PITFALLS IN THE DIAGNOSIS OF SOFT TISSUE TUMOURS OF CHILDHOOD

Dr.E Kiran KumarAssociate Professor of Pathology

MIMS, Vizianagaram

• Tumours in children and adolescents differ from that in adults in many ways

Most malignant tumours in adults are Carcinomas, and classifed acc to site to origin.

But Childhood tumors are histologically diverse. The same tumor can occur at many sites.

There are 12 main diagnostic groups, with variation in the age distribution.

• The outcome for most types of childhood tumors has dramatically improved over the last 20 yrs, unlike the small improvement in common adult cancers.

• The most important long-term serious side effects of therapy in childhood tumors are – growth failure, reduced fertility, major organ damage and significant increase in risk of secondary cancer in adulthood.

• Hence, more attention is needed to minimise the toxic effects of therapy and also to prevent the misdiagnosis in pediatric oncology.

• The potential pitfalls encountered in soft tissue tumor pathology is worth knowing, to avoid misdiagnosis.

• Due to significant overlap in the histology and immunoprofile of many tumors, cytogenetic or molecular genetic confirmation of the characteristic translocations is very valuable.

• FISH & RT-PCR assays can be reliably performed on paraffin-embedded tissue. Tumor cells can also be retrieved from fixed tissue by Laser capture microdissection, for RT-PCR analysis, for the signature translocations.

• Soft tissue sarcomas constitute 5-8% of childhood tumors. Diagnostic challenges are due the infrequent occurrence and histological diversity.

Potential pitfalls

1) Misclassification of specific sarcomas :Diagnosis of the precise type of sarcoma has major

therapeutic significance. Pediatric STSs are classifed as – RMS, NRSTS and USTS (Undifftd STS).

Rhabdomyosarcoma (RMS) – It is the most common sarcoma (50-60%) of the STS in childhood. Primitive mesenchymal tumor.2/3rd of cases diagnosed before 10 yrs age.

Typing into embryonal and alveolar RMS is of extreme importance, since alveolar RMS has adverse outcome.

The intl classfn of RMS used currently places RMS into 3 prognostic groups –

The superior prognostic group ,includes the botryoid and spindle-cell variants of embryonal RMS.

Embryonal RMS-NOS – Intermediate groupAlveolar RMS – Poor prognosis.Typing is not recommended on poor quality or small

sample – in such cases, the term ‘RMS-NOS’ should be used.

• Mixed RMS,composed of both embryonal and alveolar RMS, is classified as alveolar RMS.

• Embryonal RMS is a spindle-cell tumor( to be difftd from SS, MPNST, FS), while alveolar RMS is round cell tumor ( to be difftd from EFT, DRCT, MRT, NHL/ALL, Neuroblastoma, blastemal component of WT), by using difft sets of antibodies in IHC, whenever there is overlap in histology.

• More than 95% of RMSs show expression of myogenic TFs, myogenin/myf4 & myoD1/myf3( To diffte from non RMS lesions)

The extent and intensity of staining of Myogenin and myoD1 is used as an aid in distinguishing between the two main types of RMS.

Most alveolar RMS display extensive(>50%) nuclear staining for Myogenin, whereas embryonal RMS show a focal pattern of staining.

Myogenin expression is reliable in separating RMS from MPNST. Also to exclude Nodular fasciitis, inflammatory myofibroblastic tumor, smooth muscle and neural tumors.

Myogenin expression should be interpreted with caution, since focal immunoreactivity is also seen in blastemal component of WT, SS & Infantile FS.

Myogenin positivity also seen in some benign nuclei in nodes and in entrapped, non-neoplastic or regenerating skeletal muscle fibres ( along with myoD1 positivity) – A potential pitfall, while assessing immunoreactivity at the infiltrative edges of round or spindle cell tumor.

Several other immunocytochemical markers are identified in RMS – Can lead to diagnostic errors, if interpreted, out of context of the clinical and histological findings. E.g – desmin, CD99, anaplastic lymphoma kinase, CD56, SMA, CK, S100 &NF protein.

EMBRYONAL RMS

They show cytological characteristics that range from primitive stellate or fusiform mesenchymal cells to well-differentiated forms with extensive rhabdomyoblastic diffn.

Hypocellular areas around blood vessels, alternate with mucoid matrix-rich hypocellular areas.

Adequate sampling is crucial, since wide variety of myxoid tumors can mimic hypocellular embryonal RMS.

ERMS

SPINDLE CELL VARIANT OF ERMS

• Arises in the paratesticular & head/neck regions. It has cells resembling smooth muscle cells, arranged in intersecting fascicles. May display BFH / NF-like pattern.

• May be mistaken for Leiomyosarcoma, and differentiated from it by Positive Myogenin & Negative h-caldesmon immunoprofile.

ERMS – spindle cell variant

ERMS-spindle cell variant

BOTYROID VARIANT OF ERMS

• Arises in the hollow cavities of GU, biliary, auditory and upper aerodigestive tract.

• Definite diagnosis requires the presence of subepithelial cellular, ‘cambium layer’ of neoplastic rhabdomyoblasts.

• Its endoscopic finding of grape-like or polypoid appearance, can be mimicked by fetal rhabdomyoma & inflammatory myofibroblastic tumor(IMT).

Botyroid ERMS

• Fetal rhabdomyoma, also overlaps with E-RMS in – age at diagnosis, location & histology. Head/neck are the common site for both. It can be distinguished from E-RMS by an absence of cellular pleomorphism.

• Submucosal tumors are differentiated from botyroid variant of ERMS by the absence of cambium layer.

Fetal rhabdomyoma

Fetal rhabdomyoma – plump pink cells

Inflamm myofibroblastic tumor(IMT)

• Overdiagnosed as ERMS, due to similarity in gross appearance & infiltrative growth pattern.

• Also the myofibroblastic/fibroblastic cells may show high mitotic count, moderate atypia & vascular bulging.

May show desmin and ERMS may show ALK positivity, causing further confusion.

Distinguished from ERMS by a mixture of three growth patterns, absence of atypical mitotic figures and Myogenin expression

IMT

ALVEOLAR RMS

• Defined by the cytological appearance of round cells with frequent mitosis, hyperchromatic nuclei, coarse chromatin pattern & distinct smooth nuclear membrane.

• Classic variant – anastomosing fibrous septa, which delineate cell aggregates.

• Solid variant – Paucity of fibrous septa.Reticulin stain is helpful, as it encircles the cell aggregates. Even a single focus of alveolar morphology is sufficient to

type it as ARMS.

ARMS

• ANAPLASTIC VARIANT OF RMS : Has a poor prognosis. Shows large hyperchromatic nuclei and multipolar mitotic figures. More common in ERMS.

• Sclerosing matrix-rich subtype of RMS – Usually in head/neck region.

Difficulties in diagnosis are – abundant stroma obscures the smaller component of small, blue, tumor cells, causing microalveolar architectures, resembling angiosarcoma or carcinoma.

• MyoD1 strong diffuse positivity of alveolar component and and strong Myogenin positivity of embryonal component is helpful in diagnosis.

Anaplastic RMS

Sclerosing RMS

RMS-strong diffuse desmin positivity

Pediatric NonRMS STS

Ewings sarcoma family of tumors(ESFT) : 2nd common sarcoma in childhood. Usually in chest wall, paraspinal tissues and abdominal wall.

A range of appearances are seen, depending on the degree of neural differentiation.

ES represents undifftd end of the spectrum & PNET shows varying degrees of neural diffn.

A large number of histological variants of ES are present, like adamantinoma-like, sclerosing-type and spindle-cell type - Thereby creating a potential diagnostic trap.

EWINGS SARCOMA

EWINGS SARCOMA

EWING’S -PAS positivity

• CD99 &FLI-1, seen in 90% of these tumors, but not specific for EFT. Hence other antibodies also should be used to exclude hematolymphoid, neuroblastic and myogenic tumors.

• Lymphoblastic lymphoma is notorious for its mimicry of EFT, since it displays the uniform strong membranous reactivity of CD99 and nuclear reactivity of FLI-1.

Another prob is, they can also be negative for CD45(LCA). • So a large panel of antibodies are reqd for its diffl

diagnosis.

ALL-FLI1 positivity

ALL- CD99 positivity

Desmoplastic round cell tumor (DRCT)

• Highly aggressive, well-defined entity, mainly of young adults, and also in children.

• Displays striking diversity in location, and so, in clinical presentation.

• Has wide histological spectrum, with several morphological variants and polyphenotypic immunoreactivity.

• In difficult cases, confirmation of the specific chromosomal rearrangement and chimeric transcript is done by FISH & RT-PCR.

DRCT

DSRCT- CK20 positivity

DSRCT- CD56 positivity

DSRCT- MAP2 POSITIVITY

DSRCT-DESMIN positivity

MALIGNANT RHABDOID TUMOR(MRT)

• Very aggressive neoplasm of infancy and childhood, with tendency for wide spread metastasis. Difft locations are – Renal, CNS, etc.

• Hallmark ‘ rhabdoid cell’ has large, round or oval nuclei, with distinct central eosinophilic nucleoli & abundant cytoplasm, with sometimes, paranuclear cytoplasmic globules.

• Typical arrangement – Patternless sheets or cords. • Variety of morphological patterns can occur. Complex

immunophenotype is present.• Most specific antibody is INI1/BAF1, to differentiate from

other similar appearing tumors.

MRT

MRT - lung

ADULT TYPE NRSTS

• Heterogenous group of tumors, and includes the entities, usually seen in adults.

• Synovial sarcoma(SS) : In extremitis & also in visceral sites like mediastinum.MPNST is the most impt tumor in the D/D of biphasic SS, since it also shows glandular diffn.

• A characteristic feature of MPNST is the presence of intestinal-type epith diffn with goblet-cells and microvilli in glands, not seen in SS.

• The monophasic spindle-cell type of SS should be difftd from MPNST & fibrosarcoma, which can be v.difficult.

• Epith component and foci of calcification are more common in SS than in fibrosarcoma. CK/EMA positivity in both the spindle and epith cells is the most reliable indicator of SS, in this context.

• Poorly difftd variant of SS has to be difftd from SRCTs/MPNST.• Myxoid variant of SS has to be difftd from ERMS.• HPCmatous variant of SS has to be difftd from Infantile HPC.• Specific translocation in SS is t(X;18), which has to be

demonstrated.

Synovial sarcoma

Synovial sarcoma - biphasic

MPNST

• An impt tumor to consider in the D/D of pediatric MPNST is – plexiform(multinodular) cellular schwannoma, which is a rare type of peripheral nerve sheath tumor, mistaken as malignancy, due to its rapid growth & locally aggressive behaviour.

• Plexiform schwannomas occur most commonly in the first decade, No a/w NF1, superficial location, in subcutis of extremities, uniform positivity for S100 and no p53 positivity.

• E/M shows uniform popln of wd schwann cells.

MPNST- spindle cells, storiform, glandular, neuroendocrine

PLEXIFORM SCHWANNOMA – discontiguous encapsulated nodules,with typical schwannoma pattern

PLEXIFORM SCHWANNOMA

SARCOMAS MISDIAGNOSED AS BENIGN LESIONS

• Myxoid tumors are the most vulnerable group, for misdiagnosis in pediatric soft tissue tumors.

• MYXOFIBROSARCOMA: It is the malignant myofibroblastic tumor, and rare in children.

Arises in bone/soft tissue, mostly in head/neck region. Metastasis can occur after a very long period of primary occurrence.

Infiltrative pattern of spindle cells, in a herring-bone pattern, in a collagenous background. Low to high grade cytomorphology. Just like in ERMS, foci of low cellularity may be misleading to know the grade.

MYXOFIBROSARCOMA

MYXOFIBROSARCOMA

MYXOFIBROSARCOMA

Low grade Fibromyxoid sarcoma(LGFMS)

Along with the entity, ‘Hyalinising spindle cell tumor with giant rosettes’(HSCTWGR) – Both are intermediate-grade tumors, of the same entity.

Have a common translocation, t(7;16)(q34;p11).Present as slow growing deep soft tissue mass,

commonly in shoulder/extremity. Late metastasis is impt feature.Mixture of fibrous/myxoid tissue; swirling pattern;

Impt feature is arcades of curvilinear blood vessels.

• LGFMS shows a bland appearance due to low cellularity, infrequent mitosis & absence of nuclear pleomorphism; well-circumscribed borders .

Hence, deceptively appear benign due to resemblance to benign tumors like nodular fascitis / benign nerve sheath tumor.

• Lack of awareness of LGFMS and its rarity contribute to its misdiagnosis.

• Deep location and size(9-10cms) are pointers towards high-grade sarcoma;

• Also chimeric FUS/CREB3L2 gene & its transcripts detected by RT-PCR.

LGFMS

LGFMS

HSCTWGR – large confluent collagen rosettes with intervening spindle cells

HSCTWGR

NON STTs MISDIAGNOSED AS STS• JUVENILE XANTHOGRANULOMA(JXG): Most common non-

Langerhan’s histiocytic disorder. • Occurs mostly as solitary/multiple skin lesions in head/neck; also

in subcut/parenchymal organs. • Deep JXG is a benign self-limited lesion. • Lipidised & non-lipidised, multinucleated Touton cells & inflam

cells. • Difficulty in diagnosis is, when it is located in deep soft

tissues/skeletal muscle; where they are poorly circumscribed, grow rapidly, and mimic a sarcoma.

• Hence, JXG should be kept in mind, in infants & children, with a histiocytic lesion in soft tissues. CD68/FXIIa is helpful.

JXG

JXG

HEMATOLYMPHOID MALIGNANCIES

• Extranodal non-Hodgkin’s lymphomas are more common in children, in contrast to adults.

• Extramedullary myeloid tumors are also more common in children.

• Hence these should be considered in D/D of round cell tumors, to avoid pitfalls in the diagnosis.