Long-term risk of radionecrosis and imaging changes after stereotactic radiosurgery for brain metastases

Radiotoxicity in robotic radiosurgery: proposing a new quality index for optimizing the treatment planning of brain metastases

BACKGROUND

As irradiated brain volume at 12 Gy (V12) is a predictor for radionecrosis, the purpose of the study was to develop a model for Cyberknife (CK) plans that is able to predict the lowest achievable V12 at a given tumor size and prescription dose (PD), and to suggest a new quality index regarding V12 for optimizing the treatment planning of brain metastases.

METHOD

In our model V12 was approximated as a spherical shell around the tumor volume. The radial distance between tumor surface and the 12 Gy isodose line was calculated using an approximation of the mean dose gradient in that area. Assuming a radially symmetrical irradiation from the upper half space, the dose distribution is given by the superposition of single fields. The dose profiles of a single field were derived by the measured off-center ratios (OCR) of the CK system. Using the calculated gradients of the sum dose profiles, minimal-V12 was estimated for different tumor sizes. The model calculation was tested using a phantom dataset and retrospectively applied on clinical cases.

RESULTS

Our model allows the prediction of a best-case scenario for V12 at a given tumor size and PD which was confirmed by the results of the isocentric phantom plans. The results of the non-isocentric phantom plans showed that an optimization of coverage caused an increase in V12. This was in accordance with the results of the retrospective analysis. V12 s of the clinical cases were on average twice that of the predicted model calculation. A good agreement was achieved for plans with an optimal conformity index (nCI). Re-planning of cases with high V12 showed that lower values could be reached by selecting smaller collimators and by allowing a larger number of total MU and more MU per beam.

CONCLUSIONS

V12 is a main parameter for assessing plan quality in terms of radiotoxicity. The index f12 defined as the ratio of V12 from the actual plan with the evaluated V12 from our model describes the conformity of an optimally possible V12 and thus can be used as a new quality index for optimizing treatment plans.

Differentiating radiation necrosis from tumor progression in brain metastases treated with stereotactic radiotherapy: utility of intravoxel incoherent motion perfusion MRI and correlation with histopathology

Radiation necrosis is a serious potential adverse event of stereotactic radiosurgery that cannot be reliably differentiated from recurrent tumor using conventional imaging techniques. Intravoxel incoherent motion (IVIM) is a magnetic resonance imaging (MRI) based method that uses a diffusion-weighted sequence to estimate quantitative perfusion and diffusion parameters. This study evaluated the IVIM-derived apparent diffusion coefficient (ADC) and perfusion fraction (f), and compared the results to the gold standard histopathological-defined outcomes of radiation necrosis or recurrent tumor. Nine patients with ten lesions were included in this study; all lesions exhibited radiographic progression after stereotactic radiosurgery for brain metastases that subsequently underwent surgical resection due to uncertainty regarding the presence of radiation necrosis versus recurrent tumor. Pre-surgical IVIM was performed to obtain f and ADC values and the results were compared to histopathology. Five lesions exhibited pathological radiation necrosis and five had predominantly recurrent tumor. The IVIM perfusion fraction reliably differentiated tumor recurrence from radiation necrosis (f_mean = 10.1 ± 0.7 vs. 8.3 ± 1.2, p = 0.02; cutoff value of 9.0 yielding a sensitivity/specificity of 100%/80%) while the ADC did not distinguish between the two (ADC_mean = 1.1 ± 0.2 vs. 1.2 ± 0.4, p = 0.6). IVIM shows promise in differentiating recurrent tumor from radiation necrosis for brain metastases treated with radiosurgery, but needs to be validated in a larger cohort.

The radiosurgery fractionation quandary: single fraction or hypofractionation?

Stereotactic radiosurgery (SRS), typically administered in a single session, is widely employed to safely, efficiently, and effectively treat small intracranial lesions. However, for large lesions or those in close proximity to critical structures, it can be difficult to obtain an acceptable balance of tumor control while avoiding damage to normal tissue when single-fraction SRS is utilized. Treating a lesion in 2 to 5 fractions of SRS (termed "hypofractionated SRS" [HF-SRS]) potentially provides the ability to treat a lesion with a total dose of radiation that provides both adequate tumor control and acceptable toxicity. Indeed, studies of HF-SRS in large brain metastases, vestibular schwannomas, meningiomas, and gliomas suggest that a superior balance of tumor control and toxicity is observed compared with single-fraction SRS. Nonetheless, a great deal of effort remains to understand radiobiologic mechanisms for HF-SRS driving the dose-volume response relationship for tumors and normal tissues and to utilize this fundamental knowledge and the results of clinic studies to optimize HF-SRS. In particular, the application of HF-SRS in the setting of immunomodulatory cancer therapies offers special challenges and opportunities.

Radiation-induced brain injury: low-hanging fruit for neuroregeneration

Brain radiation is a fundamental tool in neurooncology to improve local tumor control, but it leads to profound and progressive impairments in cognitive function. Increased attention to quality of life in neurooncology has accelerated efforts to understand and ameliorate radiation-induced cognitive sequelae. Such progress has coincided with a new understanding of the role of CNS progenitor cell populations in normal cognition and in their potential utility for the treatment of neurological diseases. The irradiated brain exhibits a host of biochemical and cellular derangements, including loss of endogenous neurogenesis, demyelination, and ablation of endogenous oligodendrocyte progenitor cells. These changes, in combination with a state of chronic neuroinflammation, underlie impairments in memory, attention, executive function, and acquisition of motor and language skills. Animal models of radiation-induced brain injury have demonstrated a robust capacity of both neural stem cells and oligodendrocyte progenitor cells to restore cognitive function after brain irradiation, likely through a combination of cell replacement and trophic effects. Oligodendrocyte progenitor cells exhibit a remarkable capacity to migrate, integrate, and functionally remyelinate damaged white matter tracts in a variety of preclinical models. The authors here critically address the opportunities and challenges in translating regenerative cell therapies from rodents to humans. Although valiant attempts to translate neuroprotective therapies in recent decades have almost uniformly failed, the authors make the case that harnessing human radiation-induced brain injury as a scientific tool represents a unique opportunity to both successfully translate a neuroregenerative therapy and to acquire tools to facilitate future restorative therapies for human traumatic and degenerative diseases of the central nervous system.

A dosimetric comparison between CyberKnife and tomotherapy treatment plans for single brain metastasis

PURPOSE

Radiosurgery (RS) is a well-established treatment in selected patients with brain metastasis. The aim of this study is to compare the differences between CyberKnife (CK) and TomoTherapy (HT) treatment plans of RS of single brain metastasis (BM) to define when HT should be used in cases beyond Cyberknife-when both systems are readily available for the radiation oncologist.

METHODS AND MATERIALS

Nineteen patients with single brain metastasis treated with CK were re-planned for radiosurgery using TomoTherapy Hi-ART system. Two planning approaches have been used for TomoTherapy plans: the classical one (HT) and the improved conformity (icHT) that produces dose distributions more similar to those of RS plans. PTV coverage, Conformity Index (CI), Paddick Conformity Index (nCI), Homogeneity Index (HI), Gradient Index (GI), and beam on time of CK, HT, and icHT plans were evaluated and compared.

RESULTS

A good coverage was found for CK, HT, and icHT plans. A difference between mean HI of CK and icHT plans was observed (p = 0.007). Better dose gradients compared to both icHT and HT modalities were observed in CK plans. icHT modality showed improved mean CI respect to HT modality, similar to that obtained in CK plans.

CONCLUSIONS

CK plans show higher conformity and lower GI than icHT and HT plans. TomoTherapy demonstrates the advantage of being a device capable to reach different clinical objectives depending on the different planning modality employed. CyberKnife and TomoTherapy are both optimal RS devices, the choice to use one over another has to be clinically guided.

Differentiation between Radiation Necrosis and Tumor Progression Using Chemical Exchange Saturation Transfer

Purpose: Stereotactic radiosurgery (SRS) is a common treatment used in patients with brain metastases and is associated with high rates of local control, however, at the risk of radiation necrosis. It is difficult to differentiate radiation necrosis from tumor progression using conventional MRI, making it a major diagnostic dilemma for practitioners. This prospective study investigated whether chemical exchange saturation transfer (CEST) was able to differentiate these two conditions.Experimental Design: Sixteen patients with brain metastases who had been previously treated with SRS were included. Average time between SRS and evaluation was 12.6 months. Lesion type was determined by pathology in 9 patients and the other 7 were clinically followed. CEST imaging was performed on a 3T Philips scanner and the following CEST metrics were measured: amide proton transfer (APT), magnetization transfer (MT), magnetization transfer ratio (MTR), and area under the curve for CEST peaks corresponding to amide and nuclear Overhauser effect (NOE).Results: Five lesions were classified as progressing tumor and 11 were classified as radiation necrosis (using histopathologic confirmation and radiographic follow-up). The best separation was obtained by NOE_MTR (NOE_MTR,necrosis = 8.9 ± 0.9%, NOE_{MTR,progression} = 12.6 ± 1.6%, P < 0.0001) and Amide_MTR (Amide_MTR,necrosis = 8.2 ± 1.0%, Amide_{MTR,progression} = 12.0 ± 1.9%, P < 0.0001). MT (MT_necrosis = 4.7 ± 1.0%, MT_progression = 6.7 ± 1.7%, P = 0.009) and NOE_AUC (NOE_AUC,necrosis = 4.3 ± 2.0% Hz, NOE_{AUC,progression} = 7.2 ± 1.9% Hz, P = 0.019) provided statistically significant separation but with higher P values.Conclusions: CEST was capable of differentiating radiation necrosis from tumor progression in brain metastases. Both NOE_MTR and Amide_MTR provided statistically significant separation of the two cohorts. However, APT was unable to differentiate the two groups. Clin Cancer Res; 23(14); 3667-75. ©2017 AACR.

Understanding the continuum of radionecrosis and vascular disorders in the brain following gamma knife irradiation: An MRI study

PURPOSE

The radiation dose delivered to brain tumors is limited by the possibility to induce vascular damage and necrosis in surrounding healthy tissue. In the present study, we assessed the ability of MRI to monitor the cascade of events occurring in the healthy rat brain after stereotactic radiosurgery, which could be used to optimize the radiation treatment planning.

METHODS

The primary somatosensory forelimb area (S1FL) and the primary motor cortex in the right hemisphere of Fischer rats (n = 6) were irradiated with a single dose of Gamma Knife radiation (Leksell Perfexion, Elekta AG, Stockholm, Sweden). Rats were scanned with a small-animal 7 Tesla MRI scanner before treatment and 16, 21, 54, 82, and 110 days following irradiation. At every imaging session, T₂ -weighted (T₂ w), Gd-DTPA dynamic contrast-enhanced MRI (DCE-MRI), and T2*-weighted ( T2* w) images were acquired to measure changes in fluid content, blood vessel permeability, and structure, respectively. At days 10, 110, and 140, histopathology was performed on brain sections. Locomotion and spatial memory ability were assessed longitudinally by behavioral tests.

RESULTS

No vascular changes were initially observed. After 54 days, a small necrotic volume in the white matter below the S1FL, surrounded by an area presenting significant vascular permeability, was revealed. Between 54 and 110 days, the necrotic volume increased and was accompanied by the formation of a ring-like region, where a mixture of necrosis and permeable blood vessels were observed, as confirmed by histology. Behavioral changes were only observed after day 82.

CONCLUSION

Together, DCE-MRI and T2* w images supported by histology provided a coherent picture of the phenomena involved in the formation of new, leaky blood vessels, which was followed by the detection of radionecrosis in a preclinical model of brain irradiation. Magn Reson Med 78:1420-1431, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

[Imaging methods used in the differential diagnosis between brain tumour relapse and radiation necrosis after stereotactic radiosurgery of brain metastases: Literature review]

After stereotactic radiosurgery for a cerebral metastasis, one of the dreaded toxicities is radionecrosis. In the follow-up of these patients, it is impossible to distinguish radiation necrosis from tumour relapse either clinically or with MRI. In current practice, many imaging methods are designed such as special sequences of MRI (dynamic susceptibility contrast perfusion and susceptibility-weighted imaging, diffusion), proton magnetic resonance spectroscopy, positron emission tomography, or more seldom 201-thallium single-photon emission computerized tomography. This article is a required literature analysis in order to establish a decision tree with the analysis of retrospective and prospective data.

Possible Interaction of Anti-PD-1 Therapy with the Effects of Radiosurgery on Brain Metastases

Delayed radiation-induced vasculitic leukoencephalopathy related to stereotactic radiosurgery (SRS) of brain metastases has been reported to manifest clinically 9 to 18 months after treatment. Immune-modulating therapies have been introduced to treatment regimens for malignancies with metastatic predilection to the brain. The interaction of these systemic therapies with other modalities of treatment for brain metastases, namely, SRS, has not been fully characterized. We report two patients with metastatic malignancies to the brain who received SRS followed by immunotherapy with monoclonal antibodies (mAb) to programmed death 1 (PD-1). Both patients appeared to have early clinical and radiologic progression of their treated lesions, which was highly suspicious for tumor progression. Both patients underwent surgical resection of their lesions and the material was submitted for histopathologic examination. Pathologic examination in both cases showed predominantly radiation-induced changes characterized by reactive astrocytosis and vascular wall infiltration by T lymphocytes. The accelerated response to SRS in these two patients was temporally related to the initiation of immunotherapy. We propose a possible biologic interaction between SRS and the PD-1 mAbs. Additionally, awareness of this potential occurrence is critical for accurate interpretation and proper management of clinical and radiologic findings in these patients. Cancer Immunol Res; 4(6); 481-7. ©2016 AACR.

[Surgery for metastases, anatomical and ethical limits. Special aspect: oligometastases]

The surgical resection of metastases is nowadays feasible in selected patients with multifocal metastatic disease due to the implementation of interdisciplinary multimodal therapeutic options. Anatomical limitations do not seem to represent obstacles which cannot be overcome because of the development of new surgical techniques. The cornerstone of the selection of patients is the correct staging diagnosis achieved through modern diagnostic tools; however, surgery alone does not always offer acceptable survival and recurrence-free rates. Furthermore, in every complex surgical procedure there is the risk of morbidity and mortality; therefore, parameters such as alternative therapeutic modalities, the individual situation of the patient and tumor biology have to be considered in order to make the correct selection of patients. This is one of the major future challenges and should never be driven by unfounded hopes and expectations of the patients. The same principle also applies for brain metastases, which represent the most common brain tumors. Approximately 70 % of patients with brain metastases have 1-3 lesions (oligometastases). Treatment is now individualized and the goal of therapy has shifted towards long-term survival (≥ 24 months) and improved quality of life. Under this aspect surgery is one of the important treatment options, particularly in patients with a single metastasis or oligometastases. Furthermore, approximately 20 % of patients who have recurrent brain metastases, successfully undergo a complete resection of tumors and with a Karnofsky performance status (KPS) score > 70 show a long-term survival of ≥ 24 months.