Impact of the Number of Metastatic Tumors Treated by Stereotactic Radiosurgery on the Dose to Normal Brain: Implications for Brain Protection

Optimizing treatment of brain metastases in an era of novel systemic treatments: a single center consecutive series

BACKGROUND

The multidisciplinary management of patients with brain metastases consists of surgical resection, radiation treatment and systemic treatment. Tailoring and timing these treatment modalities is challenging. This study presents real-world data from consecutively treated patients and assesses the impact of all treatment strategies and their relation with survival. The aim is to provide new insights to improve multidisciplinary decisions towards individualized treatment strategies in patients with brain metastases.

METHODS

A retrospective consecutive cohort study was performed. Patients with brain metastases were included between June 2018 and May 2020. Brain metastases of small cell lung carcinoma were excluded. Overall survival was analyzed in multivariable models.

RESULTS

676 patients were included in the study, 596 (88%) received radiotherapy, 41 (6%) awaited the effect of newly started or switched systemic treatment and 39 (6%) received best supportive care. Overall survival in the stereotactic radiotherapy group was 14 months (IQR 5-32) and 32 months (IQR 11-43) in patients who started or switched systemic treatment and initially did not receive radiotherapy. In patients with brain metastases without options for local or systemic treatment best supportive care was provided, these patients had an overall survival of 0 months (IQR 0-1). Options for systemic treatment, Karnofsky Performance Score ≥ 70 and breast cancer were prognostic for a longer overall survival, while progressive extracranial metastases and whole-brain-radiotherapy were prognostic for shorter overall survival.

CONCLUSIONS

Assessing prognosis in light of systemic treatment options is crucial after the diagnosis of brain metastasis for the consideration of radiotherapy versus best supportive care.

How many brain metastases can be treated with stereotactic radiosurgery before the radiation dose delivered to normal brain tissue rivals that associated with standard whole brain radiotherapy?

INTRODUCTION

Clinical trial data comparing outcomes after administration of stereotactic radiosurgery (SRS) or whole-brain radiotherapy (WBRT) to patients with brain metastases (BM) suggest that SRS better preserves cognitive function and quality of life without negatively impacting overall survival. Here, we estimate the maximum number of BM that can be treated using single and multi-session SRS while limiting the dose of radiation delivered to normal brain tissue to that associated with WBRT.

METHODS

Multiple-tumor SRS was simulated using a Monte Carlo - type approach and a pre-calculated dose kernel method. Tumors with diameters ≤36 mm were randomly placed throughout the contoured brain parenchyma until the brain mean dose reached 3 Gy, equivalent to the radiation dose delivered during a single fraction of a standard course of WBRT (a total dose of 30 Gy in 10 daily fractions of 3 Gy). Distribution of tumor sizes, dose coverage, selectivity, normalization, and maximum dose data used in the simulations were based on institutional clinical metastases data.

RESULTS

The mean number of tumors treated, mean volume of healthy brain tissue receiving > 12 Gy (V12) per tumor, and total tumor volume treated using mixed tumor size distributions were 12.7 ± 4.2, 2.2 cc, and 12.9 cc, respectively. Thus, we estimate that treating 12-13 tumors per day over 10 days would deliver the dose of radiation to healthy brain tissue typically associated with a standard course of WBRT.

CONCLUSION

Although in clinical practice, treatment with SRS is often limited to patients with ≤15 BM, our findings suggest that many more lesions could be targeted while still minimizing the negative impacts on quality of life and neurocognition often associated with WBRT. Results from this in silico analysis require clinical validation.

Cerebral metastases

Brain metastases are common and deadly. Over the last 25 years GKNS has been established as an invaluable treatment. It may be used as a primary treatment or after either surgery or WBRT. Patients are assessed using one of a number of available scales. GKNS may be repeated for new metastases and for unresponsive tumors. Prescription doses are usually between 18 and 20Gy. The use of advanced MR techniques to highlight sensitive structures like the hippocampi have extended the efficacy of the treatment. More recently GKNS has been used with different target therapies with improved results. More recently frameless treatments have become more popular in this group of very sick patients. GKNS controls tumors in between 80% and over 95% of cases and may even be used for brainstem tumors.

A multidisciplinary management algorithm for brain metastases

The incidence of brain metastases continues to present a management issue despite the advent of improved systemic control and overall survival. While the management of oligometastatic disease (ie, 1-4 brain metastases) with surgery and radiation has become fairly straightforward in the era of radiosurgery, the management of patients with multiple metastatic brain lesions can be challenging. Here we review the available evidence and provide a multidisciplinary management algorithm for brain metastases that incorporates the latest advances in surgery, radiation therapy, and systemic therapy while taking into account the latest in precision medicine-guided therapies. In particular, we argue that whole-brain radiation therapy can likely be omitted in most patients as up-front therapy.

Radiation therapy for brain metastases

We present the update of the recommendations of the French society for radiation oncology on radiation therapy for the management of brain metastases. It has evolved in recent years and has become more complex. As the life expectancy of patients has increased and retreatments have become more frequent, side effects must be absolutely avoided. Cognitive side effects must in particular be prevented, and the most modern radiation therapy techniques must be used systematically. New prognostic classifications specific to the primary tumour of patients, advances in imaging and radiation therapy technology and new systemic therapeutic strategies, are making treatment more relevant. Stereotactic radiation therapy has supplanted whole-brain radiation therapy both for patients with metastases in place and for those who underwent surgery. Hippocampus protection is possible with intensity-modulated radiation therapy. Its relevance in terms of cognitive functioning should be more clearly demonstrated but the requirement for its use is constantly increasing. New targeted cancer treatment therapies based on the nature of the primitive have complicated the notion of the place and timing of radiation therapy and the discussion during multidisciplinary care meeting to indicate the best sequences is becoming a challenging issue as data on the interaction between treatments remain to be documented. In the end, although aimed at patients in the palliative phase, the management of brain metastases is one of the locations for which technical reflection is the most challenging and treatment become increasingly personalized.

Is Real-Time Inverse Planning Optimizing Dose to the Normal Brain? A Prospective Comparative Trial in a Series of Brain Metastases Treated by Stereotactic Radiosurgery

BACKGROUND

Radiosurgery has demonstrated good safety and efficacy in the treatment of multiple brain metastases (BMs). However, multi-target dose planning can be challenging and time-consuming. A recently developed real-time inverse treatment planning (IP) by convex optimization has been demonstrated to produce high-quality treatment plans with good conformity and selectivity in single-target plans. We intended to test the capacity of this IP to rapidly generate efficient plans while optimizing the preservation of normal tissue in multiple BM.

METHODS

Seventy-nine patients (mean age 62.4, age range 22-85) with a total of 272 BMs were treated by Gamma Knife Radiosurgery. All subjects were treated using a forward planning (FP) technique by an expert neurosurgeon. The new Intuitive Plan was applied and able to automatically generate an alternative plan for each patient. All planning variables were collected from the IP to be compared with the corresponding measurements obtained from the FP. A paired sample t test was applied to compare the 2 plans for the following variables: brain volumes receiving 10 Gy (V10) (primary endpoint), and 12 Gy (V12), planning indices (selectivity, coverage, gradient, and Paddick Conformity Index [PCI]), beam-on time (BOT), and integral doses. Additionally, the noninferiority margin for each item was calculated, and the 2 plans were compared for noninferiority using a paired t test.

RESULTS

The mean age of patients was 62.4 years old (age range 22-85), with a sex ratio of 1.02. The average number of lesions per patient was 3.4 (range 1-12). The mean prescription dose was 21.46 Gy (range 14-24 Gy). Noninferiority of the IP was concluded for V10, V12, prescription isodose volume, BOT, PCI, and selectivity. The V10 (and V12) was significantly lower with the IP (p < 0.001). These volumes were 8.69 cm3 ± 11.39 and 5.47 cm3 ± 7.03, respectively, for the FP and 7.57 cm3 ± 9.44 and 4.78 cm3 ± 5.86 for the IP. Only the coverage was significantly lower with the IP (-2.3%, p < 0.001), but both selectivity (+17%) and PCI (+15%) were significantly higher with the IP than FP (p < 0.001).

CONCLUSION

This IP demonstrated its capacity to generate multi-target plans rapidly, with a dose to the brain (V10) and BOT noninferior to the one of a human expert planner. These results would benefit from confirmation in a larger prospective series.

Treatment of multiple intracranial metastases in radiation oncology: a contemporary review of available technologies

The use of stereotactic radiosurgery to treat multiple intracranial metastases, frequently concurrently, has become increasingly common. The ability to accurately and safely deliver stereotactic radiosurgery treatment to multiple intracranial metastases (MIM) relies heavily on the technology available for targeting, planning, and delivering the dose. A number of platforms are currently marketed for such applications, each with intrinsic capabilities and limitations. These can be broadly categorised as cobalt-based, linac-based, and robotic. This review describes the most common representative technologies for each type along with their advantages and current limitations as they pertain to the treatment of multiple intracranial metastases. Each technology was used to plan five clinical cases selected to represent the clinical breadth of multiple metastases cases. The reviewers discuss the different strengths and limitations attributed to each technology in the case of MIM as well as the impact of disease-specific characteristics (such as total number of intracranial metastases, their size and relative proximity) on plan and treatment quality.

Evaluation of the correlation between dosimetric, geometric, and technical parameters of radiosurgery planning for multiple brain metastases

Purpose

To evaluate the correlation between dosimetric, geometric, and technical parameters for radiosurgery planning of multiple brain metastasis treatments treated with a linear accelerator with volumetric modulated arc therapy (VMAT) technique.

Materials and methods

Data were collected retrospectively from 55 patients who underwent radiosurgery in a single institution from August 2017 to February 2020. Patients presented 4–21 brain metastases were treated with a single fraction with doses between 18 and 20 Gy. Dosimetric variables were collected including V5Gy, V8Gy, V10Gy, V12Gy, V14Gy, conformity index (CI), heterogeneity index (HI), maximum dose (Dmax), and the CI_R50. Geometric variables including the number of lesions, target volumes, the smallest target volume, the largest target volume, and the distance between the isocenter and the most distant lesion (DIL) and technical variables such as the numbers of total arcs, noncoplanar arcs, and isocenters were collected for analysis.

Results

The number of lesions had a moderate positive correlation with V5Gy, V8Gy, V10Gy, V12Gy, V14Gy, HI, Dmax, and with the number of total arcs. The target volumes had a positive medium–high correlation with V5Gy, V8Gy, V10Gy, V12Gy, V14Gy, and moderate positive correlation with HI, Dmax, number of arcs and noncoplanar arcs. The CI and CI_R50 had a negative correlation with all volumes related to the target: the target volumes, the smallest, and the largest lesion. A positive correlation was observed between the distance of the isocenter and the most DIL with V5Gy, V8Gy, V10Gy, V12Gy, V14Gy, HI, Dmax, and the number of isocenters.

Conclusion

It was found that the number of lesions and the target volumes are good predictors of dosimetric indexes of plan evaluation and that the distance between the isocenter and the most DIL harms them.

Gamma Knife radiosurgery: Scenarios and support for re-irradiation

Stereotactic radiosurgery (SRS) involves the focal delivery of large, cytotoxic doses of radiation to small targets within the brain, often located in close proximity to radiosensitive normal tissue structures and requiring very low procedural uncertainties to perform safely. Historically, neurosurgeons considered SRS as a one-time, single session procedure. However therapeutic advances and a better understanding of the clinical response to SRS have caused a renewal of interest in a variety of re-irradiation scenarios; including re-irradiation of the same target after prior SRS, SRS treatments after prior broad-field radiation, hypofractionated treatments, and volume-staged treatments. Re-irradiation may in some cases require even greater effort towards minimizing treatment uncertainties as compared to one-time-only treatments. Gamma Knife radiosurgery (GKRS) has evolved over time in ways that directly supports many re-irradiation scenarios while helping to minimize overall procedural uncertainty.

Advances in the management of brain metastases from cancer of unknown primary

Cancer of unknown primary accounts for 3-5% of all cancers for which an adequate investigation does not identify the primary tumor. The particular subset of brain metastasis in cancer of unknown primary (BMCUP) is a clinical challenge that lacks standardized diagnostic and therapeutic options. It is diagnosed predominantly in male patients in the sixth decade of age with complaints of headache, neurological dysfunction, cognitive and behavioral disturbances and seizures. The therapeutic approach to patients with BMCUP relies on local control and systemic treatment. Surgery or stereotactic radiosurgery and/or whole brain radiation therapy seems to be the cornerstone of the treatment approach to BMCUP. Systemic therapy remains essential as cancers of unknown primary are conceptually metastatic tumors. The benefits of chemotherapy were disappointing whereas those of targeted therapies and immune checkpoint inhibitors remain to be evaluated. In this Review, we address the advances in the diagnosis and treatment of BMCUP.