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

Advances in Surgery for Metastatic Disease of the Spine: An Update for Oncologists

STUDY DESIGN

Narrative review.

OBJECTIVE

Metastatic spine disease is an increasingly common clinical challenge that requires individualised multidisciplinary care from spine surgeons and oncologists. In this article, the authors describe the recent surgical advances in patients presenting with spinal metastases.

METHODS

We present an overview of the presentation, assessment, and management of spinal metastases from the perspective of the spine surgeon, highlighting advances in surgical technology and techniques, to facilitate multidisciplinary care for this complex patient group. Neither institutional review board approval nor patient consent was needed for this review.

RESULTS

Advances in radiotherapy delivery and systemic therapy (including immunotherapy and targeted therapy) have refined operative indications for decompression of neural structures and spinal stabilisation, while advances in surgical technology and technique enable these goals to be achieved with reduced morbidity. Formulating individualised management strategies that optimise outcome, while meeting patient goals and expectations, requires a comprehensive understanding of the factors important to patient management.

CONCLUSION

Spinal metastases require prompt diagnosis and expert management by a multidisciplinary team. Improvements in systemic, radiation, and surgical therapies have broadened operative indications and increased operative candidacy, and future advances are likely to continue this trend.

Total dose, fraction dose and respiratory motion management impact adrenal SBRT outcome

Purpose/Objectives

Stereotactic body radiotherapy (SBRT) is an effective treatment for oligometastatic disease in multiple sites. However, the optimal radiation dose for long-term local control of adrenal metastases has yet to be determined. The aim of this study is to evaluate outcomes of adrenal SBRT and to evaluate factors that correlate with local control.

Materials/Methods

After IRB approval, a retrospective data review of patients treated with SBRT for adrenal metastases at a medical center in Israel between 2015 and 2021 was conducted. A biological effective dose was calculated using an alpha beta ratio of 10. Kaplan Meier and Cox regression were calculated using SPSS software to describe the hazard ratio for local control and survival.

Results

83 cases of adrenal SBRT were identified. The average age was 67 (range 42-92 years old). Non-small cell lung cancer was the primary site in 44 % of patients. A total of 70 % of the patients had oligometastatic disease (less than five lesions), and the rest were polymetastatic, responding to systemic therapy with oligo progression in the adrenal. The average gross tumor volume (GTV) was 42 ml. Respiratory control was applied in 88 % of cases; 49.3 % used 4-D/ITV, and 38.5 % used breath-hold or continuous positive airway pressure (CPAP) with free breathing. On multivariable analysis, Dose above 75 Gy (biological effective Dose) (HR = 0.41, p = 0.031), Dose above 8 Gy per fraction (HR = 0.53p = 0.038), and breath-holds or CPAP (HR = 0.65, p = 0.047) were significant for local control. From multivariable analysis, we computed a predicted nomogram curve using seven clinical parameters to evaluate local control odds.

Conclusion

In this single institution series reported to date, we found unilateral adrenal SBRT safe, yet bilateral treatment harbors a risk of adrenal insufficiency. Biological effective Dose > 75 Gy (BED), motion management with breath-hold or CPAP, and Dose per fraction > 8 Gy were the enhanced local controls. We propose a nomogram to help in decision-making regarding total Dose and Dose per fraction when treating adrenal SBRT.

Histology-driven hypofractionated radiation therapy schemes for early-stage lung adenocarcinoma and squamous cell carcinoma

BACKGROUND AND PURPOSE

Histology was found to be an important prognostic factor for local tumor control probability (TCP) after stereotactic body radiotherapy (SBRT) of early-stage non-small-cell lung cancer (NSCLC). A histology-driven SBRT approach has not been explored in routine clinical practice and histology-dependent fractionation schemes remain unknown. Here, we analyzed pooled histologic TCP data as a function of biologically effective dose (BED) to determine histology-driven fractionation schemes for SBRT and hypofractionated radiotherapy of two predominant early-stage NSCLC histologic subtypes adenocarcinoma (ADC) and squamous cell carcinoma (SCC).

MATERIAL AND METHODS

The least-χ2 method was used to fit the collected histologic TCP data of 8510 early-stage NSCLC patients to determine parameters for a well-developed radiobiological model per the Hypofractionated Treatment Effects in the Clinic (HyTEC) initiative.

RESULTS

A fit to the histologic TCP data yielded independent radiobiological parameter sets for radiotherapy of early-stage lung ADC and SCC. TCP increases steeply with BED and reaches an asymptotic maximal plateau, allowing us to determine model-independent optimal fractionation schemes of least doses in 1-30 fractions to achieve maximal tumor control for early-stage lung ADC and SCC, e.g., 30, 44, 48, and 51 Gy for ADC, and 32, 48, 54, and 58 Gy for SCC in 1, 3, 4, and 5 fractions, respectively.

CONCLUSION

We presented the first determination of histology-dependent radiobiological parameters and model-independent histology-driven optimal SBRT and hypofractionated radiation therapy schemes for early-stage lung ADC and SCC. SCC requires substantially higher radiation doses to maximize tumor control than ADC, plausibly attributed to tumor genetic diversity and microenvironment. The determined optimal SBRT schemes agree well with clinical practice for early-stage lung ADC. These proposed optimal fractionation schemes provide first insights for histology-based personalized radiotherapy of two predominant early-stage NSCLC subtypes ADC and SCC, which require further validation with large-scale histologic TCP data.

Stereotactic Body Radiotherapy as a Curative Treatment for De Novo Mucosal Carcinoma of the Head and Neck: A Feasible Alternative Option for Fragile Patients with Small Lesion: A Systematic Review

Stereotactic body radiotherapy (SBRT) is characterized by a high dose per fraction, well-defined small targets, superior dose conformity, and a steep off-target dose gradient. A literature search was conducted to examine the experience with SBRT as a curative treatment for newly diagnosed mucosal carcinoma of the head and neck (MCHN). Four retrospective case series and one prospective phase I clinical trial published between 2012 and 2020 described 124 patients. SBRT was mainly performed in older patients with different tumor sites. The median size of the planning target volumes ranged from 5.3 to 41 cm3. Different approaches were used to create margins. In two studies, limited elective nodal irradiation was performed. The equivalent doses used were 60-83.33 Gy delivered in five fractions. Considerable heterogeneity was observed in the radiation dose specification. The incidence of grade ≥3 late toxicity was 0-8.3%, with local and regional control ranging from 73% to 100%. Improved or stable quality of life after SBRT was reported in two studies. Curative-intent SBRT for de novo MCHN appears to be an effective and relatively safe treatment for small tumor targets, preferably without concomitant elective tissue irradiation. Standardization of SBRT practice and well-designed prospective clinical trials are needed to better define the role of SBRT in this setting.

Biological Insights and Radiation-Immuno-Oncology Developments in Primary and Secondary Brain Tumors

Malignant central nervous system (CNS) cancers include a group of heterogeneous dis-eases characterized by a relative resistance to treatments and distinguished as either primary tumors arising in the CNS or secondary tumors that spread from other organs into the brain. Despite therapeutic efforts, they often cause significant mortality and morbidity across all ages. Radiotherapy (RT) remains the main treatment for brain cancers, improving associated symptoms, improving tumor control, and inducing a cure in some. However, the ultimate goal of cancer treatment, to improve a patient's survival, remains elusive for many CNS cancers, especially primary tumors. Over the years, there have thus been many preclinical studies and clinical trials designed to identify and overcome mechanisms of resistance to improve outcomes after RT and other therapies. For example, immunotherapy delivered concurrent with RT, especially hypo-fractionated stereotactic RT, is synergistic and has revolutionized the clinical management and outcome of some brain tumors, in particular brain metastases (secondary brain tumors). However, its impact on gliomas, the most common primary malignant CNS tumors, remains limited. In this review, we provide an overview of radioresistance mechanisms, the emerging strategies to overcome radioresistance, the role of the tumor microenviroment (TME), and the selection of the most significant results of radiation-immuno-oncological investigations. We also identify novel therapeutic opportunities in primary and secondary brain tumors with the purpose of elucidating current knowledge and stimulating further research to improve tumor control and patients' survival.

Mathematical modeling of the synergistic interplay of radiotherapy and immunotherapy in anti-cancer treatments

Introduction

While radiotherapy has long been recognized for its ability to directly ablate cancer cells through necrosis or apoptosis, radiotherapy-induced abscopal effect suggests that its impact extends beyond local tumor destruction thanks to immune response. Cellular proliferation and necrosis have been extensively studied using mathematical models that simulate tumor growth, such as Gompertz law, and the radiation effects, such as the linear-quadratic model. However, the effectiveness of radiotherapy-induced immune responses may vary among patients due to individual differences in radiation sensitivity and other factors.

Methods

We present a novel macroscopic approach designed to quantitatively analyze the intricate dynamics governing the interactions among the immune system, radiotherapy, and tumor progression. Building upon previous research demonstrating the synergistic effects of radiotherapy and immunotherapy in cancer treatment, we provide a comprehensive mathematical framework for understanding the underlying mechanisms driving these interactions.

Results

Our method leverages macroscopic observations and mathematical modeling to capture the overarching dynamics of this interplay, offering valuable insights for optimizing cancer treatment strategies. One shows that Gompertz law can describe therapy effects with two effective parameters. This result permits quantitative data analyses, which give useful indications for the disease progression and clinical decisions.

Discussion

Through validation against diverse data sets from the literature, we demonstrate the reliability and versatility of our approach in predicting the time evolution of the disease and assessing the potential efficacy of radiotherapy-immunotherapy combinations. This further supports the promising potential of the abscopal effect, suggesting that in select cases, depending on tumor size, it may confer full efficacy to radiotherapy.

Efficient quality assurance for isocentric stability in stereotactic body radiation therapy using machine learning

This study aims to predict isocentric stability for stereotactic body radiation therapy (SBRT) treatments using machine learning (ML), covers the challenges of manual assessment and computational time for quality assurance (QA), and supports medical physicists to enhance accuracy. The isocentric parameters for collimator (C), gantry (G), and table (T) tests were conducted with the RUBY phantom during QA using TrueBeam linac for SBRT. This analysis combined statistical features from the IsoCheck EPID software. Five ML models, including logistic regression (LR), decision tree (DT), random forest (RF), naive Bayes (NB), and support vector machines (SVM), were used to predict the outcome of the QA procedure. 247 Winston-Lutz (WL) tests were collected from 2020 to 2022. In our study, both DT and RF achieved the highest score on test accuracy (Acc. test) ranging from 93.5% to 99.4%, and area under curve (AUC) values from 90 to 100% on three modes (C, G, and T). The precision, recall, and F1 scores indicate the DT model consistently outperforms other ML models in predicting isocenter stability deviation in QA. The QA assessment using ML models can assist error prediction early to avoid potential harm during SBRT and ensure safe and effective patient treatments.

Optimal Radiation Therapy Fractionation Regimens for Early-Stage Non-Small Cell Lung Cancer

PURPOSE

A series of radiobiological models were developed to study tumor control probability (TCP) for stereotactic body radiation therapy (SBRT) of early-stage non-small cell lung cancer (NSCLC) per the Hypofractionated Treatment Effects in the Clinic (HyTEC) working group. This study was conducted to further validate 3 representative models with the recent clinical TCP data ranging from conventional radiation therapy to SBRT of early-stage NSCLC and to determine systematic optimal fractionation regimens in 1 to 30 fractions for radiation therapy of early-stage NSCLC that were found to be model-independent.

METHODS AND MATERIALS

Recent clinical 1-, 2-, 3-, and 5-year actuarial or Kaplan-Meier TCP data of 9808 patients from 56 published papers were collected for radiation therapy of 2 to 4 Gy per fraction and SBRT of early-stage NSCLC. This data set nearly triples the original HyTEC sample, which was used to further validate the HyTEC model parameters determined from a fit to the clinical TCP data.

RESULTS

TCP data from the expanded data set are well described by the HyTEC models with α/β ratios of about 20 Gy. TCP increases sharply with biologically effective dose and reaches an asymptotic maximal plateau, which allows us to determine optimal fractionation schemes for radiation therapy of early-stage NSCLC.

CONCLUSIONS

The HyTEC radiobiological models with α/β ratios of about 20 Gy determined from the fits to the clinical TCP data for SBRT of early-stage NSCLC describe the recent TCP data well for both radiation therapy of 2 to 4 Gy per fraction and SBRT dose and fractionation schemes of early-stage NSCLC. A steep dose response exists between TCP and biologically effective dose, and TCP reaches an asymptotic maximum. This feature results in model-independent optimal fractionation regimens determined whenever safe for SBRT and hypofractionated radiation therapy of early-stage NSCLC in 1 to 30 fractions to achieve asymptotic maximal tumor control, and T2 tumors require slightly higher optimal doses than T1 tumors. The proposed optimal fractionation schemes are consistent with clinical practice for SBRT of early-stage NSCLC.

Neoadjuvant stereotactic ablative body radiotherapy combined with surgical treatment for renal cell carcinoma and inferior vena cava tumor thrombus: a prospective pilot study

BACKGROUND

Surgical treatment for renal cell carcinoma (RCC) and inferior vena cava (IVC) tumor thrombus (TT) is difficult, and the postoperative complication rate is high. This study aimed to explore the safety and oncologic outcomes of neoadjuvant stereotactic ablative body radiotherapy (SABR) combined with surgical treatment for RCC and IVC-TT.

METHODS

Patients with RCC and IVC-TTs were enrolled in this study. All patients received neoadjuvant SABR focused on the IVC at a dose of 30 Gy in 5 fractions, followed by 2 ~ 4 weeks of rest. Then, radical nephrectomy and IVC tumor thrombectomy were performed for each patient. Adverse effects, perioperative outcomes, and long-term prognoses were recorded.

RESULTS

From June 2018 to January 2019, 8 patients were enrolled-4 with Mayo grade II TT and 4 with Mayo grade III TT. Four (50%) patients had complicated IVC wall invasion according to CT/MRI. All patients received neoadjuvant SABR as planned. Short-term local control was observed in all 8 patients. Only Grade 1-2 adverse events were reported. In total, 3 (37.5%) laparoscopic surgeries and 5 (62.5%) open surgeries were performed. The median operation time was 359 (IQR: 279-446) min, with a median intraoperative bleeding volume of 750 (IQR: 275-2175) ml. The median postoperative hospital stay was 7 (5-10) days. With a 26-month (range: 5-41) follow-up period, the estimated mean overall survival was 30.67 ± 5.38 months.

CONCLUSIONS

This is the first preoperative radiotherapy study in Asia that focused on patients with TT. This study revealed the considerable safety of neoadjuvant SABR for RCC with IVC-TT.

TRIAL REGISTRATION

This study was registered in the Chinese Clinical Trials Registry on 2018-03-08 (ChiCTR1800015118). For more information, please see the direct link ( https://www.chictr.org.cn/showproj.html?proj=25747 ).

A simulation study on the radiation-induced immune response of tumors after single fraction high-dose irradiation

PURPOSE

We investigated radiation-induced antitumor immunity and its suppression by hypoxia-inducible factor (HIF-1α) for radiosurgery (SRS) using an improved cellular automata (CA) model.

METHOD

A two-dimensional Cellular Automata (CA) model was employed to simulate the impact of radiation on cancer cell death and subsequent immune responses. Cancer cells died from direct cell death from radiation and indirect cell death due to radiation-induced vascular damage. The model also incorporated radiation-induced immunity and immuno-suppression. It was incorporated into the model assuming that the death of cancer cells generates effector cells, forming complexes with cancer cells, and high radiation doses lead to vascular damage, inducing tumor hypoxia and increasing HIF-1α expression. The model was validated and subjected to sensitivity analysis by evaluating tumor volume changes post-irradiation and exploring the effects and sensitivity of radiation-induced immune responses.

RESULTS

The ratios of the tumor volume at 360 days post-irradiation and the SRS day (rTV) decreased with a higher PME, a higher Pcomp, and a lower ThHIF. The rTVs were 4.6 and 2.0 for PME = 0.1 and 0.9, 12.0 and 2.2 for Pcomp = 0.1 and 0.9, and 1.5 and 15.3 for ThHIF = 0.1 and 10.0, respectively.

CONCLUSIONS

By modeling the activation and deactivation of the effectors, the improved CA model showed that the radiation-induced immunogenic cell death in the tumor caused a decrease in the post-irradiation volume by a factor of four for the therapeutic doses relative to non-immune reaction cases. Furthermore, the suppressive effects of HIF-1α induced by hypoxia decreased radiation-induced immune effects by more than 50.

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.

Does Vascular Collapse Occur After Treatment of Hepatocellular Cancer With Stereotactic Body Radiation Therapy?

There is debate about why stereotactic body radiation therapy (SBRT) produces superior control of hepatocellular cancer (HCC) compared to fractionated treatment. Both preclinical and clinical evidence has been presented to support a "classic" biological explanation: the greater BED of SBRT produces more DNA damage and tumor cell kill. More recently, preclinical evidence has supported the concept of a "new biology", particularly radiation-induced vascular collapse, which increases hypoxia and free radical activation. This is hypothesized to cause much greater tumor cell death than was produced by the initial radiation-induced DNA damage to the tumor. We decided to investigate if vascular collapse occurs after standard SBRT for patients with HCC. Eight patients with 10 lesions underwent dynamic contrast enhanced MRI at the time of simulation and either 48 or 96 hours after the first fraction. Only three of 10 tumors showed a decrease in blood flow. These findings suggest that vascular collapse does not typically occur after SBRT for HCC.

Single-Fraction Stereotactic Ablative Body Radiotherapy for Primary and Extracranial Oligometastatic Cancers

Stereotactic ablative body radiotherapy (SABR) consists of delivering high doses of ionising radiation, typically across three to eight fractions with high precision and conformity. SABR has become increasingly commonplace throughout the last quarter of a century and is offered for the treatment of various primary and metastatic tumour types. Delivering SABR in a single fraction has arisen as an appealing possibility for several reasons. These include fewer hospital visits, greater patient convenience, improved sustainability and lower costs. However, these factors must be balanced against considerations such as toxicity, side-effects and, most importantly, progression-free and overall survival. In this review we seek to analyse the results of studies looking at the efficacy of single-fraction SABR for lung, prostate, renal and pancreas primary tumours, as well as oligometastases. The tumour type to be most widely treated with single-fraction SABR is lung, but its remit continues to expand. We also look at the biological rationale underpinning SABR and how this can be extended to single-fraction regimens. Finally, we turn our attention towards the future directions of SABR and specifically single-fraction regimens. These include the possibility of combining SABR with immunotherapy and technological advances in the field, which could serve to expand the scope of SABR. We conclude by summarising the current clinical studies of single-fraction SABR.

A retrospective study of sinonasal tumors in 182 dogs treated with stereotactic radiotherapy (3 × 10 Gy) (2010-2015)

BACKGROUND

Stereotactic radiotherapy (SRT) is an emerging treatment for sinonasal tumors in dogs. Reported results regarding tumor control and incidence of acute and late radiation morbidities are inconsistent.

OBJECTIVES

To determine treatment efficacy and prognostic indicators of SRT in dogs with sinonasal tumors and to quantify acute and late radiation morbidities.

ANIMALS

One hundred and eighty-two client-owned dogs with sinonasal tumors diagnosed cytologically, histologically, or radiographically that underwent SRT.

METHODS

Single-arm retrospective study by reviewing medical records of dogs treated with SRT (10 Gy × 3) between 2010 and 2015. Kaplan-Meier analysis was used to determine overall survival (OST; from the first day of SRT to death by any cause) and disease-specific survival times (DSST; OST but censoring tumor/treatment-unrelated death). Tumors were staged using modified Adams criteria.

RESULTS

Median OST and DSST of dogs treated with 1 course of SRT was 441 (95% CI: 389-493 days) and 482 (428-536 days) days, respectively with skin/oral cavity acute morbidities observed in 3% of dogs. DSST in dogs with stage 4 disease showed no statistical difference compared to other stages (P = .64). Oro-nasal (n = 2) or naso-cutaneous (n = 11) fistula development occurred in 7.1% of dogs with median time of 425 days (range: 83-1733 days). Possible chronic rhinitis after SRT was recorded in 54 of 88 dogs (61%) where information was available.

CONCLUSIONS AND CLINICAL IMPORTANCE

Results are comparable to other reports of treatment of SRT. Acute morbidities were minimal. Modified Adams stage scheme appeared to be inappropriate for prognostication for dogs with sinonasal tumors treated with SRT.

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.

Using immunotherapy to enhance the response of a C3H mammary carcinoma to proton radiation

BACKGROUND

The benefit of combining immunotherapy with photon irradiation has been shown pre-clinically and clinically. This current pre-clinical study was designed to investigate the anti-tumour action of combining immunotherapy with protons.

MATERIALS AND METHODS

Male CDF1 mice, with a C3H mammary carcinoma inoculated on the right rear foot, were locally irradiated with single radiation doses when tumours reached 200mm3. Radiation was delivered with an 83-107MeV pencil scanning proton beam in the centre of a 3 cm spread out Bragg peak. Following irradiation (day 0), mice were injected intraperitoneal with anti-CTLA-4, anti-PD-1, or anti-PD-L1 (10 mg/kg) twice weekly for two weeks. Endpoints were tumour growth time (TGT3; time to reach 3 times treatment volume) or local tumour control (percent of mice showing tumour control at 90 days). A Student's T-test (tumour growth) or Chi-squared test (tumour control) were used for statistical analysis; significance levels of p < 0.05.

RESULTS

Untreated tumours had a mean (± 1 S.E.) TGT3 of 4.6 days (± 0.4). None of the checkpoint inhibitors changed this TGT3. A linear increase in TGT3 was seen with increasing radiation doses (5-20 Gy), reaching 17.2 days (± 0.7) with 20 Gy. Anti-CTLA-4 had no effect on radiation doses up to 15 Gy, but significantly enhanced 20 Gy; the TGT3 being 23.0 days (± 1.3). Higher radiation doses (35-60 Gy) were investigated using a tumour control assay. Logit analysis of the dose response curve, resulted in a TCD50 value (radiation dose causing 50% tumour control; with 95% confidence intervals) of 48 Gy (44-53) for radiation only. This significantly decreased to 43 Gy (38-49) when mice were treated with anti-CTLA-4. Neither anti-PD-1 nor anti-PD-L1 significantly affected tumour control.

CONCLUSION

Checkpoint inhibitors enhanced the response of this C3H mammary carcinoma to proton irradiation. However, this enhancement depended on the checkpoint inhibitor and radiation dose.

Liver parenchymal changes detected by MR elastography and diffusion-weighted imaging after stereotactic body radiotherapy for hepatocellular carcinoma

BACKGROUND

Stereotactic body radiotherapy (SBRT) is a local treatment option for hepatocellular carcinoma (HCC). SBRT-induced focal reactions on the liver parenchyma have not been thoroughly evaluated using quantitative magnetic resonance imaging (MRI).

PURPOSE

To quantitatively evaluate liver parenchymal changes caused by SBRT for HCC using magnetic resonance elastography (MRE) and diffusion-weighted imaging (DWI).

METHOD

We retrospectively evaluated 22 adult patients who received SBRT for HCC and 27 who received locoregional therapy other than SBRT (controls). Liver stiffness by MRE and apparent diffusion coefficient (ADC) values by DWI of the liver parenchyma were measured before and after SBRT. Regions of interest (ROIs) were drawn on the two areas of radiation dose distribution levels, > 30 Gy and ≤ 30 Gy; a ROI was drawn in the control group. The two indices were compared before and after SBRT using a Wilcoxon matched-pairs signed-rank test.

RESULTS

Liver stiffness and ADC values were significantly increased after SBRT in the dose areas of > 30 Gy compared with those before SBRT (4.05 vs 4.85 kPa; p < 0.05 in liver stiffness, and 1.10 vs 1.40 ×10-3 s/mm2; p < 0.05 in ADC values). In the dose area of ≦ 30 Gy, liver stiffness showed a significant increase in one reader (p = 0.033) but not in another reader (p = 0.085); ADC value showed no significant difference before and after SBRT as per both readers (p > 0.05). The control group demonstrated no significant differences before and after treatment (p > 0.05).

CONCLUSION

MRE and DWI can be used to detect SBRT-induced liver parenchymal changes.

Historical Progress of Stereotactic Radiation Surgery

Radiosurgery and stereotactic radiotherapy have established themselves as precise and accurate areas of radiation oncology for the treatment of brain and extracranial lesions. Along with the evolution of other methods of radiotherapy, this type of treatment has been associated with significant advances in terms of a variety of modalities and techniques to improve the accuracy and efficacy of treatment. This paper provides a comprehensive overview of the progress in stereotactic radiosurgery (SRS) over several decades, and includes a review of various articles and research papers, commencing with the emergence of stereotactic techniques in radiotherapy. Key clinical aspects of SRS, such as fixation methods, radiobiology considerations, quality assurance practices, and treatment planning strategies, are presented. In addition, the review highlights the technological advancements in treatment modalities, encompassing the transition from cobalt-based systems to linear accelerator-based modalities. By addressing these topics, this study aims to offer insights into the advancements that have shaped the field of SRS, that have ultimately enhanced the accuracy and effectiveness of treatment.

Outcomes of Isocitrate Dehydrogenase Wild Type Glioblastoma after Re-irradiation

Purpose

Glioblastomas (GBM) are the most common malignant primary brain tumors in adults and have a dismal prognosis. Patients frequently suffer from local tumor recurrences, with limited therapeutic options. Re-irradiation represents a possible intervention, but given the recent 5th edition of the World Health Organization classification of central nervous system tumors, studies in isocitrate dehydrogenase wild type (IDH-wt) cohorts undergoing a second course of radiotherapy remain limited. Herein, we sought to describe our institutional experience and outcomes after GBM IDH-wt re-irradiation.

Materials and Methods

GBM patients with confirmed IDH-wt status undergoing re-irradiation were included in this single-center, retrospective analysis.

Results

A total of 88 patients were analyzed. The median clinical and radiographic follow-up periods were 4.6 months and 4.4 months, respectively. Most patients had a Karnofsky performance status of at least 80% (n = 57). The median biologically effective dose and 2 Gy equivalent dose (EQD2) for re-irradiations, assuming an α/β ratio of 10 Gy for GBM, were 51.4 and 42.8 Gy, respectively. In total, 71 deaths were recorded. The median overall survival (OS) was 8.0 months. Multivariable Cox regression of OS revealed a positive influence of gross total resection vs. biopsy or no resection (hazard ratio: 0.43, p = 0.02). The median progression-free survival (PFS) was 5.9 months. The multivariable Cox regression for PFS did not detect any significant factors. No clear evidence of radiation necrosis was recorded during the available follow-up. However, only a minority (n = 4) of patients underwent surgery after re-irradiation, none showing histopathological proof of radiation necrosis.

Conclusion

The prognosis for recurrent IDH-wt GBM after re-irradiation is poor. Patients who are amenable and able to undergo re-resection may have a favorable OS. A second course of radiotherapy with a moderate cumulative EQD2 and small- to medium-sized planning target volumes appeared safe regarding the occurrence of radiation necrosis.

Neoadjuvant Arterial Embolization of Spine Metastases Associated With Improved Local Control in Patients Receiving Surgical Decompression and Stereotactic Body Radiotherapy

BACKGROUND

Spine metastases often cause significant pain, instability, and/or neurological morbidity. Local control (LC) of spine metastases has been augmented with advances in systemic therapies, radiation, and surgical technique. Prior reports suggest an association between preoperative arterial embolization and improved LC and palliative pain control.

OBJECTIVE

To further elucidate the role of neoadjuvant embolization on LC of spine metastases and the potential for improved pain control in patients receiving surgery and stereotactic body radiotherapy (SBRT).

METHOD

A retrospective single-center review between 2012 and 2020 identified 117 patients with spinal metastases from various solid tumor malignancies managed with surgery and adjuvant SBRT with or without preoperative spinal arterial embolization. Demographic information, radiographic studies, treatment characteristics, Karnofsky Performance Score, Defensive Veterans Pain Rating Scale, and mean daily doses of analgesic medications were reviewed. LC was assessed using magnetic resonance imaging obtained at a median 3-month interval and defined as progression at the surgically treated vertebral level.

RESULTS

Of 117 patients, 47 (40.2%) underwent preoperative embolization, followed by surgery and SBRT and 70 (59.8%) underwent surgery and SBRT alone. Within the embolization cohort, the median LC was 14.2 months compared with 6.3 months among the nonembolization cohort ( P = .0434). Receiver operating characteristic analysis suggests ≥82.5% embolization predicted significantly improved LC (area under the curve = 0.808; P < .0001). Defensive Veterans Pain Rating Scale mean and maximum scores significantly decreased immediately after embolization ( P < .001).

CONCLUSION

Preoperative embolization was associated with improved LC and pain control suggesting a novel role for its use. Additional prospective study is warranted.

Radiobiological Meta-Analysis of the Response of Prostate Cancer to Different Fractionations: Evaluation of the Linear-Quadratic Response at Large Doses and the Effect of Risk and ADT

The purpose of this work was to investigate the response of prostate cancer to different radiotherapy schedules, including hypofractionation, to evaluate potential departures from the linear-quadratic (LQ) response, to obtain the best-fitting parameters for low-(LR), intermediate-(IR), and high-risk (HR) prostate cancer and to investigate the effect of ADT on the radiobiological response. We constructed a dataset of the dose-response containing 87 entries/16,536 patients (35/5181 LR, 32/8146 IR, 20/3209 HR), with doses per fraction ranging from 1.8 to 10 Gy. These data were fit to tumour control probability models based on the LQ model, linear-quadratic-linear (LQL) model, and a modification of the LQ (LQmod) model accounting for increasing radiosensitivity at large doses. Fits were performed with the maximum likelihood expectation methodology, and the Akaike information criterion (AIC) was used to compare the models. The AIC showed that the LQ model was superior to the LQL and LQmod models for all risks, except for IR, where the LQL model outperformed the other models. The analysis showed a low α/β for all risks: 2.0 Gy for LR (95% confidence interval: 1.7-2.3), 3.4 Gy for IR (3.0-4.0), and 2.8 Gy for HR (1.4-4.2). The best fits did not show proliferation for LR and showed moderate proliferation for IR/HR. The addition of ADT was consistent with a suppression of proliferation. In conclusion, the LQ model described the response of prostate cancer better than the alternative models. Only for IR, the LQL model outperformed the LQ model, pointing out a possible saturation of radiation damage with increasing dose. This study confirmed a low α/β for all risks.

Radiation Therapy in the Treatment of Localized and Advanced Renal Cancer

Renal cell carcinoma (RCC) has historically been considered resistant to radiotherapy. However, advances in the field of radiation oncology have led to safe delivery of higher radiation doses through the use of stereotactic body radiotherapy (SBRT) that have shown significant activity against RCC. SBRT has now been shown to be a highly effective modality for management of localized RCC for nonsurgical candidates. Increasing evidence also points to a role for SBRT in the management of oligometastatic RCC as a means for not only providing palliation but prolonging time to progression and potentially improving survival.

Implications of the Organ-Specific Immune Environment for Immune Priming Effect of Radiotherapy in Metastatic Setting

With the development of immune checkpoint inhibitors (ICIs), the tumour immune microenvironment (TIME) has been increasingly considered to improve cancer management. The TIME of metastatic lesions is strongly influenced by the underlying immune contexture of the organ in which they are located. The metastatic location itself appears to be an important prognostic factor in predicting outcomes after ICI treatment in cancer patients. Patients with liver metastases are less likely to respond to ICIs than patients with metastases in other organs, likely due to variations in the metastatic TIME. Combining additional treatment modalities is an option to overcome this resistance. Radiotherapy (RT) and ICIs have been investigated together as an option to treat various metastatic cancers. RT can induce a local and systemic immune reaction, which can promote the patient's response to ICIs. Here, we review the differential impact of the TIME according to metastatic location. We also explore how RT-induced TIME modifications could be modulated to improve outcomes of RT-ICI combinations.

Precision Radiation for Brain Metastases With a Focus on Hypofractionated Stereotactic Radiosurgery

There are multiple published randomized controlled trials supporting single-fraction stereotactic radiosurgery (SF-SRS) for patients presenting with 1 to 4 brain metastases, with the benefit of minimizing radiation-induced neurocognitive sequelae as compared to whole brain radiotherapy . More recently, the dogma of SF-SRS as the only means of delivering an SRS treatment has been challenged by hypofractionated SRS (HF-SRS). The ability to deliver 25-35 Gy in 3-5 HF-SRS fractions is a direct consequence of the evolution of radiation technologies to allow image guidance, specialized treatment planning, robotic delivery and/or patient positioning corrections in all 6 degrees-of-freedom, and frameless head immobilization. The intent is to mitigate the potentially devastating complication of radiation necrosis and improve rates of local control for larger metastases. This narrative review provides an overview of outcomes specific to HF-SRS in addition to the more recent developments of staged SRS, preoperative SRS, and hippocampal avoidance-whole brain radiotherapy with simultaneous integrated boost.

Effect of Stereotactic Body Radiation Therapy on Diverse Organ Lesions in Advanced Non-Small Cell Lung Cancer Patients Receiving Immune Checkpoint Inhibitors

OBJECTIVE

The combination of stereotactic body radiation therapy (SBRT) and immune checkpoint inhibitors (ICIs) is actively being explored in advanced non-small-cell lung cancer (NSCLC) patients. However, little is known about the optimal fractionation and radiotherapy target lesions in this scenario. This study investigated the effect of SBRT on diverse organ lesions and radiotherapy dose fractionation regimens on the prognosis of advanced NSCLC patients receiving ICIs.

METHODS

The medical records of advanced NSCLC patients consecutively treated with ICIs and SBRT were retrospectively reviewed at our institution from Dec. 2015 to Sep. 2021. Patients were grouped according to radiation sites. Progression-free survival (PFS) and overall survival (OS) were recorded using the Kaplan-Meier method and compared between different treatment groups using the log-rank (Mantel-Cox) test.

RESULTS

A total of 124 advanced NSCLC patients receiving ICIs combined with SBRT were identified in this study. Radiation sites included lung lesions (lung group, n=43), bone metastases (bone group, n=24), and brain metastases (brain group, n=57). Compared with the brain group, the mean PFS (mPFS) in the lung group was significantly prolonged by 13.3 months (8.5 months vs. 21.8 months, HR=0.51, 95%CI: 0.28-0.92, P=0.0195), and that in the bone group prolonged by 9.5 months with a 43% reduction in the risk of disease progression (8.5 months vs. 18.0 months, HR=0.57, 95%CI: 0.29-1.13, P=0.1095). The mPFS in the lung group was prolonged by 3.8 months as compared with that in the bone group. The mean OS (mOS) in the lung and bone groups was longer than that of the brain group, and the risk of death decreased by up to 60% in the lung and bone groups as compared with that of the brain group. When SBRT was concurrently given with ICIs, the mPFS in the lung and brain groups were significantly longer than that of the bone group (29.6 months vs. 16.5 months vs. 12.1 months). When SBRT with 8-12 Gy per fraction was combined with ICIs, the mPFS in the lung group was significantly prolonged as compared with that of the bone and brain groups (25.4 months vs. 15.2 months vs. 12.0 months). Among patients receiving SBRT on lung lesions and brain metastases, the mPFS in the concurrent group was longer than that of the SBRT→ICIs group (29.6 months vs. 11.4 months, P=0.0003 and 12.1 months vs. 8.9 months, P=0.2559). Among patients receiving SBRT with <8 Gy and 8-12 Gy per fraction, the mPFS in the concurrent group was also longer than that of the SBRT→ICIs group (20.1 months vs. 5.3 months, P=0.0033 and 24.0 months vs. 13.4 months, P=0.1311). The disease control rates of the lung, bone, and brain groups were 90.7%, 83.3%, and 70.1%, respectively.

CONCLUSION

The study demonstrated that the addition of SBRT on lung lesions versus bone and brain metastases to ICIs improved the prognosis in advanced NSCLC patients. This improvement was related to the sequence of radiotherapy combined with ICIs and the radiotherapy fractionation regimens. Dose fractionation regimens of 8-12 Gy per fraction and lung lesions as radiotherapy targets might be the appropriate choice for advanced NSCLC patients receiving ICIs combined with SBRT.

Stereotactic Body Radiation Therapy Versus Conventional Radiation Therapy in Pain Relief for Bone Metastases: A Systematic Review and Meta-Analysis

PURPOSE

This study aimed to investigate the difference in pain relief between stereotactic body radiation therapy (SBRT) and conventional radiation therapy (cRT) for patients with bone metastases.

METHODS AND MATERIALS

Clinical trials and observational studies comparing SBRT versus cRT for bone metastases were retrieved. The main endpoint was pain relief after radiation therapy; the secondary endpoints were pain score change, local progression-free survival, reirradiation rate, and toxic events. When there was a significant heterogeneity, the random-effects model was applied. Otherwise, the fixed-effects model was used. Analyses of all included studies were performed first, followed by analyses of randomized controlled trials (RCTs) only.

RESULTS

Six RCTs, 1 prospective cohort study, and 3 retrospective observational studies were enrolled. Between 2004 and 2019, 448 patients received SBRT, and 445 patients received cRT. All prospective studies defined the lesions as oligometastatic. Pooled results based on all included studies indicated that SBRT was generally associated with a higher overall relief rate (P < .001 at 3 months; P = .015 at 6 months) and complete relief rate (P = .029 at 1 month; P < .001 at 6 months). Pooled results based on RCTs indicated that at 3 and 6 months, SBRT was associated with a higher overall relief rate (P < .001 and P = .017, respectively) and complete relief rate (P < .001 and P < .00, respectively). Subgroup analyses indicated that in more cases, the analgesic advantage of SBRT was more obvious when spinal lesions were irradiated, when the difference in the mean biological effective dose (BED) was less, or when intensity modulated radiation therapy was used to deliver SBRT.

CONCLUSIONS

Excessive elevation of BED introduces the risk of diminishing the analgesic effect of SBRT. SBRT delivered using intensity modulated radiation therapy is preferred for pain relief in spinal oligometastases. More RCTs are required to determine the most appropriate BED or dose regimen for SBRT.

How Stereotactic Radiotherapy Changed the Landscape in Cancer Care

The term “stereotactic body radiotherapy” (SBRT) refers to high-precision radiotherapy techniques using numerous beams converging in a small target volume, allowing the delivery of high doses per fraction (>6–7 Gy) in a very few number of fractions [...]

Uncertainties in the dosimetric heterogeneity correction and its potential effect on local control in lung SBRT

Objective. Dose calculation in lung stereotactic body radiation therapy (SBRT) is challenging due to the low density of the lungs and small volumes. Here we assess uncertainties associated with tissue heterogeneities using different dose calculation algorithms and quantify potential associations with local failure for lung SBRT.Approach. 164 lung SBRT plans were used. The original plans were prepared using Pencil Beam Convolution (PBC, n = 8) or Anisotropic Analytical Algorithm (AAA, n = 156). Each plan was recalculated with AcurosXB (AXB) leaving all plan parameters unchanged. A subset (n = 89) was calculated with Monte Carlo to verify accuracy. Differences were calculated for the planning target volume (PTV) and internal target volume (ITV) Dmean[Gy], D99%[Gy], D95%[Gy], D1%[Gy], and V100%[%]. Dose metrics were converted to biologically effective doses (BED) usingα/β= 10Gy. Regression analysis was performed for AAA plans investigating the effects of various parameters on the extent of the dosimetric differences. Associations between the magnitude of the differences for all plans and outcome were investigated using sub-distribution hazards analysis.Main results. For AAA cases, higher energies increased the magnitude of the difference (ΔDmean of -3.6%, -5.9%, and -9.1% for 6X, 10X, and 15X, respectively), as did lung volume (ΔD99% of -1.6% per 500cc). Regarding outcome, significant hazard ratios (HR) were observed for the change in the PTV and ITV D1% BEDs upon univariate analysis (p = 0.042, 0.023, respectively). When adjusting for PTV volume and prescription, the HRs for the change in the ITV D1% BED remained significant (p = 0.039, 0.037, respectively).Significance. Large differences in dosimetric indices for lung SBRT can occur when transitioning to advanced algorithms. The majority of the differences were not associated with local failure, although differences in PTV and ITV D1% BEDs were associated upon univariate analysis. This shows uncertainty in near maximal tumor dose to potentially be predictive of treatment outcome.

Oncological outcome of vocal cord-only radiotherapy for cT1-T2 glottic laryngeal squamous cell carcinoma

PURPOSE

Early-stage glottic cancer can be treated with radiotherapy only. Modern radiotherapy solutions allow for individualized dose distributions, hypofractionation and sparing of organs at risk. The target volume used to be the entire voice box. This series describe the oncological outcome and toxicity of individualized vocal cord-only hypofractionated radiotherapy for early stage (cT1a-T2 N0).

METHODS

Retrospective cohort study with patients treated in a single center between 2014 and 2020.

RESULTS

A total of 93 patients were included. Local control rate was 100% for cT1a, 97% for cT1b and 77% for cT2. Risk factor for local recurrence was smoking during radiotherapy. Laryngectomy-free survival was 90% at 5 years. Grade III or higher late toxicity was 3.7%.

CONCLUSION

Vocal cord-only hypofractionated radiotherapy appears to be oncologically safe in early-stage glottic cancer. Modern, image-guided radiotherapy led to comparable results as historical series with very limited late toxicity.

Is single fraction the future of stereotactic body radiation therapy (SBRT)? A critical appraisal of the current literature

Stereotactic Body Radiation Therapy (SBRT) is a standard of care for many localizations but the question of the optimal fractionation remains a matter of concern. If single fraction sessions are routinely used for intracranial targets, their utilization for mobile extracranial lesions is a source of debate and apprehension. Single session treatments improve patient comfort, provide a medico-economic benefit, and have proven useful in the context of the SARS-CoV 2 pandemic. However, both technical and radiobiological uncertainties remain. Experience from intracranial radiosurgery has shown that the size of the target, its proximity to organs at risk, tumor histology, and the volume of normal tissue irradiated are all determining factors in the choice of fractionation. The literature on the use of single fraction for extracranial sites is still scarce. Only primary and secondary pulmonary tumors have been evaluated in prospective randomized trials, allowing the integration of these fractionation schemes in daily practice, for highly selected cases and in trained teams. The level of evidence for the other organs is mainly based on dose escalation or retrospective trials and calls for caution, with further studies being needed before routine use in clinical practice.

First treatments for Lattice stereotactic body radiation therapy using magnetic resonance image guided radiation therapy

Two abdominal patients were treated with Lattice stereotactic body radiation therapy (SBRT) using magnetic resonance guided radiation therapy (MRgRT). This is one of the first reported treatments of Lattice SBRT with the use of MRgRT. A description of the treatment approach and planning considerations were incorporated into this report. MRgRT Lattice SBRT delivered similar planning quality metrics to established dosimetric parameters for Lattice SBRT. Increased signal intensity were seen in the MRI treatments for one of the patients during the course of treatment.

Vasculitis, CA19-9, and Perineural Invasion Differentially Predict Response and Surgical Outcome in Pancreatic Ductal Adenocarcinoma

PURPOSE

Curative intent treatment of pancreatic adenocarcinoma (PDAC) relies on surgical resection. Modern treatment protocols focus on optimizing neoadjuvant therapy to increase resectability and improve oncologic outcomes. To elucidate differences in outcomes, we investigated the relationship between neoadjuvant chemotherapy (NAC), either with or without stereotactic body radiation therapy (SBRT), and vascular inflammation, surgical outcomes, and the resultant transcriptomic changes.

METHODS AND MATERIALS

Clinical data were collected from patients with borderline resectable PDAC (clinical T3-T4N0-1) who underwent NAC or NAC-SBRT followed by curative intent resection between 2014 and 2019. Vascular structures on surgical specimens were histologically evaluated for vasculitis. RNA sequencing was used to evaluate differential gene expression and to generate enrichment maps. Multivariate analysis was used to analyze the relationship between patient characteristics and oncological outcome.

RESULTS

In total, 46 patients met inclusion criteria (n = 12 NAC, n = 34 NAC-SBRT) with a median follow-up of 20.1 months. All patients underwent curative resection, with 91.3% achieving R0. There was no significant difference in patterns of failure, overall survival, or progression-free survival between NAC and NAC-SBRT groups. Patients with vasculitis had a lower median overall survival compared with those without (14.5 vs 28.3 months; hazard ratio, 12.96; 95% confidence interval, 3.55-47.28; P < .001). There was no significant correlation between inflammation and surgical complications or pathologic response. Neoadjuvant therapy did not have a significant effect on development of vasculitis (odds radio, 1.64 for NAC-SBRT; 95% confidence interval, 0.40-8.43; P = .52). Predictors of poor survival included perineural invasion and high baseline carbohydrate antigen 19-9 (CA19-9) (>191 U/mL). Patients with robust CA19-9 (>20% decrease) responses to neoadjuvant therapy had enrichment in immune response, chemotaxis, and cytotoxic T-cell and natural killer-cell proliferation.

CONCLUSIONS

Vasculitis predicts for poor survival outcomes in patients with PDAC; NAC-SBRT did not increase the rate of vasculitis compared with NAC. Perineural invasion and CA19-9 remain strong prognosticators. Understanding and optimizing immune interactions remain a crucial hurdle in achieving response in pancreatic cancer.

Clinical Radiobiology for Radiation Oncology

This chapter is focused on radiobiological aspects at the molecular, cellular, and tissue level which are relevant for the clinical use of ionizing radiation (IR) in cancer therapy. For radiation oncology, it is critical to find a balance, i.e., the therapeutic window, between the probability of tumor control and the probability of side effects caused by radiation injury to the healthy tissues and organs. An overview is given about modern precision radiotherapy (RT) techniques, which allow optimal sparing of healthy tissues. Biological factors determining the width of the therapeutic window are explained. The role of the six typical radiobiological phenomena determining the response of both malignant and normal tissues in the clinic, the 6R’s, which are Reoxygenation, Redistribution, Repopulation, Repair, Radiosensitivity, and Reactivation of the immune system, is discussed. Information is provided on tumor characteristics, for example, tumor type, growth kinetics, hypoxia, aberrant molecular signaling pathways, cancer stem cells and their impact on the response to RT. The role of the tumor microenvironment and microbiota is described and the effects of radiation on the immune system including the abscopal effect phenomenon are outlined. A summary is given on tumor diagnosis, response prediction via biomarkers, genetics, and radiomics, and ways to selectively enhance the RT response in tumors. Furthermore, we describe acute and late normal tissue reactions following exposure to radiation: cellular aspects, tissue kinetics, latency periods, permanent or transient injury, and histopathology. Details are also given on the differential effect on tumor and late responding healthy tissues following fractionated and low dose rate irradiation as well as the effect of whole-body exposure.

The minimal FLASH sparing effect needed to compensate the increase of radiobiological damage due to hypofractionation for late-reacting tissues

PURPOSE

Normal tissue (NT) sparing by ultra-high dose rate (UHDR) irradiations compared to conventional dose rate (CONV) irradiations while being isotoxic to the tumor has been termed "FLASH effect" and has been observed when large doses per fraction (d ≳ 5 Gy) have been delivered. Since hypofractionated treatment schedules are known to increase toxicities of late-reacting tissues compared to normofractionated schedules for many clinical scenarios at CONV dose rates, we developed a formalism based on the biologically effective dose (BED) to assess the minimum magnitude of the FLASH effect needed to compensate the loss of late-reacting NT sparing when reducing the number of fractions compared to a normofractionated CONV treatment schedule while remaining isoeffective to the tumor.

METHODS

By requiring the same BED for the tumor, we derived the "break-even NT sparing weighting factor" W_BE for the linear-quadratic (LQ) and LQ-linear (LQ-L) models for an NT region irradiated at a relative dose r (relative to the prescribed dose per fraction d to the tumor). W_BE was evaluated numerically for multiple values of d and r, and for different tumor and NT α/β-ratios. W_BE was compared against currently available experimental data on the magnitude of the NT sparing provided by the FLASH effect for single fraction doses.

RESULTS

For many clinically relevant scenarios, W_BE decreases steeply initially for d > 2 Gy for late-reacting tissues with (α/β)_NT ≈ 3 Gy, implying that a significant NT sparing by the FLASH effect (between 15% and 30%) is required to counteract the increased radiobiological damage experienced by late-reacting NT for hypofractionated treatments with d < 10 Gy compared to normofractionated treatments that are equieffective to the tumor. When using the LQ model with generic α/β-ratios for tumor and late-reacting NT of (α/β)_T  = 10 Gy and (α/β)_NT  = 3 Gy, respectively, most currently available experimental evidence about the magnitude of NT sparing by the FLASH effect suggests no net NT sparing benefit for hypofractionated FLASH radiotherapy (RT) in the high-dose region when compared with W_BE . Instead, clinical indications with more similar α/β-ratios of the tumor and dose-limiting NT toxicities [i.e., (α/β)_T  ≈ (α/β)_NT ], such as prostate treatments, are generally less penalized by hypofractionated treatments and need consequently smaller magnitudes of NT sparing by the FLASH effect to achieve a net benefit. For strongly hypofractionated treatments (>10-15 Gy/fraction), the LQ-L model predicts, unlike the LQ model, a larger W_BE suggesting a possible benefit of strongly hypofractionated FLASH RT, even for generic α/β-ratios of (α/β)_T  = 10 Gy and (α/β)_NT  = 3 Gy. However, knowledge on the isoeffect scaling for high doses per fraction (≳10 Gy/fraction) and its modeling is currently limited and impedes accurate and reliable predictions for such strongly hypofractionated treatments.

CONCLUSIONS

We developed a formalism that quantifies the minimal NT sparing by the FLASH effect needed to compensate for hypofractionation, based on the LQ and LQ-L models. For a given hypofractionated UHDR treatment scenario and magnitude of the FLASH effect, the formalism predicts if a net NT sparing benefit is expected compared to a respective normofractionated CONV treatment.

Effect of Ablative Dose Irradiation on Redistribution and Radioresponse in a Mouse Xenograft Model

We investigated the effects of ablative dose irradiation on redistribution and radioresponse after the second irradiation in a mouse xenograft model, assuming stereotactic body radiotherapy (SBRT). A human tongue cancer cell line, SAS-Fucci, expressing the fluorescent ubiquitination-based cell cycle indicator (Fucci) that visualizes the cell cycle, was employed in this study. Tumor xenografts formed subcutaneously in nude mice (approximately 6 mm in diameter), with essentially no hypoxic regions, were irradiated at 10 Gy and G2 arrest kinetics were determined using histology sections and a real-time detection method. The second irradiation (10 Gy) was given at intervals of 0 h, 3 h, 1 day, and 4 days after the first irradiation, and tumor regrowth curves were obtained. It was revealed that the ratio of G2-arrested cells showed a much higher peak at 1 day postirradiation compared to 2 Gy, assuming conventional radiotherapy, and gradually decreased thereafter up to 4 days. Tumors irradiated at intervals of 0 h and 1 day demonstrated significantly higher radioresponses than other timings. We conclude that redistribution could contribute to the efficacy of SBRT.

Recommendations for stereotactic body radiation therapy for spine and non-spine bone metastases. A GETUG (French society of urological radiation oncolgists) consensus using a national two-round modified Delphi survey

Background and purpose

The relevance of metastasis-directed stereotactic body radiation therapy (SBRT) remains to be demonstrated through phase III trials. Multiple SBRT procedures have been published potentially resulting in a disparity of practices. Therefore, the french society of urological radiation oncolgists (GETUG) recognized the need for joint expert consensus guidelines for metastasis-directed SBRT in order to standardize practice in trials carried out by the group.

Materials and methods

After a comprehensive literature review, 97 recommendation statements were created regarding planning and delivery of spine bone (SBM) and non-spine bone metastases (NSBM) SBRT. These statements were then submitted to a national online two-round modified Delphi survey among main GETUG investigators. Consensus was achieved if a statement received ≥ 75 % agreements, a trend to consensus being defined as 65-74 % agreements. Any statement without consensus at round one was re-submitted in round two.

Results

Twenty-one out of 29 (72.4%) surveyed experts responded to both rounds. Seventy-five statements achieved consensus at round one leaving 22 statements needing a revote of which 16 achieved consensus and 5 a trend to consensus. The final rate of consensus was 91/97 (93.8%). Statements with no consensus concerned patient selection (3/19), dose and fractionation (1/11), prescription and dose objectives (1/9) and organs at risk delineation (1/15). The voting resulted in the writing of step-by-step consensus guidelines.

Conclusion

Consensus guidelines for SBM and NSBM SBRT were agreed upon using a validated modified Delphi approach. These guidelines will be used as per-protocole recommendations in ongoing and further GETUG clinical trials.

Imaging Features in the Liver after Stereotactic Body Radiation Therapy

Historically, radiation therapy was not considered in treatment of liver tumors owing to the risk of radiation-induced liver disease. However, development of highly conformed radiation treatments such as stereotactic body radiation therapy (SBRT) has increased use of radiation therapy in the liver. SBRT is indicated in treatment of primary and metastatic liver tumors with outcomes comparable to those of other local therapies, especially in treatment of hepatocellular carcinoma. After SBRT, imaging features of the tumor and surrounding background hepatic parenchyma demonstrate a predictable pattern immediately after treatment and during follow-up. The goals of SBRT are to deliver a lethal radiation dose to the targeted liver tumor and to minimize radiation dose to normal liver parenchyma and other adjacent organs. Evaluation of tumor response after SBRT centers on changes in size and enhancement; however, these changes are often delayed secondary to the underlying physiologic effects of radiation. Knowledge of the underlying pathophysiologic mechanisms of SBRT should allow better understanding of the typical imaging features in detection of tumor response and avoid misinterpretation from common pitfalls and atypical imaging findings. Imaging features of radiation-induced change in the surrounding liver parenchyma are characterized by a focal liver reaction that can potentially be mistaken for no response or recurrence of tumor. Knowledge of the pattern and chronology of this phenomenon may allay any uncertainty in assessment of tumor response. Other pitfalls related to fiducial marker placement or combination therapies are important to recognize. The authors review the basic principles of SBRT and illustrate post-SBRT imaging features of treated liver tumors and adjacent liver parenchyma with a focus on avoiding pitfalls in imaging evaluation of response. Online supplemental material is available for this article. ^©RSNA, 2022.

The effect of ionizing radiation through cardiac stereotactic body radiation therapy on myocardial tissue for refractory ventricular arrhythmias: A review

Cardiac stereotactic body radiation therapy (cSBRT) is a non-invasive treatment modality that has been recently reported as an effective treatment for ventricular arrhythmias refractory to medical therapy and catheter ablation. The approach leverages tools developed and refined in radiation oncology, where experience has been accumulated in the treatment of a wide variety of malignant conditions. However, important differences exist between rapidly dividing malignant tumor cells and fully differentiated myocytes in pathologically remodeled ventricular myocardium, which represent the respective radiation targets. Despite its initial success, little is known about the radiobiology of the anti-arrhythmic effect cSBRT. Pre-clinical data indicates a late fibrotic effect of that appears between 3 and 4 months following cSBRT, which may result in conduction slowing and block. However, there is clear clinical evidence of an anti-arrhythmic effect of cSBRT that precedes the appearance of radiation induced fibrosis for which the mechanism is unclear. In addition, the data to date suggests that even the late anti-arrhythmic effect of cSBRT is not fully attributable to radiation.-induced fibrosis. Pre-clinical data has identified upregulation of proteins expected to result in both increased cell-to-cell coupling and excitability in the early post cSBRT period and demonstrated an associated increase in myocardial conduction velocity. These observations indicate a complex response to radiotherapy and highlight the lack of clarity regarding the different stages of the anti-arrhythmic mechanism of cSBRT. It may be speculated that in the future cSBRT therapy could be planned to deliver both early and late radiation effects titrated to optimize the combined anti-arrhythmic efficacy of the treatment. In addition to these outstanding mechanistic questions, the optimal patient selection, radiation modality, radiation dose and treatment planning strategy are currently being investigated. In this review, we consider the structural and functional effect of radiation on myocardium and the possible anti-arrhythmic mechanisms of cSBRT. Review of the published data highlights the exciting prospects for the development of knowledge and understanding in this area in which so many outstanding questions exist.

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.

HIF-1α Inhibition Improves Anti-Tumor Immunity and Promotes the Efficacy of Stereotactic Ablative Radiotherapy (SABR)

Simple Summary

Stereotactic ablative radiotherapy (SABR), which irradiates tumors with high-dose radiation per fraction, promotes anti-tumor immunity by stimulating various immune processes. SABR also induces vascular damage and obstructs blood flow, thereby increasing tumor hypoxia and upregulation of hypoxia-inducible factors HIF-1α and HIF-2α, master transcription factors for the cellular response to hypoxia. HIF-1α and HIF-2α are key players in the upregulation of immune suppression in hypoxia. Therefore, the radiation-induced increase in anti-tumor immunity is masked by the HIF-mediated immune suppression. Pre-clinical experiments show that inhibition of HIF-1α effectively prevents immune suppression and improves anti-tumor immunity. A combination of HIF-1α inhibitors with immunotherapy with checkpoint blocking antibodies may represent a novel approach to boost anti-tumor immunity and enhance the efficacy of SABR.

Abstract

High-dose hypofractionated radiation such as SABR (stereotactic ablative radiotherapy) evokes an anti-tumor immune response by promoting a series of immune-stimulating processes, including the release of tumor-specific antigens from damaged tumor cells and the final effector phase of immune-mediated lysis of target tumor cells. High-dose hypofractionated radiation also causes vascular damage in tumors, thereby increasing tumor hypoxia and upregulation of hypoxia-inducible factors HIF-1α and HIF-2α, the master transcription factors for the cellular response to hypoxia. HIF-1α and HIF-2α are critical factors in the upregulation of immune suppression and are the master regulators of immune evasion of tumors. Consequently, SABR-induced increase in anti-tumor immunity is counterbalanced by the increase in immune suppression mediated by HIFα. Inhibition of HIF-1α with small molecules such as metformin downregulates immunosuppressive pathways, including the expression of immune checkpoints, and it improves or restores the anti-tumor immunity stimulated by irradiation. Combinations of HIFα inhibitors, particularly HIF-1α inhibitors, with immune checkpoint blocking antibodies may represent a novel approach to boost the overall anti-tumor immune profile in patients and thus enhance outcomes after SABR.

One-Week Dynamic Changes in Cardiac Proteomes After Cardiac Radioablation in Experimental Rat Model

Background

Recently, stereotactic ablative radiotherapy (SABR) has been adopted to non-invasively treat catheter ablation-refractory ventricular tachycardia (VT). VT episodes have been dramatically reduced after SABR, within weeks; however the underlying mechanisms of these clinical effects and potential mediators of early anti-arrhythmic effect remain unclear.

Methods

In this study, cardiac tissue was harvested from non-irradiated control (0 Gy), conventional irradiated control (2 Gy), and radioablative test (25 Gy) rat groups after 3 and 7 days of irradiation. The samples were proteomically analyzed to identify the differentially expressed proteins (DEP) between different groups. Validation experiments were performed similar to validation in profiling where Data independent acquisition and parallel reaction monitoring methods were used. Data are available via ProteomeXchange with identifier PXD030878.

Results

Functional enrichment analysis of 25 Gy sample showed that among the downregulated proteins, “intracellular signal transduction” and “cell to cell adhesion” proteins were significantly affected at day 3 while “Ras protein signal transduction,” “GTPase regulation,” and “actin filament-based process” proteins were majorly affected at day 7. GO analysis demonstrated that most of the upregulated proteins belonged to the classes “cellular stress response,” “endomembranal organization,” or “endoplasmic reticulum stress response” at day 3. At day 7, 42 proteins, mainly associated with response to drug, organic substance, or radiation, were specifically upregulated in 25 Gy. DEP analysis of cardiac conduction showed Ryr2 and Cav1 upregulation and Cacna2d2, Gja3, Scnb2, and Kcnn3 downregulation in the 25 Gy group compared to 0 Gy. In validation experiments, four proteins (Gsta1, Myot, Ephx1, and Capg) were repeatedly detected with 25 Gy-specific patterns at day 7.

Conclusions

25 Gy single fractional irradiation induces considerable cardiac proteome changes within the first 7 days, distinct from 2 Gy. Several candidate proteins displayed 25 Gy-specific changes and were related to oxidative stress-induced innate response or cardiac remodeling processes. Future studies should explore the specific role of these proteins upon cardiac radioablation.

Stereotactic Radiotherapy: An Alternative Option for Refractory Ventricular Tachycardia to Drug and Ablation Therapy

Refractory ventricular tachycardia (VT) often occurs in the context of organic heart disease. It is associated with significantly high mortality and morbidity rates. Antiarrhythmic drugs and catheter ablation represent the two main treatment options for refractory VT, but their use can be associated with inadequate therapeutic responses and procedure-related complications. Stereotactic body radiotherapy (SBRT) is extensively applied in the precision treatment of solid tumors, with excellent therapeutic responses. Recently, this highly precise technology has been applied for radioablation of VT, and its early results demonstrate a favorable safety profile. This review presents the potential value of SBRT in refractory VT.

Dynamic 18F-FET PET/CT to differentiate recurrent primary brain tumor and brain metastases from radiation necrosis after single-session robotic radiosurgery

OBJECTIVE

Cyberknife robotic radiosurgery (RRS) provides single-session high-dose radiotherapy of brain tumors with a steep dose gradient and precise real-time image-guided motion correction. Although RRS appears to cause more radiation necrosis (RN), the radiometabolic changes after RRS have not been fully clarified. ¹⁸F-FET-PET/CT is used to differentiate recurrent tumor (RT) from RN after radiosurgery when MRI findings are indecisive. We explored the usefulness of dynamic parameters derived from ¹⁸F-FET PET in differentiating RT from RN after Cyberknife treatment in a single-center study population.

METHODS

We retrospectively identified brain tumor patients with static and dynamic ¹⁸F-FET-PET/CT for suspected RN after Cyberknife. Static (tumor-to-background ratio) and dynamic PET parameters (time-activity curve, time-to-peak) were quantified. Analyses were performed for all lesions taken together (TOTAL) and for brain metastases only (METS). Diagnostic accuracy of PET parameters (using mean tumor-to-background ratio >1.95 and time-to-peak of 20 min for RT as cut-offs) and their respective improvement of diagnostic probability were analyzed.

RESULTS

Fourteen patients with 28 brain tumors were included in quantitative analysis. Time-activity curves alone provided the highest sensitivities (TOTAL: 95%, METS: 100%) at the cost of specificity (TOTAL: 50%, METS: 57%). Combined mean tumor-to-background ratio and time-activity curve had the highest specificities (TOTAL: 63%, METS: 71%) and led to the highest increase in diagnosis probability of up to 16% p. - versus 5% p. when only static parameters were used.

CONCLUSIONS

This preliminary study shows that combined dynamic and static ¹⁸F-FET PET/CT parameters can be used in differentiating RT from RN after RRS.

T-Cell Repertoire in Tumor Radiation: The Emerging Frontier as a Radiotherapy Biomarker

Simple Summary

Radiotherapy constitutes an essential component of the treatment for malignant disease. Besides its direct effect on cancer cells, namely, DNA damage and cell death, ionizing irradiation also mediates indirect antitumor effects that are mostly mediated by the immune system. Investigations into the processes underlying the interaction between radiotherapy and the immune system have uncovered mechanisms that can be exploited to promote the antitumor efficacy of radiotherapy both locally in the irradiated primary tumor and also at distant lesions in non-irradiated tumors. Because of its capacity to stimulate antitumor immunity, radiotherapy is also applied in combination with immune-checkpoint-inhibition-based immunotherapy. This review discusses the important pathways that govern the synergistic interactions between ionizing radiation and antitumor immune reactivity. Unravelling these involved mechanisms is mandatory for the successful application of anticancer radiotherapy and immunotherapy. We also place emphasis on the need for biomarkers that will aid in the selection of patients most likely to benefit from such combined treatments.

Abstract

Radiotherapy (RT) is a therapeutic modality that aims to eliminate malignant cells through the induction of DNA damage in the irradiated tumor site. In addition to its cytotoxic properties, RT also induces mechanisms that result in the promotion of antitumor immunity both locally within the irradiation field but also at distant tumor lesions, a phenomenon that is known as the “abscopal” effect. Because the immune system is capable of sensing the effects of RT, several treatment protocols have been assessing the synergistic role of radiotherapy combined with immunotherapy, collectively referred to as radioimmunotherapy. Herein, we discuss mechanistic insights underlying RT-based immunomodulation, which also enhance our understanding of how RT regulates antitumor T-cell-mediated immunity. Such knowledge is essential for the discovery of predictive biomarkers and for the improvement of clinical trials investigating the efficacy of radio-immunotherapeutic modalities in cancer patients.