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Bone Cancer Staging: TNM Classification System

Bone cancer staging helps to determine the disease prognosis and to select an appropriate treatment strategy. TNM is the most commonly used system for staging bone cancer by the medical community.

“T” stands for “Tumor Size”, “N” for “Lymph Nodes”, and “M” for “Metastasis”. Numbers and/or letters after T (1, 2, and 3), N (0 and 1), and M (0 and 1) provide more details about each of these factors.

Additionally, based on the appearance under the microscope (level of differentiation), cancer cells are assigned a grade (represented by letter ‘G’) from 1 to 3, where 1 indicates most cancer cells look like normal cells (well-differentiated cancer cells) and 3 indicates very few cancer cells look like normal cells (poorly-differentiated cancer cells).

Once T, N, and M categories and grade of cancer cells are determined, this information is combined to assign an overall stage (from I to IV).

Following table describes the characteristics of bone cancer according to different TNM stages:

STAGE TNM DESCRIPTION
IA T1a N0 M0 G1 The tumor in the bone measures </=8 cm in largest dimension. Tumor is well differentiated. No spread of disease to nearby lymph nodes or distant body parts.
IB T2-3 N0 M0 G1 The tumor in the bone measures >8 cm in largest dimension or multiple tumors are present in the same bone. Tumor is well differentiated. No spread of disease to nearby lymph nodes or distant body parts.
IIA T1 N0 M0 G2‑3 The tumor in the bone measures </=8 cm in largest dimension. Tumor is moderately to poorly differentiated. No spread of disease to nearby lymph nodes or distant body parts.
IIB T2 N0 M0 G2‑3 The tumor in the bone measures >8 cm in largest dimension. Tumor is moderately to poorly differentiated. No spread of disease to nearby lymph nodes or distant body parts.
III T3 N0 M0 G2‑3 Multiple tumors are present in same bone. Tumor is moderately to poorly differentiated. No spread of disease to nearby lymph nodes or distant body parts.
IVA Any T N0 M1a Any G Cancer has spread to lungs, irrespective of its size and grade. No spread to regional lymph nodes
IVB Any T N1 Any M Any G Cancer has spread to nearby lymph nodes, irrespective of its size, grade and spread to distant sites.
OR
Any T Any N M1b Any G Cancer has spread to other bones or distant site(s).

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