Analyses of distribution and dosimetry of brain metastases in small cell lung cancer with relation to the neural stem cell regions: feasibility of sparing the hippocampus in prophylactic cranial irradiation

Incidence of hippocampal and perihippocampal brain metastases and impact on hippocampal-avoiding radiotherapy: A systematic review and meta-analysis

BACKGROUND AND PURPOSE

In patients requiring prophylactic cranial irradiation (PCI) or whole-brain radiotherapy (WBRT) for brain metastases (BMs), hippocampal avoidance (HA) has been shown to preserve neurocognitive function and quality of life. Here, we aim to estimate the incidence of hippocampal and perihippocampal BMs and the subsequent risk of local undertreatment in patients undergoing hippocampal sparing radiotherapy.

MATERIALS AND METHODS

MEDLINE, Embase, and Scopus were searched with the terms "Hippocampus", "Brain Neoplasms", and related terms. Trials reporting on the incidence of hippocampal and/or perihippocampal BMs or hippocampal failure rate after PCI or WBRT were included.

RESULTS

Forty records were included, encompassing a total of 5,374 patients with over 32,570 BMs. Most trials employed a 5 mm margin to define the HA zone. In trials reporting on BM incidence, 4.4 % (range 0 - 27 %) and 9.2 % (3 - 41 %) of patients had hippocampal and perihippocampal BMs, respectively. The most common risk factor for hippocampal BMs was the total number of BMs. The reported failure rate within the HA zone after HA-PCI or HA-WBRT was 4.5 % (0 - 13 %), salvageable with radiosurgery in most cases. SCLC histology was not associated with a higher risk of hippocampal failure (OR = 2.49; p = 0.23). In trials comparing with a conventional (non-HA) PCI or WBRT group, HA did not increase the hippocampal failure rate (OR = 1.90; p = 0.17).

CONCLUSION

The overall incidence of hippocampal and perihippocampal BMs is considerably low, with a subsequent low risk of local undertreatment following HA-PCI or HA-WBRT. In patients without involvement, the hippocampus should be spared to preserve neurocognitive function and quality of life.

Sparing the Hippocampus in Prophylactic Cranial Irradiation Using Three Different Linear Accelerators: A Comparative Study

Hippocampus protection, as an organ at risk in brain radiotherapy, might protect patients' quality of life. Prophylactic cranial irradiation (PCI) has been used traditionally in small cell lung cancer (SCLC) patients as it increases survival. This study aimed to discover the contributing parameters for a successful PCI with simultaneous protection of the hippocampus by using three different treatment machines. For this purpose, treatment plans were generated for 45 SCLC patients using three half-arcs in three linear accelerators (LINACs; Elekta Infinity, Synergy, and Axesse; Elekta Ltd, Stockholm, Sweden) with different radiation field sizes and multileaf collimator (MLC) leaf thickness characteristics. The prescribed dose was 25 Gy in 10 fractions. Thresholds for the hippocampus were calculated based on the Radiation Therapy Oncology Group 0933 dose constraints. The planning and treatment system templates were common to all three LINACs. Plan evaluation was based on the dosimetric target coverage by the 95% isodose, the maximum dose of the plan, the conformity index (CI), the degree of plan modulation (MOD), and the patient-specific quality assurance (QA) pass rate. The mean target coverage was highest for Infinity (97.3%), followed by Axesse (96.6%) and Synergy (95.5%). The mean maximum dose was higher for Synergy (27.5 Gy), followed by Infinity (27.0 Gy) and Axesse (26.9 Gy). Axesse plans had the highest CI (0.93), followed by Infinity (0.91) and Synergy (0.88). Plan MOD was lower for Synergy (2.88) compared with Infinity (3.07) and Axesse (3.69). Finally, patient-specific QA was successful in all Infinity plans, in all but one Synergy plan, and in 17/45 Axesse plans, as was expected from the field size in that treatment unit. Based on overall performance, the most favorable combination of target coverage, hippocampus sparing, and plan deliverability was obtained with the LINAC, which has the largest field opening and thinnest MLC leaves.

Prophylactic cranial irradiation in small cell lung cancer: an update

PURPOSE OF REVIEW

The current review presents recent updates in the seminal literature of research on prophylactic cranial irradiation (PCI) in small cell lung cancer (SCLC).

RECENT FINDINGS

Brain MRI restaging before the administration of PCI reveals a substantial proportion of brain metastasis in baseline brain metastasis free extensive-stage SCLC (ES-SCLC) and limited-stage SCLC (LS-SCLC). Posthoc analyses from the CASPIAN and IMpower133 trials revealed decreases in brain metastasis rates in ES-SCLC treated with chemoimmunotherapy relative to the brain metastasis rates in ES-SCLC treated with chemotherapy alone. A recent meta-analysis of literature published after the landmark 1999 Auperin meta-analysis confirmed the survival benefit of PCI in LS-SCLC patients. A recent study employing PET before and after PCI demonstrated that hippocampal avoidance -PCI (HA-PCI) preserved the metabolic activity of the hippocampi compared with regular PCI. Two phase III trials evaluating neurocognitive functions after HA-PCI versus PCI have yielded conflicting results. Ongoing clinical trials (MAVERICK, PRIMALung, NRG CC003, NCT04535739, NCT04829708 and NCT03514849) regarding PCI versus MRI surveillance and HA-PCI versus PCI were also discussed.

SUMMARY

Currently, the indications for PCI in SCLC are under question in the modern MRI era. Result from prospective phase III, MRI staged and MRI monitored RCTs are expected to elucidate the role of PCI in LS-SCLC and ES-SCLC. Preliminary results indicated that adding immunotherapy to chemotherapy may reduce brain metastasis rate in SCLC. Further data to this aspect are warranted to determine the role of PCI in the immuno-chemotherapy era. The future direction for PCI should be the comprehensive integration of personalized patient selection, HA-PCI utilization and potential employment of other neurocognitive preservation strategies.

The Risk of Hippocampal Metastasis and the Associated High-Risk Factors in 411 Patients With Brain Metastases

Background and Aims

To retrospectively analyze the incidence of hippocampal metastasis and the associated high-risk factors in patients with brain metastases and evaluate the safety of hippocampal avoidance whole-brain radiation therapy (HA-WBRT).

Methods

We retrospectively analyzed the data of patients with brain metastases diagnosed by contrast-enhanced cranial Magnetic resonance imaging (MRI) at the First Hospital of Lanzhou University from 2017 to 2020. The boundaries of the hippocampus, hippocampus + 5 mm area, hippocampus + 10 mm area, and hippocampus + 20 mm area were delineated, and the distances from the brain metastases to the hippocampus were measured. Univariate and multivariate logistic regressions were adopted to analyze the high-risk factors of hippocampal metastasis.

Results

A total of 3,375 brain metastases in 411 patients were included in the analysis. The metastasis rates in the hippocampus and surrounding areas of the entire group were as follows: 7.3% (30/411) in the hippocampus, 16.5% (68/411) in the hippocampus + 5 mm area, 23.8% (98/411) in the hippocampus + 10 mm area, and 36.5% (150/411) in the hippocampus + 20 mm area. Univariate logistic regression showed that the pathological type, the number of metastases, the maximum diameter of metastases, and the volume of brain metastases were all correlated with hippocampal metastasis. Multivariate logistic regression showed that the pathological type, the number of metastases, and the total volume of metastases were correlated with hippocampal metastasis.

Conclusion

The pathological type, the number of metastases, and the total volume of metastases are the high-risk factors associated with hippocampal metastasis. Small cell lung cancer (SCLC) has a significantly higher rate of hippocampal metastasis than other tumor types. The greater the number and total volume of metastases, the more likely the hippocampal metastasis. For patients with SCLC or a greater number and total volume of brain metastases, the implementation of HA-WBRT may bring a higher risk of tumor recurrence.

Treatment planning of volumetric modulated arc therapy and positioning optimization for hippocampal-avoidance prophylactic cranial irradiation

BACKGROUND

Hippocampal-avoidance prophylactic cranial irradiation (HA-PCI) offers potential neurocognitive benefits but raises technical challenges to treatment planning. This study aims to improve the conventional planning method using volumetric modulated arc therapy (VMAT) technique and investigate a better patient's head positioning to achieve a high quality of HA-PCI treatment plans.

METHODS

The improved planning method set a wide expansion of hippocampus as a special region for dose decline. The whole brain target was divided into two parts according to whether the slice included hippocampus and their optimization objectives were set separately. Four coplanar full arcs with partial field sizes were employed to deliver radiation dose to different parts of the target. The collimator angle for all arcs was 90°. Tilting patient's head was achieved by rotating CT images. The improved planning method and tilted head positioning were verified using datasets from 16 patients previously treated with HA-PCI using helical tomotherapy (HT).

RESULTS

For the improved VMAT plans, the max and mean doses to hippocampus were 7.88 Gy and 6.32 Gy, respectively, significantly lower than those for the conventional VMAT plans (P < 0.001). Meanwhile, the improved planning method significantly improved the plan quality. Compared to the HT plans, the improved VMAT plans result in similar mean dose to hippocampus (P > 0.1) but lower max dose (P < 0.02). Besides, the target coverage was the highest for the improved VMAT plans. The tilted head positioning further reduced the max and mean doses to hippocampus (P < 0.05), significantly decreased the max dose to lens (P < 0.001) and resulted in higher plan quality as compared to nontilted head positioning.

CONCLUSIONS

The improved planning method enables the VMAT plans to meet the clinical requirements of HA-PCI treatment with high plan quality and convenience. The tilted head positioning provides superior dosimetric advantages over the nontilted head positioning, which is recommended for clinical application.

Hippocampal avoidance in prophylactic cranial irradiation for small cell lung cancer: benefits and pitfalls

Small cell lung cancers (SCLC) are a group of cancers that are clinically and pathologically different from other lung cancers. They are associated with high recurrence rates and mortality, and many patients present with metastatic disease. Approximately ten percent of SCLC patients have brain metastases at time of diagnosis, and the cumulative incidence of brain metastases increases to more than fifty percent at two years, even with optimal treatment. Hence, in patients without brain metastases at presentation, prophylactic cranial irradiation (PCI) is an important component of treatment along with systemic chemotherapy and radiotherapy. The goal of PCI is to decrease the incidence of subsequent symptomatic brain metastases in patients who show an initial response to the systemic treatment. Various clinical trials have evaluated the utility of PCI and found substantial benefit. Unfortunately, the long-term toxicity associated with PCI, namely the neuro-cognitive impairment that may develop in patients as a result of the radiation toxicity to the hippocampal areas of the brain, has raised concern both for patients and their treating physicians. Various techniques have been tried to ameliorate the neuro-cognitive impairment associated with PCI, including pharmacological agents and highly conformal hippocampal avoidance radiation. All of these have shown promise, but there is a lack of clarity about the optimal way forward. Hippocampal avoidance PCI appears to be an excellent option and a number of groups are currently evaluating this technique. Although there is clear benefit with this specialized radiation treatment, there are also concerns about the risk of disease recurrence in the undertreated hippocampal areas. This review attempts to compile the available data regarding the benefits and pitfalls associated with hippocampal avoidance PCI in the setting of SCLC.

A low percentage of metastases in deep brain and temporal lobe structures

BACKGROUND

Whole-brain radiotherapy (WBRT) in patients with brain metastases (BM) is associated with neurocognitive decline. Given its crucial role in learning and memory, efforts to mitigate this toxicity have mostly focused on sparing radiation to the hippocampus. We hypothesized that BM are not evenly distributed across the brain and that several additional areas may be avoided in WBRT based on a low risk of developing BM.

METHODS

We contoured 2757 lesions in a large, single-institution database of patients with newly diagnosed BM. BM centroids were mapped onto a standard brain atlas of 55 anatomic subunits and the observed percentage of BM was compared with what would be expected based on that region's volume. A region of interest (ROI) analysis was performed in a validation cohort of patients from 2 independent institutions using equivalence and one-sample hypothesis tests.

RESULTS

The brainstem and bilateral thalami, hippocampi, parahippocampal gyri, amygdala, and temporal poles had a cumulative risk of harboring a BM centroid of 4.83% in the initial cohort. This ROI was tested in 157 patients from the validation cohort and was found to have a 4.1% risk of developing BM, which was statistically equivalent between the 2 groups (P < 1 × 10-6, upper bound).

CONCLUSION

Several critical brain structures are at a low risk of developing BM. A risk-adapted approach to WBRT is worthy of further investigation and may mitigate the toxicities of conventional radiation.

Should Stereotactic Radiosurgery Be Considered for Salvage of Intracranial Recurrence after Prophylactic Cranial Irradiation or Whole Brain Radiotherapy in Small Cell Lung Cancer? A Population-Based Analysis and Literature Review

BACKGROUND

Prophylactic cranial irradiation (PCI) improves survival and prevents intracranial recurrence (IR) in limited stage (LS) and extensive stage (ES) small cell lung cancer (SCLC). However, despite PCI, IR affects 12%-45%, and limited data exist regarding salvage brain reirradiation (ReRT). We performed a population-based review of IR in SCLC.

METHODS

Demographic, treatment, and outcome data of consecutive patients (N = 371) with SCLC assessed at a tertiary cancer centre (01/2013-12/2015) were abstracted, and summary statistics calculated. Kaplan-Meier estimates and univariate and multivariate analysis (MVA) via the Cox proportional hazard model were performed.

RESULTS

Median age was 66.1 years, and 59.8% were Eastern Cooperative Oncology Group (ECOG) performance status 0-2. Median survival was 24 months (95% CI 18.3-29.7 months) for LS (N = 103) and 7 months (95% CI 6.1-7.9 months) for ES (N = 268). 72 of 103 patients with LS and 97 of 214 of those with ES received PCI. 54 of 268 ES presented with brain metastases (BM) of whom 46 of 54 received whole brain RT (WBRT). 18.9% (32/169) recurred post-PCI (13 LS; 19 ES) and 30.4% (14/46) recurred after WBRT. Of those who recurred/progressed after cranial RT, 56.5% (26/46) had <5 BM, 39.1% had no extracranial disease, and 50% were ECOG 0-2. In retrospect, 17 of 46 would have been candidates for salvage stereotactic radiosurgery: 13 post-PCI and 4 post-WBRT.

CONCLUSIONS

This cohort challenges commonly held beliefs that IR is always diffuse, associated with clinical deterioration, and synchronous with systemic failure. Approximately 1 in 3 SCLC patients with IR after PCI or WBRT appear clinically appropriate for salvage stereotactic radiosurgery.