The surgical management of giant anterior mediastinal tumors in children remains challenging because adequate exposure must be balanced against the risk of long-term chest wall deformity. Conventional clamshell thoracotomy with transverse sternotomy provides excellent visualization but may compromise sternal growth in pediatric patients. Tumors associated with pathogenic variants of the DICER1 gene represent a rare but important diagnostic category. While pleuropulmonary blastoma (PPB) is the most recognized manifestation, DICER1-associated sarcomas can arise at extrapulmonary sites and mimic high-grade PPB morphologically 1, 2. We report a pediatric case of a primary mediastinal DICER1-associated sarcoma successfully resected using a sternum-sparing modified clamshell thoracotomy. A 6-year-old boy presented with right chest pain and persistent cough. Chest radiography revealed a large right-sided intrathoracic mass (Figure 1a), and contrast-enhanced computed tomography demonstrated a well-defined, heterogeneous anterior mediastinal tumor measuring approximately 110 mm in diameter, containing cystic components and punctate calcifications. The mass compressed the right atrium and superior vena cava (SVC), with leftward displacement of the heart and compressive atelectasis of the right lung (Figure 1b). Magnetic resonance imaging showed mixed cystic and solid components with fluid–fluid levels and restricted diffusion in the solid areas. No lymphadenopathy or distant lesions were identified (Figure 1c). Given the tumor size and cardiopulmonary compression, surgical resection was planned. Under general anesthesia, a sternum-sparing modified clamshell thoracotomy was performed. This approach was selected to achieve wide bilateral exposure while preserving the structural integrity of the sternal body. Bilateral anterior thoracic incisions were made with division of multiple costal cartilages (2nd–4th superiorly and 5th–7th inferiorly) while maintaining sternal continuity (Figure 1d,e). This approach provided wide, multidirectional exposure of the mediastinum. The tumor was densely adherent to the lung, SVC, thymus, and pericardium. Combined pericardial resection was required because of strong adhesion, although tumor boundaries remained grossly discernible (Figure 1f). Both phrenic nerves were preserved. The tumor was completely resected with an R0 margin, confirmed both macroscopically and histologically (Video S1). At closure, the divided costal cartilages and perichondrium were meticulously reapproximated using interrupted nonabsorbable sutures to facilitate cartilage regeneration and maintain chest wall stability (Figure 1g). The postoperative course was uneventful, and the patient was discharged on postoperative day 9. At 6-month follow-up, there was no evidence of paradoxical chest wall movement or instability. Grossly, the resected tumor measured 110 × 97 mm and weighed 470 g, showing mixed soft, chondroid, and cystic areas. Histologically, the tumor showed primitive mesenchymal proliferation with chondroid differentiation and focal rhabdomyoblastic features (Figure 1h). Immunohistochemistry supported a diagnosis of high-grade sarcoma. Although the tumor histologically resembled pleuropulmonary blastoma (PPB) type III, no continuity with lung tissue was identified. This finding prompted molecular evaluation, and PCR sequencing detected a pathogenic DICER1 mutation. Although biallelic inactivation was not fully assessed, these findings supported the diagnosis of a DICER1-associated sarcoma. PCR sequencing detected a pathogenic DICER1 point mutation, supporting the diagnosis of a primary mediastinal DICER1-associated sarcoma. The thymus showed no metastatic involvement. The patient received adjuvant chemotherapy according to the high-risk protocol of the International PPB Registry (vincristine, dactinomycin, and cyclophosphamide alternating with ifosfamide, etoposide, and adriamycin), followed by maintenance therapy. Following two cycles of each, the regimen was switched to maintenance ifosfamide and etoposide (IE). At 6-month follow-up, the patient remains in complete remission with no evidence of chest wall instability. DICER1-associated sarcomas represent part of a spectrum of neoplasms sharing morphological, genetic, and epigenetic features with high-grade PPB 2, 3. Although PPB typically arises in the lung, extrapulmonary tumors with similar histology have been reported in the mediastinum, kidney, uterus, and other sites 3. The absence of lung continuity in this case supports classification as a primary mediastinal DICER1-associated sarcoma rather than classic PPB. Surgical exposure is critical for safe resection of giant mediastinal tumors involving major vascular and cardiac structures. While conventional clamshell thoracotomy offers excellent visualization, transverse sternotomy has been associated with potential impairment of sternal growth in pediatric patients. Preservation of the sternal body may help maintain symmetric skeletal development 4. The sternum-sparing clamshell approach used in this case preserved the sternal body while providing wide bilateral access to the mediastinum, facilitating meticulous dissection around the SVC and pericardium with minimal blood loss. Although the extensive cartilage division is reminiscent of the Ravitch procedure, preservation of the sternal body combined with meticulous reapproximation of the costal cartilages and perichondrium helps maintain chest wall stability and supports cartilage regeneration. Identification of a DICER1 mutation has important clinical implications. Solid, high-grade DICER1-associated tumors are associated with aggressive behavior and require careful oncologic management 5. Moreover, individuals with DICER1-associated tumors may develop additional neoplasms, underscoring the importance of genetic counseling and long-term surveillance 6. Further evaluation for germline DICER1 alterations was therefore recommended in this patient. Sternum-sparing clamshell thoracotomy enabled safe and complete resection of a giant mediastinal DICER1-associated sarcoma in a child while preserving chest wall integrity. This approach represents a valuable surgical option for selected pediatric mediastinal tumors. M.Y., H.O., S.I., and M.K. contributed to the assessment and surgical management of the patient. C.N. and H.F. were responsible for the oncological management and chemotherapy. T.I. performed the pathological and molecular analyses. M.Y. drafted the manuscript. M.Y. and M.K. revised the manuscript. All authors approved the final version of the manuscript. The authors thank Drs Yuichi Takama, Marie Todo, Shohei Kishida, and Ken Suzuki. Written informed consent was obtained from the patient's parents for the publication of the patient's data and images. All patient information was anonymized to protect confidentiality. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request. Video S1: Sternum-sparing clamshell thoracotomy for pediatric mediastinal tumor resection. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
Yamoto et al. (Thu,) studied this question.