•In this cohort, the cumulative incidence of psPD increases from 13% in the first year to 28% at 10 years.•psPD is asymptomatic in 83% of patients.•1p/19q codeletion and biopsy are associated with an increased risk of psPD.•When analyzed as a time-dependent covariate, psPD is not associated with overall survival.•An uniform definition and analysis method for psPD in diffuse glioma is needed for comparisons between photon - and proton beam radiotherapy. The interpretation of new enhancing lesions after radiotherapy for diffuse glioma remains a clinical challenge. We sought to characterize and classify new contrast enhancing lesions in a historical multicenter cohort of patients with IDH mutated grade 2 diffuse glioma treated with photon therapy. We reviewed all follow-up MRI’s of all patients treated with radiotherapy for histologically confirmed, IDH mutated diffuse grade 2 glioma between 1–1-2007 and 31–12-2018 in two tertiary referral centers. Disease progression (PD) was defined in accordance with the RANO criteria for progressive disease in low grade glioma. Pseudoprogression (psPD) was defined as any transient contrast-enhancing lesion between the end of radiotherapy and PD, or any new contrast-enhancing lesion that remained stable over a period of 12 months in patients who did not exhibit PD. A total of 860 MRI’s of 106 patients were reviewed. psPD was identified in 24 patients (23%) on 76 MRI’s. The cumulative incidence of psPD was 13% at 1 year, 22% at 5 years, and 28% at 10 years. The mean of the observed maximal volume of psPD was 2.4 cc. The median Dmin in psPD lesions was 50.1 Gy. The presence of an 1p/19q codeletion was associated with an increased risk of psPD (subhazard ratio 2.34, p = 0.048). psPD was asymptomatic in 83% of patients. The cumulative incidence of psPD in grade 2 diffuse glioma increases over time. Consensus regarding event definition and statistical analysis is needed for comparisons between series investigating psPD.