Tumor Deposits
Definition
Tumor deposits are discontinuous nodules of tumor in the mesorectal fat with no identifiable nodal or vascular tissue (AJCC 8th edition). The definition has changed between the 5th and 8th AJCC editions and continues to evolve.
Origin and Pathogenesis
Originally described by Gabriel (1935) as in-transit tumor thrombi that form a nodular mass, occurring in conjunction with EMVI.
MRI Appearance
Comet Tail Sign
- Heterogeneous nodule with irregular/spiculated borders contiguous with tumor thrombus in a vessel
- This appearance = tumor deposit (identifiable by association with EMVI)
Spiculated Nodule Without Comet Tail
- Round heterogeneous spiculated nodules in the mesorectal fat
- Cannot distinguish between tumor deposits and completely replaced lymph nodes on imaging
- Both should be treated the same for MRF assessment purposes
MRF Involvement
- Tumor deposits <1 mm from MRF = MRF involvement (expert consensus)
- MRF involvement by tumor deposits → 40.9% local recurrence rate (Birbeck et al.)
- Even higher LR rate than EMVI-related MRF involvement (30.4%)
Distinction from Lymph Nodes
| Feature | Tumor Deposit / Spiculated LN | Smooth Encapsulated LN |
|---|---|---|
| Border | Spiculated, irregular | Smooth capsule |
| MRF assessment | YES — report if <1 mm from MRF | NO — do not report as MRF involvement |
| Rationale | Cannot distinguish on imaging; high LR risk | Even tumor-bearing smooth nodes do not increase LR risk |
Board Pearl
Heterogeneous spiculated nodules in the mesorectal fat should be assessed for MRF involvement regardless of whether they represent tumor deposits or replaced lymph nodes. Smooth encapsulated nodes — even if heterogeneous and potentially metastatic — should NOT be reported as MRF involvement.