RETROSPECTIVE STUDY OF THE RADIOLOGICAL FINDINGS OF THE MEDIASTINUM AFTER END-OF-TREATMENT FOR A MEDIASTINAL LYMPHOMATOUS MASS IN PEDIATRIC ONCOLOGY
Authors: Tibout Pauline , Ferguson Natasha , St-Laurent Guillaume , Rondeau-Legault Judith , Simonyan David , Larouche Valérie 
Published: May 2018

Abstract

BACKGROUND
Reactive thymic hyperplasia, or rebound thymus, is a well-known phenomenon following chemotherapy. While rebound thymus has been described after treatment for many different malignancy, it has been more often noted after treatment for lymphomas. In children and adolescents in which the primary lymphoma was located in the mediastinum, a mediastinal mass, such as a reactive thymic hyperplasia, can be misdiagnosed as a tumor relapse. It can be difficult for the clinician to differentiate between a tumor relapse and a reactive thymic hyperplasia, which may cause unnecessary additional imaging and invasive diagnostic procedures as well as anxiety for the patient and his family.

OBJECTIVES
The main objective was to measure the incidence of reactive thymic hyperplasia following treatment for a paediatric mediastinal lymphoma. The secondary objectives were to describe the radiologic findings which may help differentiate thymic hyperplasia and tumor relapse and to analyse if the finding of a mediastinal mass changed the clinical management.

DESIGN/METHODS
We conducted a retrospective cohort study. The consent for reviewing medical files was obtained from the institutional ethics board. We obtained data from the archives at the Centre mère-enfant Soleil, CHU de Québec.
The medical and radiologic files of 72 paediatric patients which completed two years of follow-up after the treatment for mediastinal lymphoma were reviewed. The radiologic imaging reports were analysed and the patients were classified depending if they developed a mediastinal mass during follow-up or not. If a mass was developed, its characteristics were described to differentiate between a tumor relapse and a thymic hyperplasia. Statistical analyses were performed using SAS 9.4 Statistical Software (Institute SAS, Cary, NC, USA). Descriptive analysis includes mean ± standard deviation, range and median, interquartile range for continuous variables, and frequency and percentage for categorical. Bivariate tests were used to compare the rebound thymic hyperplasia group with the group without this problem.

RESULTS
The patients were followed for a mean of 27.7 ± 28.0 months (95% CI 21.1–34.2) and a median of 12.6 months. Of seventy-two patients reviewed, thirty-nine (54.2%) developed a mediastinal mass at follow-up. Of them, three had a mediastinal relapse of their tumor. One patient had a lymphoma relapse located elsewhere than the mediastinum and a benign rebound mediastinal mass. Thirty-five out of the 72 patients (48.6%, 95% CI 37.3%-60%) developed a benign mediastinal mass and were diagnosed with having rebound thymic hyperplasia. Of those thirty-five patients, twelve were investigated with additional imaging, and one had a mediastinal biopsy showing true thymic hyperplasia. The other twenty-three were followed-up according to the clinician, with no modification to their follow-up because of the mediastinal mass. These results are shown in Table 2.
The majority of the rebound thymic hyperplasia were a mass of triangular shape, with well-defined margins and homogenous density. It remained unchanged or minimized at follow-up, but it was noted that three patients had an augmentation of the hyperplasia at follow-up, while remaining disease-free. The age <14 years old was a risk factor in our population (Hazard Ratio (HR) 1.95, p=.0491).

CONCLUSION
Reactive thymic hyperplasia is a common phenomenon showed in half of our cohort of patients. Some radiological findings, including triangular shape, well defined margins and mild homogenous enhancement, oriented towards a rebound thymic hyperplasia. Additional imaging study should be limited to patients whose rebound mass or symptoms make the clinician suspect a tumor relapse. A prospective cohort study with standardized care should be conducted to better characterize the rebound thymic hyperplasia and help the clinician approach a mediastinal mass at follow-up.