Fractionated RT is the standard treatment for all patients with newly-diagnosed grade III and IV gliomas after surgical resection, and may be applied at a later stage for patients with grade II glioma who show signs of progression or recurrence. Total radiation dose is limited by late toxicity, which can manifest as neurocognitive decline. Understanding the temporal evolution of the effects of radiation on normal-appearing brain parenchyma is therefore an important consideration in the management of such patients and the determination of endpoints that describe outcome.
Because of their longer progression-free survival of 3–7 years (25
) and earlier age at onset of disease, patients with grade II and grade III glioma are more likely to experience neurocognitive deficits from RT than patients with more aggressive tumors such as grade IV glioma (26
). It has been argued that the standard clinical trial endpoints of survival and time to progression may be inadequate for quantifying outcomes for patients with glioma (27
). Both the tumor itself and treatment effects can impact brain function and may compromise the patient's QOL. This is especially critical for patients who have long survival and for whom the late effects of radiation become relevant. It has been reported that glioma survivors maintain a relatively good performance status in the absence of recurrence but that they may experience a progressive decline in neurocognitive and psychological function following RT, with patients treated with more focal brain irradiation having improved outcome (28
). The current neurocognitive frameworks for understanding changes in QOL over time requires further refinement and would benefit from the development of imaging biomarkers of radiation effects in order to gain additional insights into the pathogenesis of treatment-related brain injury. To date, there have been no definitive studies to identify specific brain regions that are most vulnerable to radiotherapy, although some authors have suggested that the hippocampus may be particularly sensitive to RT (29
), and that visual attention and memory are disproportionately impaired in late radiation damage (28
Phase-sensitive and SWI techniques are powerful tools for imaging microvasculature and iron-containing hemorrhagic foci (13
). Microbleeds appear as hypointense foci in T2*-weighted magnitude images due to intravoxel dephasing in and around the microbleed. The dephasing is caused by B0
field distortion from iron containing compounds in venous blood. SWI is a post-processing technique that multiplies a masked, high-pass filtered phase image into the magnitude image to increase contrast by reducing signal intensity in susceptibility-shifted structures such as veins and microbleeds. Although the phase image is more accurate in quantifying the size of a structure if it is larger than the voxel size, when the anatomy of interest is smaller than the voxel size, the “blooming” effect in the magnitude image aids in detection. Using SWI for the current application is advantageous in that it utilizes information from both the magnitude and phase images, allowing the detection of large and small microbleeds without the need to vary the voxel size. At 7T, heightened susceptibility-effects provide enhanced sensitivity to these lesions (20
), which we have shown is useful for examining the onset of subtle changes in normal brain tissue after exposure to radiotherapy.
Paramagnetic and diamagnetic field effects, which distinguish iron-laden blood products from calcification, can be differentiated using phase imaging but not with SWI. This is because both paramagnetic and diamagnetic field shifts cause signal dropout and therefore hypointense signal on SWI images, while diamagnetic calcifications appear hyperintense on phase images. Distinguishing calcium from blood products is clinically relevant in cases where apparent blood products are actual pathological calcification or where apparent benign physiological calcification is actually pathological hemorrhage, but there is no biological evidence that would suggest the presence of calcification due to treatment effects. In this study, we used phase images to confirm that our lesions were in fact iron-containing microbleeds and not calcification. Only one of the 812 total identified lesions was determined to be a calcifying lesion and was therefore excluded from the analysis. This lesion was also much larger than all the other true microbleeds.
The preliminary results from high-resolution 7T SWI images presented in this manuscript suggest that radiation induced-damage to microvasculature results in the formation of microbleeds. These microbleeds began appearing approximately 2 years after the completion of radiotherapy in all patients scanned around this time, and only 2 patients had a single microbleed before 2 years. This time course of microbleed evolution is similar to histologic vascular changes observed in a group of healthy rodent brains irradiated with a single dose of 30–50Gy using a converging photon beam technique (37
), and other reported delayed radiation vascular injuries (38
). The vast majority (87%) of the 17 total microbleeds observed in patients that received RT within 2.5 years were located within the T2-lesion. This is most likely due to the fact that the standard definition of the treatment planning target volume contains the T2-hyperintense region plus a 2cm margin and doses are specified such that at least 95% of the PTV receives the prescribed dose. Thus, it is not surprising that this high dose region is where microbleeds initially appear. After 3 years post-RT, microbleeds begin to emerge beyond the T2-hyperintensity, most likely where the tissue was treated with a lower dose of radiation and would thus require a longer period of time for the vasculature to experience the same damage. It is also possible that normal-appearing brain parenchyma surrounding the current T2-lesion was initially hyperintense at the onset of treatment, and therefore received a higher dose as well.
A larger variation in the number of microbleeds among patients was observed after 7 years post-RT, suggesting that differences in the extent of radiation dose received to healthy brain tissue plays a major role in microbleed formation. The degree to which a treatment plan can conform to the margins of the tumor, and thus spare normal brain tissue, is highly dependent on the anatomical location of the tumor, its shape, and the capabilities of the radiotherapy system. Because this was a retrospective study, patients were treated over the course of a 20-year time period at various institutions, often before some of the recent advances in RT technology such as intensity-modulated RT. Therefore, patients treated more than 7 years ago likely experienced greater variations in the amount of dose administered to healthy tissue than those treated more recently. It is also possible that some anatomical structures are more susceptible to RT than others, as determined to be the case for the hippocampus (39
) and neurogenic centers in the subgranular and subventricular zones (41
Besides the inherent variation among RT treatment plans, the main source of error which contributed to the variability of microbleed counts over time was the differences in scan coverage due to the retrospective nature of this study. Images that were acquired earlier on in this study did not have the same full brain coverage later achievable as the study progressed with the development of more robust parallel imaging routines that allowed for increased coverage within the same scan time. This was an issue for 7 patients who had microbleeds that were at the edge of the imaged volume and, therefore, possibly more that extended beyond the covered region. Despite this limitation, the volumetric nature of SWI ensured that differences in microhemorrhage counts at different time points in the same patient were not simply due to partial volume averaging. Moreover, imaging in all cases was targeted to the tumor and surrounding T2 signal abnormality, which included the largest number of microhermorrhages.
A less likely contributor to the variation in microbleed counts is human error in lesion identification. The high conspicuity of both veins and microbleeds made it challenging to separate microbleeds from perpendicular veins using automated methods. Although care was taken to avoid these vessels by examining consecutive slices above and below the plane of each microbleed, misidentification of perpendicular veins as microbleeds was possible. The contiguous slice coverage of the 3D scan and minimum-intensity projections limited this source of error. Additionally, all scans were angled in parallel with the anterior commissure-posterior commissure, or AC-PC, line of the corpus callosum on a sagittal slice of the brain. This way, the percentage of perpendicular vessels, and therefore any errors in categorization, would be relatively consistent among patients.
In conclusion, we have demonstrated that high-field SWI has potential for visualizing the appearance of hemosiderin containing microbleeds associated with long-term effects of radiotherapy on brain tissue. The prevalence of these lesions increases over time since receiving RT. The ability to visualize these lesions in normal brain tissue may be important in further understanding the utility of this treatment in patients with grade II and grade III tumors with longer survival time. Future studies will involve longitudinal follow-up with serial imaging to investigate additional physiological changes due to treatment, and correlate the number and location of microbleeds and other imaging parameters with radiation dose and neurocognitive decline. The knowledge gained will be instrumental in planning future target volumes for irradiation, as well as understanding the temporal effects of radiation on the brain and which brain areas are more susceptible to radiation injury.