Is lymph node density a better predictor of cancer-specific survival than number of positive lymph nodes?

This study compared the predictive ability of lymph node density and the absolute number of positive lymph nodes in patients with prostate cancer and lymph node invasion.  

Radical prostatectomy is the surgical removal of the prostate gland in prostate cancer. Chemotherapy may be used following surgery to increase cancer-specific survival rates (the percentage of people who have not died from a specific cancer). However, there is a need for predictive indicators of survival outcomes in order to avoid unnecessary chemotherapy in those for whom it would hold no benefit. Lymph node invasion (spread of cancer into the lymph nodes) is suggested to be a potential predictive indicator.

Laproscopic pelvic lymph node dissection is a minimally invasive surgery used to detect lymph node invasion in prostate cancer. This is a surgical procedure in which the lymph nodes are removed and a sample of tissue is examined under a microscope for signs of cancer. Nodal metastasis (spread of cancer) is indicated when a sample tests positive for cancer.

Several studies have reported excellent cancer-specific survival outcomes for patients with a lower absolute number of positive nodes as compared to men with a higher absolute number of positive nodes. However, another method of sample analysis is lymph node density; the ratio of samples that are positive compared to the total samples tested.

This study aimed to test whether lymph node density was superior to the absolute number of nodal metastases in predicting long-term cancer-specific survival.

This study involved evaluation of the records of 568 patients with lymph node invasion treated with radical prostatectomy and extended pelvic lymph node dissection. The average number of nodes removed was 17. The average follow-up was 48.3 months.

The average number of positive lymph nodes was 2. The average lymph node density was 11.1%

Patients with 2 or less positive lymph nodes had an average 5-year cancer-specific survival rate of 94.6% compared to 87.6% for patients with greater than 2 positive lymph nodes. Patients with 2 or less positive lymph nodes had an average 8-year and 10-year cancer-specific survival rate of 87% compared to an 8-year of 76.7% and a 10-year of 73% for patients with greater than 2 positive lymph nodes. 

The authors determined that the optimal predictive cut-off mark for lymph node density was 30%. Patients with lymph node density less than 30% had an average 5-year cancer-specific survival rate of 94.1% compared to 83.8% in patients with lymph node density greater than 30%. Patients with lymph node density less than 30% had an average 8-year cancer-specific survival rate of 86.4% and an average 10-year cancer-specific survival rate of 85.2% compared to 71% at both 8 and 10 years in patients with lymph node density greater than 30%. 

Both the absolute number of positive lymph nodes and lymph node density were deemed to be independent predictors of cancer-specific survival.

The authors suggest that either greater than 2 positive lymph nodes or a lymph node density of greater than 30% may be used to stratify patients for chemotherapy following radical prostatectomy.

Surgical experience may influence how many lymph nodes were dissected, which may affect the results.