Treated Gliomas

Immediate Postoperative Period: Pearls and Pitfalls

The aims of imaging in the postoperative period include detection of residual tumor and assessment of postoperative complications. One potential imaging pitfall is the presence of hemorrhage. In the setting of subacute hemorrhage shortly after surgery, the presence of intracellular methemoglobin demonstrates T1 shortening which may be mistaken for enhancement on gadolinium-enhanced T1-weighted images. The acquisition of a precontrast T1-weighted sequence is helpful for clarifying this because residual enhancing tumor would show relative T1 shortening between the precontrast and postcontrast images ( Fig. 15.3 ).

A variety of nontumoral processes can cause apparent enhancement, including postsurgical changes. For example, postoperative ischemic changes presenting with diffusion restriction along the margins of the surgical site may have associated enhancement in the subacute period. As such, it is generally suggested that contrast-enhanced magnetic resonance imaging (MRI) should be performed within 48 hours after surgery.

One potential pitfall is patients who have received electrocoagulation at the surgical site. Solid parenchymal enhancement that can be indistinguishable from residual tumor may appear even on intraoperative imaging.

Laser-Induced Thermotherapy

Laser-induced thermotherapy (LITT) allows for both biopsy and laser-guided ablation of lesions. A laser probe is placed into the core of the lesion and induces thermal coagulation, creating a region of coagulative necrosis. The typical early postsurgical change after this procedure is characterized by five concentric areas, including a peripheral rim of abnormal enhancement and perilesional edema ( Fig. 15.4 ).

Local Chemotherapy or Brachytherapy

Local delivery of chemotherapeutic agents and radiation therapy (RT) are also options in the treatment of glial tumors. Examples of these therapeutic options include Gliasite for localized RT ( ¹²⁵ I) into the surgical site and Gliadel wafers for intracavitary chemotherapy. A significant proportion of cases treated with Gliadel wafers can experience transient worsening of imaging findings during the first 4 weeks after placement ( Figs. 15.5 and 15.6 ).

Foreign Body Reactions and Textilomas

Packing or hemostatic material (e.g., Gelfoam, Surgicel) left inside a surgical cavity during the resection of a glial tumor can trigger a foreign body reaction in some patients. The resulting pseudotumoral lesion is often called gossypiboma (when triggered by cotton material), textiloma, and gauzoma, among other terms. Importantly, this reaction can exhibit imaging features that overlap with tumor recurrence, such as contrast enhancement. However, in contrast to high-grade gliomas, follow-up imaging should demonstrate relatively stable mass effect and adjacent T2 prolongation except in the setting of complications (i.e., secondary infections) ( Fig. 15.7 ).

Early Postradiation Changes (Pseudoprogression)

Definition and Epidemiology

Pseudoprogression is the apparent worsening of abnormal enhancement, T2-hyperintense areas, or local mass effect that is often seen in patients with glial tumors after radiation treatment. This phenomenon usually occurs weeks to 3 months after treatment, whereas radiation necrosis often occurs 18 to 24 months to years afterward ( Table 15.1 , Fig. 15.8 ).

TABLE 15.1

Summary of Treatment-Related Changes in High-Grade Gliomas

	Time Course	Imaging Findings	Outcomes
True progression	Anytime	↑ Enhancement ↑ T2-hyperintense signal ↑ Mass effect ↑ Perfusion ↑ Cho:Cr ratio	Progresses
Pseudoprogression (radiation therapy)	≤3 months	↑ Enhancement ↑ T2-hyperintense signal ↑ Mass effect ↓ Perfusion	Resolves Stabilizes
Radiation necrosis	≥6 months	↑ Enhancement ↑ T2-hyperintense signal ↑ Mass effect ↓ Perfusion ↑ Lipid/lactate peak	Progresses Stabilizes Resolves
Pseudoresponse (antiangiogenic therapy, i.e., bevacizumab)	Weeks to months	↓ Enhancement ↓ T2-hyperintense signal ↓ Mass effect ↑ Diffusion restriction ↓ Perfusion
Pseudoprogression (immunotherapy, i.e., checkpoint inhibitors)	≤6 months	↑ Enhancement ↑ T2-hyperintense signal ↑ Mass effect	Resolves Stabilizes

Although pseudoprogression can occur after isolated RT, it has also been described after combined RT and TMZ. There is also an association with methylguanine methyltransferase (MGMT) gene promoter methylation. Although the incidence is thought to be between 20% and 25% among all patients, those with MGMT gene promoter methylation have a significantly higher incidence, at approximately 35% to 40%. Along these lines, MGMT methylation predicts pseudoprogression in approximately 90% of cases.

An important clinical implication is that the presence of radiologic pseudoprogression is associated with improved 1- and 2-year progression-free survival. Conversely, patients with MGMT-unmethylated tumors have relatively higher rates of progressive disease.