Outcomes of the transformation of follicular lymphoma to diffuse large B‐cell lymphoma in the rituximab era: A population‐based study

Abstract Background Histological transformation (HT) to diffuse large B‐cell lymphoma (DLBCL) is a common complication of follicular lymphoma (FL) and is usually associated with a dismal outcome. However, the survival rate of these patients has improved over the last 20 years with the introduction of rituximab. This study aimed to access the outcome of transformation to DLBCL (t‐DLBCL) from FL in a retrospective series that began after the widespread use of rituximab use. In addition, we also compared survival between t‐DLBCL and primary DLBCL (p‐DLBCL) in the same timeframe. Methods We utilized the Surveillance, Epidemiology, and End Results (SEER) database to identify patients with primary FL and patients with p‐DLBCL between 2000 and 2020. Patients who had a subsequent diagnosis of DLBCL at least 2 months after FL diagnosis were identified as t‐DLBCL. Results Finally, we identified 50,332 FL and 95,933 p‐DLBCL. With a median follow‐up of 119 months, 1631 patients developed t‐DLBCL. The median time from FL diagnosis to t‐DLBCL was approximately 4 years. The post‐transformation survival (PTS) rate at 5 years was 49.6%, with a median PTS of 56 months. Older age, advanced stage, and early transformation were associated with worse PTS. Furthermore, t‐DLBCL receiving chemotherapy or combined modality as initial therapy before HT was also associated with worse PTS, while the result was inverse when taking the impact of initial management strategy at HT into account. Taking t‐DLBCL and p‐DLBCL as a whole, comparable survival was observed between p‐DLBCL and t‐DLBCL receiving radiation or watch‐and‐wait as initial therapy prior to HT. Conclusion The outcome of t‐DLBCL in the rituximab era was better than historical series before the rituximab era. Due to the good prognosis, we did not recommend autologous stem cell transplantation for t‐DLBCL receiving watch‐and‐wait or radiation as initial therapy before HT.


| INTRODUCTION
Follicular lymphoma (FL) is the most common subtype of indolent non-Hodgkin lymphoma.It is characterized by an indolent clinical course accompanied by a continuous pattern of relapses.In recent decades, the survival of FL has improved significantly due to the extensive use of anti-CD20 monoclonal antibody rituximab, approved for the treatment of FL in late 1997, in standard induction and maintenance regimens, [1][2][3][4] Nevertheless, histological transformation (HT), the most common diffuse large B-cell lymphoma (DLBCL), still remains a critical event in the natural history of FL.According to studies conducted after the introduction of rituximab, [5][6][7][8][9] the prognosis of transformed FL (t-FL), previously considered very poor with a median survival of approximately 1 year, [10][11][12][13][14] has improved in the rituximab era.6][7][8][9] Therefore, we conducted this population-based analysis based on Surveillance, Epidemiology, and End Results (SEER) database to further clarify the outcome of transformation to DLBCL from FL (t-DLBCL) in the rituximab era.In addition, we also compared the survival outcome between t-DLBCL and primary DLBCL (p-DLBCL), which has been rarely reported.

| Patients
Data of this study were obtained from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database, which is an authoritative source of population-based cancer statistics in the United States (US) and currently covers approximately ~30% of the US population. 15Our analysis was limited to the SEER 17 registry database, released in April 2023 based on the November 2022 submission.
In the SEER cancer registry, all FL cases were identified using the third edition of the International Classification of Diseases for Oncology (ICD-O-3) code 9690, 9691, 9695, and 9698, and all p-DLBCL cases were identified using the ICD-O-3 code 9680.Patients who had a subsequent diagnosis of DLBCL at least 2 months after FL diagnosis were identified as t-DLBCL, which can avoid ascertainment bias.The exclusion criteria were: (1) FL or p-DLBCL diagnosis confirmed only by autopsy or death certificate; (2) age <18 years; and (3) patients who were not the primary malignancy.Histology was divided into grades 1, 2, 3, and unknown according to the World Health Organization histological classification.The stage was divided into early stage (stage I-II), advanced stage (III/IV) and unknown.We use summary stage for lymphoma to determine the exact stage.Age was divided into four age groups: 18-59, 60-69, 70-79, and ≥80 years.Involvement of extranodal sites was divided into "yes" and "no."Radiation therapy (RT) and chemotherapy (CT) were classified as "yes" and "no/unknown."The SEER database records first-course of therapy data, which is defined as a treatment plan initiated within 12 months of diagnosis.Second-line therapies are not recorded.Therefore, patients receiving RT and CT simultaneously were defined as patients receiving combined modality (CM) as initial treatment strategy, and patients receiving neither RT nor CT were defined as patients receiving watch and wait (WW) as an initial treatment strategy.

| Statistical analysis
Overall survival (OS) for FL and p-DLBCL was defined as the date of diagnosis to the date of death (all causes) or last follow-up.Post-transformation survival (PTS) was defined as the date of HT to the date of death (all causes) or last follow-up.The latency period was defined as the date of FL diagnosis to the date of HT.In exploratory analyses, we assessed early versus late transformation using an a priori defined cutoff of 18 months.Survival curves were reported using Kaplan-Meier estimates, and statistical comparisons between curves were peformed using Logrank test.To evaluate the influence of various covariates on survival, we used Cox proportional hazards regression model, including all significant variables in the univariate analysis (p < 0.05), to calculate hazard ratios (HR) and 95% confidence intervals (CI).
Data were extracted using SEER*Stat (version 8.4.1.2).Statistical analysis was conducted using R 4.2.3 software (R Development Core Team).All p values were calculated as two-sided, and p values less than 0.05 were considered statistically significant.

K E Y W O R D S
diffuse large B-cell lymphoma, follicular lymphoma, population-based, SEER, survival, transformation 3.1 | Patients' characteristics of t-DLBCL Finally, 50,332 patients with FL and 95,933 patients with p-DLBCL were identified during 2000-2020.Of the patients with FL, 48,868 patients survived ≥2 months, with 1631 patients (3.3%) developing t-DLBCL after a median follow-up of 119 months (range, 2 to 251 months).The median time from FL diagnosis to development of t-DLBCL was 48 months (range, 2 to 233 months).Of the patients with t-DLBCL, most were 60-69 years old 520 (31.9%) at HT, male 892 (54.7%), advanced stage 903 (55.4%) at HT, and no involvement of extranodal sites 1314 (82.2%) at HT.For the latency period, most patients developed HT after 18 months 1220 (74.8%).Regarding the initial management strategy before HT, 679 (41.6%) patients receiving WW, 149 (9.1%) patients receiving RT, 727 (44.6%) patients receiving CT, and 76 (4.7%) patients receiving CM.At HT, the proportion of patients receiving WW decreased from 41.6% to 18.9%, while the proportion patients receiving CT increased from 44.6% to 66.2%.The patients' main presenting features are summarized in Table 1.

| Impact of HT on overall survival
Regarding the influence of HT on the outcome of patients with FL, Log-rank test showed that the appearance of HT significantly decreased patient's OS (p < 0.001, Figure 1A).For patients with t-DLBCL, the OS rate at 10-year was 56.6% (95% CI, 53.9%-59.3%)with a median OS of 137 months (range, 2 to 251 months), while the OS rate at 10-year was 64.8% (95% CI, 64.3%-65.3%)with a median OS of 194 months (range, 2 to 251 months) for patients without t-DLBCL.

| Comparison between t-DLBCL and p-DLBCL
When taking t-DLBCL and p-DLBCL as a whole, Log-rank test showed that t-DLBCL had inferior 5-year OS (OS5) in comparison with p-DLBCL (p = 0.002, Figure (1B).The OS5 was 54.3% (95% CI, 54.0%-54.7%)with a median OS of 81 months (range, 0-251 months) for patients with p-DLBCL.However, multivariate analysis, adjusted by age, stage, extronodal sites, and initial management strategy, showed that t-DLBCL (HR = 0.971, p = 0.102) was not an independent risk factor for 10-year OS (0S10) in comparison with p-DLBCL.Considering the PTS differences of t-DLBCL based on the initial management strategy before HT, we performed a subgroup analysis.When taking p-DLBCL as reference, t-DLBCL receiving CT (Log-rank: p < 0.001; multivariate: HR = 1.349, p < 0.001) or CM (Logrank: p = 0.034; multivariate: HR = 1.564, p = 0.003) had inferior OS10, while the OS10 of t-DLBCL receiving RT (Log-rank: p = 0.049; multivariate: HR = 0.987, p = 0.050) or receiving WW (Log-rank: p = 0.870; multivariate: HR = 0.724, p = 0.020) were comparable to p-DLBCL.The results of the univariate and multivariate analysis of OS10 are summarized in   T A B L E 2 Univariate and multivariate analysis of overall survival for DLBCL.

| DISCUSSION
t-DLBCL is a heterogeneous disease and survival outcomes are controversial in the rituximab era, meaning more information is needed to discriminate the subgroup of patients with different prognosis.In this comprehensive retrospective series of patients with t-DLBCL, we explored clinical characteristics affecting PTS and compared OS between t-DLBCL and p-DLBCL.To our knowledge, this is the largest dataset of patients with t-DLBCL published to date.In our study, FL with t-DLBCL had significantly worse OS in comparison with FL without t-DLBCL, which was similar to the results of previous studies. 5,16These results reflected the transformation of FL from indolent to aggressive, which strengthened the necessity of performing a biopsy at the recurrence of FL.There was a significant difference in the prognosis of t-FL, the most common DLBCL, between patients before and after the rituximab era.1][12][13][14] In the Swiss series, in which a minority received rituximab, the median PTS improved to 2.7 years. 16The median PTS of our observation was 56 months, which was comparable to previous reports in the rituximab era with a median PTS nearly 5 years. 5,7,17,18In addition, we also compared the outcome between p-DLBCL and t-DLBCL diagnosed in the same timeframe by taking t-DLBCL and p-DLBCL as a whole.Although p-DLBCL had superior OS in comparison with t-DLBCL, the difference was small (OS5, 54.3% vs. 49.6%)and had not statistically significant in multivariate analysis.Compared with the dismal prognosis of t-FL described before the rituximab era, our study confirmed a good prognosis of t-DLBCL in the rituximab era, which was even comparable to p-DLBCL in the same timeframe.
We further explored the prognostic factors associated with PTS10 of t-DLBCL.Initial treatment strategy before HT was a crucial prognostic factor for PTS10.Patients receiving WW (PTS5, 53.0%; median PTS, 73 months) or RT (PTS5, 66.3%; median PTS, not reached) had a significant PTS10 advantage in comparison with patients receiving CT (PTS5, 43.9%; median PTS, 36 months) or CM (PTS5, 41.7%; median PTS, 27 months).This result is consistent with previous studies, in which treatment-naïve patients before HT had better PTS. 5,7,8,19Furthermore, our study shown that patients receiving RT before HT also had a good prognosis, which has rarely been reported rarely in previous studies.After HT, patients usually received treatment regimens similar to induction regimens for p-DLBCL, with R-CHOP being the most common.For initial treatment strategy at HT, patients receiving CT (PTS5, 51.6%; median PTS, 69 months) or CM (PTS5, 56.6%; median PTS, 84 months) were associated with better PTS10 in comparison with patients receiving WW (PTS5, 40.5%; median PTS, 18 months) or RT (PTS5, 30.7%; median PTS, 19 months), which was inverse to the result of initial treatment strategy before HT.In addition, older age at HT and advanced stage at HT were also associated with worse PTS10.Gender, grade, extranodal sites before HT and at HT, and stage before HT had no influence on PTS10.
One interesting finding of our study was that t-DLBCL with early HT (≥ 18 months) (PTS5, 46.9%; median PTS, 41 months) had a significant PTS10 advantage in comparison with patients with late HT (<18 months) (PTS5, 50.5%; median PTS, 64 months).This phenomenon was also observed in previous two reports. 17,20One plausible rationale for the observed disparities in PTS between early and late HT is that early HT may represent a more aggressive process, possibly enhanced by the aggressive component that is not yet recognized.Early and late HT may represent different biologic processes that require molecular characterization for further understanding.
For the choice of treatment after HT, autologous stem cell transplantation (ASCT) was widespread recommended prior to the rituximab era. 21,22However, the role of ASCT in t-FL in the rituximab era is debated, as some studies demonstrated a PTS advantage for t-FL receiving ASCT, 18,23,24 while others did not. 17,19Considering the heterogeneous prognosis of t-DLBCL, we performed a subgroup analysis according to the reception of initial treatment strategy before HT.Taking t-DLBCL and p-DLBCL as a whole, the OS10 of t-DLBCL receiving RT or WW was comparable to that of p-DLBCL, while t-DLBCL receiving CT or CM had significantly worse OS10 than p-DLBCL.Therefore, we identified a cohort of t-DLBCL who do relatively well in the rituximab era.At the same time, some studies also identified a favorable subgroup of t-FL, mainly treatment-or anthracycline-naïve patients before HT, with excellent outcome without ASCT, 8,17 which is consistent with our study.By combining our and previous studies in the rituximab era, the initial treatment strategy for t-FL should be tailored and individualized.Our analysis suggested that ASCT should be avoided in t-DLBCL receiving RT or WW before HT and should be strongly considered in t-DLBCL receiving CT or CM before HT.
Several limitations exist due the nature of the SEER registry and lack of central review of pathology slides.As no reassurance exists to the quality of pathological review used to label each patient included in the dataset, we aimed to minimize the risk of incorporating patients who were incorrectly diagnosed with t-DLBCL without a true culprit HT event by incorporating a delay of 2 months between the diagnosis of FL and t-DLBCL in the registry.The strength of our study is that SEER registries are population-based, avoiding potential selection bias that could arise from institution-based studies.In addition, the large size of the cohort and its long-running nature are also key strengths of this study.
In conclusion, the outcome of t-DLBCL shown significant improvement in the rituximab era and was even comparable to p-DLBCL in the same timeframe.In multivariate analysis, initial treatment strategies of CT or CM before HT, initial treatment strategies of RT or WW at HT, older age at HT, advanced stage at HT, and early HT were associated with worse PTS10 of t-DLBCL.Given the heterogenous prognosis of t-DLBCL, we did not recommend ASCT for t-DLBCL receiving WW or RT due to their comparable survival to p-DLBCL.These results provided important evidences for the outcome and management strategy of t-DLBCL.

F I G U R E 1
Kaplan-Meier overall survival curves.(A) FL with histological transformation (HT) versus FL without HT, (B) t-DLBCL versus p-DLBCL, (C) p-DLBCL versus t-DLBCL receiving different initial management strategies before HT.

F I G U R E 2
Kaplan-Meier post-transformation survival curves for t-DLBCL according to (A) Age at histological transformation (HT), (B) Stage at HT, (C) Latency period, (D) Initial management strategy before HT, (E) Initial management strategy at HT.

Table 2
, and the OS10 curves are shown in Figure (1C).Univariate and multivariate analysis of post-transformation survival for t-DLBCL.

Clinical features Univariate analysis p Multivariate analysis p
Abbreviations: CI, confidence interval; CM, combined modality; CT, chemotherapy; RT, radiotherapy; WW, watch and wait.