1 Tumor Overview
Below is a “big-picture” map that links embryologic germ layer → mature tissue derivative → major pathologic tumor class, then gives you one or two prototypical tumors per class, their favoured location, and the signature imaging appearance you are most likely to quote at the workstation or in the oral boards.
1.1 1. Germ-Layer Overview (one-liner)
| Germ layer | Dominant mature tissues | Dominant tumour classes |
|---|---|---|
| Ectoderm | Surface epithelium, neural crest, neuro-ectoderm | Carcinomas of surface epithelium, melanocytic tumours, CNS tumours |
| Mesoderm | Connective tissue, bone, cartilage, muscle, endothelium, blood & lymphoid cells | Sarcomas, mesothelioma, haematolymphoid tumours |
| Endoderm | Gut & respiratory epithelium, hepatobiliary & pancreatic ducts, endocrine gut derivatives | Adeno-carcinomas (GI, hepato-biliary-pancreatic, thyroid, lung) |
1.2 2. Ectoderm-Derived Tumours
| Tumour class (pathology) | Prototype tumour & typical site | CT / MRI hallmarks you should mention |
|---|---|---|
| Surface-epithelial carcinoma | Squamous Cell Carcinoma, upper aerodigestive tract or lung hilum | Solid mass with central necrosis, lobulated margins; CT hyperattenuating (keratin), T1 iso / T2 hyper; avid FDG uptake. |
| Invasive Ductal Ca Breast (surface ectodermal gland) | Mammography spiculated high-density mass; US irregular hypoechoic; MRI irregular mass with rapid wash-in/ wash-out and low ADC. | |
| Neuro-ectodermal (CNS) tumours | Glioblastoma, cerebral hemispheres | T1 hypointense, T2/FLAIR hyperintense, thick irregular ring enhancement, central necrosis, elevated rCBV, extensive vasogenic oedema crossing CC. |
| Medulloblastoma, midline cerebellar vermis (children) | CT hyperdense; MRI T1 hypo, T2 iso-hyper, strong homogeneous enhancement, marked DWI restriction. | |
| Neural-crest tumours | Melanoma (cutaneous; brain/liver mets) | Melanin ⇒ T1 hyperintense ± susceptibility (blood), solid enhancing nodule; lung nodules often cannon-ball. |
| Phaeochromocytoma, adrenal medulla | “Light-bulb” very T2-bright, avid arterial enhancement ± wash-out; positive MIBG/PET. |
Pearl: Ectoderm loves either keratin (hyperdense SCC), melanin (T1 bright), or very cellular “small-round-blue” embryonal tumours that restrict on DWI.
1.3 3. Mesoderm-Derived Tumours
| Tumour class | Prototype tumour & site | Signature imaging |
|---|---|---|
| Bone-forming sarcoma | Osteosarcoma, metaphysis of distal femur/prox tibia | Mixed lytic–sclerotic lesion, cloud-like osteoid matrix, aggressive sunburst periosteal reaction, Codman triangle, soft-tissue mass enhancing on MRI. |
| Cartilage-forming sarcoma | Chondrosarcoma, pelvis or proximal femur | Rings-and-arcs chondroid calcifications on CT, T2 very high signal, lobulated cartilage matrix; endosteal scalloping. |
| Adipocytic sarcoma | Well-diff Liposarcoma, retro-peritoneum | Large mass with ≥75 % fat + thick (> 2 mm) septa or nodular non-fat components; may displace kidney/IVC. |
| Smooth-muscle sarcoma | Leiomyosarcoma, uterus or IVC | Heterogeneous necrotic soft-tissue mass, T2 hetero high, irregular peripheral arterial enhancement. |
| Skeletal-muscle tumour | Rhabdomyosarcoma, orbit or bladder/prostate (kids) | Iso- to hyperdense soft-tissue mass, T2 bright, avid enhancement, tends to engulf rather than displace. |
| Vascular sarcoma | Angiosarcoma, scalp/face elderly | Infiltrative soft-tissue thickening with haemorrhagic nodules; MRI T1/T2 mixed; blooming on GRE/SWI. |
| Haemato-lymphoid | Diffuse Large B-cell Lymphoma, nodal ± extranodal | CT homogeneously hypo- to iso-dense, minimal necrosis; MRI T2 intermediate, DWI very restricted; “sandwich sign” in mesentery. |
| Mesothelial | Malignant Pleural Mesothelioma | Diffuse pleural rind thickening ± nodules encasing lung; calcified plaques in background of asbestosis. |
Pearl: Think “meso = meat”: sarcomas drop fat, bone, cartilage or muscle signal; lymphoma stays surprisingly homogeneous.
1.4 4. Endoderm-Derived Tumours
| Tumour class | Prototype tumour & site | Key imaging features |
|---|---|---|
| Hepatocellular tumour | Hepatocellular Carcinoma (cirrhotic liver) | Arterial-phase hyper-enhancement with portal/venous wash-out; capsule in delayed, intralesional fat or blood; DWI restriction; corona enhancement on CT. |
| Biliary carcinoma | Cholangiocarcinoma, perihilar (Klatskin) | Delayed progressive fibrosis-type enhancement, biliary ductal dilatation upstream; capsular retraction in intra-hepatic variant. |
| Pancreatic ductal adenocarcinoma | Pancreas head | Ill-defined hypoattenuating mass, upstream duct dilatation & “double duct” sign; T1 hypo, T2 slightly hyper, low enhancement, DWI restricted; encases vessels without early occlusion. |
| Colorectal adenocarcinoma | Sigmoid/rectum | “Apple-core” annular thickening, heterogeneous enhancement, T2 intermediate, DWI restricted; hepatic FDG-avid mets. |
| Pulmonary adenocarcinoma (endodermal bronchial ep.) | Peripheral lung | Ground-glass to part-solid nodule; air-bronchogram, pleural retraction; PET-avid when solid. |
| Thyroid follicular-cell carcinoma (papillary/follicular) | Thyroid lobe | Solid hypo-echoic nodule with micro-calcifications (“dots”), taller-than-wide, hypervascular; CT shows coarse nodal calcifications; on MRI T1/T2 variable but DWI restricted. |
Pearl: Endodermal adenocarcinomas usually show gland-like or desmoplastic enhancement patterns, plus duct obstruction in foregut derivatives.
1.5 5. “Beyond the Germ Layers” – Totipotent Primordial Germ-Cell Tumours
| Prototype tumour | Typical site | Imaging clue |
|---|---|---|
| Seminoma | Testis, midline retro-peritoneum | Homogeneous, iso- to slightly hyperdense mass; T2 intermediate, strong uniform enhancement, avid FDG; “lobulated sheet of grey”. |
| Mature Cystic Teratoma | Ovary, anterior mediastinum | Fat–fluid level, Rokitansky tooth, calcified mural nodule; on MRI fat-suppressed dropout. |
| Embryonal Ca / Yolk-sac tumour | Testis | Heterogeneous hemorrhagic mass, infiltrative, rapid β-hCG / AFP rise; DWI restricted solid elements. |
1.6 6. High-Yield Board-Style Take-Home Points
Ectoderm → surface epithelium & neural derivatives → “C-M-C” Carcinoma (squamous/ductal), Melanoma (melanin T1 bright), CNS embryonal tumours (DWI bright small-round-blue).
Mesoderm → connective tissues → SARCOMA spectrum Bone = osteoid cloud; Cartilage = rings-and-arcs; Fat = macroscopic fat with thick septa; Muscle = necrotic soft mass; Lymphoma stays homogeneous.
Endoderm → glandular/ductal epithelium → ADENOCARCINOMA Look for duct cut-off, progressive fibrotic enhancement (cholangioca), or arterial wash-in/wash-out (HCC).
Neural-crest exceptions (though ectodermal): pheochromocytoma (T2 bright), neuroblastoma (calcified adrenal mass), paraganglioma (“salt-and-pepper” T2).
Totipotent germ-cell tumours break the rules—contain multiple tissue signals (fat, calcium, cartilage) and often occur along the midline.
Mnemonic for imaging dominance
- “Bright Bone, Dark Desmoplasia, Fatty Fluff, Light-bulb Pheo, Melanin Makes T1 bright.”
With this germ-layer-to-pathology roadmap, you can logically categorise almost any tumour you meet in practice and instantly recall which imaging clues point to its embryologic roots.