The relationship between spine SBRT outcomes and the extent of epidural cauda equina compression (ECEC) by malignant epidural disease has yet to be reported. Our objective was to determine clinical, anatomic and dosimetric factors that predict for local failure (LF) and overall survival (OS) specific to ECEC. Consecutive patients with ECEC treated with spine SBRT were retrospectively reviewed. ECEC parameters including anatomic measurements, lumbar stenosis grading, anatomic disease position and various dosimetric factors, were analyzed for their prognostic utility. Covariates with a p-value ≤ 0.20 on univariate analysis were selected for multivariable analysis (MVA), and those statistically significant (p < 0.05) were included in the final model. Ninety-four spinal segments (79 patients) with ECEC were identified, 69 (73

INTRODUCTION:There is little research regarding patient engagement (PE) in Continuing Professional Development (CPD) programs in radiation oncology. This study aims to understand the barriers and enablers to PE in the design and implementation process of CPD programs, and advance PE in these programs moving forward.METHODS:This qualitative study involved 17 semi-structured interviews, with 5 cancer patients and 12 educators, conducted from June 2019 to April 2020. Interview data identified common themes, such as: the current state of PE in CPD programming, and key barriers and recommendations on how to engage patients in meaningful and practical ways.RESULTS:Six themes were identified related to PE: the concept of PE, ethical considerations, barriers, key considerations in planning resources, and the anticipated impact of PE on curriculum planning.CONCLUSION:Both patients and educators emphasized that creating and sustaining meaningful educator-patient relationships and giving patients an active and effective role in CPD planning would improve curriculum content. The University of Toronto Department of Radiation Oncology (UTDRO) should consider building this initiative into its strategic CPD priorities and ensure the appropriate infrastructure is in place.

Purpose/Objective(s) The most common complication following spine stereotactic body radiotherapy (SBRT) is vertebral compression fracture (VCF). The purpose of this analyses was to determine if predictors of VCF vary according to presence or absence of a pre-existing (baseline) VCF. Materials/Methods A retrospective review of a prospectively maintained institutional database of patients treated with SBRT for spinal metastases performed. The primary outcome was VCF. Clinical, dosimetric and radiographic factors were reported with descriptive statistics. The cumulative incidence of VCF was estimated using a competing risk analysis method. The impact of covariates was estimated with Cox proportional hazards model and hazard ratios (HR) generated. Results From 2008 – 2022, 744 patients with 1813 spinal segments were treated with spine SBRT. The median age was 64.9 years, and 19.1% had a baseline VCF. The majority of segments were treated with 24Gy (42.8%) or 28Gy (27.4%) in 2 fractions, the median dose-per fraction was 12Gy (range = 5-24), 68% were de novo (no prior radiation exposure) and 235 (13.0%) had prior surgical stabilization. 254 VCF event were observed (14%), 179 were iatrogenic and 75 associated with concurrent tumor progression. The 1-, 2- and 5-year VCF rates were 8.3%, 12%, and 15.2%, respectively. 85/254 (33.5%) occurred in those with a baseline VCF and 169/254 (66.5%) in those without. On multivariable analysis (MVA), tumor progression (HR = 2.24, 95% CI = 1.66-3.04, P<0.01), baseline VCF (HR = 1.91, 95% CI = 1.35-2.71, P<0.01), mechanical pain (HR = 2.01, 1.31-3.08, P<0.01), increasing age (HR = 1.015, 95% CI = 1.002-1.03, P<0.02), fewer consecutive segments treated (HR = 0.80, 95% CI = 0.65-0.98, P<0.03), increasing dose to 90% (D90) of the clinical target volume (CTV) as equivalent dose in 2-Gy fractions (EQD2) (CTV D90 EQD2, HR = 1.007, 95% CI = 1.002-1.01, P<0.02) predicted for VCF. Predictors common to segments treated with or without a baseline VCF included concurrent tumor progression and an increasing CTVD90 EQD2. No prior stabilization surgery, involvement of the posterior elements, fewer consecutive segments treated were unique to the baseline VCF cohort, while histology, mechanical pain and higher dose per fraction were unique predictors in the no prior VCF cohort. Conclusion We report predictive factors for VCF following spine SBRT and observe distinct risk factors according to presence or absence of a baseline VCF that can guide treatment decisions.

Purpose/Objective(s) Diffusion-weighted imaging (DWI) enables in vivo diffusivity measurement through the ADC. Aggressive hypercellular tumors exhibit restricted water diffusion and low-ADC values. We hypothesized that areas of glioblastoma that have a high likelihood of recurrence would exhibit smaller ADC change during concurrent CRT than those areas responsive to treatment. Our study aims to compare ADC changes within the areas of the gross tumor volume (GTV) that developed recurrence versus those that remained recurrence-free. Materials/Methods We reviewed a prospectively collected cohort of patients with glioblastoma imaged between Dec 2017 and Apr 2021 with DWI at planning (Fx0), fraction 10 (Fx10), fraction 20 (Fx20), and 1 month after a standard 6-week course of concurrent CRT (P1M). The GTV was contoured at all time points and included the surgical cavity and any residual enhancing tumor. The contrast-enhancing recurrence was contoured at the first magnetic resonance imaging (MR) timepoint showing progression per RANO 2.0 criteria. The intersection of the GTV and the recurrence volume was labelled resistant-GTV (R-GTV), while the GTV that did not intersect with the recurrence was labelled sensitive-GTV (S-GTV). Patients who did not experience failure within the GTV and had a minimum follow-up of 36 months were included in the analysis, and the entire GTV was labelled S-GTV. The interior of the surgical cavity was contoured at each time point and excluded from measurement of ADC values to avoid bias. Absolute ADC values and ADC changes (%) at each time point relative to Fx0 were compared between R-GTV and S-GTV using one-sided Wilcoxon rank-sum tests. Temporal changes were assessed using a linear mixed-effects model (fixed effects: region, time; random effects: subject). Results A total of 51 patients were included (median age 56y, range 20-69). The median absolute ADC values for R-GTV and S-GTV were 0.91 (IQR = 0.84-1.08) vs 0.94 (IQR = 0.87-1.10) at Fx0 (P = 0.358), 1.02 (IQR = 0.88-1.18) vs 1.17 (IQR = 0.96-1.35) at Fx10 (P = 0.011), 1.09 (IQR = 0.95-1.22) vs 1.16 (IQR = 1.00-1.44) at Fx20 (P = 0.058), and 1.19 (IQR = 1.07-1.33) vs 1.37 (IQR = 1.17-1.52) at P1M (P = 0.008), respectively. The median relative ADC change for R-GTV and S-GTV was 0.9% (IQR = -2.3-20.4) vs 15.3% (IQR = 7.5-28.5) at Fx10 (P = 0.006), 11.2% (IQR = 2.7-27.4) vs 23.1% (IQR = 11.8-27.8) at Fx20 (P = 0.048), and 22.5% (IQR = 10.1-39.0) vs 33.3% (IQR = 21.1-50.2) at P1M (P = 0.034), respectively. The linear mixed-effects model revealed a significant difference in relative ADC changes between R-GTV and S-GTV (P<0.001). Conclusion Early ADC change during CRT is an imaging biomarker for treatment response and recurrence prediction in glioblastoma. Regions exhibiting smaller ADC changes during CRT indicate potential sites of recurrence, suggesting utility for MR-guided biologically adapted radiation clinical trials.

Purpose/Objective(s) To assess the feasibility and safety of weekly on-line MR-Linac (MRL) adaptive radiotherapy with concurrent temozolomide (chemoRT), with a reduced 5 mm clinical target volume (CTV) margin, for patients with HGG (grade 4 astrocytoma or IDH-wildtype glioblastoma). Materials/Methods In this single arm Phase 2 trial (NCT04726397), patients with newly diagnosed HGG planned for chemoRT with either 60 Gy/30 fractions (long course) or 40 Gy/15 fractions (short course) were eligible. Gross tumor volume (GTV) was defined as the surgical cavity and residual tumor; the CTV was a 5 mm uniform expansion plus involved T2-hyperintense FLAIR signal at the discretion of the oncologist; PTV was 3 mm. All patients were treated using a 1.5T integrated MRL. At fraction 1, and each subsequent fifth fraction, an on-line contrast-enhanced MR was acquired, volumes were re-contoured and treatment was re-planned. For the remaining fractions an on-line workflow used non-enhanced MR images to account for positional shifts. The primary endpoint was the risk of a marginal failure (MF) defined as 20-80% of the recurrent GTV (at the time of failure) within the 95% treatment isodose line and/or within a standard 15 mm CTV envelope. Using a historical 11.1% MF event risk from previously reported series using non-adaptive chemoRT with standard (15-20 mm) CTV margins, non-inferiority would be demonstrated if 13 or fewer MF events were observed within a sample size of 98, assuming a 11.9% non-inferiority margin (95% upper boundary of historical outcomes). Secondary endpoints included progression-free survival (PFS) and overall survival (OS) according to treatment schedule (long versus short course). Results A total of 108 patients were consented between April 2021 and May 2023 of which 98 completed the treatment protocol (59 long course and 39 short course). All tumors were IDH-wt, and 52/98 (53%) were MGMT methylated (MGMT-m), 41/98 (42%) unmethylated (MGMT-um) and 5/98 (5%) indeterminate. Median follow up was 23.3 months (mo). MF events were observed in 4/98 (4.1%, 95% CI: 1.6-10%) patients, establishing non-inferiority (p<0.001); the most common pattern of failure was central (52%). Median PFS was 11.6 mo for a long course (14.1 mo for MGMT-m and 8.5 mo for MGMT-um), and 6.8 mo for those treated with a short course (9.4 mo for MGMT-m and 5.1 mo for MGMT-um). Median OS was 18.5 mo after a long course (31.9 mo for MGMT-m and 13.0 mo for MGMT-um) and 10.6 mo after short course (15.3 mo for MGMT-m and 8.9 mo for MGMT-um). Conclusion We present the first trial evaluating on-line MRL weekly adaptive chemoRT for HGG with a limited CTV. Safety and feasibility was demonstrated with a low risk of MF (4%) without compromising PFS or OS. Further trials are required to test whether the reduction in irradiated normal brain tissue using this approach results in neurocognitive and quality of life benefits.

Purpose/Objective(s) The most common complication following spine stereotactic body radiotherapy (SBRT) is vertebral compression fracture (VCF). Iatrogenic VCF has been widely reported, but it has not yet been reported in the context of tumor progression. We hypothesized that predictors of VCF vary with cause, whether iatrogenic or in conjunction with local failure. Materials/Methods A prospectively maintained institutional database of patients treated with SBRT for spinal metastases was retrospectively reviewed. The primary outcome was VCF, with or without local failure. Clinical, dosimetric and radiographic factors were reported with descriptive statistics. The cumulative incidence of iatrogenic VCF and VCF with tumor progression was estimated using a competing risk analysis method. The impact of covariates was estimated with Cox proportional hazards model and hazard ratios (HR) generated. Results From 2008 – 2022, 744 patients with 1813 spinal segments were treated with spine SBRT. The median age was 64.9 years and median follow up was 19.9 months. Of the 254 VCF events (14%), 179 (70.5%) were iatrogenic and 75 (29.5%) associated with concurrent tumor progression. Median time to iatrogenic VCF was 10.1 months and 9.0 months in those with tumor progression. The majority of segments that had an iatrogenic VCF or with tumor progression were treated with 24Gy in 2 fractions (41.8% and 41.3%, respectively) and the median dose per fraction was 12Gy in both (range = 5-24). The 1- and 2-year iatrogenic VCF rates were 6.8% and 9.9%, respectively, and 15.6% and 22.0% in those concurrent with tumor progression, respectively. On multivariable analysis (MVA), spinal level and increasing dose to 90% (D90) of the clinical target volume (CTV) as equivalent dose in 2-Gy fractions (EQD2) were the only predictors common to both iatrogenic VCF and those secondary to tumor progression. Risk factors that uniquely increased the risk of iatrogenic VCF were presence of a baseline VCF (HR = 1.85, 95% CI = 1.25-2.74, P = 0.002), increasing consecutive segments treated (HR = 0.76, 95% CI = 0.64-0.90, P<0.01), and older age (HR = 1.02, 95% CI = 1.002-1.03, P = 0.03). Meanwhile, epidural disease (HR = 2.84, 95% CI = 1.68-4.79, P<0.001), mechanical pain (HR = 3.33, 95% CI = 1.53-7.25, P<0.01) and fewer SBRT fractions (HR = 0.65, 95% CI = 0.45-0.93, P<0.02) were predictive of VCF in those concurrent with tumor recurrence. Conclusion This first report of predictive factors for VCF following spine SBRT, according to an iatrogenic cause or concurrent with tumor progression, may improve clinical decision making in VCF management.

Patients with HER2-positive (HER2+) and triple-negative (TNBC) metastatic breast cancer (BC) have a high propensity to develop brain metastases (BrM), but BrM incidence among patients who present with early-stage BC has not been well studied. In this population-based cohort study, we used linked administrative databases in Ontario, Canada to identify adult patients diagnosed with stage I-III BC between 2009-2021. Cumulative incidence of surgery and/or radiotherapy for BrM (surrogate for BrM) accounting for competing risk of death, time from primary BC diagnosis to local BrM therapy, and survival time from BrM diagnosis date as stratified by stage and subtype were analyzed using SAS. Of 91, 517 BC patients, 1, 858 (2%) received local treatment for BrM. Median [IQR] age at BrM diagnosis was 52.5 [45-62] years and initial stage at BC presentation was as follows: stage I (n=272; 15%), stage II (n=767, 41%), and stage III (n=819, 44%). With a median follow-up of 6.4 years [IQR:3.7-9.6], the incidence proportion of BrM was: 0.6% among patients with stage I; 2.4% with stage II; and 6.9% with stage III BC. Subtypes included 406 (22%) hormone-receptor-positive (HR+)/HER2-negative (HER2-), 161 (9%) HER2+/HR+, 126 (7%) HER2+/HR-, 288 (15%) TNBC; unknown subtype, n=877 (47%). Among stage III patients, the incidence proportion of BrM was 16% (TNBC), 13% (HER2+/HR-), 9% (HER2+/HR+), and 4% (HR+/HER2-); corresponding median time from initial BC diagnosis to local BrM therapy was 1.6, 2.0, 2.9, and 3.7 years, respectively. Among patients with stage I-III BC treated locally for BrM, median (m) OS from date of BrM treatment was 5.8 (2-16) months. Survival by BC subtype was as follows, irrespective of initial stage at diagnosis: HER2+ BC, mOS=13.1 (4.6-28) months; HR+/HER2- BC, mOS=5.1 (1.7-15) months; TNBC, mOS=3.6 (1.5-8.7) months. A high incidence of BrM among patients with stage III BC was identified particularly among those with TNBC or HER2+ BC. The true incidence is likely higher as screening for asymptomatic BrM is not typically performed. In addition, mOS may be over-estimated because untreated patients were not included in this analysis.

Purpose/objectives:The growing use of stereotactic body radiotherapy (SBRT) in metastatic cancer has led to its use in varying anatomic locations. The objective of this study was to review our institutional SBRT experience for axillary metastases (AM), focusing on outcomes and process. Materials/methods:Patients treated with SBRT to AM from 2014 to 2022 were reviewed. Cumulative incidence functions were used to estimate the incidence of local failure (LF), with death as competing risk. Kaplan-Meier method was used to estimate progression-free (PFS) and overall survival (OS). Univariate regression analysis examined predictors of LF. Results:We analyzed 37 patients with 39 AM who received SBRT. Patients were predominantly female (60 %) and elderly (median age: 72). Median follow-up was 14.6 months. Common primary cancers included breast (43 %), skin (19 %), and lung (14 %). Treatment indication included oligoprogression (46 %), oligometastases (35 %) and symptomatic progression (19 %). A minority had prior overlapping radiation (18 %) or surgery (11 %). Most had prior systemic therapy (70 %).Significant heterogeneity in planning technique was identified; a minority of patient received 4-D CT scans (46 %), MR-simulation (21 %), or contrast (10 %). Median dose was 40 Gy (interquartile range (IQR): 35-40) in 5 fractions, (BED10 = 72 Gy). Seventeen cases (44 %) utilized a low-dose elective volume to cover remaining axilla.At first assessment, 87 % had partial or complete response, with a single progression. Of symptomatic patients (n = 14), 57 % had complete resolution and 21 % had improvement. One and 2-year LF rate were 16 % and 20 %, respectively. Univariable analysis showed increasing BED reduced risk of LF. Median OS was 21.0 months (95 % [Confidence Interval (CI)] 17.3-not reached) and median PFS was 7.0 months (95 % [CI] 4.3-11.3). Two grade 3 events were identified, and no grade 4/5. Conclusion:Using SBRT for AM demonstrated low rates of toxicity and LF, and respectable symptom improvement. Variation in treatment delivery has prompted development of an institutional protocol to standardize technique and increase efficiency. Limited followup may limit detection of local failure and late toxicity.

Preliminary results indicate that a once weekly adaptive approach for small margin MR-guided RT improves tumor coverage for progressive tumors compared to a static (baseline) plan without adaptation.

PD involving adjacent rib and muscle may be associated with worse LF following SBRT. Further expansion of the cohort and dosimetric analyses are ongoing.

Purpose/Objective(s) To analyze the predictors of survival and radiation necrosis in adult patients (pts) with recurrent high-grade gliomas (rHGG) who have undergone re-irradiation (ReRT). Materials/Methods All adult pts with rHGG who had ReRT from 2009 to 2020 at one institution were retrospectively reviewed. Demographic, clinical, dosimetric, and radiological data were obtained from the electronic medical records. The primary outcome was to identify predictors of overall survival (OS) and radiation necrosis (RN). The secondary outcome was to identify patterns of failure after ReRT, which was defined as in-field, marginal or distant if >95% of the recurrence volume was in the 80% isodose line (IDL), between 80%-20% IDL, or outside 20% IDL, respectively. OS, progression-free survival (PFS), and RN were estimated by the Kaplan-Meier method. Toxicity was recorded according to CTCAE V5.0. Results Were included 79 pts with a median age of 52 yrs (range, 19-79), 62% were male, 85% had grade 4 glioma at presentation and 98% at ReRT. IDH was wildtype/mutated/unknown in 73%/11%/15% of pts. 92% had concurrent/adjuvant temozolamide at the primary treatment. 34% had re-resection prior to ReRT. 16% had concurrent bevacizumab at ReRT. The most common fractionation schedules at the primary treatment were 40 Gy/15Fx (9%) and 50-60 Gy/28-33Fx (91%), and at ReRT were 17-24 Gy/1Fx (9%), 20-35 Gy/5Fx (38%), 25-35 Gy/10Fx (48%) and 36-54 Gy/18-30Fx (5%). The median cumulative equivalent dose in 2 Gy fractions (EQD2, a/b=2) was 103 Gy (range, 81-216). The median OS and PFS were 9.9 (95% CI 8.3-11.6) and 4.1 mos (95% CI 3.6-5.4), respectively. The OS and PFS rate at 6/12 months were 69.6%/34.2% and 29.1%/7.5%, respectively. The prognostic factors for OS on multivariate analyses (MVA) were interval from initial treatment to first progression ≥16.3mos (HR=0.35, 95% CI 0.20-0.59, p<0.001), re-resection prior ReRT (HR=0.43, 95% CI 0.25-0.73, p=0.002), ECOG ≥1 at ReRT (HR=1.90, 95% CI 1.10-3.30, p=0.022) and PTV volume at ReRT ≥112cc (HR=2.63, 95% CI 1.55-4.44, p≤0.001). Toxicities G2 and G3 were 22% (8.8% RN) and 5% (2.5% RN), respectively. Concurrent use of bevacizumab (p<0.001) and EQD2 ≤98 Gy (p<0.001) were predictors for lower incidence of RN on MVA. Exploratory analysis suggested three risk groups for RNs based on cumulative EQD2: ≤100 Gy (RN = 4%), 100-111 Gy (RN = 13%), and ≥111 Gy (RN = 20%). The failures after ReRT were in-field, marginal or distant in 67%, 6%, and 27% of pts who had follow-up MRI, respectively. Conclusion Re-irradiation is a safe and effective treatment for GBM. We describe predictive factors for OS and RN to guide patient section. Focus on pts with the most favorable OS may aid in identifying pts most likely to benefit from ReRT.

Purpose/Objective(s)The management of locally recurrent brain metastases is a clinical challenge. A second course of radiosurgery may be delivered as the primary salvage treatment or as postoperative radiosurgery to the surgical cavity if the recurrence resected. We report our institutional experience of repeat radiosurgery for locally recurrent brain metastases, with a focus on local control and radionecrosis (RN) outcomes.Materials/MethodsWe report a single institution retrospective cohort study specific to patients with recurrent brain metastases treated with repeat radiosurgery. Clinical and dosimetric details were collected. Overall survival was estimated using the Kaplan Meier method. Local recurrence and RN rates were estimated using the Aalen-Johnson method, with death from any cause as the competing risk for both endpoints. Univariable competing risk regression using Fine and Gray's methods were performed, and subsequent multivariable (MVA) regression based on a priori variable selection generated final adjusted models.ResultsWe identified 96 patients and 130 brain metastases re-treated with radiosurgery (either single fraction or hypofractionation between July 2010 to April 2020. The most common primary sites were lung (N = 44, 33.8%), breast, (N = 38, 29.2%), and melanoma (N = 26, 20%). More than half of the retreated lesions were postoperative cavities (N = 68, 52.3%). Re-treatment was most commonly delivered with 25 Gy in 5 fractions (N = 77, 59.2%) followed by 27.5 Gy in 5 fractions (N = 23, 17.7%). The mean BED10 was 39.71 Gy (standard deviation [SD] 6.16) with a mean treatment volume of 14.94 cm3 (SD, 20.15). Nearly half (N = 64, 49.2%) of lesions were treated in the absence of systemic therapy, while 43.8% (N = 57) and 6.9% (N = 9) were treated while receiving targeted therapy/immunotherapy and chemotherapy, respectively. The median time between treatment courses was 13.7 months (IQR 10.88). With a median follow up of 34.9 months (interquartile range [IQR] 25-41.9), the 1 and 2-year overall survival rates were 59.8% (95% confidence interval [95% CI] 50.4-70.9) and 36.2% (95% CI, 27.2-48.2), respectively. The risk of local failure at 1 and 2 years was 26.2% (95%CI, 18.5-34) and 28% (95%CI, 20.1-35.9), respectively. MVA identified increasing target volume (hazard ratio [HR] 1.03; 95% CI, 1.02-1.04), and a shorter time-interval between radiosurgery courses (HR 0.29; 95% CI, 0.19-0.45) to be associated with worse local control. The crude incidence of RN was 18.5% (N = 24), of which 8 events (6.15%) were symptomatic. The 1 and 2-year rates of RN were 17.6% (95% CI, 10.9-34) and 19.3% (95%CI, 12.3-35.9), respectively.ConclusionRepeat radiosurgery for locally recurrent brain metastases results in good local control with a moderate risk of RN. Larger target volume and a shorter timeframe between radiosurgery treatments are associated with worse control.

Purpose/Objective(s) At our institution, 30 Gy in 4 spine stereotactic body radiotherapy (SBRT) fractions is typically delivered for larger volume when multiple segments are included in the treatment volume and/or for the retreatment of spinal metastases. We report MRI-based local failure (LF) and vertebral compression fracture (VCF) rates for patients (pts) treated with 30 Gy/4Fx of spine SBRT. Materials/Methods A retrospective analysis of all pts with spine metastases treated with 30 Gy/4Fx from 2010 to 2021 from an institutional registry was performed. Demographic, clinical, dosimetric, and radiological data were summarized, and the primary endpoint was the MRI-based LF rate. Secondary endpoints included the incidence of VCF and overall survival (OS). Kaplan Meier was used to calculate LF and VCF per segment and OS per patient. Results Were included 116 pts with 245 treated segments in this analysis. The median number of consecutive segments in the treatment volume was 3 (1-7), and the median clinical target volume (CTV) was 126 cc (range, 10-863). Kidney (25%), lung (20%), breast (19%), prostate (19%), and colon (10%) cancer were the most common primary histologic types. 38% of pts had oligometastatic disease and 24% spine metastases only. 15% (17/116) of the patients were treated with postoperative SBRT. 31% of segments were re-irradiated for conventional palliative radiation, and 26% prior SBRT, failures. 25% of segments had a baseline VCF, 46% epidural disease, and 53% paraspinal tumor extension. The median follow-up per patient was 18.5 (range, 0.1-61) and per segment 10.7 mos (range, 0.1-59). The LF rates at 12 and 24 mos were 10.7% (95% CI 7.1-15.2) and 16% (95% CI 11.5-21.2), and for VCF were 7.3% (95% CI 4.4-11.2) and 11.2% (95% CI 7.5-15.8), respectively. 60% of failures had an epidural, and 23% a paraspinal, component. Age <68y (HR=0.43, 95% CI 0.19-0.95, p=0.038), volume of CTV <72cc (HR=0.09, 95% CI 0.01-0.70, p=0.021), and prior surgical stabilization (HR=0.25, 95% CI 0.07-0.81, p=0.021) were protective predictors for VCF on multivariable analyses (MVA). In particular, the VCF rate at 24 mos was 1.8% for pts with CTV volume <72 vs 14.6% for ≥72cc. Median OS was 20.3 mos (95% CI 14.8-27.1). Low grade or no epidural disease (HR=0.42, 95% CI 0.21-0.84, p=0.014), paraspinal disease (HR=3.07, 95% CI 1.68-5.62, p<0.001) and kidney primary cancer (HR=2.43, 95% CI 1.08-5.44, p=0.031) were predictors for OS on MVA, with a trend for oligometastatic disease (HR=0.56, 95% CI 0.31-1.02, p=0.059). No radiation-induced myelopathy was observed. Conclusion This first report for 30 Gy/4Fx as a novel fractionation for spine SBRT suggests high rates of local control and a low rate of VCF, particularly for those target volumes with a CTV<72 cc. However, our rate of VCF in very large treatment volumes (≥72cc) is comparable to the SBRT-induced VCF literature, where optimal management remains an active area of investigation.

C. Tseng: Advisory Board; Sanofi. H. Chen: Employee; North York General Hospital. J. Stewart: None. A. Lau: None. R. Chan: None. L.S. Lawrence: Student affiliated with University of Toronto; Sunnybrook Research Institute. M. Campbell: None. S.D. Myrehaug: Advisory Board; Novartis AG.H. Soliman: None. Z.A. Husain: Independent Contractor; RadOncQuestions LLC. Research Grant; Merck. Travel Expenses; Elekta; NIH Head and neck PULA task force. J. Detsky: None. P. Maralani: None. B.M. Keller: None. M.E. Ruschin: Patent/License Fees/Copyright; Elekta AB.A. Sahgal: Research Grant; Elekta.

Clinically meaningful changes in target dynamics were observed during the course of cavity irradiation, supporting the use of MR image guided radiation therapy and treatment adaptation. We also observed variability in CTV delineation between T1c and T2f sequences, highlighting the importance of further work in this field.

Purpose Gamma Knife Icon (GKI) enables a user-defined gating threshold for intrafraction motion during stereotactic radiosurgery (SRS). An optimal threshold would ensure dosimetric fidelity of the planned distribution and minimize treatment time extension by gating. A prediction of motion characteristics for a patient based on a retrospective database of motion traces could be beneficial to evaluating the choice of gating threshold. A short acquisition of motion may help to define a personalized threshold that balances dosimetric accuracy and treatment length. This study aims to evaluate the performance of a prediction of motion and the resultant dosimetric consequences for a range of motion gating thresholds. Methods A database of 2552 motion traces (776 patients) was analyzed using previously published methods to characterize patient intrafraction motion on the GKI. For a selection of six fractionated SRS patient cases (two patients with single brain metastasis, four vestibular schwannomas), a 10-min sample of motion was used to classify motion and identify traces in the database with similar metrics. The similar motion traces were used to perform a predictive reconstruction of the selected patient's delivered dose for a range of motion thresholds. The remaining fractions were reconstructed and compared to that predicted. From the six cases, 26 fractions were used to predict the number of interruptions (n = 26), change in target coverage (n = 26), and change in brainstem maximum dose (vestibular cases only, n = 20). The difference between mean predicted and reconstructed values was compared for accuracy. Results The difference between mean prediction and reconstructed values was 0.32 +/- 0.38% in target coverage, 2.36 +/- 5.06 interruptions, and 0.15 +/- 0.24 Gy for the brainstem maximum dose. Sixty-seven of the 72 predictions (26 coverage, 26 interruptions, and 20 brainstem maximum dose) were within one standard deviation of the predicted mean. Conclusions Large databases of motion traces were used to characterize patient performance and predict motion performance. Dosimetric deterioration due to motion and extension of treatment duration can be predicted in some cases using only a short acquisition of motion and the treatment plan. This reconstruction may provide benefit in generating a patient-specific motion threshold.

We have quantified residual setup and motion uncertainties for a large series of treated MR-Linac brain tumor patients, generated a model for a population-based PTV, and recommend a minimum PTV of 2-3 mm for this indication. This work is a crucial step towards developing an adaptive brain treatment based on dynamic tumor changes and tumor response.

Improved overall survival rates among brain metastases (BM) patients have led to higher rates of salvage re-irradiation in patients with local failure. Hypofractionated stereotactic regimens are an attractive option due to the possibility of lower rates of radionecrosis (RN). This study aims to determine the incidence and risk factors of RN following re-irradiation with hypofractionated stereotactic radiotherapy (hSRT) and compares the results to the incidence of RN following re-irradiation with single fraction SRS reported in literature. Retrospective chart review was conducted on patients from a single institution who received re-irradiation using hSRT to BMs that previously received stereotactic radiotherapy. Patients who received additional whole brain radiation (WBRT), prior to, or after stereotactic radiotherapy were included. Patient, tumor, treatment characteristics, and follow-up data were collected. MRI and pathology of any lesions resected post-radiotherapy were reviewed to assess for the development of RN and the response to treatment. RN was confirmed through MRI or by surgical resection. Associations between RN and target volume, as well as number of months between stereotactic treatments were examined using logistic regression. Association between RN and use of additional WBRT was examined using relative risk and chi-square tests. A total of 110 metastatic brain lesions in 90 patients received hSRT reirradiation between August 2010 and December 2019. 26.7% (n = 24) of patients also received additional WBRT. The most common histologies were breast (n = 31), NSCLC (n = 30), and melanoma (n = 29). The median volume of lesions was 12.25cc (IQR 4.97 – 25.07cc). The most common fractionation scheme was 25 Gy in 5 fractions (72 lesions). The incidence of local failure was 11.6% and 17.4% at 6 and 12 months, respectively. Incidence of symptomatic RN was 6.15% and 15% at 6 and 12 months, respectively. There was no statistically significant association between symptomatic RN at 12-months and radiation dose (p = 0.40), target volume (p = 0.96), use of additional WBRT (p = 0.74), or number of months between SRT (p = 0.99). The incidence of symptomatic RN following re-irradiation with hSRT is lower compared to re-irradiation with single fraction SRS as reported in literature. hSRT provides an alternative salvage option for treating local failure of BMs to reduce risk of RN while achieving local control.