Management of Suspected Pseudoprogression in Immunotherapy

Introduction to Pseudoprogression Management

The European Society for Medical Oncology recommends continuing checkpoint inhibitor therapy in clinically stable patients with suspected pseudoprogression and obtaining confirmatory imaging 4-8 weeks later to distinguish true progression from pseudoprogression 1, 2

The American Society of Clinical Oncology suggests that clinical stability be established using specific criteria, including no worsening of performance status from baseline, no clinically relevant increases in disease-related symptoms, and no requirement for intensified symptom management 1, 2
Clinically unstable patients should discontinue immunotherapy immediately, as continuing treatment beyond progression is not recommended for these patients 1

The National Comprehensive Cancer Network recommends repeat imaging in 4-8 weeks after initial progression is detected to confirm pseudoprogression or true progression 1, 2
The 4-8 week window ensures patients remain fit for salvage therapies if true progression is confirmed 1

Confirmed progression is established when additional new lesions appear on follow-up imaging, or when new lesion target measurements increase ≥5 mm from initial progression, according to the Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 1, 2

Pseudoprogression occurs in fewer than 10% of patients treated with checkpoint inhibitors, according to the European Society for Medical Oncology 3
In melanoma specifically, atypical progression or pseudoprogression using RECIST criteria has been observed in only 7% of patients treated with anti-PD-1 therapy 4, 5, 6

The European Society for Medical Oncology consensus guidelines state that continuing immunotherapy beyond confirmed progression is NOT generally recommended, with Level III evidence 4, 5, 6
However, FDA analysis of 2,624 patients treated with pembrolizumab or nivolumab showed subsequent responses in 19% of patients treated beyond progression versus 16% who were not 4, 5, 6

The American Society of Clinical Oncology recommends discussing imaging findings and the recommendation to continue treatment with the patient before proceeding, covering the uncertainty of pseudoprogression versus true progression, the risk of continuing ineffective therapy, and the low probability of pseudoprogression 1, 2, 4, 3

The European Society for Medical Oncology explicitly states that treatment past true progression "could reduce subsequent effective therapies if the patient is no longer fit enough to tolerate any further treatment" 3
Waiting longer than 8 weeks for confirmatory imaging in most cases is not recommended, as it balances the need to identify pseudoprogression against the risk of disease progression making patients unfit for salvage therapy 1

Pseudoprogression is an early radiologic increase in tumor size or appearance of new lesions that mimics disease progression but is subsequently followed by tumor stabilization or shrinkage, reflecting a true therapeutic response rather than treatment failure. 7, 8, 9
It occurs when lesions meet RECIST 1.1 criteria for progressive disease (new target lesions, unequivocal progression of non‑target lesions, or ≥20 % increase in sum of target measurements) yet later imaging shows stabilization or regression of the same lesions. 10, 7
Patients typically remain symptomatically stable or even clinically improved despite the radiographic worsening. 8, 9
The underlying mechanism involves infiltration of inflammatory immune cells and immune‑mediated edema that temporarily enlarge lesions, followed by delayed tumor cell death. 11, 12, 13

In non‑small cell lung cancer, pooled analyses show that up to 8 % of patients who continued checkpoint‑inhibitor therapy beyond RECIST‑defined progression later achieved a partial response, indicating pseudoprogression. 7

Stable or improved clinical condition (e.g., unchanged or better performance status) is a key indicator suggesting pseudoprogression. 13
Absence of severe laboratory abnormalities (e.g., no new significant organ dysfunction) supports a pseudoprogression interpretation. 12
Lack of clinically significant additional progression on confirmatory imaging (no new lesions or further measurable growth) favors pseudoprogression over true progression. 11, 12

The iRECIST criteria (developed by the RECIST Working Group) were specifically created to capture pseudoprogression by requiring a confirmatory scan (CT2) before declaring definitive progressive disease. 14, 8, 9