Stereotactic ablative radiation therapy for brain metastases with volumetric modulated arc therapy and flattening filter free delivery: feasibility and early clinical results

An exploratory analysis of MR-guided fractionated stereotactic radiotherapy in patients with brain metastases

Purpose

To assess the feasibility and potential benefits of online adaptive MR-guided fractionated stereotatic radiotherapy (FSRT) in patients with brain metastases (BMs).

Methods and materials

Twenty-eight consecutive patients with BMs were treated with FSRT of 30 Gy in 5 fractions on the 1.5 T MR-Linac. The FSRT fractions employed daily MR scans and the contours were utilized to create each adapted plan. The brain lesions and perilesional edema were delineated on MR images of pre-treatment simulation (Fx0) and all fractions (Fx1, Fx2, Fx3, Fx4 and Fx5) to evaluate the inter-fractional changes. These changes were quantified using absolute/relative volume, Dice similarity coefficient (DSC) and Hausdorff distance (HD) metrics. Planning target volume (PTV) coverage and organ at risk (OAR) constraints were used to compare non-adaptive and adaptive plans.

Results

A total of 28 patients with 88 lesions were evaluated, and 23 patients (23/28, 82.1%) had primary lung adenocarcinoma. Significant tumor volume reduction had been found during FSRT compared to Fx0 for all 88 lesions (median -0.75%, -5.33%, -9.32%, -17.96% and -27.73% at Fx1, Fx2, Fx3, Fx4 and Fx5, p < 0.05). There were 47 (47/88, 53.4%) lesions being accompanied by perilesional edema and the inter-fractional changes were significantly different compared to those without perilesional edema (p < 0.001). Patients with multiple lesions (13/28, 46.4%) had more significant inter-fractional tumor changes than those with single lesion (15/28, 53.6%), including tumor volume reduction and anatomical shift (p < 0.001). PTV coverage of non-adaptive plans was below the prescribed coverage in 26/140 fractions (19%), with 12 (9%) failing by more than 10%. All 140 adaptive fractions met prescribed target coverage. The adaptive plans also had lower dose to whole brain than non-adaptive plans (p < 0.001).

Conclusions

Significant inter-fractional tumor changes could be found during FSRT in patients with BMs treated on the 1.5 T MR-Linac. Daily MR-guided re-optimization of treatment plans showed dosimetric benefit in patients with perilesional edema or multiple lesions.

Dosimetric Parameters in Hypofractionated Stereotactic Radiotherapy for Brain Metastases: Do Flattening Filter-Free Beams Bring Benefits? A Preliminary Study

PURPOSE

This study aimed to compare the dosimetric results of flattening filter-free (FFF) vs. flattened (FF) treatment plans for fractionated stereotactic radiotherapy (fSRT), with the goal to highlight potential advantages of FFF beams.

METHODS

A group of 18 patients with brain metastases treated with fSRT (30 Gy delivered in 5 fractions) were included. The dosimetric parameters evaluated were: (1) physical dosimetric parameters (number of monitor units (MUs), conformity index (CI), dose gradient index (DGI), beam on time (BOT)); (2) clinical dosimetric parameters pertaining to target volume (PTV) and organs at risk (OARs). Two treatment plans were performed for all patients: one used 6 MV FFF beams and the other used 6 MV flattened beams.

RESULTS

A slight increase in MUs was observed for the FFF mode (+23.3 MUs). The CI showed a difference of -2.7% for the FF plans (p = 0.28), correlated with a poorer coverage of the PTV. DGI values reported in terms of PTV are in line with international recommendations and showed a +1.9% difference for FFF plans. An average BOT of 90.3 s was reported for FFF plans, which was 2.3 times shorter than that required for FF plans delivery (p ≤ 0.001). A slight decrease of PTV coverage (-1.26%, p = 0.036) for FF plans can be considered relevant, but no other significant differences were observed between the two optimizations. No statistically significant benefit of using FFF beams to reduce V₂₀ for normal brain could be demonstrated.

CONCLUSION

These dosimetric results encourage the implementation of fSRT with standard flattened beams in centers where FFF linacs are not available.

Planning issues on linac-based stereotactic radiotherapy

This work aims to summarize and evaluate the current planning progress based on the linear accelerator in stereotactic radiotherapy (SRT). The specific techniques include 3-dimensional conformal radiotherapy, dynamic conformal arc therapy, intensity-modulated radiotherapy, and volumetric-modulated arc therapy (VMAT). They are all designed to deliver higher doses to the target volume while reducing damage to normal tissues; among them, VMAT shows better prospects for application. This paper reviews and summarizes several issues on the planning of SRT to provide a reference for clinical application.

Radiosurgery for multiple brain metastases using volumetric modulated arc therapy: a single institutional series

Background

Patients with brain metastases (BM) live longer due to improved diagnosis and oncologic treatments. The association of volumetric modulated arc therapy (VMAT) and image-guided radiation therapy (IGRT) with brain radiosurgery (SRS) allows complex dose distributions and faster treatment delivery to multiple lesions.

Materials and methods

This study is a retrospective analysis of SRS for brain metastasis using VMAT. The primary endpoints were local disease-free survival (LDFS) and overall survival (OS). The secondary outcomes were intracranial disease-free survival (IDFS) and meningeal disease-free survival (MDFS).

Results

The average number of treated lesions was 5.79 (range: 2-20) per treatment in a total of 113 patients. The mean prescribed dose was 18 Gy (range: 12-24 Gy). The median LDFS was 46 months. The LDFS in 6, 12, and 24 months was for 86%, 79%, and 63%, respectively. Moreover, brain progression occurred in 50 patients. The median overall survival was 47 months. The OS in 75%, 69%, and 61% patients was 6, 12, and 24 months, respectively. IDFS was 6 and 24 months in 35% and 14% patients, respectively. The mean MDFS was 62 months; it was 6 and 24 months for 87% and 83% of patients. Acute severe toxicity was relatively rare. During follow-up, the rates of radionecrosis and neurocognitive impairment were low (10%).

Conclusion

The use of VMAT-SRS for multiple BM was feasible, effective, and associated with low treatment-related toxicity rates. Thus, treatment with VMAT is a safe technique to plan to achieve local control without toxicity.

Comparison between flattening filter-free (FFF) and flattened photon beam VMAT plans for the whole brain radiotherapy (WBRT) with hippocampus sparing

Purpose

To evaluate and investigate the feasibility of flattening filter‐free (FFF) beam for the whole‐brain radiotherapy (WBRT) with hippocampus sparing.

Methods

Eighteen patients with volumetric‐modulated arc therapy (VMAT) plans in FFF and conventional beam modes were included in this study. The prescribed dose was 30 Gy in 10 fractions. The conformity index (CI), heterogeneity index reported by TPS (HI‐M), and homogeneity index (HI) for planning target volume (PTV) were evaluated. Subsequently, the following parameters for PTV were calculated and compared: D_2%, D_98%; the mean dose, maximum dose, and minimal dose for OARs. Plan modulation index, total MUs, and the gamma index were used to evaluate the plan quality.

Results

HI‐M results were similar for the two techniques (1.239 vs. 1.247, respectively, p = 0.048); FFF beam plans yielded lower D2% compared to FF beam plans (3,416.3 cGy vs. 3,437.2 cGy, p = 0.22), mean dose (3,177.5 cGy vs. 3,195.2 cGy, p = 0.009), and CI (0.884 vs. 0.876, p = 0.001) for PTV. Significant differences were observed between the two beam modes (FF model vs. FFF model) for the maximum dose (1,612.9 cGy vs. 1,470.2 cGy, respectively, p < 0.001), minimum dose (987.6 cGy vs. 898.8 cGy, respectively, p < 0.001), and the mean dose (1144.4 cGy vs. 1047.3 cGy, respectively, p < 0.001) to the hippocampus, and the maximum dose to the eyes (2,792.6 cGy vs. 2,751.3 cGy, respectively, p < 0.001). The average total MUs for FFF‐VMAT plans was significantly greater than FF‐VMAT plans. However, differences for the plan modulation index and the gamma index were negligible.

Conclusion

In comparison with FF beam, the FFF beam mode offers a clear benefit with respect to WBRT with hippocampal sparing.

Progress and prospects of flattening filter free beam technology in radiosurgery and stereotactic body radiotherapy

The aim of this work is to summarize and evaluate the current status of knowledge on flattening filter free (FFF) beams and their applications in stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). A PubMed search was undertaken in order to identify relevant publications using FFF and stereotactic radiotherapy as keywords. On a clinical aspect, lung tumors treated with FFF SBRT show promising results in terms of local control and overall survival with acute toxicities consistent with those that occur with standard radiotherapy. Beside lung, SBRT is suitable for different anatomical sites such as liver, prostate, cervix, etc. offering similar results: reduced treatment time, good tumor control and mild acute toxicities. Regarding brain tumors, the employment of SRS with FFF beams significantly reduces treatment time and provides notable normal tissue sparing due to the sharp dose fall-off outside the tumor.

Linac-based STereotactic Arrhythmia Radioablation (STAR) of ventricular tachycardia: Case report and literature review

Linac-based STereotactic Arrhythmia Radioablation (STAR) is a safety and effective approach for selected patients with ventricular arrhythmias.

Single-Isocenter Volumetric Modulated Arc Therapy vs. CyberKnife M6 for the Stereotactic Radiosurgery of Multiple Brain Metastases

Introduction: Stereotactic radiosurgery (SRS) is becoming more frequently used for patients with multiple brain metastases (BMs). Single-isocenter volumetric modulated arc therapy (SI-VMAT) is an emerging alternative to dedicated systems such as CyberKnife (CK). We present a dosimetric comparison between CyberKnife M6 and SI-VMAT, planned at RayStation V8B, for the simultaneous SRS of five or more BM.

Patients and Methods: Twenty treatment plans of CK-based single-session SRS to ≥5 brain metastases were replanned using SI-VMAT for delivery at an Elekta VersaHD linear accelerator. Prescription dose was 20 or 18 Gy, conformally enclosing at least 98% of the total planning target volume (PTV), with PTV margin-width adapted to the respective SRS technique. Comparatively analyzed quality metrics included dose distribution to the healthy brain (HB), including different isodose volumes, conformity, and gradient indices. Estimated treatment time was also compared.

Results: Median HB isodose volumes for 3, 5, 8, 10, and 12 Gy were consistently smaller for CK-SRS compared to SI-VMAT (p < 0.001). Dose falloff outside the target volume, as expressed by the gradient indices GI_high and GI_low, was consistently steeper for CK-SRS compared to SI-VMAT (p < 0.001). CK-SRS achieved a median GI_high of 3.1 [interquartile range (IQR), 2.9–1.3] vs. 5.0 (IQR 4.3–5.5) for SI-VMAT (p < 0.001). For GI_low, the results were 3.0 (IQR, 2.9–3.1) for CK-SRS vs. 5.6 (IQR, 4.3–5.5) for SI-VMAT (p < 0.001). The median conformity index (CI) was 1.2 (IQR, 1.1–1.2) for CK-SRS vs. 1.5 (IQR, 1.4–1.7) for SI-VMAT (p < 0.001). Estimated treatment time was shorter for SI-VMAT, yielding a median of 13.7 min (IQR, 13.5–14.0) compared to 130 min (IQR, 114.5–154.5) for CK-SRS (p < 0.001).

Conclusion: SI-VMAT offers enhanced treatment efficiency in cases with multiple BM, as compared to CyberKnife, but requires compromise regarding conformity and integral dose to the healthy brain. Additionally, delivery at a conventional linear accelerator (linac) may require a larger PTV margin to account for delivery and setup errors. Further evaluations are warranted to determine whether the detected dosimetric differences are clinically relevant. SI-VMAT could be a reasonable alternative to a dedicated radiosurgery system for selected patients with multiple BM.

MRI appearance change during stereotactic radiotherapy for large brain metastases and importance of treatment plan modification during treatment period

Purpose

We aimed to evaluate the magnetic resonance imaging (MRI) appearance changes during stereotactic radiotherapy (SRT) for large sized brain metastases, and analyze the lesions necessitating treatment plan modification.

Materials and methods

A total of 23 patients (27 lesions, >2 cm in tumor diameter) underwent SRT and all lesions were evaluated the appearance changes which had the necessity of the treatment plan modification. The appearance change of tumor during SRT was evaluated using gadolinium-enhanced MRI. The reasons of the modification were classified into tumor reduction, tumor enlargement, displacement, and shape change.

Results

Among the 27 lesions, 55.6% required the treatment plan modification. The reasons were tumor reduction in six lesions, tumor enlargement in three lesions, displacement in three lesions, and shape change in three lesions. The planning target volume (PTV) size changed up to 43.0% and the shift of center of PTV was a maximum of 1.7 mm. The pathological status (adenocarcinoma vs others) and timing of steroid administration (prior vs after SRT start) were the predictive factors of tumor changes required the modification.

Conclusions

As tumor changes might occur even during short period of SRT, the treatment plan evaluation and modification were important in SRT for large brain metastases.

Frameless Image-Guided Radiosurgery for Multiple Brain Metastasis Using VMAT: A Review and an Institutional Experience

We undertook a structured review of stereotactic radiosurgery (SRS) using linear particle accelerator (linac) equipment, focusing on volumetric modulated arc therapy (VMAT) technology, and frameless image-guided radiotherapy (IGRT), for the treatment of brain metastases. We analyzed the role of linac SRS and its clinical applications, exploring stereotactic localization. Historically, there was a shift from fixed frames to frameless approaches, moving toward less invasive treatments. Thus, we reviewed the concepts of VMAT for multiple-target applications, comparing its dosimetric and technical features to those of other available techniques. We evaluated relevant technical issues and discussed the planning parameters that have gained worldwide acceptance to date. Thus, we reviewed the current literature on the clinical aspects of SRS, especially its main indications and how the advantages of VMAT may achieve clinical benefits in such scenarios. Finally, we reported our institutional results on IGRT-VMAT for SRS treatments for patients with multiple brain metastases.

Linac-based radiosurgery or fractionated stereotactic radiotherapy with flattening filter-free volumetric modulated arc therapy in elderly patients : A mono-institutional experience on 110 brain metastases

AIM

The aim of this study was to analyze the feasibility and clinical results of linear accelerator (linac-)based stereotactic radiosurgery (SRS) or fractionated stereotactic radiotherapy (SFRT) with flattening filter-free (FFF) volumetric modulated arc therapy (VMAT) in elderly patients affected by brain metastases (BMs).

PATIENTS AND METHODS

Patients selected for the present analysis were ≥65 years old with a life expectancy of >3 months, a controlled or synchronous primary tumor, and <10 BMs with a diameter <3 cm. All patients were treated with FFF linac-based SRS/SFRT. The prescribed total dose (15-30 Gy/1-5 fractions) was based on BM size and proximity to organs at risk (OAR). Toxicity was assessed according to the Common Terminology Criteria for Adverse Events (CTCAE) v4.0. MedCalc v18.2 (MedCalc Software, Ostend, Belgium) was used for statistical analysis.

RESULTS

From April 2014 to December 2017, 40 elderly patients with 110 BMs were treated by FFF linac-based SRS/SFRT. With a median follow-up of 28 months (range 6-50 months), median and 1‑year overall survival were 9 months and 39%, respectively; median intracranial progression-free survival was 6 months. At the time of the analysis, local control was reported in 109/110 BMs (99.1%): 12 BMs had a complete response; 51 a partial response; 46 showed stable disease. One BM (0.9%) progressed after 2 months. BM volume (<1 cc) and higher SRS/SFRT dose correlated to treatment response (p = 0.01 and p = 0.0017, respectively). No adverse events higher than grade 2 were observed.

CONCLUSION

The present findings highlight the feasibility, safety, and effectiveness of FFF linac-based SRS/SFRT in elderly patients with BMs.

Linac-based VMAT radiosurgery for multiple brain lesions: comparison between a conventional multi-isocenter approach and a new dedicated mono-isocenter technique

Background

Linac-based stereotactic radiosurgery or fractionated stereotactic radiotherapy (SRS/FSRT) of multiple brain lesions using volumetric modulated arc therapy (VMAT) is typically performed by a multiple-isocenter approach, i.e. one isocenter per lesion, which is time-demanding for the need of independent setup verifications of each isocenter. Here, we present our initial experience with a new dedicated mono-isocenter technique with multiple non-coplanar arcs (HyperArc™, Varian Inc.) in terms of a plan comparison with a multiple-isocenter VMAT approach.

Methods

From August 2017 to October 2017, 20 patients with multiple brain metastases (mean 5, range 2–10) have been treated by HyperArc in 1–3 fractions. The prescribed doses (Dp) were 18–25 Gy in single-fraction, and 21–27 Gy in three-fractions. Planning Target Volume (PTV), defined by a 2 mm isotropic margin from each lesion, had mean dimension of 9.6 cm³ (range 0.5–27.9 cm³). Mono-isocenter HyperArc VMAT plans (HA) with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°) were generated and compared to multiple-isocenter VMAT plans (RA) with 2 coplanar 360°-arcs per isocenter. A dose normalization of 100%Dp at 98%PTV was adopted, while D_2%(PTV) < 150%D_p was accepted. All plans had to respect the constraints on maximum dose to the brainstem (D_0.5cm3 < 18 Gy) as well as to the optical nerves/chiasm, eyes and lenses (D_0.5cm3 < 15 Gy). HA and RA plans were compared in terms of dose-volume metrics, by Paddick conformity (CI) and gradient (GI) index and by V₁₂ and mean dose to the brain-minus-PTV, and in terms of MU and overall treatment time (OTT) per fraction. OTT was measured for HA treatments, whereas for RA plans OTT was estimated by assuming 3 min. For initial patient setup plus 5 min. For each CBCT-guided setup correction per isocenter.

Results

Significant variations in favour of HA plans were computed for both target dose indexes, CI (p < .01) and GI (p < .01). The lower GI in HA plans was the likely cause of the significant reduction in V₁₂ to the brain-minus-PTV (p = .023). Although at low doses, below 2–5 Gy, the sparing of the brain-minus-PTV was in favour of RA plans, no significant difference in terms of mean doses to the brain-minus-PTV was observed between the two groups (p = .31). Finally, both MU (p < .01) and OTT (p < .01) were significantly reduced by HyperArc plans.

Conclusions

For linac-based SRS/FSRT of multiple brain lesions, HyperArc plans assured a higher CI and a lower GI than standard multiple-isocenter VMAT plans. This is consistent with the computed reduction in V₁₂ to the brain-minus-PTV. Finally, HyperArc treatments were completed within a typical 20 min. time slot, with a significant time reduction with respect to the expected duration of multiple-isocenters VMAT.

Hippocampal dose during Linac-based stereotactic radiotherapy for brain metastases: An observational study

INTRODUCTION

Aim of the present study is to evaluate homolateral and contralateral hippocampus (H-H, C-H, respectively) dose during Fractionated Stereotactic Radiotherapy (FSRT) or Radiosurgery (SRS) for brain metastases (BM).

MATERIALS & METHODS

Patients with BM<5, size≤30mm, KPS≥80 and a life expectancy>3months, were considered for SRS/FSRT (total dose 15-30Gy, 1-5 fractions). For each BM, a Flattening Filter Free (FFF) Volumetric Modulated Arc Therapy (VMAT) plan was generated with one or two arcs. Hippocampi were not considered during optimizations phase and were contoured and evaluated retrospectively in terms of dose: the Dmedian, Dmean, D0.1cc and the V1Gy, V2Gy, V5Gy and V10Gy were analyzed.

RESULTS

From April 2014 to December 2015, 81 BM were treated with FFF-FSRT/SRS. For the H-H, the average values of Dmedian, Dmean and D0.1cc were 1.5Gy, 1.54Gy and 2.2Gy, respectively, while the V1Gy, V2Gy, V5Gy and V10Gy values were 25%, 8.9%, 8.9% and 2.1%, respectively. For the C-H, the average Dmedian, Dmean and D0.1cc were 0.7Gy, 0.7Gy, 0.9Gy, respectively, while the average values of V1Gy, V2Gy, V5Gy and V10Gy were 18%, 10.2%, 2.8% and 1.4%, respectively. Tumor dimension, tumor cranial-caudal length and the distance between BM and H-H were correlated to Dmedian, Dmean and D0.1cc. For C-H, only the distance from PTV was correlated with a dose reduction.

CONCLUSION

During FFF-FSRT/SRS, hippocampus received a negligible dose. Despite its clinical significance is still under evaluation, in patients with a long life expectancy, H-H should be considered during Linac-based FSRT/SRS.