Markku Miettinen

The 6th edition of the World Health Organization Classification of Soft Tissue and Bone Tumors (STB6) integrates the latest scientific and molecular discoveries with decades of pathological expertise to establish a globally accepted common language for diagnosis and clinical behavior. In this volume, the “essential” and “desirable” diagnostic criteria have been refined, considering different resource settings, ensuring accuracy while maintaining flexibility. A tumor type (entity) is a tumor in which multiple parameters (eg, clinical, histopathological, and/or genetic features) differ from those of other tumor types. In total, 19 new entities met these criteria and were added. A subtype is different in at least one dimension from the main tumor type (clinical, histopathological, or genetic), preferably resulting in a different treatment or outcome. A morphological pattern (no longer “variant”) is important for pathologists to be aware of to facilitate recognition but is not associated with different clinical behavior. The classification was drafted by 203 editors and authors from 27 countries, representing genetics, medical oncology, surgical oncology, orthopedic surgery, and radiology, in addition to pathology. STB6 includes soft tissue tumors (n=116), bone tumors (n=46), vascular tumors of soft tissue and bone (n=18), undifferentiated spindle and round cell sarcomas (n=4), hematopoietic and dendritic cell neoplasms (n=6), and genetic syndromes (n=6). In total, 62 sarcomas are included. A consensus was reached among the expert editorial board. In this review, we highlight changes in tumor classification in STB6, including recently described tumor types, novel diagnostic markers, recent discoveries in molecular genetics, and improvements in prognostication.

Diagnostic classification of soft tissue tumors is based on histology, immunohistochemistry, genetic findings, and radiologic and clinical correlations. Recently, a sarcoma DNA methylation classifier was developed, covering 62 soft tissue and bone tumor entities. The classifier is based on large-scale analysis of methylation sites across the genome. It includes DNA copy number analysis and determines O⁶ methylguanine DNA methyl-transferase methylation status. In this study, we evaluated 619 well-studied soft tissue and bone tumors with the sarcoma classifier. Problem cases and typical examples of different entities were included. The classifier had high sensitivity and specificity for fusion sarcomas: Ewing, synovial, CIC-rearranged, and BCOR-rearranged. It also performed well for leiomyosarcoma, malignant peripheral nerve sheath tumors (MPNST), and malignant vascular tumors. There was low sensitivity for diagnoses of desmoid fibromatosis, neurofibroma, and schwannoma. Low specificity of matches was observed for angiomatoid fibrous histiocytoma, inflammatory myofibroblastic tumor, Langerhans histiocytosis, schwannoma, undifferentiated sarcoma, and well-differentiated/dedifferentiated liposarcoma. Diagnosis of lipomatous tumors was greatly assisted by the detection of MDM2 amplification and RB1 loss in the copy plot. The classifier helped to establish diagnoses for KIT-negative gastrointestinal stromal tumors, MPNSTs with unusual immunophenotypes, and undifferentiated melanomas. O⁶ methylguanine DNA methyl-transferase methylation was infrequent and most common in melanomas (35%), MPNSTs (11%), and undifferentiated sarcomas (11%). The Sarcoma Methylation Classifier will likely evolve with the addition of new entities and refinement of the present methylation classes. The classifier may also help to define new entities and give new insight into the interrelationships of sarcomas.

The study identifies the importance of positron emission tomographic (PET) and anatomic imaging modalities and their individual performances in detecting succinate dehydrogenase A (SDHA)-related metastatic pheochromocytoma and paraganglioma (PPGL). The detection rates of PET modalities—⁶⁸Ga-DOTATATE, ¹⁸F-FDG, and ¹⁸F-FDOPA—along with the combination of computed tomography (CT) and magnetic resonance imaging (MRI) are compared in a cohort of 11 patients with metastatic PPGL in the setting of a germline SDHA mutation. The imaging detection performances were evaluated at three levels: overall lesions, anatomic regions, and a patient-by-patient basis. ⁶⁸Ga-DOTATATE PET demonstrated a lesion-based detection rate of 88.6% [95% confidence interval (CI), 84.3–92.5%], while ¹⁸F-FDG, ¹⁸F-FDOPA, and CT/MRI showed detection rates of 82.9% (CI, 78.0–87.1%), 39.8% (CI, 30.2–50.2%), and 58.2% (CI, 52.0–64.1%), respectively. The study found that ⁶⁸Ga-DOTATATE best detects lesions in a subset of patients with SDHA-related metastatic PPGL. However, ¹⁸F-FDG did detect more lesions in the liver, mediastinum, and abdomen/pelvis anatomic regions, showing the importance of a combined approach using both PET modalities in evaluating SDHA-related PPGL.

Most gastrointestinal stromal tumors (GISTs) occur in the tubular gastrointestinal (GI) tract, but some present apparently outside the GI tract. In this study, we analyzed 112 GISTs located in the retroperitoneum. These tumors occurred in 55 women and 57 men with a median age of 65 years (range: 21 to 89 y). On the basis of clinically or histologically detected connections to GI tract, 15 tumors were considered likely of gastric, 9 duodenal, and 13 of small intestinal origin. The remaining cases were categorized by location as peripancreatic (n=25), pelvic (n=11), mesenteric (n=4), and of unspecified/miscellaneous sites (n=35). The tumors varied in size 3 to 35 cm (median, 15 cm) and by mitotic rate per 5 mm², 0 to >100 (median, 10). Histologically the tumors apparently arising outside the GI tract had features of intestinal (n=41) and gastric GISTs (n=25); 9 cases had indeterminate histology. The histologic variants included spindled, epithelioid, vacuolated, nested, and myxoid potentially simulating other tumors such as liposarcoma and solitary fibrous tumor. Most GISTs were KIT-positive (106/112 cases), and the remaining 6 tumors were DOG1/Ano1-positive. Five cases showed focal nuclear positivity for MDM2. KIT mutations were detected in 42/59 cases, and PDGFRA mutations in 4/16 KIT wild-type and 3/5 of the KIT-negative tumors analyzed. One pelvic retroperitoneal GIST was succinate dehydrogenase deficient. All 79 patients were dead at last follow-up with a median survival of 14 months, with few survivals >5 years. Only operable versus inoperable tumor was a statistically favorable factor in univariate analysis (P50/5 mm² was significant for a shorter survival (hazard ratio, 5.25; 95% confidence interval, 1.65-16.8; P<0.01). Histologic and clinicopathologic similarity of extragastrointestinal retroperitoneal GISTs with GISTs of GI tract suggests their GI tract origin. Potentially overlapping features between GIST and other retroperitoneal tumors necessitate use of multiple diagnostic markers and molecular genetic studies.

Rarity and heterogeneity of liposarcomas (LPS) make their diagnosis difficult even for sarcoma-experts pathologists. The molecular mechanism underlying the development and progression of liposarcomas (LPS) remains only partially known. In order to identify and compare the genomic profiles, we analyzed array-based comparative genomic hybridization (array-CGH) profiles of 66 liposarcomas, including well-differentiated (WDLPS), dedifferentiated (DDLPS) and myxoid (MLPS) subtypes. Copy number aberrations (CNAs) were identified in 98% of WDLPS and DDLPS and in 95% of MLPS cases. The minimal common region of amplification at 12q14.1q21.1 was observed in 96% of WDLPS and DDLPS cases. Four regions of CNAs, including losses of chromosome 6, 11 and 13 and gains of chromosome 14 were classified as recurrent in DDLPS; at least one was identified in 74% of DDLPS tumors. The DDLPS-associated losses were much more common in tumors with increased genomic complexity. In MLPS, the most frequent CNAs were losses of chromosome 6 (40%) and gains of chromosome 1 (30%), with the minimal overlapping regions 6q14.1q22.31 and 1q25.1q32.2, respectively. Our findings show that the application of array-CGH allows to delineate clearly the genomic profiles of WDLPS, DDLPS and MLPS that reflect biological differences between these tumors. Although CNAs varied widely, the subtypes of tumors have characteristic genomic profiles that could facilitate the differential diagnosis of LPS subtypes, especially between WDLPS and DDLPS.

Loss-of-function germline mutations in the fumarase (FH) gene of the Krebs cycle characterize hereditary leiomyomatosis and renal cell cancer syndrome. Fumarase (FH) deficiency can be diagnosed by the loss of immunohistochemical expression. In this study, we investigated the occurrence and clinicopathologic features of FH-deficient uterine smooth muscle tumors (SMTs). A total of 1583 uterine and 157 nonuterine SMTs were examined using a polyclonal FH antibody and automated immunohistochemistry, and 86 uterine leiomyomas with an FH loss were identified. The frequencies of FH deficiency for subcohorts of uterine SMTs were 1.6% for unselected nonatypical leiomyomas, 1.8% for cellular leiomyomas, 37.3% for atypical leiomyomas, and 0% for leiomyosarcomas. One extrauterine, retroperitoneal estrogen receptor–positive leiomyoma was also FH deficient. The patient age of FH-deficient uterine leiomyomas was 20 to 52 years (median, 38 y). Grossly, these tumors were often soft and amorphous resembling a fibrothecoma. Histologically, the FH-deficient nonatypical leiomyomas lacked cellular packeting and distinct collagenous zones and showed chain-like or palisading nuclear arrangements, prominent staghorn-shaped blood vessels, oval nuclei with no or at most mild atypia, small eosinophilic nucleoli, and a low mitotic rate (0 to 1/10 HPF). The FH-deficient atypical leiomyomas had nuclear atypia often manifesting as multinucleation, prominent eosinophilic nucleoli, and mitotic activity up to 7/10 HPF, with atypical mitoses seen in 32% of cases. However, similar histologic changes were seen in some non–FH-deficient atypical leiomyomas. Loss-of-function FH-gene mutations including 5 whole-gene deletions and 3 frameshift mutations were identified in 8 of 16 FH-deficient nonatypical leiomyomas using multiplex ligation-dependent probe amplification and Sanger sequencing, respectively. Follow-up data on patients with FH-deficient atypical uterine leiomyomas revealed 19 patients alive (median follow-up 27 y) and 5 patients dead. Deaths occurred 9 to 30 years after surgery at a median age of 72 years; causes of death could not be determined. These results indicate that FH-deficient uterine leiomyomas occur with a high frequency among atypical leiomyomas and infrequently in nonatypical leiomyomas and are often histologically distinctive. They seem to have a low biological potential and lack any significant association with leiomyosarcoma.

Two cancer testis antigens, the New York esophageal squamous cell carcinoma-1 (NY-ESO-1) and the melanoma-antigen family A (MAGE-A), represent promising immunotherapy targets due to the low expression of these antigens in nonmalignant tissue. To assess overexpression patterns in various cancers, we performed a systematic immunohistochemical analysis for NY-ESO-1 and MAGE-A on tissue array samples of 3668 common epithelial carcinomas (CA) and germ cell tumors of high prevalence and mortality. Here, we find significantly higher expression of MAGE-A (>50% on tumor cells) compared with NY-ESO-1 in several CAs including cutaneous squamous cell carcinomas (SCC) (52.8%/2.8%), esophageal SCC (50%/0%), head and neck SCC (41.1%/<1%), bladder urothelial CA (40.4%/8.3%), cervical/anal SCC (37.5%/0%), lung SCC (34%/3.8%), lung adenocarcinomas (27.6%/3.9%), ovarian CA (26.4%/3.6%), endometrial CA (26.3%/1.3%), lung small cell CA (24.4%/2.4%), gastric adenocarcinomas (20%/4%), breast mucinous CA (19.3%/0%), hepatocellular CA (18.8%/1.2%), breast infiltrating ductal CA (16.4%/1.8%), colorectal adenocarcinomas (10.7%/<1%), cholangiocarcinomas (9.8%/0%), thymic CA (9%/4.5%), and mesotheliomas (7.9%/<1%). Furthermore, high expression of MAGE-A, but not NY-ESO-1, was seen in whole slide evaluations of an independent cohort of metastatic SCC (45.5%/3.6%) and metastatic CA (13.5%/0%) of various primaries with significantly higher expression of MAGE-A in metastatic SCC compared with other metastatic CA. MAGE-A is also more highly expressed in germ cell tumors, seminomas (69%/3.5%) and nonseminomas (40.1%/4.7%). In summary, MAGE-A is more highly expressed than NY-ESO-1 in a majority of human malignancies, and targeting MAGE-A may benefit a large number of patients.