ©2017 AACR.

Radiobiology of stereotactic body radiation therapy (SBRT)

Recent advances in the technology of radiotherapy have enabled the development of new therapeutic modalities that deliver radiation with very high accuracy, reduced margins and high dose conformation, allowing the reduction of healthy tissue irradiated and therefore minimizing the risk of toxicity. The next step was to increase the total tumor dose using conventional fractionation (which remains the best way to relatively radioprotect healthy tissues when large volumes are treated) or to use new fractionation schemes with greater biological effectiveness. Based on the experience gained in radiosurgery, the latter way was chosen for small and well-defined tumors in the body. Stereotactic body radiotherapy delivers high doses of radiation to small and well-defined targets in an extreme hypofractionated (and accelerated) scheme with a very high biological effectiveness obtaining very good initial clinical results in terms of local tumor control and acceptable rate of late complications. In fact, we realize a posteriori that it was not feasible to administer such biologically equivalent dose in a conventional fractionation because the treatment could last several months. So far, these new therapeutic modalities have been developed due to technologic advances in image guidance and treatment delivery but without a solid biological basis. It is the role of traditional radiobiology (and molecular radiobiology) to explain the effects of high doses of ionizing radiation on tumor and normal tissues. Only through a better understanding of how high doses of ionizing radiation act, clinicians will know exactly what we do, allowing us in the future to refine our treatments. This article attempts to describe through simple and understandable concepts the known aspects of the biological action of high doses of radiation on tumor and normal tissues, but it is clear that we need much more basic research to better understand the biology of high doses of radiation.

Stereotactic Ablative Body Radiotherapy for Lung Metastases: Where is the Evidence and What are We Doing With It?

This review provides an overview of the use of stereotactic ablative body radiotherapy (SABR) for pulmonary metastases. The local control rates after SABR are generally >90%. Whether this also translates into a significant improvement in overall survival is the subject of ongoing studies. New exciting opportunities including the integration of SABR with targeted and immune therapies as well as some competing treatment strategies are discussed.

The Radiation Stress Response: Of the People, By the People and For the People

The radiation stress response can have broad impact. In this Failla Award presentation it is discussed in three components using terms relevant to the current political season as to how the radiation stress response can be applied to the benefit for cancer care and as service to society. Of the people refers to the impact of radiation on cells, tissues and patients. The paradigm our laboratory uses is radiation as a drug, called "focused biology", and physics as "nano-IMRT" because at the nanometer level physics and biology merge. By the people refers to how the general population often reacts to the word "radiation" and how the Radiation Research Society can better enable society to deal with the current realities of radiation in our lives. For the people refers to the potential for radiation oncology and radiation sciences to improve the lives of millions of people globally who are now beyond benefits of cancer treatment and research.

Mathematical modeling of tumor response to radiation: radio-sensitivity correlation with BOLD, TOLD, ΔR1 and ΔR2* investigated in large Dunning R3327-AT1 rat prostate tumors

Tumor response to radiation therapy can vary highly across patients. Several factors, both tumor- and environment-specific, can influence the radio-sensitivity, one of the most well-known being hypoxia. In this work, we investigated possible correlations between the radio-sensitivity parameters determined by means of a simple mathematical model of tumor volume evolution, and the MRI-based indicators of oxygenation in Dunning R3327-AT1 rats. Prior to irradiation the rats were subjected to an oxygen-breathing challenge, which was evaluated by MRI. The tumors were administered a single irradiation dose (30 Gy), while breathing air or oxygen. Despite a poor fitting performance, the model was able to identify two different tumor volume regression patterns. Moreover, the radio-sensitivity of the oxygen-breathing group was found to correlate with the variation of the transverse relaxation rate ΔR2* (-0.89). This suggests that MRI-based indices of tumor oxygenation may provide information about radio-sensitivity.

Characterization of in vitro radiosensitization in mammalian cells using biomathematical modelling: implications for hypofractionated radiotherapy with a combined modality approach

OBJECTIVE

It is unclear whether radiosensitization is beneficial when radiotherapy is administered at a high dose per fraction. The aim of this study was to assess the impact of radiation dose on the effectiveness of a broad range of radiosensitizers.

METHODS

We analyzed 653 pairs of clonogenic survival curves in 285 published articles, in which modifications of radiosensitivity were studied using the colony-forming assay. The modifications of radiosensitivity were arbitrarily classified into 20 classes. The survival curves were fitted to two biomathematical models: the linear-quadratic model and the repair-misrepair (RMR) model.

RESULTS

We found that radiosensitization was predominantly characterized by an increase of the α value (α-sensitization) without an increase of the β value (β-sensitization). A subset analysis revealed that all 20 classes showed significant α-sensitization. In contrast, only oxygen/hypoxic sensitizers (oxygen) and poly(adenosine diphosphate-ribose) polymerase inhibition (PARPi) exhibited β-sensitization. An analysis using the RMR model revealed two major sources of radiosensitization: an increased residual DNA lesion through repair inhibition and a shift from linear repairs to quadratic misrepairs, leading to enhanced lethal chromosomal aberrations.

CONCLUSION

Oxygen and PARPi were found to show β-sensitization, which was favourable for eliciting a comparable degree of sensitization in the higher dose range. Reduced fidelity of the repair was suggested to be a possible mechanism of β-sensitization. Further study targeting β-sensitization is needed to develop a novel combined modality therapy with high-dose-per-fraction radiotherapy.

ADVANCES IN KNOWLEDGE

Radiosensitization can be classified into two groups, α- and β-sensitizations. These two phenomena may stem from distinct underlying mechanisms.

In Vitro and In Vivo Studies of a New Class of Anticancer Molecules for Targeted Radiotherapy of Cancer

There is a compelling need to develop anticancer therapies that target cancer cells and tissues. Arising from innovative femtomedicine studies, a new class of non-platinum-based halogenated molecules (called FMD molecules) that selectively kill cancer cells and protect normal cells in treatments of multiple cancers has been discovered. This article reports the first observation of the radiosensitizing effects of such compounds in combination with ionizing radiation for targeted radiotherapy of a variety of cancers. We present in vitro and in vivo studies focused on combination with radiotherapy of cervical, ovarian, head and neck, and lung cancers. Our results demonstrate that treatments of various cancer cells in vitro and in vivo mouse xenograft models with such compounds led to enhanced efficiencies in radiotherapy, while the compounds themselves induced no or little radiotoxicity toward normal cells or tissues. These compounds are therefore effective radiosensitizers that can be translated into clinical trials for targeted radiotherapy of multiple types of cancer. This study also shows the potential of femtomedicine to bring breakthroughs in understanding fundamental biologic processes and to accelerate the discovery of novel drugs for effective treatment or prevention of a variety of cancers. Mol Cancer Ther; 15(4); 640-50. ©2016 AACR.

Radiobiological mechanisms of stereotactic body radiation therapy and stereotactic radiation surgery

Despite the increasing use of stereotactic body radiation therapy (SBRT) and stereotactic radiation surgery (SRS) in recent years, the biological base of these high-dose hypo-fractionated radiotherapy modalities has been elusive. Given that most human tumors contain radioresistant hypoxic tumor cells, the radiobiological principles for the conventional multiple-fractionated radiotherapy cannot account for the high efficacy of SBRT and SRS. Recent emerging evidence strongly indicates that SBRT and SRS not only directly kill tumor cells, but also destroy the tumor vascular beds, thereby deteriorating intratumor microenvironment leading to indirect tumor cell death. Furthermore, indications are that the massive release of tumor antigens from the tumor cells directly and indirectly killed by SBRT and SRS stimulate anti-tumor immunity, thereby suppressing recurrence and metastatic tumor growth. The reoxygenation, repair, repopulation, and redistribution, which are important components in the response of tumors to conventional fractionated radiotherapy, play relatively little role in SBRT and SRS. The linear-quadratic model, which accounts for only direct cell death has been suggested to overestimate the cell death by high dose per fraction irradiation. However, the model may in some clinical cases incidentally do not overestimate total cell death because high-dose irradiation causes additional cell death through indirect mechanisms. For the improvement of the efficacy of SBRT and SRS, further investigation is warranted to gain detailed insights into the mechanisms underlying the SBRT and SRS.

Targeting tumour hypoxia to improve outcome of stereotactic radiotherapy

BACKGROUND

Hypoxia is a characteristic feature of solid tumours that significantly reduces the efficacy of conventional radiation therapy. In this study we investigated the role of hypoxia in a stereotactic radiation schedule by using a variety of hypoxic modifiers in a preclinical tumour model.

MATERIAL AND METHODS

C3H mammary carcinomas were irradiated with 3 × 15 Gy during a one-week period, followed three days later by a clamped top-up dose to produce a dose response curve; the endpoint was tumour control. The hypoxic modifiers were nimorazole (200 mg/kg), nicotinamide (120 mg/kg) and carbogen (95% O2 + 5% CO2) breathing, OXi4503 (10 mg/kg), and hyperthermia (41.5°C; 1 h).

RESULTS

The radiation dose controlling 50% of clamped tumours (TCD50) following 3 × 15 Gy was 30 Gy. Giving nimorazole or nicotinamide+ carbogen prior to the final 15 Gy fraction non-significantly (χ(2)-test; p < 0.05) reduced this TCD50 to 20-23 Gy; when administered with each 3 × 15 Gy fraction these values were significantly reduced to ≤ 2.5 Gy. Injecting OXi4503 or heating after irradiating significantly reduced the TCD50 to 9-12 Gy regardless of whether administered with one or all three 15 Gy fractions. Combining OXi4503 and heat with the final 15 Gy had a significantly larger effect (TCD50 = 2 Gy).

CONCLUSIONS

Clinically relevant modifiers of hypoxia effectively enhanced an equivalent stereotactic radiation treatment confirming the importance of hypoxia in such schedules.

Effects of low oxygen levels on G2-specific cytogenetic low-dose hyper-radiosensitivity in irradiated human cells

Low-dose hyper-radiosensitivity (HRS) has been reported in normal human lymphoblastoid cell lines for exposures at ≤ 20 cGy, but the cytogenetic effects of oxygen (O2 ) levels in tissue culture medium on HRS have not been evaluated. We asked whether HRS was lost in G2-irradiated cells grown in atmospheres of 2.5% or 5% O2 , compared to responses by cells cultured in ambient O2 (21%). The results indicate a loss of HRS when cells are cultured and irradiated either in 2.5% or 5% O2 . We then evaluated whether low O2 levels either before or after exposure were responsible for the loss of HRS. For cells irradiated in 5% O2 , subsequent immediate re-oxygenation to ambient O2 levels restored the HRS effect, while cells cultured and irradiated at ambient O2 levels and then transferred to 5% O2 exhibited little or no HRS, indicating that ambient O2 levels after, but not before, radiation substantially affect the amounts of cytogenetic damage. HRS was not observed when cells were irradiated in G1. At doses of 40-400 cGy there was significantly less cytogenetic damage when cells were recovering from radiation at low O2 levels than at ambient O2 levels. Here we provide the first cytogenetic evidence for the loss of HRS at low O2 levels in G2-irradiated cells; these results suggest that at low O2 levels for all doses evaluated there is either less damage to DNA, perhaps because of lower amounts of reactive oxygen species, or that DNA damage repair pathways are activated more efficiently.

Available evidence on re-irradiation with stereotactic ablative radiotherapy following high-dose previous thoracic radiotherapy for lung malignancies

Patients affected with intra-thoracic recurrences of primary or secondary lung malignancies after a first course of definitive radiotherapy have limited therapeutic options, and they are often treated with a palliative intent. Re-irradiation with stereotactic ablative radiotherapy (SABR) represents an appealing approach, due to the optimized dose distribution that allows for high-dose delivery with better sparing of organs at risk. This strategy has the goal of long-term control and even cure. Aim of this review is to report and discuss published data on re-irradiation with SABR in terms of efficacy and toxicity. Results indicate that thoracic re-irradiation may offer satisfactory disease control, however the data on outcome and toxicity are derived from low quality retrospective studies, and results should be cautiously interpreted. As SABR may be associated with serious toxicity, attention should be paid for an accurate patients' selection.

Colorectal Histology Is Associated With an Increased Risk of Local Failure in Lung Metastases Treated With Stereotactic Ablative Radiation Therapy

PURPOSE

Stereotactic ablative radiation therapy (SABR) is increasingly used to treat lung oligometastases. We set out to determine the safety and efficacy of this approach and to identify factors associated with outcomes.

METHODS AND MATERIALS

We conducted a retrospective study of patients treated with SABR for metastatic lung tumors at our institution from 2003 to 2014. We assessed the association between various patient and treatment factors with local failure (LF), progression, subsequent treatment, systemic treatment, and overall survival (OS), using univariate and multivariate analyses.

RESULTS

We identified 122 tumors in 77 patients meeting inclusion criteria for this study. Median follow-up was 22 months. The 12- and 24-month cumulative incidence rates of LF were 8.7% and 16.2%, respectively; the 24-month cumulative incidence rates of progression, subsequent treatment, and subsequent systemic treatment were 75.2%, 64.5%, and 35.1%, respectively. Twenty-four-month OS was 74.6%, and median OS was 36 months. Colorectal metastases had a significantly higher cumulative incidence of LF at 12 and 24 months (25.5% and 42.2%, respectively), than all other histologies (4.4% and 9.9%, respectively; P<.0004). The 24-month cumulative incidences of LF for colorectal metastases treated with a biologically effective dose at α/β = 10 (BED10) of <100 Gy versus BED10 of ≥100 Gy were 62.5% and 16.7%, respectively (P=.08). Toxicity was minimal, with only a single grade 3 or higher event observed.

CONCLUSIONS

SABR for metastatic lung tumors appears to be safe and effective with excellent local control, treatment-free intervals, and OS. An exception is metastases from colorectal cancer, which have a high LF rate consistent with a radioresistant phenotype, suggesting a potential role for dose escalation.