Standardization of terminology in stereotactic radiosurgery: Report from the Standardization Committee of the International Leksell Gamma Knife Society

Stereotactic Radiation Therapy Planning, Dose Prescription and Delivery in Veterinary Medicine: A Systematic Review on Completeness of Reporting and Proposed Reporting Items

Increasing numbers of dogs and cats with cancer are treated with stereotactic radiosurgery, stereotactic radiation therapy or stereotactic body radiotherapy (SRS, SRT or SBRT). We provide a systematic review of the current data landscape with a focus on technical and dosimetric data of stereotactic radiotherapy in veterinary oncology. Original peer-reviewed articles on dogs and cats with cancer treated with SRT were included. The systematic search included Medline via PubMed and EMBASE. The study was performed according to the Preferred Reporting Items for Systematic Reviews (PRISMA) statement. We assessed the manuscripts regarding outcome reporting, treatment planning, dose prescription, -delivery and -reporting as well as quality assurance. As of February 2024, there are 80 peer-reviewed publications on various disease entities on SRS, SRT and SBRT in veterinary medicine. Overall, we found often insufficient or highly variable technical data, with incomplete information to reproduce these treatments. While in some instances, technical factors may not impact clinical outcome, the variability found in protocols, outcome and toxicity assessments precludes accurate and reliable conclusions for a benefit of stereotactic radiotherapy for many of the treated diseases. In line with the extensive recommendations from human stereotactic radiotherapy practise, we propose a draft of reporting items for future stereotactic radiation treatments in veterinary medicine. SRS, SRT and SBRT have specific clinical and technological requirements that differ from those of standard radiation therapy. Therefore, a deep understanding of the methodologies, as well as the quality and precision of dose delivery, is essential for effective clinical knowledge transfer.

Current and Future Applications of Arterial Spin Labeling MRI in Cerebral Arteriovenous Malformations

Arterial spin labeling (ASL) has emerged as a promising noninvasive tool for the evaluation of both pediatric and adult arteriovenous malformations (AVMs). This paper reviews the advantages and challenges associated with the use of ASL in AVM assessment. An assessment of the diagnostic workup of AVMs and their variants in both adult and pediatric populations is proposed. Evaluation after treatments, whether endovascular or microsurgical, was similarly examined. ASL, with its endogenous tracer and favorable safety profile, offers functional assessment and arterial feeder identification. ASL has demonstrated strong performance in identifying feeder arteries and detecting arteriovenous shunting, although some studies report inferior performance compared with digital subtraction angiography (DSA) in delineating venous drainage. Challenges include uncertainties in sensitivity for specific AVM features. Detecting AVMs in challenging locations, such as the apical cranial convexity, is further complicated, demanding careful consideration due to the risk of underestimating total blood flow. Navigating these challenges, ASL provides a noninvasive avenue with undeniable merits, but a balanced approach considering its limitations is crucial. Larger-scale prospective studies are needed to comprehensively evaluate the diagnostic performance of ASL in AVM assessment.

Plan quality assessment of modern radiosurgery technologies in the treatment of multiple brain metastases

Stereotactic radiosurgery (SRS) of multiple brain metastases has evolved over the last 40 years allowing centres to treat an increasing number of brain metastases in a single treatment fraction. HyperArcTMplanning optimisation technique is one such development that streamlines the treatment of multiple metastases with a single isocentre. Several studies have investigated the plan quality of HyperArc compared to CyberKnife or Gamma Knife, however there are limited number of studies that include all three modalities. It is the aim of this study to provide an assessment of plan quality between the three SRS platforms across ten patients with multiple brain metastases ranging from three to eight metastases per patient. Strict planning workflows were established to avoid bias towards any particular treatment platform. Plan quality was assessed through dose to organs at risk, Paddick conformity index (PCI), gradient index (GI), global efficiency index (Gη) and dose to normal brain tissue. Results from this study found mean PCI observed across Gamma Knife plans was significantly lower than HyperArc and CyberKnife. HyperArc plans observed significantly shorter beam-on times which were 10 to 20 times faster than CyberKnife and Gamma Knife plans. Gamma Knife and CyberKnife were found to produce plans with significantly superior GI, global efficiency index and the volume of healthy brain receiving greater than 12 Gy (V12Gy) when compared to HyperArc plans. Lesion volume was seen to influence the relative difference in dose metrics between systems. The study revealed that all three treatment modalities produced high quality plans for the SRS treatment of multiple brain metastases, each with respective benefits and limitations.

Comparison of Gamma Knife (GK) and Linear Accelerator (LINAC) radiosurgery of brain metastasis resection cavity: a systematic review and proportional meta-analysis

PURPOSE

Stereotactic radiosurgery (SRS) to the resection cavity is essential in the treatment of brain metastasis (BM) amenable to surgical resection. The two most common platforms for SRS delivery include Gamma Knife (GK) and LINAC. Here we collated the available peer-reviewed literature and performed a meta-analysis on clinical outcomes after GK or LINAC resection cavity SRS.

METHODS

Following PRISMA Guidelines, a search on PUBMED and MEDLINE was performed to include all studies evaluating each post-operative SRS modality. Local control, overall survival, radiation necrosis, and leptomeningeal disease were evaluated from the available data. A proportional meta-analysis was performed via R using the metafor package to pool the outcomes of studies and a moderator effect to assess the significance between groups.

RESULTS

We identified 21 GK studies (n = 2009) and 28 LINAC studies (n = 2219). The radiosurgery doses employed were comparable between GK and LINAC studies. The pooled estimate of 1-year local control, 1-year overall survival, and risk of leptomeningeal disease were statistically comparable between GK and LINAC (81.7 v 85.8%; 61.4 v 62.7%; 10.6 v 12.5%, respectively). However, the risk of radiation necrosis (RN) was higher for LINAC resection cavity SRS (5.4% vs. 10%, p = 0.036). The volume of the resection cavity was a significant modifying factor for RN in both modalities (p = 0.007) with a 0.5% and 0.7% increase in RN risk with every 1 cm3 increase in tumor volume for GK and LINAC, respectively.

CONCLUSIONS

Our meta-analysis suggests that GK and LINAC SRS of resection cavity achieve comparable 1-year local control and survival. However, resection cavity treated with GK SRS was associated with lowered RN risk relative to those treated with LINAC SRS.

Noninvasive cardiac radioablation for ventricular tachycardia: dosimetric comparison between linear accelerator- and robotic CyberKnife-based radiosurgery systems

BACKGROUND

Few dosimetric comparisons have been published between linear accelerator (LA)-based systems and CyberKnife (CK)-based robotic radiosurgery systems for cardiac radio-ablation in ventricular tachycardia. This study aimed to compare the dosimetry of noninvasive cardiac radio-ablation deliverable on LA with that on CK.

METHODS

Thirteen patients who underwent noninvasive cardiac radio-ablation by LA were included. The prescribed dose was 25 Gy in 1 fraction, and the average planning target volume was 49.8 ± 31.0 cm3 (range, 14.4-93.7 cm3). CK plans were generated for comparison.

RESULTS

Both the CK and LA plans accomplished appropriate dose coverage and normal tissue sparing. Compared with the LA plans, the CK plans achieved significantly lower gradient indices (3.12 ± 0.71 vs. 3.48 ± 0.55, p = 0.031) and gradient measures (1.00 ± 0.29 cm vs. 1.17 ± 0.29 cm, p < 0.001). They had similar equivalent conformity indices (CK vs. LA: 0.84 ± 0.08 vs. 0.87 ± 0.07, p = 0.093) and maximum doses 2 cm from the planning target volume (PTV) in any direction (CK vs. LA: 50.8 ± 9.9% vs. 53.1 ± 5.3%, p = 0.423). The dosimetric advantages of CK were more prominent in patients with a PTV of ≤ 50 cm3 or a spherical PTV. In patients with a PTV of > 50 cm3 or a non-spherical PTV, the LA and CK plans were similar regarding dosimetric parameters. CK plans involved more beams (232.2 ± 110.8 beams vs. 10.0 ± 1.7 arcs) and longer treatment times (119.2 ± 43.3 min vs. 22.4 ± 1.6 min, p = 0.007).

CONCLUSIONS

Both CK and LA are ideal modalities for noninvasive cardiac radio-ablation. Upfront treatment should be considered based on clinical intent.

Cochlear-optimized treatment planning in photon and proton radiosurgery for vestibular schwannoma patients

Objective

To investigate the potential to reduce the cochlear dose with robotic photon radiosurgery or intensity-modulated proton therapy planning for vestibular schwannomas.

Materials and Methods

Clinically delivered photon radiosurgery treatment plans were compared to five cochlear-optimized plans: one photon and four proton plans (total of 120). A 1x12 Gy dose was prescribed. Photon plans were generated with Precision (Cyberknife, Accuray) with no PTV margin for set-up errors. Proton plans were generated using an in-house automated multi-criterial planning system with three or nine-beam arrangements, and applying 0 or 3 mm robustness for set-up errors during plan optimization and evaluation (and 3 % range robustness). The sample size was calculated based on a reduction of cochlear Dmean > 1.5 Gy(RBE) from the clinical plans, and resulted in 24 patients.

Results

Compared to the clinical photon plans, a reduction of cochlear Dmean > 1.5 Gy(RBE) could be achieved in 11/24 cochlear-optimized photon plans, 4/24 and 6/24 cochlear-optimized proton plans without set-up robustness for three and nine-beam arrangement, respectively, and in 0/24 proton plans with set-up robustness. The cochlea could best be spared in cases with a distance between tumor and cochlea. Using nine proton beams resulted in a reduced dose to most organs at risk.

Conclusion

Cochlear dose reduction is possible in vestibular schwannoma radiosurgery while maintaining tumor coverage, especially when the tumor is not adjacent to the cochlea. With current set-up robustness, proton therapy is capable of providing lower dose to organs at risk located distant to the tumor, but not for organs adjacent to it. Consequently, photon plans provided better cochlear sparing than proton plans.

Plan Assessment Metrics for Dose Painting in Stereotactic Radiosurgery

Purpose

As radiation therapy treatment precision increases with advancements in imaging and radiation delivery, dose painting treatment becomes increasingly feasible, where targets receive a nonuniform radiation dose. The high precision of stereotactic radiosurgery (SRS) makes it a good candidate for dose painting treatments, but no suitable metrics to assess dose painting SRS plans exist. Existing dose painting assessment metrics weigh target overdose and underdose equally but are unsuited for SRS plans, which typically avoid target underdose more. Current SRS metrics also prioritize reducing healthy tissue dose through selectivity and dose fall-off, and these metrics assume single prescriptions. We propose a set of metrics for dose painting SRS that would meet clinical needs and are calculated with nonuniform dose painting prescriptions.

Methods and Materials

Sample dose painting SRS prescriptions are first created from Gamma Knife SRS cases, apparent diffusion coefficient magnetic resonance images, and various image-to-prescription functions. Treatment plans are found through semi-infinite linear programming optimization and using clinically determined isocenters, then assessed with existing and proposed metrics. Modified versions of SRS metrics are proposed, including coverage, selectivity, conformity, efficiency, and gradient indices. Quality factor, a current dose painting metric, is applied both without changes and with modifications. A new metric, integral dose ratio, is proposed as a measure of target overdose.

Results

The merits of existing and modified metrics are demonstrated and discussed. A modified conformity index using mean or minimum prescription dose would be suitable for dose painting SRS with integral or maximum boost methods, respectively. Either modified efficiency index is a suitable replacement for the existing gradient index.

Conclusions

The proposed modified SRS metrics are appropriate measures of plan quality for dose painting SRS plans and have the advantage of giving equal values as the original SRS metrics when applied to single-prescription plans.

Patient-specific geometrical distortion corrections of MRI images improve dosimetric planning accuracy of vestibular schwannoma treated with gamma knife stereotactic radiosurgery

PURPOSE

To investigate the impact of MRI patient-specific geometrical distortion (PSD) on the quality of Gamma Knife stereotactic radiosurgery (GK-SRS) plans of the vestibular schwannoma (VS) tumors.

METHODS AND MATERIALS

Three open access datasets including the MPI-Leipzig Mind-Brain-Body (318 patients), the slow event-related fMRI designs dataset (62 patients), and the VS dataset (242 patients) were used. We used first two datasets to train a 3D convolution network to predict the distortion map of third dataset that were then used to calculate and correct the PSD. GK-SRS plans of VS dataset were used to evaluate dose distribution of PSD-corrected MRI images. GK-SRS prescription dose of VS cases was 12 Gy. Geometric and dosimetric discrepancies were assessed between the dose distributions and contours before and after the PSD corrections. Geometry indices were center of the contours, Dice coefficient (DC), Hausdorff distance (HD), and dosimetric indices wereD μ ${D_\mu }$ ,D m a x ${D_{max}}$ ,D m i n ${D_{min}}$ , andD 95 % ${D_{95{\mathrm{\% }}}}$ doses, target coverage (TC), Paddick's conformity index (PCI), Paddick's gradient index (GI), and homogeneity index (HI).

RESULTS

Geometric distortions of about 1.2 mm were observed at the air-tissue interfaces at the air canal and nasal cavity borders. Average center of the targets was significantly distorted along the frequency encoding direction after the PSD-correction. Average DC and HD metrics were 0.90 and 2.13 mm. AverageD μ ${D_\mu }$ ,D 95 % , ${D_{95{\mathrm{\% ,}}}}$ andD m i n ${D_{min}}$ in Gy significantly increased after PSD correction from 16.85 to 17.25, 12.30 to 12.77, and from 8.98 to 9.92.D m a x ${D_{max}}$ did not significantly change after the correction. Average TC and PCI significantly increased from 0.97 to 0.98, and 0.94 to 0.96. Average GI decreased significantly from 2.24 to 2.15 after PSD correction. However, HI did not significantly change after the correction.

CONCLUSION

The proposed method could predict and correct the PSD that indicates the importance of PSD correction before GK-SRS plans of the VS patients.

Toxicity and outcomes of melanoma brain metastases treated with stereotactic radiosurgery: the risk of subsequent symptomatic intralesional hemorrhage exceeds that of radiation necrosis

PURPOSE

We aimed to assess the outcomes and patterns of toxicity in patients with melanoma brain metastases (MBM) treated with stereotactic radiosurgery (SRS) with or without immunotherapy (IO).

METHODS

From a prospective registry, we reviewed MBM patients treated with single fraction Gamma Knife SRS between 2008 and 2021 at our center. We recorded all systemic therapies (chemotherapy, targeted therapy, or immunotherapy) administered before, during, or after SRS. Patients with prior brain surgery were excluded. We captured adverse events following SRS, including intralesional hemorrhage (IH), radiation necrosis (RN) and local failure (LF), as well as extracranial disease status. Distant brain failure (DBF), extracranial progression-free survival (PFS) and overall survival (OS) were determined using a cumulative Incidence function and the Kaplan-Meier method.

RESULTS

Our analysis included 165 patients with 570 SRS-treated MBM. Median OS for patients who received IO was 1.41 years versus 0.79 years in patients who did not (p = 0.04). Ipilimumab monotherapy was the most frequent IO regimen (30%). In the absence of IO, the cumulative incidence of symptomatic (grade 2 +) RN was 3% at 24 months and remained unchanged with respect to the type or timing of IO. The incidence of post-SRS g2 + IH in patients who did not receive systemic therapy was 19% at 1- and 2 years compared to 7% at 1- and 2 years among patients who did (HR: 0.33, 95% CI 0.11-0.98; p = 0.046). Overall, neither timing nor type of IO correlated to rates of DBF, OS, or LF. Among patients treated with IO, the median time to extracranial PFS was 5.4 months (95% IC 3.2 - 9.1).

CONCLUSION

The risk of g2 + IH exceeds that of g2 + RN in MBM patients undergoing SRS, with or without IO. IH should be considered a critical adverse event following MBM treatments.

3D dose prediction for Gamma Knife radiosurgery using deep learning and data modification

PURPOSE

To develop a machine learning-based, 3D dose prediction methodology for Gamma Knife (GK) radiosurgery. The methodology accounts for cases involving targets of any number, size, and shape.

METHODS

Data from 322 GK treatment plans was modified by isolating and cropping the contoured MRI and clinical dose distributions based on tumor location, then scaling the resulting tumor spaces to a standard size. An accompanying 3D tensor was created for each instance to account for tumor size. The modified dataset for 272 patients was used to train both a generative adversarial network (GAN-GK) and a 3D U-Net model (U-Net-GK). Unmodified data was used to train equivalent baseline models. All models were used to predict the dose distribution of 50 out-of-sample patients. Prediction accuracy was evaluated using gamma, with criteria of 4 %/2mm, 3 %/3mm, 3 %/1mm and 1 %/1mm. Prediction quality was assessed using coverage, selectivity, and conformity indices.

RESULTS

The predictions resulting from GAN-GK and U-Net-GK were similar to their clinical counterparts, with average gamma (4 %/2mm) passing rates of 84.9 ± 15.3 % and 83.1 ± 17.2 %, respectively. In contrast, the gamma passing rate of baseline models were significantly worse than their respective GK-specific models (p < 0.001) at all criterion levels. The quality of GK-specific predictions was also similar to that of clinical plans.

CONCLUSION

Deep learning models can use GK-specific data modification to predict 3D dose distributions for GKRS plans with a large range in size, shape, or number of targets. Standard deep learning models applied to unmodified GK data generated poorer predictions.

Cochlear sparing in LINAC-based radiosurgery for vestibular schwannoma: a dosimetric comparison of dynamic conformal arc, IMRT and VMAT treatment plans

BACKGROUND

Stereotactic radiosurgery (SRS) is the preferred treatment for vestibular schwannoma (VS) in patients with preserved hearing and tumour diameter < 3 cm. Emerging evidence suggests restricting cochlear dose could preserve hearing. This retrospective replanning study aims to compare dynamic conformal arc therapy (DCAT), intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans for superiority of cochlear dose sparing without compromising tumour coverage.

METHODS

Eligibility criteria included sporadic VS, serviceable hearing and availability of CT and MRI for planning. The original gross tumour volume and brainstem OAR volume were retained; the cochlea was newly contoured on the planning CT scan (bone window). Each case was replanned using the three above techniques, prescribing 12 Gy to the 80% isodose line. No dose constraint was applied to the cochlea.

RESULTS

Eighteen patients were replanned. Mean tumour volume was 2.25 cc. Tumour coverage and tumour mean dose (DCAT: 14.2, IMRT: 14.6, VMAT: 14.5 Gy) were comparable. Paddick and RTOG conformity indices were better for DCAT (0.66 and 1.6) and VMAT (0.69 and 1.5) compared to IMRT (0.56 and 1.9). DCAT had superior gradient index (3.0) compared to VMAT (3.4) and IMRT (3.4). VMAT delivered the lowest mean brainstem maximum dose (8.3 Gy) and decreased the mean cochlear dose (3.4 Gy) by 2.3 and 2.1 Gy, and the mean cochlear maximum dose (3.6 Gy) by 2.4 and 2.5 Gy relative to DCAT and IMRT, respectively.

CONCLUSION

LINAC-based SRS treatment using VMAT can achieve better cochlear dose sparing than DCAT or IMRT while maintaining tumour coverage.

The effect of tumor shape irregularity on Gamma Knife treatment plan quality and treatment outcome: an analysis of 234 vestibular schwannomas

The primary aim of Gamma Knife (GK) radiosurgery is to deliver high-dose radiation precisely to a target while conforming to the target shape. In this study, the effects of tumor shape irregularity (TSI) on GK dose-plan quality and treatment outcomes were analyzed in 234 vestibular schwannomas. TSI was quantified using seven different metrics including volumetric index of sphericity (VioS). GK treatment plans were created on a single GK-Perfexion/ICON platform. The plan quality was measured using selectivity index (SI), gradient index (GI), Paddick's conformity index (PCI), and efficiency index (EI). Correlation and linear regression analyses were conducted between shape irregularity features and dose plan indices. Machine learning was employed to identify the shape feature that predicted dose plan quality most effectively. The treatment outcome analysis including tumor growth control and serviceable hearing preservation at 2 years, were conducted using Cox regression analyses. All TSI features correlated significantly with the dose plan indices (P < 0.0012). With increasing tumor volume, vestibular schwannomas became more spherical (P < 0.05) and the dose plan indices varied significantly between tumor volume subgroups (P < 0.001 and P < 0.01). VioS was the most effective predictor of GK indices (P < 0.001) and we obtained 89.36% accuracy (79.17% sensitivity and 100% specificity) for predicting PCI. Our results indicated that TSI had significant effects on the plan quality however did not adversely affect treatment outcomes.

Direct dosimetric comparison of linear accelerator vs. Gamma Knife fractionated stereotactic radiotherapy (fSRT) of large brain tumors

The purpose of this study was to directly compare the plan quality of Gamma Knife (GK) (Elekta, Stockholm, Sweden)- vs linear accelerator (LINAC)-based delivery techniques for fractionated stereotactic radiotherapy (fSRT) of large brain metastases. Eighteen patients with clinical target volumes (CTVs) larger than 9.5 cc were selected to generate comparative plans for the prescription dose of 9 Gy × 3 fractions, utilizing the Eclipse (Varian, Palo Alto, US) vs Leksell GammaPlan (LGP) (Elekta, Stockholm, Sweden) treatment planning systems (TPS). Each GK plan was first developed using LGP's automatic planning, followed by manual adjustments/refinements. The same MRI and structures, including CTVs and organs at risk, were then DICOM-transferred to the Eclipse TPS. Volumetric Modulated Arc Therapy (VMAT) and Dynamic Conformal Arc (DCA) plans for a Truebeam, with high-definition multi-leaf collimators (MLCs), were developed on these MR images and structures using a single isocenter and 3 non-coplanar arcs. No planning target volume (PTV) margins were added, and no heterogeneity correction was used for either TPS. GK plans were prescribed to the 50% isodose line, and Eclipse VMAT and DCA plans allowed a maximum dose up to 170% and ∼125%, respectively. Gradient index (GI), Paddick Conformity Index (PCI), V_20GyRind, and V_4GyRind of all 3 techniques were calculated and compared. One-way analysis of variance (ANOVA) was performed to determine the statistical significance of the differences of these planning indices for the 3 planning techniques. A total of eighteen treatment targets were analyzed. Median CTV volume was 14.4 cc (range 9.5 cc - 55.9 cc). Mean ± standard deviation of PCI were 0.85 ± 0.03, 0.90 ± 0.03, and 0.72 ± 0.11 for GK, VMAT and DCA plans, respectively. They were respectively 2.64 ± 0.17, 2.46 ± 0.18, and 2.83 ± 0.48 for GI; 15.33 ± 8.45 cc, 10.47 ± 4.32 cc and 23.51 ± 16 cc for V_20GyRind; and 316.28 ± 138.35 cc, 317.81 ± 108.21 cc, and 394.85 ± 142.16 cc for V_4GyRind. The differences were statistically significant with p < 0.01 for all indices, except for V_4GyRind (p > 0.129). In conclusion, a direct dosimetric comparison using the same MRI scan and contours was performed to evaluate the plan quality of various fSRT delivery techniques for CTV > 9.5 cc. LINAC VMAT plans provided the best dosimetric outcome in regard to PCI, GI, and V_20GyRind. GK outcomes were similar to LINAC VMAT plans while LINAC DCA outcomes were significantly worse. Even though GK has a smaller physical penumbra, LINAC VMAT outperformed GK in this study due to enhanced penumbra sharpening and better beam optimization.

Zero Setup Margin Mask versus Frame Immobilization during Gamma Knife® Icon™ Stereotactic Radiosurgery for Brain Metastases

We compared the clinical outcomes of BM treated with mask immobilization with zero-SM (i.e., zero-PTV) to standard zero-SM frame immobilization SRS. Consecutive patients with BM, 0.5−2.0 cm in maximal diameter, treated with single-fraction SRS (22−24 Gy) during March 2019−February 2021 were included. Univariable and multivariable analysis were performed using the Kaplan−Meier method and Cox proportional hazards regression. A total of 150 patients with 453 BM met inclusion criteria. A total of 129 (28.5%) lesions were treated with a zero-SM mask immobilization and 324 (71.5%) with zero-SM frame immobilization. Frame immobilization treatments were associated with a higher proportion of gastrointestinal and fewer breast-cancer metastases (p = 0.024), and a higher number of treated lesions per SRS course (median 7 vs. 3; p < 0.001). With a median follow up of 15 months, there was no difference in FFLF between the mask and frame immobilization groups on univariable (p = 0.29) or multivariable analysis (p = 0.518). Actuarial FFLF at 1 year was 90.5% for mask and 92% for frame immobilization (p = 0.272). Radiation necrosis rates at 1 year were 12.5% for mask and 4.1% for frame immobilization (p = 0.502). For BM 0.5−2.0 cm in maximal diameter treated with single-fraction SRS using 22−24 Gy, mask immobilization with zero SM produces comparable clinical outcomes to frame immobilization. The initial findings support omitting a SM when using mask immobilization with this treatment approach on a Gamma Knife® Icon™.

Single-isocenter multiple-target stereotactic radiosurgery for multiple brain metastases: dosimetric evaluation of two automated treatment planning systems

Purpose

Automated treatment planning systems are available for linear accelerator (linac)-based single-isocenter multi-target (SIMT) stereotactic radiosurgery (SRS) of brain metastases. In this study, we compared plan quality between Brainlab Elements Multiple Brain Metastases (Elements MBM) software which utilizes dynamic conformal arc therapy (DCAT) and Varian HyperArc (HA) software using a volumetric modulated arc therapy (VMAT) technique.

Patients and methods

Between July 2018 and April 2021, 36 consecutive patients ≥ 18 years old with 367 metastases who received SIMT SRS at UPMC Hillman Cancer San Pietro Hospital, Rome, were retrospectively evaluated. SRS plans were created using the commercial software Elements MBM SRS (Version 1.5 and 2.0). Median cumulative gross tumor volume (GTV) and planning tumor volume (PTV) were 1.33 cm³ and 3.42 cm³, respectively. All patients were replanned using HA automated software. Extracted dosimetric parameters included mean dose (D_mean) to the healthy brain, volumes of the healthy brain receiving more than 5, 8,10, and 12 Gy (V_5Gy, V_8Gy, V_10Gy and V_12Gy), and doses to hippocampi.

Results

Both techniques resulted in high-quality treatment plans, although Element MBM DCAT plans performed significantly better than HA VMAT plans, especially in cases of more than 10 lesions). Median V_12Gy was 13.6 (range, 1.87–45.9) cm³ for DCAT plans and 18.5 (2.2–62,3) cm³ for VMAT plans (p < 0.0001), respectively. Similarly, V_10Gy, V_8Gy, V_5Gy (p < 0.0001) and median dose to the normal brain (p = 0.0001) were favorable for DCAT plans.

Conclusions

Both Elements MBM and HA systems were able to generate high-quality plans in patients with up to 25 brain metastases. DCAT plans performed better in terms of normal brain sparing, especially in patients with more than ten lesions and limited total tumor volume.

A Volume-Independent Conformity Index for Stereotactic Radiosurgery

PURPOSE

To develop a volume-independent conformity metric called the Gaussian Weighted Conformity Index (GWCI) to evaluate stereotactic radiosurgery/radiotherapy (SRS/SRT) plans for small brain tumors.

METHODS

A signed bi-directional local distance (BLD) between the prescription isodose line and the target contour is determined for each point along the tumor contour (positive distance represents under-coverage). A similarity score function is derived from Gaussian function, penalizing under- and over-coverage at each point by assigning standard deviations of the Gaussian function. Each point along the dose line contour is scored with this score function. The average of the similarity scores determines the GWCI. A total of 40 targets from 18 patients who received Gamma-Knife SRS/SRT treatments were analyzed to determine appropriate penalty criteria. The resulting GWCIs for test cases already deemed clinically acceptable are presented and compared to the same cases scored with the New Conformity Index to determine the influence of tumor volumes on the two conformity indices.

RESULTS

A total of four penalty combinations were tested based on the signed BLDs from the 40 targets. A GWCI of 0.9 is proposed as a cutoff for plan acceptability. The GWCI exhibits no target volume dependency as designed.

CONCLUSION

A limitation of current conformity indices, volume dependency, becomes apparent when applied to SRS/SRT plans. The GWCI appears to be a more robust index which penalizes over- and under-coverage of tumors and is not skewed by the tumor volume. This article is protected by copyright. All rights reserved.

Systematic Clinical Evaluation of A Deep Learning Method for Medical Image Segmentation: Radiosurgery Application

We systematically evaluate a Deep Learning model in a 3D medical image segmentation task. With our model, we address the flaws of manual segmentation: high inter-rater contouring variability and time consumption of the contouring process. The main extension over the existing evaluations is the careful and detailed analysis that could be further generalized on other medical image segmentation tasks. Firstly, we analyze the changes in the inter-rater detection agreement. We show that the model reduces the number of detection disagreements by 48% (p < 0.05). Secondly, we show that the model improves the inter-rater contouring agreement from 0.845 to 0.871 surface Dice Score (p < 0.05). Thirdly, we show that the model accelerates the delineation process between 1.6 and 2.0 times (p < 0.05). Finally, we design the setup of the clinical experiment to either exclude or estimate the evaluation biases; thus, preserving the significance of the results. Besides the clinical evaluation, we also share intuitions and practical ideas for building an efficient DL-based model for 3D medical image segmentation.

Total workflow uncertainty of frameless radiosurgery with the Gamma Knife Icon cone-beam computed tomography

Objective

Frameless treatment with the Gamma Knife Icon is still relatively new as a treatment option. As a result, additional confidence/knowledge about the uncertainty that exists within each portion of the treatment workflow could be gained especially regarding steps that have not been previously studied in the literature.

Methods

The Icon base delivery device (Perfexion) uncertainty is quantified and validated. The novel portions of the Icon such as mask immobilization, cone‐beam computed tomography image guidance, and the intrafraction motion management methods are studied specifically and to a greater extent to determine a total workflow uncertainty of frameless treatment with the Icon.

Results

The uncertainty of each treatment workflow step has been identified with the total workflow uncertainty being identified in this work as 1.3 mm with a standard deviation of 0.51 mm.

Conclusion

The total uncertainty of frameless treatment with the Icon has been evaluated and this data may indicate the need for setup margin in this setting with data that could be used by other institutions to calculate needed setup margin per their preferred recipe after validation of this data in their context.

Dosimetry

The dosimeters used to measure radiation dose produce a value which has to be calibrated to be in keeping with the values in an approved laboratory, which will be one of an international network of such laboratories at the center of which is the Bureau International des Poids et Mesures in France (BIPM). Dosimeters work by producing a quantitatively proportional change in status to the intensity of the radiation being measure. Amongst the techniques in use are thermoluminescent devices, radiographic film, radiochromic film, semiconductors, ionization chambers, silicon diodes and gel dosimeters. The Gamma Knife radiation has been difficult to measure directly because the beams have been to fine for accurate measurement by commonly available dosimeters. For more modern dosimeters this is less of a problem. During the treatment of a patient, a variety of indices are recorded to assist in the standardization and accuracy of treatment. Having determined the dose in the beams, it is necessary to calculate how much energy is lost during the passage of radiation from the source to the target. There has been a steady evolution of these calculations to make them more accurate.

Radiobiology

New understandings of the biology of radiosurgery are considered. Differences from the radiobiology of fractionated radiotherapy are outlined. It is noted DNA damage alone is insufficient to account for the tissue changes which occur. Changes in blood vessels and immunological mechanisms are also involved. Tissue repair is more rapid than previously thought so that dose rate (the rate of delivery of radiation to the tissues) has been seen to be more important. The value of fractionation is examined. The effect of radiosurgery on normal brain (so called functional radiosurgery) is considered. The desired effects may be achieved by a focal stable destruction of brain from a high radiation dose. They may also be achieved using a lower dose which acts through the mechanism known as radiosurgical neuromodulation.

Significance of Dosimetric Parameters in Patients Undergoing Gamma Knife Radiosurgery for Vestibular Schwannoma

Vestibular schwannoma (VS) is a benign, encapsulated, and slow-growing tumor of the myelin-forming cells of the 8th cranial nerve. Gamma Knife radiosurgery (GKRS) has become a widely accepted primary treatment modality for small- to medium-sized VSs. In the case of VS, highly conformal, precisely focused radiation is delivered to the acoustic tumor in a single session under the direct supervision of a radiosurgery team

Tufts Medical Center Experience With Long-Term Follow-Up of Vestibular Schwannoma Treated With Gamma Knife Stereotactic Radiosurgery: Novel Finding of Delayed Pseudoprogression

Purpose

Our purpose was to evaluate the long-term outcomes of patients with vestibular schwannoma (VS) treated with Gamma Knife stereotactic radiosurgery (GKSRS) with modern techniques, with attention to posttreatment tumor growth dynamics, dosimetric predictors, and late toxicities.

Methods and Materials

One hundred twelve patients with VS were treated with GKSRS with a median dose of 12.5 Gy to the 50% isodose line treated between 2004 and 2015, with patients followed up to 15 years. Target and organ-at-risk doses were recorded, and tumor diameter/volume, audiologic decline, and trigeminal/facial nerve preservation were tracked from treatment onward.

Results

GKSRS yielded local control of 5, 10, and 15 years at 96.9%, 90.0%, and 87.1% respectively. Pseudoprogression was found in 45%, with a novel pattern detected with peak swelling at 31 months. Pseudoprogression was associated with smaller tumor diameter at treatment and fewer treatment isocenters, but not with the development of any toxicity, nor was it predicted by any dosimetric factor. Median time to hearing loss was 3.4 years with actuarial hearing preservation at 2, 5, and 10 years of 66.5%, 43.1%, and 37.6%, with rate of hearing loss correlating with maximum cochlea and modiolus doses. Trigeminal and facial nerve preservation rates were 92.7% and 97.6%, respectively. Increasing maximum tumor dose was associated with facial paresthesia.

Conclusions

Modern GKSRS is a safe and effective treatment for VS on long-term follow-up, with high levels of facial and trigeminal nerve preservation. A novel pattern of pseudoprogression has been identified suggesting longer imaging follow-up may be needed before initiating salvage in those without symptomatic progression. Several tumor and dosimetric predictors have been suggested for the development of different toxicities, requiring further evaluation.

Pituitary adenoma treatment plan quality comparison between linear accelerator volumetric modulated arc therapy and Leksell Gamma Knife® radiosurgery

The aim of this study was to compare radiosurgical treatment plan quality of a linear accelerator with Leksell Gamma Knife (LGK) for pituitary adenoma irradiation. Thirty pituitary adenoma patients were evaluated in this study. Treatment plans were prepared on LGK and stereotactic linear accelerator Varian TrueBeam STx. Volumetric Modulated Arc Therapy (VMAT) plans (21 plans with 2 coplanar arcs and 9 plans with 4 non-coplanar arcs) were calculated for linear accelerator. All the plans were evaluated in terms of conformity, selectivity, gradient index and organ at risk (OAR) sparing. VMAT produced dosimetrically comparable treatment plans to LGK regarding conformity and selectivity (New Conformity Index (NCI): 1.76 ± 0.65 for 4 arc VMAT, 2.33 ± 1,16 for 2 arc VMAT and 1.96 ± 0.71 for LGK; Selectivity Index (SI): 0.63 ± 0.16 for 4 arc VMAT, 0.51 ± 0.16 for 2 arc VMAT and 0.58 ± 0.17 for LGK). Gradient index (GI) was superior for LGK plans (GI: 2.74 ± 0.20 for LGK and 5.28 ± 2.29 for 4 arc VMAT). OAR sparing for optics, brainstem, and hypophysis was similar for both modalities while target volume coverage was maintained the same. Finally, treatment time resulted in favor of VMAT plans (in this study VMAT plans were almost 5 times faster than LGK treatment regarding beam on time). According to the results of this study stereotactic linear accelerator with VMAT treatment could be used as a reasonable alternative to LGK for pituitary adenoma radiosurgery but only if the same head fixation method accuracy and target volume delineation are maintained for both modalities.

Radiobiological Evaluation of Combined Gamma Knife Radiosurgery and Hyperthermia for Pediatric Neuro-Oncology

Simple Summary

This study proposes a novel strategy in brain cancer management. Stereotactic radiosurgery delivered by the Gamma Knife was combined with hyperthermia. For the radiobiological modelling of this synergistic treatment modality, we used the linear-quadratic model with temperature-dependent parameters to assess the potential enhancement of the therapeutic outcome. The results indicate that focused intracranial heating can be used to boost the dose to the target. Alternatively, one can conclude that for the same therapeutic effect, hyperthermia can help to minimize the dose undesirably delivered to healthy tissues. This study is also the first to advocate a combination of stereotactic radiosurgery with focused heating and motivates the future development of hyperthermia systems for brain cancer treatment.

Abstract

Combining radiotherapy (RT) with hyperthermia (HT) has been proven effective in the treatment of a wide range of tumours, but the combination of externally delivered, focused heat and stereotactic radiosurgery has never been investigated. We explore the potential of such treatment enhancement via radiobiological modelling, specifically via the linear-quadratic (LQ) model adapted to thermoradiotherapy through modulating the radiosensitivity of temperature-dependent parameters. We extend this well-established model by incorporating oxygenation effects. To illustrate the methodology, we present a clinically relevant application in pediatric oncology, which is novel in two ways. First, it deals with medulloblastoma, the most common malignant brain tumour in children, a type of brain tumour not previously reported in the literature of thermoradiotherapy studies. Second, it makes use of the Gamma Knife for the radiotherapy part, thereby being the first of its kind in this context. Quantitative metrics like the biologically effective dose (BED) and the tumour control probability (TCP) are used to assess the efficacy of the combined plan.

The Hippocampus: A New Organ at Risk for Postoperative Radiation Therapy for Bucco-alveolar Cancer? A Dosimetric and Biological Analysis

PURPOSE

A significant proportion of patients with bucco-alveolar cancer are long-term survivors, warranting attention to survivorship issues. Decline in neurocognitive function after cranial irradiation for brain tumors correlates with a hippocampal maximum dose (Dmax) of more than 16 Gy, minimum dose (Dmin) of more than 9 Gy, and dose to 40% of the hippocampal volume (D40%) exceeding 7.3 Gy in 2-Gy equivalent dose (EQD2), respectively. We analyzed the utility of sparing the hippocampus in postoperative radiation therapy (PORT) for patients with bucco-alveolar cancer, given the proximity of target volumes to the hippocampus, by virtue of inclusion of the infratemporal fossa.

METHODS AND MATERIALS

We instituted hippocampal sparing for patients with bucco-alveolar cancer receiving PORT in March 2018. Ten prior and 10 subsequent consecutive patients with pathologically staged I-IVA cancers of the buccal mucosa, alveolus, and retromolar trigone formed the control group (no hippocampal sparing) and the study group (hippocampal sparing), respectively. The brain and temporal lobes were prescribed dose constraints in both groups. Patients received doses of 60 to 66 Gy at 2 Gy per fraction using the image-guided intensity modulated radiation therapy / volumetric modulated arc therapy technique. Treatment plans were evaluated for (1) hippocampal dosimetric parameters, (2) planning target volume dosimetry and plan-quality indices, and (3) biological indices of equivalent uniform dose (EUD) and normal-tissue complication probability (NTCP) for impaired neurocognitive function.

RESULTS

Hippocampal sparing significantly reduced the hippocampal DmaxEQD2, DmeanEQD2, and D40%EQD2 from 27 Gy to 10.9 Gy (P = .002), 14.3 Gy to 6.4 Gy (P = .002), and 15.5 Gy to 6.6 Gy (P = .005), respectively, with comparable plan-quality indices. The radiobiologically robust endpoints of ipsilateral hippocampal EUD (P = .005) and NTCP (P = .01) were statistically significantly improved.

CONCLUSIONS

Meaningful dosimetric benefit, corroborated with radiobiological indices, was observed with hippocampal sparing. The feasibility and benefit of hippocampal sparing supports our view that the hippocampus should be incorporated as an organ at risk and attention should be given to neurocognitive function in patients with bucco-alveolar cancer who are receiving PORT.

Analysis of treatment planning parameters in the Gamma Knife® technique for different prescription isodoses and volumes of meningiomas

The following Indexes: Homogeneity, Gradient, Conformity, Paddick Conformity and New Conformity of the dose distribution were compared. The parameters to assess a high dose to the organs at risk: V₁₀/TV, V_90%/TV and the Integral Dose were discussed. The higher the prescription isodose, the more uniform the dose distribution in the target, which is highly beneficial in the case of larger tumor sizes due to the lower risk of complications. For smaller tumors, higher dose heterogeneity is desirable. This can be obtained with a 40% prescription isodose.

Feasibility of a mini-pig model of radiation-induced brain injury to one cerebral hemisphere

BACKGROUND

Radiation-induced brain injury is a common concern for survivors of adult and pediatric brain cancer. Pre-clinically, rodent models are the standard approach to evaluate mechanisms of injury and test new therapeutics for this condition. However, these rodent models fail to recapitulate the radiological and histological characteristics of the clinical disease.

METHODS

Here we describe a hemispheric mini-pig model of radiation-induced brain injury generated with a clinical 6 MV photon irradiator and evaluated with a clinical 3T MRI. Two pairs of Yucatan mini-pigs each received either 15 Gy or 25 Gy to the left brain hemisphere. Quality of intensity modulated radiation therapy treatment plans was evaluated retrospectively with parameters reported according to ICRU guidelines. The pigs were observed weekly to check for any outright signs of neurological impairment. The pigs underwent anatomical MRI examination before irradiation and up to 6 months post-irradiation. Immediately after the last imaging time point, the pigs were euthanized and their brains were collected for histopathological assessment.

RESULTS

Analysis of the dose volume histograms showed that 93% of the prescribed dose was delivered to at least 93% of the target volume in the left hemisphere. Organs at risk excluded from the target volume received doses below clinical safety thresholds. For the pigs that received a 25 Gy dose, progressive neurological impairment was observed starting at 2 months post-irradiation leading to the need for euthanasia by 3-4 months. On MRI, these two animals presented with diffuse white matter pathology consistent with the human disease that progressed to outright radiation necrosis and severe brain swelling. Histology was consistent with the final MRI evaluation. The pigs that received a 15 Gy dose appeared normal all the way to 6 months post-irradiation with no obvious neurological impairment or lesions on MRI or histopathology.

CONCLUSION

Based on our results, a mini-pig model of radiation-induced brain injury is feasible though some optimization is still needed. The mini-pig model produced lesions on MRI that are consistent with the human disease and which are not seen in rodent models. Our data shows that the ideal radiation dose for this model likely lies between 15 and 25 Gy.

Metastatic renal cell carcinoma to the brain: optimizing patient selection for gamma knife radiosurgery

INTRODUCTION

The effects of single-fraction gamma knife radiosurgery (sf-GKRS) on patients with renal cell carcinoma (RCC) brain metastases (BM) in the era of targeted agents (TA) and immune checkpoint inhibitors (ICI) are insufficiently studied.

METHODS AND MATERIALS

Clear cell metastatic RCC patients treated with sf-GKRS due to BM in 2005-2014 at three European centres were retrospectively analysed (n = 43). Median follow-up was 56 months. Ninety-five percent had prior nephrectomy, 53% synchronous metastasis and 86% extracranial disease at first sf-GKRS. Karnofsky performance status (KPS) ranged from 60 to 100%. Outcome measures were overall survival (OS), local control (LC) and adverse radiation effects (ARE).

RESULTS

One hundred and ninety-four targets were irradiated. The median number of targets at first sf-GKRS was two. The median prescription dose was 22.0 Gy. Thirty-seven percent had repeated sf-GKRS. Eighty-eight percent received TA. LC rates at 12 and 18 months were 97% and 90%. Median OS from the first sf-GKRS was 15.7 months. Low serum albumin (HR for death 5.3), corticosteroid use pre-sf-GKRS (HR for death 5.8) and KPS < 80 (HR for death 9.1) were independently associated with worse OS. No further prognostic information was gleaned from MSKCC risk group, synchronous metastasis, age, number of BM or extracranial metastases. Other prognostic scores for BM radiosurgery, including DS-GPA, renal-GPA, LLV-SIR and CITV-SIR, again, did not add further prognostic value. ARE were seldom symptomatic and were associated with tumour volume, 10-Gy volume and pre-treatment perifocal oedema. ARE were less common among patients treated with TA within 1 month of sf-GKRS.

CONCLUSIONS

We identified albumin, corticosteroid use and KPS as independent prognostic factors for sf-GKRS of clear cell RCC BM. Studies focusing on the prognostic significance of albumin in sf-GKRS are rare. Further studies with a larger number of patients are warranted to confirm the above analytical outcome. Also, in keeping with previous studies, our data showed optimal rates of local tumour control and limited toxicity post radiosurgery, rendering GKRS the tool of choice in the management of RCC BM.

Stereotactic Radiosurgery for Vestibular Schwannomas: Tumor Control Probability Analyses and Recommended Reporting Standards

PURPOSE

We sought to investigate the tumor control probability (TCP) of vestibular schwannomas after single-fraction stereotactic radiosurgery (SRS) or hypofractionated SRS over 2 to 5 fractions (fSRS).

METHODS AND MATERIALS

Studies (PubMed indexed from 1993-2017) were eligible for data extraction if they contained dosimetric details of SRS/fSRS correlated with local tumor control. The rate of tumor control at 5 years (or at 3 years if 5-year data were not available) were collated. Poisson modeling estimated the TCP per equivalent dose in 2 Gy per fraction (EQD2) and in 1, 3, and 5 fractions.

RESULTS

Data were extracted from 35 publications containing a total of 5162 patients. TCP modeling was limited by the absence of analyzable data of <11 Gy in a single-fraction, variability in definition of "tumor control," and by lack of significant increase in TCP for doses >12 Gy. Using linear-quadratic-based dose conversion, the 3- to 5-year TCP was estimated at 95% at an EQD2 of 25 Gy, corresponding to 1-, 3-, and 5-fraction doses of 13.8 Gy, 19.2 Gy, and 21.5 Gy, respectively. Single-fraction doses of 10 Gy, 11 Gy, 12 Gy, and 13 Gy predicted a TCP of 85.0%, 88.4%, 91.2%, and 93.5%, respectively. For fSRS, 18 Gy in 3 fractions (EQD2 of 23.0 Gy) and 25 Gy in 5 fractions (EQD2 of 30.2 Gy) corresponded to TCP of 93.6% and 97.2%. Overall, the quality of dosimetric reporting was poor; recommended reporting guidelines are presented.

CONCLUSIONS

With current typical SRS doses of 12 Gy in 1 fraction, 18 Gy in 3 fractions, and 25 Gy in 5 fractions, 3- to 5-year TCP exceeds 91%. To improve pooled data analyses to optimize treatment outcomes for patients with vestibular schwannoma, future reports of SRS should include complete dosimetric details with well-defined tumor control and toxicity endpoints.

ESTRO ACROP guideline for target volume delineation of skull base tumors

BACKGROUND AND PURPOSE

For skull base tumors, target definition is the key to safe high-dose treatments because surrounding normal tissues are very sensitive to radiation. In the present work we established a joint ESTRO ACROP guideline for the target volume definition of skull base tumors.

MATERIAL AND METHODS

A comprehensive literature search was conducted in PubMed using various combinations of the following medical subjects headings (MeSH) and free-text words: "radiation therapy" or "stereotactic radiosurgery" or "proton therapy" or "particle beam therapy" and "skull base neoplasms" "pituitary neoplasms", "meningioma", "craniopharyngioma", "chordoma", "chondrosarcoma", "acoustic neuroma/vestibular schwannoma", "organs at risk", "gross tumor volume", "clinical tumor volume", "planning tumor volume", "target volume", "target delineation", "dose constraints". The ACROP committee identified sixteen European experts in close interaction with the ESTRO clinical committee who analyzed and discussed the body of evidence concerning target delineation.

RESULTS

All experts agree that magnetic resonance (MR) images with high three-dimensional spatial accuracy and tissue-contrast definition, both T2-weighted and volumetric T1-weighted sequences, are required to improve target delineation. In detail, several key issues were identified and discussed: i) radiation techniques and immobilization, ii) imaging techniques and target delineation, and iii) technical aspects of radiation treatments including planning techniques and dose-fractionation schedules. Specific target delineation issues with regard to different skull base tumors, including pituitary adenomas, meningiomas, craniopharyngiomas, acoustic neuromas, chordomas and chondrosarcomas are presented.

CONCLUSIONS

This ESTRO ACROP guideline achieved detailed recommendations on target volume definition for skull base tumors, as well as comprehensive advice about imaging modalities and radiation techniques.

Outcome evaluation of patients treated with fractionated Gamma Knife radiosurgery for large (> 3 cm) brain metastases: a dose-escalation study

OBJECTIVE

Fractionated Gamma Knife radiosurgery (GKS) represents a feasible option for patients with large brain metastases (BM). However, the dose-fractionation scheme balanced between local control and radiation-induced toxicity remains unclear. Therefore, the authors conducted a dose-escalation study using fractionated GKS as the primary treatment for large (> 3 cm) BM.

METHODS

The exclusion criteria were more than 3 lesions, evidence of leptomeningeal disease, metastatic melanoma, poor general condition, and previously treated lesions. Patients were randomized to receive 24, 27, or 30 Gy in 3 fractions (8, 9, or 10 Gy per fraction, respectively). The primary endpoint was the development of radiation necrosis assessed by a neuroradiologist blinded to the study. The secondary endpoints included the local progression-free survival (PFS) rate, change in tumor volume, development of distant intracranial progression, and overall survival.

RESULTS

Between September 2016 and April 2018, 60 patients were eligible for the study, with 46 patients (15, 17, and 14 patients in the 8-, 9-, and 10-Gy groups, respectively) available for analysis. The median follow-up duration was 9.6 months (range 2.5-25.1 months). The 6-month estimated cumulative incidence of radiation necrosis was 0% in the 8-Gy group, 13% (95% confidence interval [CI] 0%-29%) in the 9-Gy group, and 37% (95% CI 1%-58%) in the 10-Gy group. Being in the 10-Gy group was a significant risk factor for the development of radiation necrosis (p = 0.047; hazard ratio [HR] 7.2, 95% CI 1.1-51.4). The 12-month local PFS rates were 65%, 80%, and 75% in the 8-, 9-, and 10-Gy groups, respectively. Being in the 8-Gy group was a risk factor for local treatment failure (p = 0.037; HR 2.5, 95% CI 1.1-29.6). The mean volume change from baseline was a 47.5% decrease in this cohort. Distant intracranial progression and overall survival did not differ among the 3 groups.

CONCLUSIONS

In this dose-escalation study, 27 Gy in 3 fractions appeared to be a relevant regimen of fractionated GKS for large BM because 30 Gy in 3 fractions resulted in unacceptable toxicities and 24 Gy in 3 fractions was associated with local treatment failure.

High incidence of transient perifocal edema following upfront radiosurgery for intraventricular meningiomas

INTRODUCTION

Intraventricular trigonal meningiomas (ITM) seem to have a tendency for extensive perifocal edema formation following radiosurgery (RS). To further investigate this hypothesis, we undertook the following study.

METHODS

We retrospectively reviewed records of patients who underwent RS for intraventricular meningiomas at our institution.

RESULTS

From 1999 until 2019, 5 patients underwent single-session RS as primary treatment for ITM. Patients were treated either with a Gamma Knife or a CyberKnife. The mean prescription dose (PD) was 13.0 Gy ± 0.9, the mean tumor volume was 5.8 cc ± 3.1, and the mean follow-up (FU) was 8.9 years ± 5.6. Perifocal edema developed in 4/5 patients after a mean interval of 6.4 months ± 1.2. It was symptomatic in 2/5 patients. The edema regressed spontaneously in 4/5 patients. One of the patients underwent RS for the same ITM twice. One patient's edema was treated medically with steroids, and none of the patients underwent surgery following RS.

CONCLUSION

Even though the number of patients is low, there seems to be a comparably high risk for the formation of a perifocal edema following RS for ITM. Single-session RS as primary treatment for ITM seems to be safe and effective even though a perifocal edema is likely to develop. The perifocal edema and the ensuing neurological deficits were transient and could be managed conservatively in all of our 5 cases.

Initial Experience With Single-Isocenter Radiosurgery to Target Multiple Brain Metastases Using an Automated Treatment Planning Software: Clinical Outcomes and Optimal Target Volume Margins Strategy

Purpose

Our purpose was to assess the clinical outcomes and target positioning accuracy of frameless linear accelerator single-isocenter multiple-target (SIMT) dynamic conformal arc (DCA) stereotactic radiosurgery (SRS) for multiple brain metastases (BM).

Methods and Materials

Between October 2016 and September 2018, 31 consecutive patients ≥18 years old with 204 BM <3 cm in maximum size receiving SIMT DCA SRS were retrospectively evaluated. All plans were created using a dedicated automated treatment planning software (Brainlab, Munich, Germany), and treatments were performed with a Truebeam STx or a Novalis Tx (Brainlab and Varian Medical Systems, CA). The accuracy of setup and interfraction patient repositioning was assessed by Brainlab ExacTrac radiograph 6-dimensional image system and the risk of compromised target dose coverage evaluated. Brain control and overall survival were estimated by Kaplan-Meier method calculated from the time of SRS.

Results

Fourteen patients were treated for 4 to 6 and 17 patients for 7 to 10 BM. The mean gross tumor volume (GTV) was 0.65 cm³ and the mean planning target volume (PTV) was 0.89 cm³. Mean V95 (the volume of the PTV covered by 95% of the prescription dose) and D95 (the prescription dose covering 95% of the PTV) were 99.5% and 21.1 Gy, respectively. With a median clinical follow-up of 11 months (range, 4-26 months), the 1-year survival was 68% and local control was 89%. As a consequence of plan isocenter residual errors, a loss of target coverage, defined as V95 < 95%, occurred in 28 PTVs (10 patients); using a 1 mm GTV-to-PTV margin, adequate dose coverage was maintained for all lesions.

Conclusions

SIMT DCA SRS represents a fast and effective approach for patients with up to 10 BM. The dosimetric effects of residual set-up and intrafraction positioning errors are modest, although a GTV-to-PTV margin of 1 mm is recommended.

Use of uniform shots for robust planning of mask-based treatment in Gamma Knife Icon

PURPOSE

To verify whether Icon automatic correction is robust in preserving plan quality.

MATERIALS/METHODS

An end-to-end phantom was used to verify Icon's correction accuracy qualitatively. For quantitative assessment, two plans, a composite- and a uniform-shot-only, were created for an elliptical- (E) and a sausage-shaped (S) lesion inside a PseudoPatient head phantom with a film insert. The phantom was irradiated in the planned and three other positions under each plan: 14° pitch (B); 14° rotation + 8° pitch (C); 95° rotation + 4-cm shift (D).

RESULTS

Icon accurately corrects the locations of the shots. For the uniform-shot plans: all gamma index passing rates were >97%, and the differences between the planned and the delivery doses (minimum, maximum, and mean) were all ≤0.1 Gy. For the composite-shot plans, however, the dose differences increased as the phantom was shifted through positions B-D, with a gamma index passing rate of 61% for lesion-E in position D, and 92%, 79%, and 45% for lesion-S in positions B, C, and D, respectively.

CONCLUSIONS

Plans using only uniform shots are more robust to deviations in treatment position. The tolerance for such deviations may be lower for plans using composite shots.

Outcomes following stereotactic radiosurgery for small to medium-sized brain metastases are exceptionally dependent upon tumor size and prescribed dose

BACKGROUND

At our institution, we have historically treated brain metastasis (BM) ≤2 cm in eloquent brain with a radiosurgery (SRS) lower prescription dose (PD) to reduce the risk of radionecrosis (RN). We sought to evaluate the impact of this practice on outcomes.

METHODS

We analyzed a prospective registry of BM patients treated with SRS between 2008 and 2017. Incidences of local failure (LF) and RN were determined and Cox regression was performed for univariate and multivariate analyses (MVAs).

RESULTS

We evaluated 1533 BM ≤2 cm. Median radiographic follow-up post SRS was 12.7 months (1.4-100). Overall, the 2-year incidence of LF was lower for BM treated with PD ≥21 Gy (9.3%) compared with PD ≤15 Gy (19.5%) (sub-hazard ratio, 2.3; 95% CI: 1.4-3.7; P = 0.0006). The 2-year incidence of RN was not significantly higher for the group treated with PD ≥21 Gy (9.5%) compared with the PD ≤15 Gy group (7.5%) (P = 0.16). MVA demonstrated that PD (≤15 Gy) and tumor size (>1 cm) were significantly correlated (P < 0.05) with higher rates of LF and RN, respectively. For tumors ≤1 cm, when comparing PD ≤15 Gy with ≥21 Gy, the risks of LF and RN are equivalent. However, for lesions >1 cm, PD ≥21 Gy is associated with a lower incidence of LF without significantly increasing the risk of RN.

CONCLUSION

Our results indicate that rates of LF or RN following SRS for BM are strongly correlated with size and PD. Based on our results, we now, depending upon the clinical context, consider increasing PD to 21 Gy for BM in eloquent brain, excluding the brainstem.

Gamma Knife® icon CBCT offers improved localization workflow for frame-based treatment

Object

The purpose of this study was to compare two methods of stereotactic localization in Gamma Knife treatment planning: cone beam computed tomography (CBCT) or fiducial. While the fiducial method is the traditional method of localization, CBCT is now available for use with the Gamma Knife Icon. This study seeks to determine whether a difference exists between the two methods and then whether one is better than the other regarding accuracy and workflow optimization.

Methods

Cone beam computed tomography was used to define stereotactic space around the Elekta Film Pinprick phantom and then treated with film in place. The same phantom was offset known amounts from center and then imaged with CBCT and registered with the reference CBCT image to determine if measured offsets matched those known. Ten frameless and 10 frame‐based magnetic resonance imaging (MRI) to CBCT patient fusions were retrospectively evaluated using the TG‐132 TRE method. The stereotactic coordinates defined by CBCT and traditional fiducials were compared on the Elekta 8 cm Ball phantom, an anthropomorphic phantom, and actual patient data. Offsets were introduced to the anthropomorphic phantom in the stereotactic frame and CBCT's ability to detect those offsets was determined.

Results

Cone beam computed tomography defines stereotactic space well within the established limits of the mechanical alignment system. The CBCT to CBCT registration can detect offsets accurately to within 0.1 mm and 0.5°. In all cases, some disagreement existed between fiducial localization and that of CBCT which in some cases was small, but also was as high as 0.43 mm in the phantom domain and as much as 1.54 mm in actual patients.

Conclusion

Cone beam computed tomography demonstrates consistent accuracy in defining stereotactic space. Since both localization methods do not agree with each other consistently, the more reliable method must be identified. Cone beam computed tomography can accurately determine offsets occurring within stereotactic space that would be nondiscernible utilizing the fiducial method and seems to be more reliable. Using CBCT localization offers the opportunity to streamline workflow both from a patient and clinic perspective and also shows patient position immediately prior to treatment.

Systematic Review of Hearing Preservation After Radiotherapy for Vestibular Schwannoma

OBJECTIVE

To determine the long-term hearing preservation rate for spontaneous vestibular schwannoma treated by primary radiotherapy.

DATA SOURCES

The MEDLINE/PubMed, Web of Science, Cochrane Reviews, and EMBASE databases were searched using a comprehensive Boolean keyword search developed in conjunction with a scientific librarian. English language papers published from 2000 to 2016 were evaluated.

STUDY SELECTION

Inclusion criteria: full articles, pretreatment and posttreatment audiograms or audiogram based scoring system, vestibular schwannoma only tumor type, reported time to follow-up, published after 1999, use of either Gamma Knife or linear accelerator radiotherapy.

EXCLUSION CRITERIA

case report or series with fewer than five cases, inadequate audiometric data, inadequate time to follow-up, neurofibromatosis type 2 exceeding 10% of study population, previous treatment exceeding 10% of study population, repeat datasets, use of proton beam therapy, and non-English language.

DATA EXTRACTION

Two reviewers independently analyzed papers for inclusion. Class A/B, 1/2 hearing was defined as either pure tone average less than or equal to 50 db with speech discrimination score more than or equal to 50%, American Academy of Otolaryngology-Head & Neck Surgery (AAO-HNS) Hearing Class A or B, or Gardner-Robertson Grade I or II. Aggregate data were used when individual data were not specified.

DATA SYNTHESIS

Means were compared with student t test.

CONCLUSIONS

Forty seven articles containing a total of 2,195 patients with preserved Class A/B, 1/2 hearing were identified for analysis. The aggregate crude hearing preservation rate was 58% at an average reporting time of 46.6 months after radiotherapy treatment. Analysis of time-based reporting shows a clear trend of decreased hearing preservation extending to 10-year follow-up. This data encourages a future long-term controlled trial.

Safety and efficacy of multisession gamma knife radiosurgery for residual or recurrent pituitary adenomas

PURPOSE

To define the efficacy and complications of multisession Gamma Knife radiosurgery (MGKRS) delivered in three consecutive sessions for the treatment of residual or recurrent pituitary adenomas (PAs).

METHODS

This was a retrospective study of data from the Neurosurgery and Gamma Knife Radiosurgery Department at San Raffaele Hospital between May 2008 and September 2017. We recruited 47 consecutive patients undergoing MGKRS in three consecutive fractions for residual or recurrent PA with a distance from the anterior optic pathway inferior to 2-3 mm.

RESULTS

Thirty-eight (80.8%) patients had a nonfunctioning-PA (NFPA) while 9 (19.2%) had a hormone-secreting PA (HSPA). Tumor control was achieved in 100% of patients. Tumor shrinkage was seen in 33 out of 44 (75.0%) patients with a radiological follow-up. Mean tumor volume before MGKRS was 3.93 cm³. The mean tumor volume at last follow-up was 2.11 cm³, with a mean tumor shrinkage of 50.2%, as compared with baseline. One case of suspected radiation-induced optic neuropathy (RION) was documented while new-onset hypopituitarism for any axis occurred in 12 of the 31 (38.7%) patients at risk. The mean follow-up was 44.6 ± 4.0 months (range, 6-111 months).

CONCLUSIONS

MGKRS is a valid alternative to external fractionated radiotherapy and other types of stereotactic radiosurgery for the treatment of PAs, achieving a high tumor control rate with a low risk of visual deterioration. Moreover, the majority of patients showed a significant reduction of tumor size in the long term.

A linear programming approach to inverse planning in Gamma Knife radiosurgery

PURPOSE

Leksell Gamma Knife® is a stereotactic radiosurgery system that allows fine-grained control of the delivered dose distribution. We describe a new inverse planning approach that both resolves shortcomings of earlier approaches and unlocks new capabilities.

METHODS

We fix the isocenter positions and perform sector-duration optimization using linear programming, and study the effect of beam-on time penalization on the trade-off between beam-on time and plan quality. We also describe two techniques that reduce the problem size and thus further reduce the solution time: dualization and representative subsampling.

RESULTS

The beam-on time penalization reduces the beam-on time by a factor 2-3 compared with the naïve alternative. Dualization and representative subsampling each leads to optimization time-savings by a factor 5-20. Overall, we find in a comparison with 75 clinical plans that we can always find plans with similar coverage and better selectivity and beam-on time. In 44 of these, we can even find a plan that also has better gradient index. On a standard GammaPlan workstation, the optimization times ranged from 2.3 to 26 s with a median time of 5.7 s.

CONCLUSION

We present a combination of techniques that enables sector-duration optimization in a clinically feasible time frame.

Retrospective dosimetric analysis of brain lesions planned in Pinnacle 9.8 via a HDMLC linac

The University of Toledo Medical Center's Eleanor N. Dana Cancer Center located in northwest Ohio currently utilizes the Edge Radiosurgery System (Varian Medical Systems Inc., Palo Alto, CA) to deliver stereotactic radiosurgery for the treatment of brain lesions. The purpose of this study is to determine the quality of conformal arc radiotherapy in treating patients with brain lesions at The University of Toledo Medical Center and to provide more data for conformity and gradient indices (due to a lack of current data) to hopefully improve national standards by allowing centers to compare among each other. Treatment plans were assessed using the Pinnacle³ v9.8 Radiation Therapy Planning System (Philips Healthcare, Amsterdam, Netherlands). For patients (n = 41) presenting with small brain lesions (n = 82) and treated with conformal arc radiotherapy via the Edge Radiosurgery System, the RTOG conformity index, Paddick conformity index, conformity gradient index, gradient index, and dose gradient index were determined for each plan. This study additionally provides data to suggest the more accurate method of volume derivation provided by the Pinnacle³ v9.8 software. Using this method, average values for each of the following indices were calculated: RTOG conformity index = 1.36 ± 0.29; Paddick conformity index = 0.72 ± 0.12; conformity gradient index = 214.67 ± 12.35; gradient index = 3.64 ± 1.09; dose gradient index = -0.11 ± 0.16. Thus, The University of Toledo Medical Center provides favorable conformity of dose to intracranial target lesions.

Trigeminal neuropathy in vestibular schwannoma: a treatment algorithm to avoid long-term morbidity

BACKGROUND

Trigeminal neuropathy (TGN) can occur as a presenting feature of vestibular schwannoma (VS) or as an adverse effect of radiosurgery. This study was designed to evaluate a treatment algorithm for presenting symptoms of TGN in patients with VS, and a new radiosurgery dosimetric tolerance to avoid TGN after treatment. Outcome was measured after microsurgery (MS), stereotactic radiosurgery (SRS), hypofractionated stereotactic radiotherapy (HSRT), and fractionated radiotherapy (FRT).

METHODS

A prospectively held VS database was retrospectively analysed from 2011 to 2016 at a tertiary university hospital. All patients who underwent MS from 2011 and all patients who underwent radiotherapy (SRS, HSRT, FRT) from 2015 were studied. Patients on surveillance and neurofibromatosis type 2 patients were not included. Patient demographic data, tumour characteristics, presenting symptoms, and post-treatment outcomes were analysed.

RESULTS

Eighty-eight patients were included in the study (43 microsurgery, 45 radiotherapy). Twenty-seven (31%) patients presented with TGN symptoms. The median age of patients included was 56.5 (range 6-72 years), with a median follow-up for MS and SRS of 38 and 20 months, respectively (range 10-80 months). All 27 patients with TGN were offered MS as per protocol. Three patients declined, or were not fit for surgery, and received FRT. Complete resolution of TGN symptoms was achieved in all 24 patients who underwent MS and 33% (1/3) of patients with FRT. Eleven patients experienced transient post-operative complications (pseudomeningocele (6), meningitis (3), venous sinus thrombosis, cerebellar haemorrhagic contusion, and posterior fossa haematoma). Of the 45 patients in the radiotherapy cohort, 36 were suitable for SRS, of which 30 patients who met the dose-volume constraints for trigeminal nerve underwent single-fraction SRS and 6 patients who did not meet the constraints received HSRT. Nine patients (20%) received FRT including three patients with pre-treatment TGN. None of the patients developed new TGN symptoms following SRS or HSRT.

CONCLUSIONS

Our algorithm to select the optimal treatment modality appears to achieve comparable or better long-term outcome. Microsurgical resection in our cohort resulted in complete resolution of symptoms in all patients. None of our SRS- or HSRT-treated patients developed TGN during the follow-up period. The adherence to strict trigeminal nerve dose-volume constraints for SRS remains critical to minimise TGN post treatment. Fractionated radiotherapy is an alternative for patients who refuse surgery or those who are unfit for surgery.

Stereotactic radiosurgery for benign brain tumors: Results of multicenter benchmark planning studies

PURPOSE

Stereotactic radiosurgery (SRS) is strongly indicated for treatment of surgically inaccessible benign brain tumors. Various treatment platforms are available, but few comparisons have included multiple centers. As part of a national commissioning program, benchmark planning cases were completed by all clinical centers in the region.

METHODS AND MATERIALS

Four benign cases were provided, with images and structures predelineated, including intracanalicular vestibular schwannoma (VS), larger VS, skull base meningioma, and secreting pituitary adenoma. Centers were asked to follow their local practice, and plans were reviewed centrally using metrics for target coverage, selectivity, gradient falloff, and normal tissue sparing.

RESULTS

Sixty-eight plans were submitted using 18 different treatment platforms. Fourteen plans were subsequently revised following feedback, and review of 5 plans led to a restriction of service on 2 platforms (2 centers). Prescription doses were consistent for VS and meningioma submissions, but a wide range of doses were used for the pituitary case. All centers prioritized coverage, with the prescription isodose covering ≥95% of 78/82 target volumes. Lower values may be expected next to air cavities when using advanced algorithms, and in general may be acceptable for some benign lesions. Selectivity was much more variable, and in some cases this was combined with high gradient index and/or >1 mm margin, resulting in large volumes of normal tissue being irradiated. Normal tissue doses were more variable across linear accelerator (LINAC)-based plans than with Gamma Knife or CyberKnife, and dose spillage seemed independent of prescription isodose (inhomogeneity). This may reflect the variety of LINAC-based approaches represented or the necessary tradeoff between different objectives.

CONCLUSIONS

These benchmarking exercises have highlighted areas of different clinical practice and priorities and potential for improvement. The subsequent sharing of plan data and margin philosophies between the neurosurgery and oncology communities allowed for meaningful comparison between centers and their peers.

Stereotactic radiosurgery for multiple brain metastases: Results of multicenter benchmark planning studies

PURPOSE

Stereotactic radiosurgery is indicated for treatment of multiple brain metastases. Various treatment platforms are available, but most comparisons are limited to single-center studies. As part of a national commissioning program, benchmark planning cases were completed by 21 clinical centers, providing a unique dataset of current practice across a large number of providers and equipment platforms.

METHODS AND MATERIALS

Two brain metastases cases were provided, with images and structures predrawn, involving 3 and 7 lesions. Centers produced plans according to their local practice, which were reviewed centrally using metrics for target coverage, selectivity, gradient fall-off, and normal tissue sparing.

RESULTS

Fifty plans were submitted, using 24 treatment platforms. Eleven plans were revised following feedback, including 2 centers that acquired a new platform; 1 other center accepted a restriction of service. All centers prioritized coverage, with the prescription isodose covering ≥95% of 233 of 235 target volumes. Selectivity was much more variable, especially for smaller lesions, and when combined with poor gradient indices resulted in large volumes of normal tissue being irradiated. Tomotherapy submissions were outliers for either selectivity or gradient index, but other platforms could produce plans with relatively low gradient indices for larger lesion volumes. There was more variation among Varian and Elekta LINAC plans than for Gamma Knife and CyberKnife, and larger differences for smaller targets, both inter- and intratreatment platform. Doses to normal brain and brainstem were highest when margins were applied, but improvements were possible by replanning alone.

CONCLUSIONS

Multicenter benchmarking exercises have highlighted some variation in clinical practice and priorities, with a few outliers. Most platforms are able to achieve comparable plans, except for the smallest volumes and when larger planning margins are used. The data will be used to advance standardization and quality improvement of national services and can provide useful guidance for centers worldwide.

Hypopituitarism after gamma knife radiosurgery for pituitary adenoma

PURPOSE

The aim of the study was to investigate the incidence of and risk factors for hypopituitarism after gamma knife radiosurgery (GKRS) for pituitary adenoma.

MATERIALS AND METHODS

We conducted a retrospective analysis of the pituitary function of 90 patients who underwent GKRS for pituitary adenoma at the University Hospital Centre Zagreb between 2003 and 2014. Twenty seven of them met the inclusion criteria and the others were excluded from the study due to pituitary insufficiency which was present before GKRS. Eighteen patients had non-functioning and 9 patients had secretory adenomas. Median patients' age was 56 years (24-82). GKRS was performed using the Leksell gamma knife Model C. The median prescription radiation dose was 20 Gy (15-25) and the median tumor volume size was 3.4 cm³ (0.06-16.81). New onset hypopituitarism was defined as a new deficit of one of the three hormonal axes (corticotroph, thyreotroph, or gonadotroph) ≥3 months following GKRS. SPSS was used for statistical analysis, with the significance level at P<0.05.

RESULTS

During the median follow-up period of 72 months (range 6-144), 30% of patients developed new hypopituitarism after GKRS. This corresponds to incidence of one new case of hypopituitarism per 15 patient-years. Age, gender, tumor function, tumor volume, suprasellar extension, prescription dose of radiation, as well as dose-volume to the pituitary gland, stalk and hypothalamus were not predictive factors for the development of hypopituitarism.

CONCLUSIONS

In our cohort of patients with pituitary tumors who underwent GKRS, 30% developed new hypopituitarism during the follow-up period.

Finer leaf resolution and steeper beam edges using a virtual isocentre in concurrence to PTV-shaped collimators in standard distance - a planning study

Purpose

Investigation of a reduced source to target distance to improve organ at risk sparing during stereotactic irradiation (STX).

Methods

The authors present a planning study with perfectly target-volume adapted collimator compared with multi-leaf collimator (MLC) at reduced source to virtual isocentre distance (SVID) in contrast to normal source to isocentre distance (SID) for stereotactic applications. The role of MLC leaf width and 20–80% penumbra was examined concerning the healthy tissue sparing. Several prescription schemes and target diameters are considered.

Results

Paddick’s gradient index (GI) as well as comparison of the mean doses to spherical shells at several distances to the target is evaluated. Both emphasize the same results: the healthy tissue sparing in the high dose area around the planning target volume (PTV) is improved at reduced SVID ≤ 70 cm. The effect can be attributed more to steeper penumbra than to finer leaf resolution. Comparing circular collimators at different SVID just as MLC-shaped collimators, always the GI was reduced. Even MLC-shaped collimator at SVID 70 cm had better healthy tissue sparing than an optimal shaped circular collimator at SID 100 cm.

Regarding penumbra changes due to varying SVID, the results of the planning study are underlined by film dosimetry measurements with Agility™ MLC.

Conclusion

Penumbra requires more attention in comparing studies, especially studies using different planning systems. Reduced SVID probably allows usage of conventional MLC for STX-like irradiations.

Electronic supplementary material

The online version of this article (doi:10.1186/s13014-017-0826-8) contains supplementary material, which is available to authorized users.