Antiangiogenic therapy with bevacizumab results in high radiologic response rates and prolongs PFS in patients with glioblastoma.1–4,20
However, the mechanisms of bevacizumab action are not completely understood. Increasing interest in radiographic findings in bevacizumab-treated patients has developed in recent years with the particular aim of finding prognostic surrogate markers.
The present study identified T1 hyperintense lesions on the precontrast MRI as a marker of better outcome in patients with glioblastoma treated with bevacizumab. Of the 36 included patients, 22 (61.1%) showed new or markedly increased T1 hyperintense lesions after bevacizumab treatment. Microhemorrhages as a cause of these lesions were unlikely because the signal intensity of these lesions did not change on additional MRIs. Even if hemorrhages can persist for months in individual cases, the T1 hyperintense lesions of intracellular or extracellular methemoglobin should disappear with advancing hemoglobin degradation and hemosiderin accumulation.21
However, 14 (63.6%) of these 22 patients had CTs without evidence of hemorrhage and confirming the existence of tumor calcifications in the region of the T1 hyperintense lesions. Finally, we exclude the possibility that T1 shortening was associated with subacute hemorrhage, but the correlation with calcifications on CT and the persistence of the MRI signal increase over time strongly argue against this thesis. Regarding the colocalization of alterations on CT with those on MRI, Figs. , , and S1
illustrate that the T1 hyperintense lesions are confined to the region of calcifications and vice-versa, notwithstanding subtle differences in angulation between CT and MRI and the time intervals between acquisition of MRIs and CTs. These very probably reflect the same bevacizumab-induced process. Nevertheless, there is some heterogeneity within the lesions, with some areas more prominent on CT and others more prominent on MRI. We hypothesize that this reflects different stages of the calcifying process.
The T1 hyperintense lesions were apparent after a median time of only 55 days, (ie, on the first post-treatment MRI), and then remained stable. Therefore, the development of these lesions is not simply a consequence of prolonged treatment with bevacizumab, but rather an early and time-limited event during the first weeks of treatment. Because the appearance of T1 hyperintense lesions was highly correlated with objective response to bevacizumab, it appears plausible that these lesions are a result of bevacizumab-induced biological processes in the tumor tissue.
Usually, calcifications are easily depicted on normal CTs, but MRI has replaced CT in the follow-up of patients with primary brain tumors. In contrast, the detection of calcifications on standard MRI sequences is more difficult and the signal alterations are heterogeneous. Hyperintense lesions on nonenhanced T1-weighted MRIs in the brain (areas of T1 shortening) may be related to the presence of intra- or extracellular methemoglobin,22–24
as well as to the deposition of minerals, such as calcium and manganese,23,25–27
or proteinaceous material28,32
or to free radical generation.33,34
High signal intensities from calcified areas may be explained by the shortening of the T1 relaxation time of protons next to the surface of the calcium crystals.35
It could be shown that the hyperintense T1 signal corresponds to calcifications depicted on cranial CT, even though the extent of signal increase may be smaller or even may be missed.25,35,36
Standard T2* weighted images are sensitive to the susceptibility effects of calcifications; however, the detection of T2* signal decrease is quite unspecific. Even the advantages of not-so-widespread susceptibility-weighted imaging (SWI) can finally not replace CTs as the gold standard. SWI is a T2* imaging process that includes phase information that makes it possible to differentiate paramagnetic substances (eg, deoxyhemoglobin, hemosiderin, and ferritin) from diamagnetic calcium. Because we used a common standard MRI protocol with “simple” T2* imaging lacking phase information, SWI was not part of the protocol. Congruent with our findings, high T1 signal intensities from calcified areas, depending on the calcium concentration and on the surface of the calcium particles, have been reported.25,27,35–37
Because T1 hyperintense lesions in precontrast MRI turned out to be the most feasible marker for the observed tumor calcifications in our cohort, they were used for additional analyses.
Cerebral calcifications are known to occur in some types of brain tumors. They are most common in oligodendrogliomas, but rare in astrocytic neoplasms, including glioblastoma. Importantly, calcifications in the aforementioned tumors occur spontaneously. In addition, the proportion of patients with tumors displaying a oligodendroglial component in our cohort is low (8.6%) and matches the ones from previous studies, thus excluding the possibility that this might affect the study results. Furthermore, although oligodendroglial tumors are associated with calcifications, it is noteworthy that this association is valid only with regard to the incidence of spontaneously occurring calcifications. We are not aware of work describing therapy-induced calcifications in the literature in oligodendroglial tumors. Thus far, therapy-induced calcifications have only been recognized as rather long-term sequelae of radiotherapy, typically for childhood tumors, such as medulloblastoma. The pathogenetic mechanisms of postirradiation cerebral calcifications are also unclear. However, it has been proposed that these are due to mineralising microangiopathy.38
On the basis of reports on the effects of irradiation on cerebral blood vessels,39
it has also been hypothesized that irradiation-induced vasculopathy results in hypoxia, with hypoxia-induced tissue damage then presenting as calcification.40
This is intriguing, given the potential of bevacizumab to induce both vessel regression and hypoxia in glioblastomas.15,41,42
Furthermore, cerebrovascular/ischemic events and the postanoxic state can be associated with cerebral calcifications.43,44
Another disease showing cerebral calcifications is the MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes).45
Sue et al.45
found that the common basal ganglia calcifications in that syndrome were found to be located in the pericapillary regions of the globus pallidus, with no neuronal involvement.
In addition to the phenomenon of vascular normalization, there have been inconsistent reports whether or when bevacizumab treatment results in augmented tumor hypoxia. Two recently published studies by de Groot et al.41
and our group suggest that bevacizumab treatment can lead to hypoxia.10,11
Bevacizumab possibly induces both early vascular normalization in the whole tumor volume and chronic hypoxia at least focally.
In the current study, we were not able to define the mechanisms underlying the formation of T1 hyperintensities. However, the available CTs and the characteristics and time course of the MRI findings are highly suggestive of a calcificating process. Regarding some of the supposed mechanisms in other forms of brain tissue calcifications (postanoxic calcifications, postirradiation calcifications, and calcifications associated with MELAS syndrome) with a central role for blood vessels and hypoxia, a link between blockade of VEGF and the formation of calcifications due to therapy-induced changes in tumor blood vessels appears possible.38–40
In conclusion, this is the first report on bevacizumab-induced T1 hyperintense lesions, or rather tumor calcifications, in patients with glioblastoma that are related to a better outcome. We hypothesize that there might be a link between bevacizumab-induced changes in tumor blood vessels and/or focal induction of hypoxia and the formation of these tumor calcifications. In addition, this study reveals that hyperintense lesions on precontrast T1-weighted MRIs in this patient population do not necessarily correspond to microhemorrhages, and tumor calcifications should be kept in mind as a cause of these imaging findings. New T1 hyperintense lesions or tumor calcifications detected on CTs in bevacizumab-treated patients may also prove to be valuable as a predictive biomarker in this patient population.