Stereotactic Ablative Body Radiotherapy Research Articles

MLC tracking for lung SABR is feasible, efficient and delivers high-precision target dose and lower normal tissue dose

Background and purposeThe purpose of this work is to present the clinical experience from the first-in-human trial of real-time tumor targeting via MLC tracking for stereotactic ablative body radiotherapy (SABR) of lung lesions. Methods and materialsSeventeen patients with stage 1 non-small cell lung cancer (NSCLC) or lung metastases were included in a study of electromagnetic transponder–guided MLC tracking for SABR (NCT02514512). Patients had electromagnetic transponders inserted near the tumor. An MLC tracking SABR plan was generated with planning target volume (PTV) expanded 5 mm from the end-exhale gross tumor volume (GTV). A clinically approved comparator plan was generated with PTV expanded 5 mm from a 4DCT-derived internal target volume (ITV). Treatment was delivered using a standard linear accelerator to continuously adapt the MLC based on transponder motion. Treated volumes and reconstructed delivered dose were compared between MLC tracking and comparator ITV-based treatment. ResultsAll seventeen patients were successfully treated with MLC tracking (70 successful fractions). MLC tracking treatment delivery time averaged 8 minutes. The time from the start of CBCT to the end of treatment averaged 22 minutes. The MLC tracking PTV for 16/17 patients was smaller than the ITV-based PTV (range −1.6% to 44% reduction, or −0.6 to 18 cc). Reductions in mean lung dose (27 cGy) and V20Gy (50 cc) were statistically significant (p < 0.02). Reconstruction of treatment doses confirmed a statistically significant improvement in delivered GTV D98% (p < 0.05) from planned dose compared with the ITV-based plans. ConclusionThe first treatments with lung MLC tracking have been successfully performed in seventeen SABR patients. MLC tracking for lung SABR is feasible, efficient and delivers high-precision target dose and lower normal tissue dose.

PO-1912: Clinical experience of using stereotactic ablative body radiotherapy on inoperable pancreatic cancer

PO-1912: Clinical experience of using stereotactic ablative body radiotherapy on inoperable pancreatic cancer

Prevalence of Oligometastatic Disease of Solid Tumors and Stereotactic Ablative Body Radiotherapy Adverse Events

There is a growing evidence of better outcome of oligometastatic disease (OMD) of solid tumors compared to those with diffuse metastases when treated with stereotactic Ablative body radiotherapy (SABR) and or Stereotactic Radiosurgery (SRS). We reviewed our institution database about the prevalence of OMD in solid tumors, patients’ characteristics and the adverse events due to SABR/SRS. A retrospective review was performed for all solid tumor patients who treated with SABR and/or SRS at our institution from January 2017 to August 2019. Patients were defined to have OMD if ≤ 5 sites of disease were identified at time of diagnosis or first time of development of metastases. univariate analysis was performed to determine the association between treatment toxicity and patient characteristics, treated lesion volume and dose. The chi-square test or Fisher exact test were used as appropriate depending on sample being tested for comparing proportions in categorical variables and the t-test or ANOVA for continuous variables. Late toxicity was graded using the “Common Terminology Criteria for Adverse Events” (CTCAE V.5.0). Of 986 patients with solid tumors reviewed, 109 (11.05%) found to have OMD and treated with SABR and /or SRS for 225 metastatic lesions. The mean age of our OMD population was 57 years. Majority of them had Performance Status (PS) of one (53%) and two (30%) with Charlson Comorbidity index (CCI) of six (67%). Among OMD patients, 47 (43%) had primary colorectal cancer (CRC) followed by non-small cell lung cancer (NSCLC) 21(19%) and breast cancer 17(16%). Of 225 treated sites, 64 (28%) were brain metastatic sites followed by liver and lung (23%) and (21%), respectively. Median SABR dose was 48 Gy (range: 30-54 Gy) in 3 to 5 fractions and the median SRS dose was 21 Gy. Median volume of extracranial lesions was 22cc and 1.8cc for cranial lesions. SABR was well tolerated with infrequent late adverse events. No Grade 4 or 5 toxicity was encountered. Four treated sites (1.8%) in four patients (3.7%) had grade 3 toxicities; 16 treated sites (7%) in 12 treated patients (11%) had grade 2 toxicities. Toxicity correlation with PS, dose or volume was not statistically significant. OMD patients constitute a significant portion of oncology patients. SABR is a safe local treatment modality in OMD patient population. A prospective study with larger sample size is required to confirm these finding and evaluate any correlation between toxicity to SABR/SRS dose and treated lesion volume.

Pre-operative Stereotactic Ablative Body Radiotherapy (SABR) for Early-Stage Breast Cancer: A Novel Approach to Accelerated Partial Breast Radiotherapy

To evaluate pathologic and radiologic responses to pre-operative stereotactic ablative body radiotherapy (SABR) in early-stage breast carcinoma as well as treatment-related toxicity and cosmesis outcomes. To test hypothesis that pre-operative SABR will produce at least 10% probability of pathologic complete response. This investigation was conducted as a phase-2 prospective single-institution trial. Study population included women over 50 years of age with low-risk breast cancer defined as requiring all the following: unifocal tumor less than 2cm in size, ER+, Her-2 non-amplified, clinically node-negative, and non-lobular histology. Eligible patients underwent pre-operative SABR to 28.5 Gy in 3 fractions of 9.5 Gy/fraction followed by breast conserving surgery and sentinel lymph node (SLN) evaluation 6-8 weeks later. Diagnostic MRI was registered to simulation CT in all patients to define target volumes. The GTV was expanded by 15mm for CTV and 3mm was added for PTV. All patients were simulated and treated in the prone position. Primary endpoint was pathologic response. Secondary endpoints included radiologic responses on MRI, toxicity and cosmesis. Twenty patients were enrolled and treated. No pathologic complete responses were observed. With median follow-up of 13.5 (4-24) months, no local recurrences have been observed. Four patients were incidentally node-positive following SLN evaluation, two micro-metastases and two macro-metastases, and underwent post-SABR whole-breast radiotherapy. In patients who only had SABR, 15% (3) and 5% (1) developed late grade 2 or 3 soft-tissue toxicity, respectively. Of the 4 node-positive cases who received both SABR and whole-breast radiotherapy, two developed Gr 3 late soft-tissue toxicity. Pre-operative SABR failed to achieve pathologic complete response at 6-8 weeks following treatment although was well tolerated and improved convenience for most patients. Significant risk remains even in this low-risk cohort for positive nodes; 20% in our series. The resultant addition of whole breast radiotherapy in these cases may increase risk of grade 3 late toxicity.

Stereotactic Ablative Radiation Therapy (CyberKnife) for Lung Oligometastases from Head and Neck Squamous Cell Carcinoma (HNSCC): Bi-Institutional Study

The aim of this study was to assess clinical benefit of Stereotactic Ablative Body Radiation (SABR) for lung oligometastases from head and neck squamous cell carcinoma (HNSCC). Outcomes of patients with HNSCC with controlled primary tumor and presenting up to two evolutive lung metastases (<5cm) treated with SABR (CyberKnife) were retrospectively reviewed from our two French institutions. Lung metastases could have been screened by CT or PET- CT only and histological proof was not mandatory. Multidisciplinary tumor board was required to validate the indication of curative intent SABR in all cases. Between November 2007 and February 2018, 57 patients were identified in our database. Median age was 67 (50-83). Median time from treatment of the primary HNSCC to development of lung metastases was 18 months (0-93). Most had metachronous metastases (93%) rather than synchronous. The majority of patients received 60Gy in 3 fractions. 45 patients (79%) presented a solitary lung metastasis while 12 patients (21%) had 2 lung secondary lesions. With a median follow-up of 25.3 months, crude local and metastatic recurrences rates were 27.3 % and 61.8% respectively. 1 year and 2-year progression free survival (PFS) were 52.3% and 28% respectively. Median PFS was 14 months. 1 year and 2-year Overall Survival (OS) 87% and 66.8%. The median OS was 48.3 months. The treatment was well tolerated with only one patient who reported ≥ grade 3 toxicity (1.7%). In patients with a controlled primary tumor and limited number of metastases, a radical approach with SABR can be considered as an effective treatment. Prospective studies are needed to identify which patients benefit for such strategy.

Phase I Dose-Escalation Trial of Stereotactic Ablative Body Radiotherapy (SAbR) for Men with High-Risk Prostate Cancer

Phase I Dose-Escalation Trial of Stereotactic Ablative Body Radiotherapy (SAbR) for Men with High-Risk Prostate Cancer

Stereotactic radiotherapy with focal boost for intermediate and high-risk prostate cancer: Initial results of the SPARC trial

IntroductionDose escalation to dominant intraprostatic lesions (DILs) is a novel method to increase the therapeutic ratio in localised prostate cancer. The Stereotactic Prostate Augmented Radiotherapy with Cyberknife (SPARC) trial was designed to determine the feasibility of a focal boost defined with multiparametric magnetic resonance imaging (mpMRI) using stereotactic ablative body radiotherapy (SABR). Materials and methodsPatients were included with newly diagnosed intermediate to high risk prostate cancer with at least one of: Gleason score 4 + 3, stage T3a, or PSA > 20 ng/ml. Visible disease on mpMRI was mandatory and up to 2 separate nodules were allowed. All patients received androgen deprivation. Patients received 36.25 Gy in 5 fractions using CyberKnife® and the DIL received a simultaneous boost to a maximum of 47.5 Gy, as allowed by OAR constraints. Genitourinary (GU) and gastrointestinal (GI) toxicity was reported using the RTOG scoring criteria. International Index of Erectile Function (IIEF) and EQ-5D global health scores were regularly captured. ResultsAn interim safety analysis was performed on the first 8 patients, recruited between July 2013 and December 2015. Median follow up was 56 months (range 50–74). Median D95 values for the prostate PTV and boost volume were 36.55 Gy (range 35.87–36.99) and 46.62 Gy (range 44.85–48.25) respectively. Of the dose constraints, 10/80 were not achieved but all were minor dose variations. Grade 2+ acute GU and GI toxicities were 37.5% respectively while grade 2+ late GU and GI toxicities were 12.5% and 0% respectively. IIEF and quality of life scores recovered over time and all patients remain in biochemical remission. ConclusionThe first patients have been successfully treated with prostate SABR and focal boost on the SPARC trial, with excellent adherence to the planning protocol. Toxicity and efficacy results are promising and further recruitment is underway.

Assessing tumor centrality in lung stereotactic ablative body radiotherapy (SABR): the effects of variations in bronchial tree delineation and potential for automated methods

Accurate delineation of the proximal bronchial tree (PBT) is crucial for appropriate assessment of lung tumor centrality and choice of Stereotactic Ablative Body Radiotherapy (SABR) dose prescription. Here, we investigate variabilities in manual PBT delineation and their potential to influence assessing lesion centrality. A fully automatic, intensity-based tool for PBT contouring and measuring distance to the target is also described. This retrospective analysis included a total of 61 patients treated with lung SABR. A subset of 41 patients was used as a training dataset, containing clinical PBT contour and additional subsequently generated manual contours. The tool was optimized and compared against manual contours in terms of volume, distance to the target and various overlap/similarity metrics. The remaining 20 patients were used as a validation dataset to investigate the dosimetric effects of variations between manual and automatic PBT contours. Considerable interobserver variability was observed, particularly in identifying the superior and inferior borders of the PBT. Automatic PBT contours were comparable to manual contours with average Dice of 0.63 to 0.79 and mean distance to agreement of 1.78 to 3.34 mm. No significant differences in dosimetric parameters were found between automatically and manually generated contours. A moderate negative correlation was found between PBT maximum dose and distance to the lesion (p < 0.05). Variability in manual PBT delineation may result in inconsistent assessment of tumor centrality. Automatic contouring can help standardize clinical practice, support investigations into the link between SABR outcomes and lesion proximity to central airways and the development of predictive toxicity models that incorporate precise measurements of tumor location in relation to high-risk organs.

Abscopal effect in pulmonary carcinoid tumor following ablative stereotactic body radiation therapy: a case report

BackgroundThe abscopal effect was described as early as the 1950s, when untreated tumors demonstrated a response after radiation therapy was delivered to an untreated, distant site. The mechanisms underlying this global response to otherwise localized therapy remain unknown, though there is increasing evidence that increased antigen expression following ablative radiotherapy may play a role.Case presentationWe report a case of a 69-year-old African American woman with a history of metastatic typical pulmonary carcinoid with multiple lung nodules who had a significant decrease in size of an untreated left upper lobe nodule after stereotactic body radiation therapy to an oligoprogressive left lower lobe lesion.ConclusionsTo our knowledge, this report describes the first case of an abscopal effect in a typical pulmonary carcinoid. Further research is needed regarding the mechanisms responsible for this finding and the role of combining radiation therapy and cancer immunotherapy in patients with pulmonary carcinoid tumors.

FASTRACK II magnetic resonance imaging sub-study Diffusion and perfusion biomarkers for renal SABR response

Stereotactic ablative body radiotherapy (SABR) is demonstrating good local control for patients with inoperable primary renal cell carcinoma. In a previous pilot study we identified magnetic resonance imaging (MRI) early response biomarkers that correlate with later morphological changes in computed tomography (CT) images. These early functional changes in diffusion and perfusion following radiotherapy were observed on MRI and have the potential to identify non-responders who may benefit from adjuvant or salvage therapies. Here we detail the imaging protocol for an MRI sub-study of the Focal Ablative STereotactic Radiosurgery for Cancers of the Kidney (FASTRACK II) trial. A preliminary patient case demonstrates the high quality of the imaging data, with discussion of the improvements made from the pilot protocol for improved motion management and correction. We aim to validate the previously identified early response MRI biomarkers with this rich prospective multi-centre dataset.

Automatic evaluation of contours in radiotherapy planning utilising conformity indices and machine learning.

Peer-review of Target Volume (TV) and Organ at Risk (OAR) contours in radiotherapy planning are typically conducted visually; this can be time consuming and subject to interobserver variation. This study investigated automatic evaluation of contouring using conformity indices and supervised machine learning. A total of 393 contours from 253 Stereotactic Ablative Body Radiotherapy (SABR) benchmark cases (adrenal gland, liver, pelvic lymph node and spine), delineated by 132 clinicians from 25 centres, were visually evaluated for conformity against gold standard contours. Contours were scored as "pass" or "fail" on visual peer review and six Conformity Indices (CIs) were applied. CI values were mapped to pass/fail scores for each contour and used to train supervised machine learning models. A 5-fold cross validation method was employed to determine the predictive accuracies of each model. The stomach structure produced models with the highest predictive accuracy overall (96% using Support Vector Machine and Ensemble models), whilst the liver GTV produced models with the lowest predictive accuracy (76% using Logistic Regression). Predictive accuracies across all models ranged from 68-96% (68-87% for TV and 71-96% for OARs). Although a final visual review by an experienced clinician is still required, the automatic contour evaluation method could reduce the time for benchmark case reviews by identifying gross contouring errors. This method could be successfully implemented to support departmental training and the continuous assessment of outlining for clinical staff in the peer-review process, to reduce interobserver variability in contouring and improve interpretation of radiological anatomy.

Technical challenges of linac-based stereotactic ablative body radiotherapy: short review for non-radiation oncologists.

Stereotactic ablative radiotherapy (SABR) is a radiation technique delivering high doses of radiation in a small number of treatments, to extracranial targets. It is standard of care in patients with inoperable early stage non-small cell lung cancer, and it is increasingly used in patients with oligometastatic disease. The main advantage of SABR is a steep dose gradient, allowing delivery of high biologically effective doses to the target, while minimizing irradiation exposure of the neighboring normal tissues. This results in high rates of local control of the treated target and minimal toxicity risks, and minimal impact on the quality of life of the patients. However, it requires high precision, accuracy and reproducibility during the entire process, from simulation to treatment planning and treatment delivery. This article will focus on general principles of SABR treatment planning and delivery, with emphasis on the strategies to reduce errors related to immobilization, respiratory management and treatment verification.

Cost-Effectiveness Analysis of Stereotactic Ablative Body Radiation Therapy Compared With Surgery and Radiofrequency Ablation in Two Patient Cohorts: Metastatic Liver Cancer and Hepatocellular Carcinoma

AimsTo compare the cost-effectiveness of stereotactic ablative body radiation therapy (SABR) with radiofrequency ablation and surgery in adult patients with metastatic liver cancer and hepatocellular carcinoma (HCC). Materials and methodsTwo patient cohorts were assessed: liver oligometastases and HCC. For each patient cohort, a decision analytic model was constructed to assess the cost-effectiveness of interventions over a 5-year horizon. A Markov process was embedded in the decision model to simulate the possible prognosis of cancer. Data on transition probabilities, survival, side-effects, quality of life and costs were obtained from published sources and the SABR Commissioning through Evaluation (CtE) scheme. The primary outcome was the incremental cost-effectiveness ratio with respect to quality-adjusted life-years. The robustness of the results was examined in a sensitivity analysis. Analyses were conducted from a National Health Service and Personal Social Services perspective. ResultsIn the base case analysis, which assumed that all three interventions were associated with the same cancer progression rates and mortality rates, SABR was the most cost-effective intervention for both patient cohorts. This conclusion was sensitive to the cancer progression rate, mortality rate and cost of interventions. Assuming a willingness-to-pay threshold of £20 000 per quality-adjusted life-year, the probability that SABR is cost-effective was 57% and 50% in liver oligometastases and HCC, respectively. ConclusionsOur results indicate a potential for SABR to be cost-effective for patients with liver oligometastases and HCC. This finding supports further investigation in clinical trials directly comparing SABR with surgery and radiofrequency ablation.

Patients’ views of routine quality of life assessment following a diagnosis of early-stage non-small cell lung cancer

There is an increasing interest in the quality of life (QoL) evaluation following video-assisted thoracoscopic anatomical lung resection or stereotactic ablative body radiotherapy for early-stage non-small-cell lung cancer (NSCLC). A qualitative interview study was conducted to gain insight into the optimal methods of assessing and discussing QoL in clinical practice. A prospective observational longitudinal study of patients with early-stage NSCLC was conducted where repeated QoL measures were administered either online or on paper. A subset of participants was invited for qualitative interviews after the 6-month assessment or at the end of the study. The semi-structured interviews were transcribed verbatim and thematically analysed. Twenty-three patients were interviewed. Generally, patients were content with recruitment and data collection procedures. Most opted to complete the assessments on paper instead of online; this choice was influenced by the level of technology literacy. Some found the questionnaires too generic to reflect their experiences. Barriers to questionnaire completion were mostly practical, and many acknowledged benefits of QoL assessment including allowing them to express problems and health issues, and following changes over time. Generally, participants would like to discuss QoL results during clinical consultations, but reported this rarely happened. Lung cancer patient interviews confirm the acceptability of repeated QoL assessments, but online data capture is limited. Patients highlight the importance of discussing QoL aspects with their clinical team. Future strategies are needed to optimize the routine collection of patient-reported outcomes in clinical practice.

Single-fraction image-guided robotic radiosurgery efficiently controls local prostate cancer recurrence after radical prostatectomy.

PurposeTo assess the therapeutic potential of single‐fraction robotic stereotactic ablative body radiotherapy (SABR) in patients with locally recurrent prostate cancer (PC) after radical prostatectomy (RP).Materials and methodsWe included 35 patients with biochemical failure after RP with single‐site local recurrence in the prostate bed diagnosed by PSMA PET/CT. About 20/35 pts had previously received post‐surgical adjuvant radiation therapy.High‐resolution multiparametric magnetic resonance imaging (mpMRI) for exact visualization of tumor tissue was performed at 1.5 (n = 23; Siemens Magnetom Aera) or 3 Tesla (n = 12; Siemens Magnetom VIDA, Siemens Healthineers, Erlangen, Germany). Using the MRI and PET/CT dataset for planning, SABR was carried out after ultrasound‐guided placement of a single gold fiducial marker at the site of tumor recurrence using a CyberKnife M6 unit (Accuray Inc., Sunnyvale, USA). Due to the high diagnostic accuracy of PSMA PET/CT and mpMRI, pre‐SABR biopsy of tumor tissue was not deemed necessary. PSMA PET/CT performed in median 88 days before SABR confirmed the absence of distant metastases. MpMRI was performed at a median of 22 days prior to the intervention. SABR was performed in a single fraction with a dose of 20 (5/35), 21 (27/35) or 22 (3/35) Gy. Follow‐up serum PSA was measured every 3 months thereafter.ResultsMedian patient age was 72 years (57‐80 years) and median time from RP to SABR was 96.8 months (IQR, 69.3‐160.2). Median serum PSA before SABR was 1.38 ng/mL (IQR 0.75‐2.72). At 3 months, median PSA had dropped significantly in 27/35 patients to a median of 0.35 ng/mL (IQR 0.25‐0.68). At 6 months, 30/35 patients showed biochemical response to SABR, while five patients were progressing: three had systemic disease on PSMA PET/CT, while two patients had rising PSA values without a visible correlate on PET/CT. The median follow‐up time was 16 months. Grade 1 genitourinary (GU) toxicity was reported in 3/35 patients (9%) and grade 1 gastrointestinal (GI) toxicity in 2/35 patients (6%), respectively.ConclusionSABR is an efficient new treatment option in the management of single‐site local recurrent PC without the evidence of systemic disease; due to its very low toxicity, it is an alternative to surgical re‐treatment or other focal therapies. It can significantly delay the onset of androgen deprivation therapy (ADT) in biochemical failure after radical prostatectomy.

SABR for metastasis-directed therapy - what we've learned and what's to come.

SABR-COMET was the first randomized controlled trial to demonstrate an overall survival benefit with the use of stereotactic ablative body radiotherapy (SABR) for the treatment of oligometastatic cancer. Considering the recently reported long-term follow-up data from SABR-COMET, we review the outcomes and limitations of this study in the context of other emerging information on therapy for oligometastatic disease.

A novel open-source software-based high-precision workflow for target definition in cardiac radioablation.

Noninvasive ablative radiotherapy of cardiac arrhythmias (stereotactic ablative body radiation) has shown promising initial results. Precise targeting of the arrhythmogenic substrate is paramount to limit adverse effects to healthy myocardium, organs at risk, and cardiac implantable electronic devices. Using electroanatomic maps for treatment planning is technically challenging. Using the free open-source 3D Slicer software platform we established a workflow for high-precision target definition based on electroanatomic maps. An import plug-in for 3D Slicer has been designed that reads electroanatomic maps generated with three mapping systems in widespread clinical use. Using our proposed workflow in a real-world patient case we were able to align the map to the computed tomography (CT) with a mean distance of 3.1 mm. Thus, points defined on the map were translated into CT space with high accuracy and a radiotherapy treatment volume was defined in CT space based on these map-derived points. We describe a novel high-precision target definition method for stereotactic ablation of cardiac arrhythmias. Multimodal integration of the electroanatomic map with the planning CT allows for highly accurate localization of previously identified electrophysiological features in CT space. It remains to be shown whether this novel planning workflow leads to superior ablation outcomes when compared with other approaches.

Stereotactic radiotherapy for hepatocellular carcinoma: Expanding the multidisciplinary armamentarium.

Hepatocellular carcinoma (HCC) is the fifth most common malignancy worldwide and the third most common cause of cancer-related death. Long-term prognosis remains poor with treatment options frequently limited by advanced tumor stage, tumor location, or underlying liver dysfunction. Stereotactic ablative body radiotherapy (SABR) utilizes technological advances to deliver highly precise, tumoricidal doses of radiation. There is an emerging body of literature on SABR in HCC demonstrating high rates of local control in the order of 80-90% at 3years. SABR is associated with a low risk of radiation-induced liver disease or decompensation in appropriately selected HCC patients with compensated liver function and is now being incorporated into guidelines as an additional treatment option. This review outlines the emerging role of SABR in the multidisciplinary management of HCC and summarizes the current evidence for its use as an alternative ablative option for early-stage disease, as a bridge to transplant, and as palliation for advanced-stage disease. We outline specific considerations regarding patient selection, toxicities, and response assessment. Finally, we compare current international guidelines and recommendations for the use of SABR and summarize ongoing studies.

Development of Best Practices of Peer Review for Lung Radiation Therapy.

Peer review (PR) is an important component in ensuring high-quality lung radiotherapy (RT) plans. However, there are inconsistencies in the extent, timing, and minimum requirements for PR. We sought to develop guidelines of best practices for PR in curative lung RT through an expert consensus process. A modified Delphi process was conducted that consisted of an initial review by a dedicated steering committee followed by a pan-Canadian, multidisciplinary Delphi panel with 3 rounds (premeeting survey, face-to-face meeting, and postmeeting ratification survey). Candidate PR elements were ranked by importance and stratified by treatment of locally advanced (LA) disease with conventional RT or stereotactic ablative body RT (SABR) for early-stage disease. For the LA case, 6 elements (indications for RT, gross tumor volume [GTV], clinical target volume [CTV], internal target volume [ITV], dose/fractionation, and normal lung dosimetry) were considered as essential PR elements. Of these, 90%-100% of the panel endorsed them to be important to PR, and 80% believed that the PR should be done by a second radiation oncologist (RO). In the SABR case, 6 PR elements (indications for RT, GTV, CTV/ITV, organs at risk contours, dose/fractionation, and composite plan review) were deemed essential. Of these, 90%-100% of panel members believed these elements to be important to PR and unanimously agreed that PR should be done by a second RO. A suite of PR elements for lung RT has been developed and endorsed with high consensus. This suite should serve as a basis to help to harmonize PR practices across centers and to help to develop novel PR approaches going forward.

Stereotactic ablative radiotherapy for colorectal cancer liver metastasis.

Survival improvement of colorectal liver metastasis (CRLM) benefits from systemic therapy and metastasis-directed local therapy. Stereotactic ablative body radiotherapy (SABR), as a new efficient metastasis-directed local therapy with a systematic impact, plays a vital role in CRLM multidisciplinary treatment. SABR leads to a dramatic immunological change in the tumor microenvironment (TME) via differential activation of cytoprotective and cytotoxic pathways in malignant and non-malignant cells, in addition to direct tumor cell death. The synergy of SABR and immunotherapy might increase the abscopal response rate of out-field lesions by targeting different steps of the immune-mediated response, in addition to direct intratumoral cell death. The clinical treatment and efficacy of SABR, its influence on TME, and potential molecular underpinnings of which are the topic of this review.