19.4 Cutaneous Lymphomas, Merkel Cell Carcinoma & Cutaneous Sarcomas

19.4 Cutaneous Lymphomas, Merkel Cell Carcinoma & Cutaneous Sarcomas

Primary Cutaneous Lymphomas: The WHO-EORTC Classification

Primary cutaneous lymphomas are defined as clonal lymphoproliferative neoplasms that manifest primarily in the skin without evidence of extracutaneous dissemination (lymph node, bone marrow, or visceral disease) at the time of initial diagnosis. In Europe, classification and management are standardized by the World Health Organization–European Organisation for Research and Treatment of Cancer (WHO-EORTC) consensus classification.

Cutaneous lymphomas are divided into two fundamental biological categories:

1. Cutaneous T-Cell Lymphomas (CTCL): Represent approximately 75% to 80% of all primary cutaneous lymphomas.
2. Cutaneous B-Cell Lymphomas (CBCL): Represent approximately 20% to 25% of cases.

                    PRIMARY CUTANEOUS LYMPHOMAS (WHO-EORTC)
                                       │
          ┌────────────────────────────┴───────────────────────────┐
          ▼                                                        ▼
   CUTANEOUS T-CELL LYMPHOMAS (75%–80%)             CUTANEOUS B-CELL LYMPHOMAS (20%–25%)
   ├── Mycosis Fungoides (MF, ~40%)                 ├── Indolent Subtypes:
   ├── Sézary Syndrome (SS, aggressive leukemic)    │   ├── Cutaneous Marginal Zone (PC-MZL)
   ├── CD30+ Lymphoproliferative Spectrum:         │   └── Cutaneous Follicle Center (PC-FCL)
   │   ├── Lymphomatoid Papulosis (LyP)             └── Aggressive Subtype:
   │   └── Anaplastic Large Cell (C-ALCL, ALK-)         └── Diffuse Large B-Cell, Leg Type
   └── Subcutaneous Panniculitis-like T-Cell                (PCDLBCL-LT, BCL2+, MUM1+)

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Mycosis Fungoides (MF)

Mycosis fungoides (MF) is the most common form of CTCL, accounting for about 40% of all primary cutaneous lymphomas and about half of cutaneous T-cell lymphomas (WHO-EORTC 2018). It is biologically defined by a clonal proliferation of small-to-medium-sized skin-homing T helper memory lymphocytes.

Immunophenotypic Hallmarks

• Canonical Profile: CD3+, CD4+, CD8−, CD45RO+, expressing cutaneous lymphocyte-associated antigen (CLA) and the chemokine receptor CCR4.
• Pan-T-Cell Marker Loss: Diagnostic confirmation rests on the aberrant loss of mature T-cell surface markers: loss of CD7 (present in >80% of cases) and loss of CD5.
• Clonality: Monoclonal rearrangement of the T-cell receptor (TCR-β or TCR-γ) genes detected by polymerase chain reaction (PCR).

Clinical Staging and Evolution

MF typically pursues an indolent, decade-long clinical course through three sequential morphological stages:

1. Patch Stage: Erythematous, finely scaly, slightly atrophic, poikilodermatous (mottled hyper-/hypopigmentation and telangiectasia) macules or patches. Characteristic predilection for non-sun-exposed "bathing suit" distribution (buttocks, lower abdomen, inner thighs, breasts). Patients are frequently misdiagnosed with eczema, parapsoriasis, or plaque psoriasis for 5 to 10 years.
2. Plaque Stage: Infiltrated, indurated, elevated, red-brown or violaceous plaques with sharply circumscribed, annular, arciform, or serpiginous contours. Clearing in the center creates bizarre annular shapes.
3. Tumour Stage: Fleshy, noduloulcerative, dome-shaped, red-to-violaceous or dark reddish-brown cutaneous tumours prone to central necrosis, deep ulceration, and secondary Staphylococcus aureus infection.

                       HISTOPATHOLOGY OF MYCOSIS FUNGOIDES
                       
  Stratum Corneum     [ Parakeratosis without Spongiosis ]
                      ────────────────────────────────────
  Epidermis           [ Epidermotropism: Atypical Cerebriform Lymphocytes ]
                      [ Pautrier Microabscess: Intraepidermal Cluster    ]
                      [ "String of Pearls" Alignment Along DEJ          ]
  ══════════════════  ════════════════════════════════════════════════════  (DEJ)
  Papillary Dermis    [ Band-like (Lichenoid) Atypical Infiltrate        ]
                      [ Coarse Bundles of Wiry Dermal Collagen           ]

Diagnostic Histopathology

• Band-Like Infiltrate: Dense, band-like (lichenoid) lymphocytic infiltrate in the upper papillary dermis.
• Epidermotropism Without Spongiosis: Invasion of atypical lymphocytes into the epidermis in the absence of significant intercellular epidermal edema (spongiosis). This distinguishes MF from spongiotic eczematous dermatitis.
• Cerebriform Nuclei: Atypical lymphocytes displaying hyperchromatic, deeply indented, folded, brain-like (cerebriform) nuclear contours.
• Pautrier Microabscesses: Pathognomonic intraepidermal aggregations of three or more atypical cerebriform lymphocytes surrounded by a clear halo. Highly specific for MF, though identified in only 20% to 30% of early-stage biopsy specimens.
• Basal Alignment: Atypical lymphocytes aligned in single file along the dermo-epidermal junction ("string-of-pearls" pattern), each surrounded by a clear halo.

TNMB Staging and Management Guidelines

Staging is categorized according to the revised ISCL/EORTC TNMB system based on Skin (T1–T4), Lymph nodes (N0–N3), Viscera (M0–M1), and Peripheral blood (B0–B2):

• Early-Stage MF (Stage IA: T1 <10% BSA; Stage IB: T2 ≥ 10% BSA; Stage IIA: N1/N2): Managed primarily with skin-directed therapies (SDT):
  • Superpotent topical corticosteroids (clobetasol propionate).
  • Phototherapy: Narrowband UVB (311 nm) for patch stage; PUVA (oral 8-MOP + UVA) for thicker plaque stage.
  • Topical chlormethine / mechlorethamine (0.016% gel, nitrogen mustard alkylating agent).
  • Localized superficial radiotherapy (electrons).
• Advanced-Stage MF (Stage IIB: Tumours T3; Stage III: Erythroderma T4; Stage IV: Nodal/Visceral): Requires systemic interventions:
  • Brentuximab vedotin: Anti-CD30 antibody-drug conjugate (auristatin E) approved for CD30-transformed MF.
  • Mogamulizumab: Defucosylated anti-CCR4 monoclonal antibody targeting skin-homing T cells.
  • Bexarotene: Oral synthetic Retinoid X Receptor (RXR) selective agonist (requires monitoring for central hypothyroidism and severe hypertriglyceridemia).
  • Systemic chemotherapy (gemcitabine, pegylated liposomal doxorubicin) or allogeneic hematopoietic stem cell transplantation (HSCT) for fit patients with refractory disease.

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Sézary Syndrome (SS)

Sézary syndrome is an aggressive, leukemic variant of CTCL historically considered the leukemic progression of MF, but now recognized as an autonomous neoplasm arising from CD4+ central memory T cells (TCM: CD45RO+, CCR7+, L-selectin/CD62L+).

Diagnostic Triad

1. Generalized Erythroderma: Exfoliative, bright red erythema covering ≥ 80% of the total body surface area.
2. Generalized Lymphadenopathy: Enlarged, firm, non-tender lymph nodes in two or more non-contiguous anatomic stations.
3. Clonal Circulating Sézary Cells: Detection of circulating clonally identical neoplastic T lymphocytes with cerebriform nuclei in peripheral blood meeting at least one of the quantitative hematologic criteria (B2 stage):
   • Absolute Sézary cell count ≥ 1000 cells/µL (1.0 × 10⁹/L).
   • CD4/CD8 ratio ≥ 10 (due to expansion of CD4+ T cells).
   • Aberrant loss of pan-T-cell antigens: ≥ 40% CD4+CD7− or ≥ 30% CD4+CD26−.
   • Identical clonal TCR rearrangement in peripheral blood and skin.

Clinical Manifestations

• Intractable, agonizing, sleep-depriving pruritus.
• Prominent palmoplantar keratoderma with painful fissures.
• Diffuse non-scarring alopecia, severe onychodystrophy, and ectropion (cicatricial eversion of lower eyelids).
• Life-threatening risk of recurrent bacterial sepsis, primarily driven by Staphylococcus aureus colonization.

European Treatment Framework

• Extracorporeal Photopheresis (ECP): The cornerstone first-line systemic therapy. Patient leukocytes are harvested via leukapheresis, incubated with 8-methoxypsoralen, exposed extracorporeally to UVA light, and reinfused to induce systemic anti-tumour immune responses.
• Combination systemic regimens: ECP combined with low-dose interferon-alpha (IFN-α), bexarotene, or mogamulizumab (anti-CCR4).

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CD30+ Cutaneous Lymphoproliferative Disorders

The CD30+ lymphoproliferative disorders comprise the second most frequent group of CTCLs (~25%), forming a biological spectrum that links self-healing inflammatory-like lesions to overt autonomous lymphomas.

                CD30+ CUTANEOUS LYMPHOPROLIFERATIVE SPECTRUM
                                     │
        ┌────────────────────────────┴────────────────────────────┐
        ▼                                                         ▼
Lymphomatoid Papulosis (LyP)              Primary Cutaneous Anaplastic Large Cell
├── Recurrent crops of papulonecrotic     ├── Solitary / localized red-violet nodule
│   lesions that spontaneously regress   ├── Sheets of large pleomorphic cells
├── Paradox: Malignant histology with     ├── >= 75% CD30+ positivity
│   benign, waxing-and-waning course      ├── ALK-Negative (favorable prognosis)
└── 10%–20% risk of second lymphoma       └── Surgical excision or radiotherapy
    (MF, Hodgkin lymphoma)

1. Lymphomatoid Papulosis (LyP)

• The Clinical Paradox: LyP displays an alarming disconnect between histopathology and clinical behavior. While biopsies demonstrate sheets of bizarre, atypical, hypermitotic, anaplastic CD30+ cells mimicking high-grade lymphoma, the lesions follow an indolent, self-healing clinical course.
• Clinical Presentation: Recurrent, rhythmic crops of asymptomatic or mildly pruritic, red-brown or violaceous papules and nodules (1–2 cm) that develop central necrosis, hemorrhage, and crusting, followed by spontaneous involution and complete healing over 3 to 12 weeks, leaving depressed, varioliform (pockmark-like) scars.
• Secondary Malignancy Risk: Patients carry a 10% to 20% lifetime risk of developing an associated second lymphoid neoplasm, most frequently mycosis fungoides, Hodgkin lymphoma, or cutaneous ALCL. Lifelong clinical surveillance is mandatory.
• Management: Reassurance and "watch-and-wait" for mild disease; low-dose oral methotrexate (10–20 mg/week) or phototherapy for cosmetically disfiguring or eruptive outbreaks.

2. Primary Cutaneous Anaplastic Large Cell Lymphoma (C-ALCL)

• Clinical Manifestation: Rapidly enlarging, solitary or localized, grouped, firm, red-to-violaceous dome-shaped tumours (>2 cm) that frequently ulcerate.
• Histopathology and Immunohistochemistry: Diffuse sheets of cohesive, large, pleomorphic, atypical anaplastic cells with abundant cytoplasm and horseshoe-shaped nuclei. Neoplastic cells must demonstrate ≥ 75% CD30 expression.
• Critical Marker: ALK-negative (Anaplastic Lymphoma Kinase negative). In contrast to systemic ALCL (which is frequently ALK-positive due to the t(2;5) NPM-ALK translocation and carries a guarded prognosis), primary cutaneous ALCL is universally ALK-negative and carries an excellent prognosis (>90% 10-year survival).
• Therapy: Local surgical excision or localized radiotherapy (30–40 Gy). Multifocal disease responds well to brentuximab vedotin.

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Primary Cutaneous B-Cell Lymphomas (CBCL)

Merkel Cell Carcinoma (MCC)

Merkel cell carcinoma (MCC) is a highly aggressive primary neuroendocrine carcinoma of the skin with a propensity for rapid local recurrence, early regional lymph node dissemination, and distant hematogenous metastases.

                           MERKEL CELL CARCINOMA (MCC)
                                       │
         ┌─────────────────────────────┴─────────────────────────────┐
         ▼                                                           ▼
  MCPyV-Positive (~80% in Europe)                             MCPyV-Negative (~20%)
  ├── Clonal integration of Merkel Cell Polyomavirus          ├── Direct UV-Induced Mutagenesis
  ├── Viral Oncoproteins (Large T & Small T Antigens)         ├── High Tumour Mutational Burden (TMB)
  └── Inactivation of p53 and Retinoblastoma (Rb)             └── Direct Somatic TP53 & RB1 Mutations
                                       │
                                       ▼
                         THE CLINICAL "AEIOU" ACRONYM
                         ├── A: Asymptomatic (non-tender)
                         ├── E: Expanding rapidly (doubling in weeks)
                         ├── I: Immune suppressed (transplants, CLL, HIV)
                         ├── O: Older (>50 years, peak 75–80)
                         └── U: UV-exposed sites (head, neck, forearms)

Etiology and Dual Pathogenesis

1. Merkel Cell Polyomavirus (MCPyV): Clonally integrated into the host genome in approximately 80% of European and North American MCC tumours. The virus expresses truncated Large T (LT) and Small T (sT) viral oncoproteins that bind and functionally inactivate tumour suppressors Rb and p53, driving autonomous oncogenic cycling.
2. Ultraviolet (UV) Radiation: Accounts for ~20% of cases (MCPyV-negative). Characterized by massive somatic mutational burdens (high TMB) containing classic UV-signature dipyrimidine transitions (C → T) with direct biallelic mutations in TP53 and RB1.

Clinical Presentation: The "AEIOU" Rule

Clinically, MCC presents as a solitary, rapidly growing, firm, painless, non-tender, dome-shaped, shiny, red-violaceous or bluish-red nodule on sun-damaged skin. More than 90% of patients satisfy at least three features of the AEIOU mnemonic:

• A: Asymptomatic / lack of tenderness.
• E: Expanding rapidly (often doubling in size within 1–3 months).
• I: Immune suppression (organ transplant recipients, chronic lymphocytic leukemia [CLL], HIV infection have a 10- to 30-fold increased risk).
• O: Older age (>50 years; median age ~75 years).
• U: UV-exposed anatomical distribution (head and neck ~50%, upper extremities ~35).

Diagnostic Histopathology and Immunohistochemistry

• H&E Architecture: Dense dermal and subcutaneous sheets, nests, or trabecular ribbons of uniform "small round blue cells" with scant cytoplasm, round-to-oval nuclei, powdery "salt-and-pepper" neuroendocrine chromatin, nuclear molding, and high mitotic/apoptotic indices.
• The Hallmark IHC Marker: Cytokeratin 20 (CK20): Exhibits pathognomonic perinuclear dot-like (crescentic) cytoplasmic positivity in >95% of cases.
• Neuroendocrine Confirmation: Strongly positive for neuroendocrine markers: Synaptophysin, Chromogranin A, and CD56 (NCAM).
• Definitive Differential Diagnosis (Key Rule): Metastatic small cell lung carcinoma (SCLC) to the skin is histologically indistinguishable on H&E. The definitive discriminator is Thyroid Transcription Factor-1 (TTF-1):
  • Merkel Cell Carcinoma: CK20+ (dot-like), TTF-1 NEGATIVE.
  • Metastatic Small Cell Lung Carcinoma: CK20 NEGATIVE, TTF-1 POSITIVE.

Staging and European Management Framework

• Mandatory Sentinel Lymph Node Biopsy (SLNB): Even in small (<1 cm) clinically node-negative tumours, occult microscopic nodal metastasis is present in up to 30% of patients. SLNB is mandatory for all clinically localized cases.
• Primary Therapy: Complete wide surgical excision with 1 to 2 cm margins down to the underlying muscle fascia.
• Routine Adjuvant Radiotherapy: MCC is exceptionally radiosensitive. European guidelines recommend adjuvant radiotherapy (50–60 Gy) to the primary tumour bed and draining nodal basin to substantially reduce locoregional recurrences.
• Metastatic Disease (Stage IV): Frontline therapy has shifted from toxic platinum chemotherapy to immune checkpoint inhibitors: Avelumab (human anti-PD-L1 IgG1 monoclonal antibody; EU- and US-approved) and Pembrolizumab (anti-PD-1; US-approved for Merkel cell carcinoma), which achieve durable objective responses in roughly 30% to 60% of patients (higher when used first line).

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Dermatofibrosarcoma Protuberans (DFSP)

Dermatofibrosarcoma protuberans (DFSP) is a locally aggressive, intermediate-grade cutaneous soft tissue sarcoma originating in the dermis with deep, destructive infiltration into the subcutis, fascia, and underlying skeletal muscle.

                      DERMATOFIBROSARCOMA PROTUBERANS
                                     │
                                     ▼
               CYTOGENETIC HALLMARK: t(17;22)(q22;q13)
                                     │
                                     ▼
                 COL1A1-PDGFB FUSION ONCOGENE
                                     │
                                     ▼
             Continuous Autocrine Stimulation of PDGFR-β
                                     │
                                     ▼
       Uncontrolled Proliferation of Monomorphic Spindle Cells

Molecular Pathogenesis and Cytogenetics

In >90% of cases, DFSP is driven by a unique reciprocal chromosomal translocation: t(17;22)(q22;q13), commonly present as a supernumerary ring chromosome [r(17;22)].

• Gene Fusion: Translocation fuses the strong promoter of the collagen type I alpha 1 gene (COL1A1 on chromosome 17q22) to the platelet-derived growth factor B-chain gene (PDGFB on chromosome 22q13).
• Oncogenic Mechanism: The constitutive COL1A1 promoter drives continuous, deregulated overproduction of the PDGF-B ligand, establishing an autonomous autocrine/paracrine loop that persistently activates Platelet-Derived Growth Factor Receptor-beta (PDGFR-β), stimulating sustained mitotic proliferation.

Clinical Presentation and Progression

• Initial Plaque Stage: Presents as an asymptomatic, firm, indurated, morphea-like or keloid-like plaque, violaceous, flesh-colored, or yellowish-brown, on the trunk (40–50%) or proximal extremities. Often neglected for years due to its indolent, non-tender nature.
• Protuberant Stage: Over decades, multiple firm, bosselated, multinodular, purplish-red or flesh-colored nodules develop within the indurated plaque ("protuberant" phase), which can ulcerate and bleed.

Histopathology and Immunohistochemistry

• The Storiform Pattern: Monomorphic, cytologically bland, slender spindle cells arranged in an intersecting, tightly whorled, storiform ("cartwheel" or "pinwheel") architecture centered around small capillaries.
• Honeycomb Subcutaneous Infiltration: Neoplastic spindle cells infiltrate deeply into subcutaneous adipose tissue, dissecting along individual fat septa and encircling solitary adipocytes in a characteristic "honeycomb" or "swiss-cheese" pattern.
• Immunohistochemical Profile:
  • CD34: Diffusely and intensely POSITIVE in >95% of cases.
  • Factor XIIIa: Strictly NEGATIVE.
  • High-Yield Board Distinction: Differentiating DFSP from a cellular dermatofibroma:
    • Dermatofibroma: CD34 negative (or patchy at rim), Factor XIIIa POSITIVE.
    • DFSP: CD34 POSITIVE, Factor XIIIa negative.

Surgical Strategy and Targeted Therapeutics

• Surgical Excision: DFSP possesses subclinical, tentacle-like microscopic extensions that extend far beyond visible margins. Conventional wide local excision (WLE) with standard 1–2 cm margins carries unacceptable local recurrence rates (20–50%).
• Gold Standard: Mohs micrographic surgery (MMS) or wide local excision with ≥ 2 to 3 cm margins including deep fascia, yielding cure rates >98%.
• Targeted Tyrosine Kinase Inhibition: For locally advanced, unresectable, recurrent, or metastatic DFSP, the selective PDGFR-β tyrosine kinase inhibitor Imatinib mesylate is EMA-approved and achieves dramatic objective tumor regression.