Intraoperative magnetic resonance imaging for pituitary neuroendocrine tumor surgery

Surgery for pituitary neuroendocrine tumors attempts the optimally possible amount of tumor resection, which ideally is total resection. However, there are limitations in the resectability and in the intraoperative assessment of the radicality of an adenomectomy. Postoperative imaging is usually performed with a few months delay after tumor resection. Intraoperative magnetic resonance imaging (MRI) is used to depict the extent of tumor removal already achieved in the operating theater during surgery. To date, there are different low- and high-field intraoperative MRI systems available. Decompression of optic pathways, preservation of the pituitary and residual tumor can be largely predicted from the intraoperative images. Several studies convincingly show that intraoperative depiction of residual tumor allows targeted attack of the remnant. Not only is the amount of tumor resected increased, but also the percentage of total tumor excisions. Intraoperative MRI provides an immediate feedback to the surgeon and is thus a valuable quality control for pituitary surgery. It also allows the acquisition of data sets for precise intraoperative navigation. However, the MRI scanners are heavy and expensive and some systems even require extensive modification of the operating theater. Imaging slightly prolongs the operation but is not associated with an increased complication rate. There are also potential artifacts which must be considered ¹⁾.

A meta-analysis demonstrates that around one fifth of patients undergoing pituitary neuroendocrine tumor resection convert from non-GTR to GTR after the use of Intraoperative magnetic resonance imaging (ioMRI). EOR and residual tumor volume (RV) can also be improved to a certain extent using ioMRI. Endoscopic versus microscopic technique or field strength does not appear to alter the impact of ioMRI. Statistical heterogeneity was high, indicating that the improvement in surgical results due to ioMRI varies considerably by center ²⁾.

However, image interpretation is not always easy and can be hindered by the presence of blood, tumor remains or the displacement of surrounding structures.

Jiménez et al present a novel technique based on using intrasellar ballons to reduce these difficulties and facilitate the surgeon's intraoperative assessment by iMRI.

Of 212 tumors treated, 131 (62%) underwent further resection based on iMRI findings, resulting in a significant increase in gross-total resection on postoperative MRI compared with iMRI (p = 0.0001) in both ETS and MTS groups. iMRI increased rates of gross-total resection for cavernous sinus invasion Knosp grades 1 and 2, but not in Knosp ≥ 3 across treatment groups (p < 0.0001). The extent of resection on postoperative MRI was significantly correlated with increased progression-free survival (p < 0.0001). Initial hormone remission off medical therapy was achieved in 64%, with a significantly higher rate of remission in tumors resected via the ETS approach (81%) compared with the MTS approach (55%) (p = 0.02). The rate of persistent new hormone deficit was low at 8%, including a 2.8% rate of permanent diabetes insipidus, and 45% of patients had improvement in preoperative hormone deficit following surgery. Serious postoperative complications including Cerebrospinal fluid fistulas requiring reoperation were rare at 1%, with no postoperative infections.

Conclusions: These results suggest that iMRI is a safe and effective method of increasing the extent of resection for pituitary neuroendocrine tumors while preserving hormone function. When paired with the endoscope, iMRI may offer the ability to tailor more aggressive removal of tumors while optimizing pituitary function, resulting in high rates of secretory hormone remission. Secretory tumors and adenomas with Knosp grade < 3 cavernous sinus invasion may benefit most from the use of iMRI ³⁾

In 300 consecutive patients, three sequential groups (groups A, B, C; n=100 each) were compared with respect to time management, complications and technical difficulties to assess improvement in these parameters with experience.

Raheja et al observed a reduction in the number of technical difficulties (p<0.001), time to induction (p<0.001) and total anesthesia time (p=0.007) in sequential groups. IOMRI was performed for neuronavigation guidance (n=252) and intraoperative validation of extent of resection (EOR; n=67). Performing IOMRI increased the EOR over and beyond the primary surgical attempt in 20.5% (29/141) and 18% (11/61) of patients undergoing glioma and pituitary surgery, respectively. Overall, EOR improved in 59.7% of patients undergoing IOMRI (40/67). Intraoperative tractography and real-time navigation using re-uploaded IOMRI images (accounting for brain shift) helps in intraoperative planning to reduce complications. IOMRI is an asset to neurosurgeons, helping to augment the EOR, especially in glioma and pituitary surgery, with no significant increase in morbidity to the patient ⁴⁾.