Hippocampal dose from stereotactic radiosurgery for 4 to 10 brain metastases: Risk factors, feasibility of dose reduction via re-optimization, and patient outcomes

Measuring out-of-field dose to the hippocampus in common radiotherapy indications

BACKGROUND

The high susceptibility of the hippocampus region to radiation injury is likely the causal factor of neurocognitive dysfunctions after exposure to ionizing radiation. Repetitive exposures with even low doses have been shown to impact adult neurogenesis and induce neuroinflammation. We address the question whether the out-of-field doses during radiotherapy of common tumour entities may pose a risk for the neuronal stem cell compartment in the hippocampus.

METHODS

The dose to the hippocampus was determined for a single fraction according to different treatment plans for the selected tumor entities: Point dose measurements were performed in an anthropomorphic Alderson phantom and the out-of-field dose to the hippocampus was measured using thermoluminescence dosimeters.

RESULTS

For carcinomas in the head and neck region the dose exposure to the hippocampal region for a single fraction ranged from to 37.4 to 154.8 mGy. The hippocampal dose was clearly different for naso-, oro- and hypopharynx, with maximal values for nasopharynx carcinoma. In contrast, hippocampal dose levels for breast and prostate cancer ranged between 2.7 and 4.1 mGy, and therefore significantly exceeded the background irradiation level.

CONCLUSION

The mean dose to hippocampus for treatment of carcinomas in the head and neck region is high enough to reduce neurocognitive functions. In addition, care must be taken regarding the out of field doses. The mean dose is mainly related to scattering effects, as is confirmed by the data from breast or prostate treatments, with a very different geometrical set-up but similar dosimetric results.

Survival and outcomes in patients with ≥ 25 cumulative brain metastases treated with stereotactic radiosurgery

OBJECTIVE

In the era in which more patients with greater numbers of brain metastases (BMs) are being treated with stereotactic radiosurgery (SRS) alone, it is critical to understand how patient, tumor, and treatment factors affect functional status and overall survival (OS). The authors examined the survival outcomes and dosimetry to critical structures in patients treated with Gamma Knife radiosurgery (GKRS) for ≥ 25 metastases in a single session or cumulatively over the course of their disease.

METHODS

A retrospective analysis was conducted at a single institution. The institution's prospective Gamma Knife (GK) SRS registry was queried to identify patients treated with GKRS for ≥ 25 cumulative BMs between June 2013 and April 2020. Ninety-five patients were identified, and their data were used for analysis. Treatment plans for dosimetric analysis were available for 89 patients. Patient, tumor, and treatment characteristics were identified, and outcomes and OS were evaluated.

RESULTS

The authors identified 1132 patients with BMs in their institutional registry. Ninety-five patients were treated for ≥ 25 cumulative metastases, resulting in a total of 3596 tumors treated during 373 separate treatment sessions. The median number of SRS sessions per patient was 3 (range 1-12 SRS sessions), with nearly all patients (n = 93, 98%) having > 1 session. On univariate analysis, factors affecting OS in a statistically significant manner included histology, tumor volume, tumor number, diagnosis-specific graded prognostic assessment (DS-GPA), brain metastasis velocity (BMV), and need for subsequent whole-brain radiation therapy (WBRT). The median of the mean WB dose was 4.07 Gy (range 1.39-10.15 Gy). In the top quartile for both the highest cumulative number and highest cumulative volume of treated metastases, the median of the mean WB dose was 6.14 Gy (range 4.02-10.15 Gy). Seventy-nine patients (83%) had all treated tumors controlled at last follow-up, reflecting the high and durable control rate. Corticosteroids for tumor- or treatment-related effects were prescribed in just over one-quarter of the patients. Of the patients with radiographically proven adverse radiation effects (AREs; 15%), 4 were symptomatic. Four patients required subsequent craniotomy for hemorrhage, progression, or AREs.

CONCLUSIONS

In selected patients with a large number of cumulative BMs, multiple courses of SRS are feasible and safe. Together with new systemic therapies, the study results demonstrate that the achieved survival rates compare favorably to those of larger contemporary cohorts, while avoiding WBRT in the majority of patients. Therefore, along with the findings of other series, this study supports SRS as a standard practice in selected patients with larger numbers of BMs.

Organs at risk radiation dose constraints

Dose constraints are essential for performing dosimetry, especially for intensity modulation and for radiotherapy under stereotaxic conditions. We present the update of the recommendations of the French society of oncological radiotherapy for the use of these doses in classical current practice but also for reirradiation.

Predicting the effect of indirect cell kill in the treatment of multiple brain metastases via single-isocenter/multitarget volumetric modulated arc therapy stereotactic radiosurgery

Purpose

Due to spatial uncertainty, patient setup errors are of major concern for radiosurgery of multiple brain metastases (m‐bm) when using single‐isocenter/multitarget (SIMT) volumetric modulated arc therapy (VMAT) techniques. However, recent clinical outcome studies show high rates of tumor local control for SIMT‐VMAT. In addition to direct cell kill (DCK), another possible explanation includes the effects of indirect cell kill (ICK) via devascularization for a single dose of 15 Gy or more and by inducing a radiation immune intratumor response. This study quantifies the role of indirect cell death in dosimetric errors as a function of spatial patient setup uncertainty for stereotactic treatments of multiple lesions.

Material and Methods

Nine complex patients with 61 total tumors (2‐16 tumors/patient) were planned using SIMT‐VMAT with geometry similar to HyperArc with a 10MV‐FFF beam (2400 MU/min). Isocenter was placed at the geometric center of all tumors. Average gross tumor volume (GTV) and planning target volume (PTV) were 1.1 cc (0.02–11.5) and 1.9 cc (0.11–18.8) with an average distance to isocenter of 5.4 cm (2.2–8.9). The prescription was 20 Gy to each PTV. Plans were recalculated with induced clinically observable patient setup errors [±2 mm, ±2^o] in all six directions. Boolean structures were generated to calculate the effect of DCK via 20 Gy isodose volume (IDV) and ICK via 15 Gy IDV minus the 20 Gy IDV. Contributions of each IDV to the PTV coverage were analyzed along with normal brain toxicity due to the patient setup uncertainty. Induced uncertainty and minimum dose covering the entire PTV were analyzed to determine the maximum tolerable patient setup errors to utilize the ICK effect for radiosurgery of m‐bm via SIMT‐VMAT.

Results

Patient setup errors of 1.3 mm /1.3° in all six directions must be maintained to achieve PTV coverage of the 15 Gy IDV for ICK. Setup errors of ±2 mm/2° showed clinically unacceptable loss of PTV coverage of 29.4 ± 14.6% even accounting the ICK effect. However, no clinically significant effect on normal brain dosimetry was observed.

Conclusions

Radiosurgery of m‐bm using SIMT‐VMAT treatments have shown positive clinical outcomes even with small residual patient setup errors. These clinical outcomes, while largely due to DCK, may also potentially be due to the ICK. Potential mechanisms, such as devascularization and/or radiation‐induced intratumor immune enhancement, should be explored to provide a better understanding of the radiobiological response of stereotactic radiosurgery of m‐bm using a SIMT‐VMAT plan.

Hippocampal Avoidance in Multitarget Radiosurgery

Brain metastases are a common complication for patients diagnosed with cancer. As stereotactic radiosurgery (SRS) becomes a more prevalent treatment option for patients with many brain metastases, further research is required to better characterize the ability of SRS to treat large numbers of metastases (≥4) and the impact on normal brain tissue and, ultimately, neurocognition and quality of life (QOL). This study serves first as an evaluation of the feasibility of hippocampal avoidance for SRS patients, specifically receiving single-isocenter multitarget treatments (SIMT) planned with volumetric modulated arc therapy (VMAT). Second, this study analyzes the effects of standard-definition (SD) multileaf collimators (MLCs) (5 mm width) on plan quality and hippocampal avoidance.

The 40 patients enrolled in this Institutional Review Board (IRB)-approved study had between four and 10 brain metastases and were treated with SIMT using VMAT. From the initial 40 patients, eight hippocampi across seven patients had hippocampal doses exceeding the maximum biologically effective dose (BED) constraint given by RTOG 0933. With the addition of upper constraints in the optimization objectives and one arc angle adjustment in one patient plan, four out of seven patient plans were able to meet the maximum hippocampal BED constraint, avoiding five out of eight total hippocampi at risk. High-definition (HD) MLCs allowed for an average decrease of 29% ± 23% (p = 0.007) in the maximum BED delivered to all eight hippocampi at risk.

The ability to meet dose constraints depended on the distance between the hippocampus and the nearest planning target volume (PTV). Meeting the maximum hippocampal BED constraint in re-optimized plans was equally likely with the use of SD-MLCs (five out of eight hippocampi at risk were avoided) but resulted in increased dose to normal tissue volumes (23.67% ± 16.3% increase in V50%[cc] of normal brain tissue, i.e., brain volume subtracted by the total PTV) when compared to the HD-MLC re-optimized plans. Comparing the effects of SD-MLCs on plans not optimized for hippocampal avoidance resulted in increases of 48.2% ± 32.2% (p = 0.0056), 31.5% ± 16.3% (p = 0.024), and 16.7% ± 8.5% (p = 0.022) in V20%[cc], V50%[cc], and V75%[cc], respectively, compared to the use of HD-MLCs. The conformity index changed significantly neither when plans were optimized for hippocampal avoidance nor when SD-MLC leaves were used for treatment. In plans not optimized for hippocampal avoidance, mean hippocampal dose increased with the use of SD-MLCs by 38.0% ± 37.5% (p = 0.01). However, the use of SD-MLCs did not result in an increased number of hippocampi at risk.

Hippocampal sparing in patients receiving radiosurgery for ≥25 brain metastases

PURPOSE/OBJECTIVES

To report our dosimetric analysis of the hippocampi (HC) and the incidence of perihippocampal tumor location in patients with ≥25 brain metastases who received stereotactic radiosurgery (SRS) in single or multiple sessions.

MATERIALS/METHODS

Analysis of our prospective registry identified 89 patients treated with SRS for ≥25 brain metastases. HC avoidance regions (HA-region) were created on treatment planning MRIs by 5 mm expansion of HC. Doses from each session were summed to calculate HC dose. The distribution of metastases relative to the HA-region and the HC was analyzed.

RESULTS

Median number of tumors irradiated per patient was 33 (range 25-116) in a median of 3 (range1-12) sessions. Median bilateral HC D_min (D₁₀₀), D₄₀, D₅₀, D_max, and D_mean (Gy) was 1.88, 3.94, 3.62, 16.6, and 3.97 for all patients, and 1.43, 2.99, 2.88, 5.64, and 3.07 for patients with tumors outside the HA-region. Multivariate linear regression showed that the median HC D₄₀, D₅₀, and D_min were significantly correlated with the tumor number and tumor volume (p < 0.001). Of the total 3059 treated tumors, 83 (2.7%) were located in the HA-region in 57% evaluable patients; 38 tumors (1.2%) abutted or involved the HC itself.

CONCLUSIONS

Hippocampal dose is higher in patients with tumors in the HA-region; however, even for patients with a high burden of intracranial disease and tumors located in the HA-regions, SRS affords hippocampal sparing. This is particularly relevant in light of our finding of eventual perihippocampal metastases in more than half of our patients.

Optimizing Adjuvant Stereotactic Radiotherapy of Motor-Eloquent Brain Metastases: Sparing the nTMS-Defined Motor Cortex and the Hippocampus

Brain metastases can effectively be treated with surgical resection and adjuvant stereotactic radiotherapy (SRT). Navigated transcranial magnetic stimulation (nTMS) has been used to non-invasively map the motor cortex prior to surgery of motor eloquent brain lesions. To date, few studies have reported the integration of such motor maps into radiotherapy planning. The hippocampus has been identified as an additional critical structure of radiation-induced deficits. The aim of this study is to assess the feasibility of selective dose reduction to both the nTMS-based motor cortex and the hippocampi in SRT of motor-eloquent brain metastases. Patients with motor-eloquent brain metastases undergoing surgical resection and adjuvant SRT between 07/2014 and 12/2018 were retrospectively analyzed. The radiotherapy treatment plans were retrieved from the treatment planning system (“original” plan). For each case, two intensity-modulated treatment plans were created: the “motor” plan aimed to reduce the dose to the motor cortex, the “motor & hipp” plan additionally reduce the dose to the hippocampus. The optimized plans were compared with the “original” plan regarding plan quality, planning target volume (PTV) coverage, and sparing of organs at risk (OAR). 69 plans were analyzed, all of which were clinically acceptable with no significant differences for PTV coverage. All OAR were protected according to standard protocols. Sparing of the nTMS motor map was feasible: mean dose 9.66 ± 5.97 Gy (original) to 6.32 ± 3.60 Gy (motor) and 6.49 ± 3.78 Gy (motor & hipp), p<0.001. In the “motor & hipp” plan, dose to the ipsilateral hippocampi could be significantly reduced (max 1.78 ± 1.44 Gy vs 2.49 ± 1.87 Gy in “original”, p = 0.003; mean 1.01 ± 0.92 Gy vs. 1.32 ± 1.07 Gy in “original”, p = 0.007). The study confirms the results from previous studies that inclusion of nTMS motor information into radiotherapy treatment planning is possible with a relatively straightforward workflow and can achieve reduced doses to the nTMS-defined motor area without compromising PTV coverage. Furthermore, we demonstrate the feasibility of selective dose reduction to the hippocampus at the same time. The clinical significance of these optimized plans yet remains to be determined. However, with no apparent disadvantages these optimized plans call for further and broader exploration.

Management of multiple brain metastases via dual-isocenter VMAT stereotactic radiosurgery

Single-isocenter volumetric modulated arc therapy (VMAT) stereotactic radiosurgery (SRS) techniques to treat multiple brain metastases simultaneously can significantly improve treatment delivery efficiency, patient compliance, and clinic workflow. However, due to large number of brain metastases sharing the same MLC pair causing island blocking, there is higher low- and intermediate-dose spillage to the normal brain and higher dose to organs-at-risk (OAR). To minimize this problem and improve plan quality, this study proposes a dual-isocenter planning strategy that groups lesions based on hemisphere location (left vs right sided) in the brain parenchyma, providing less island blocking reducing the MLC travel distance. This technique offers simplified planning while also increasing patient comfort and compliance by allowing for large number of brain metastases to be treated in 2 groups. Seven complex patients with 5 to 16 metastases (64 total) were planned with a single-isocenter VMAT-SRS technique using a 10MV-FFF beam with a prescription of 20 Gy to each lesion. The isocenter was placed at the approximate geometric center of the targets. Each patient was replanned using the dual-isocenter approach, generating 2 plans and placing each isocenter at the approximate geometric center of the combined targets of each side with corresponding non-coplanar partial arcs. Compared to single-isocenter VMAT, dual-isocenter VMAT plans provided similar target coverage and dose conformity with less spread of intermediate dose to normal brain with reduction of dose to OAR. Reduction in total monitor units and beam on time was observed, but due to the second isocenter setup and verification, overall treatment time was increased. Dual-isocenter VMAT-SRS planning for multiple brain metastases is a simplified approach that provides superior treatment options for patient compliance who may not tolerate longer traditional treatment times as with individual isocenters to each target. This planning technique significantly reduces the amount of low- and intermediate-dose spillage, further sparing OAR and normal brain, potentially improving target accuracy though localization of left vs right-sided tumors for each isocenter set up.

Dynamic conformal arcs-based single-isocenter VMAT planning technique for radiosurgery of multiple brain metastases

Multiple small beamlets in the delivery of highly modulated single-isocenter HyperArc VMAT plan can lead to dose delivery errors associated with small-field dosimetry, which can be a major concern for stereotactic radiosurgery for multiple brain lesions. Herein, we describe and compare a clinically valuable dynamic conformal arc (DCA)-based VMAT (DCA-VMAT) approach for stereotactic radiosurgery of multiple brain lesions using flattening filter free beams to minimize this effect. Original single-isocenter HyperArc style VMAT and DCA-VMAT plans were created on 7 patients with 2 to 8 brain lesions (total 35 lesions) for 10 MV- flattening filter free beam. 20 Gy was prescribed to each lesion. For identical planning criteria, DCA-VMAT utilizes user-controlled field aperture shaper before VMAT optimization. Plans were evaluated for conformity and target coverage, low- and intermediate dose spillages to brain volume that received more than 30% (V30%) and 50% (V50%) of prescription dose. Additionally, mean brain dose, V8, V12 and maximal dose to adjacent organs-at-risk (OAR) including hippocampi were reported. Total monitor units, beam modulation factor, treatment delivery efficiency, and accuracy were recorded. Comparing with original VMAT, DCA-VMAT plans provided similar tumor dose, target coverage and conformity, yet tighter radio-surgical dose distribution with lower dose to normal brain V30% (p = 0.009), V50% (p = 0.05) and other OAR including lower dose to hippocampi. Lower total number of monitor units and smaller beam modulation factor reduced beam on time by 2.82 min (p < 0.001), on average (maximum up to 3.8 min). Beam delivery accuracy was improved by 8%, on average (p < 0.001) and maximum up to 13% in some cases for DCA-VMAT plans. This novel DCA-VMAT approach provided excellent plan quality, reduced dose to normal brain, and other OAR while significantly reducing beam-on time for radiosurgery of multiple brain lesions-improving patient compliance and clinic workflow. It also provided less MLC modulation through the targets-potentially minimizing small field dosimetry errors as demonstrated by quality assurance results. Incorporating DCA-based VMAT optimization in HyperArc module for radiosurgery of multiple brain lesions merits future investigation.

Single-Isocenter Volumetric Modulated Arc Therapy (VMAT) Radiosurgery for Multiple Brain Metastases: Potential Loss of Target(s) Coverage Due to Isocenter Misalignment

Purpose A single-isocenter volumetric modulated arc therapy (VMAT) treatment to multiple brain metastatic patients is an efficient stereotactic radiosurgery (SRS) option. However, the current clinical practice of single-isocenter SRS does not account for patient setup uncertainty, which degrades treatment delivery accuracy. This study quantifies the loss of target coverage and potential collateral dose to normal tissue due to clinically observable isocenter misalignment. Methods and materials Nine patients with 61 total tumors (2-16 tumors/patient) who underwent Gamma Knife® SRS were replanned in Eclipse™ using 10 megavoltages (MV) flattening-filter-free (FFF) bream (2400 MU/min), using a single-isocenter VMAT plan, similar to HyperArc™ VMAT plan. Isocenter was placed in the geometric center of the tumors. The prescription was 20 Gy to each tumor. Average gross tumor volume (GTV) and planning target volume (PTV) were 1.1 cc (0.02-11.5 cc) and 1.9 cc (0.11-18.8 cc), respectively, derived from MRI images. The average isocenter to tumor distance was 5.5 cm (1.6-10.1 cm). Six-degrees of freedom (6DoF) random and systematic residual set up errors within [±2 mm, ±2o] were generated using an in-house script in Eclipse based on our pre-treatment daily cone-beam CT imaging shifts and recomputed for the simulated VMAT plan. Relative loss of target coverage as a function of tumor size and distance to isocenter were evaluated as well as collateral dose to organs-at risk (OAR). Results The average beam-on time was less than six minutes. However, loss of target coverage for clinically observable setup errors were, on average, 7.9% (up to 73.1%) for the GTV (p < 0.001) and 21.5% for the PTV (up to 93.7%; p < 0.001). The correlation was found for both random and systematic residual setup errors with tumor sizes; there was a greater loss of target coverage for small tumors. Due to isocenter misalignment, OAR doses fluctuated and potentially receive higher doses than the original plan. Conclusion A single-isocenter VMAT SRS treatment (similar to HyperArc™ VMAT) to multiple brain metastases was fast with < 6 min of beam-on time. However, due to small residual set up errors, single-isocenter VMAT, in its current use, is not an accurate SRS treatment modality for multiple brain metastases. Loss of target coverage was statistically significant, especially for smaller lesions, and may not be clinically acceptable if left uncorrected. Further investigation of correction strategies is underway.

Hippocampal Dosimetry and the Necessity of Hippocampal-Sparing in Gamma Knife Stereotactic Radiosurgery for Extensive Brain Metastases

PURPOSE

To characterize hippocampal dosimetry in Gamma Knife stereotactic radiosurgery (GK-SRS) for extensive brain metastases and evaluate the need for hippocampal-sparing in GK-SRS treatment planning.

METHODS AND MATERIALS

We reviewed 75 GK-SRS plans for the treatment of 4 to 30 brain metastases generated without consideration of the hippocampi. The mean dose, maximum dose to 100% of the volume (D100), maximum dose to 40% of the volume (D40), and maximum point dose (Dmax, 0.03 cm3) were obtained for the unilateral and bilateral hippocampi and compared between plans with 4 to 9 and ≥10 lesions. The rate at which plans met hippocampal dose constraints (D100 ≤ 4.21 Gy, D40 ≤ 4.50 Gy, and Dmax ≤ 6.65 Gy) was compared between groups, and each was examined for risk factors associated with excessive hippocampal dosing. For plans that exceeded constraints, we attempted replanning to spare the hippocampi.

RESULTS

Compared with those for the treatment of 4 to 9 brain metastases, GK-SRS plans with ≥10 lesions were associated with significantly greater median bilateral mean dose (1.0 vs 2.0, P = .001), D100 (0.4 vs 0.8, P = .003), D40 (0.9 vs 1.9, P = .001), and Dmax (2.0 vs 4.9, P = .0005). These plans also less frequently met hippocampal constraints, with this difference trending toward significance (80% vs 93%; P = .1382; odds ratio 0.29; 95% CI, 0.06-1.4). Risk factors for exceeding constraints included greater total disease volume and closer approach of the nearest metastasis to the hippocampi, both of which depended upon the number of metastases present. Seven plans failed to meet constraints and were successfully replanned to spare the hippocampi with minimal increases in treatment time and without compromise to target coverage or conformity.

CONCLUSIONS

Patients with extensive brain metastases treated with GK-SRS are at increased risk for excessive hippocampal dosing when ≥10 lesions are present or when lesions are in close proximity to the hippocampi and may benefit from hippocampal-avoidant treatment planning.

GammaKnife versus VMAT radiosurgery plan quality for many brain metastases

The purpose of this work was to compare dose distributions between two radiosurgery modalities, single‐isocenter volumetric modulated arc therapy (VMAT), and GammaKnife Perfexion (GK), in the treatment of a large number (≥7) of brain metastases. Twelve patients with 103 brain metastases were analyzed. The median number of targets per patient was 8 (range: 7–14). GK plans were compared to noncoplanar VMAT plans using both 6‐MV flattening filter‐free (FFF) and 10‐MV FFF modes. Parameters analyzed included radiation therapy oncology group conformity index (CI), 12, 6, and 3 Gy isodose volumes (V12 Gy, V6 Gy, V3 Gy), mean and maximum hippocampal dose, and maximum skin dose. There were statistically significant differences in CI (2.5 ± 1.6 vs 1.6 ± 0.8 and 1.7 ± 0.9, P < 0.001, P < 0.001), V12 Gy (2.8 ± 6.1 cc vs 3.0 ± 5.2 cc and 3.1 ± 5.4 cc, P = 0.003, P < 0.001), and V3 Gy (323.0 ± 294.8 cc vs, 880.1 ± 369.1 cc and 937.9 ±  vs 361.9 cc, P = 0.005, P = 0.001) between GK versus both 6‐MV FFF and 10‐MV FFF. No significant differences existed for maximum hippocampal or skin doses. In conclusion, highly optimized VMAT produced improved conformity at the expense of a higher V12 Gy and V3 Gy volume when compared with highly optimized GK.