Platinum Priority – Prostate CancerEditorial by R. Jeffrey Karnes on pp. 202–203 of this issueExtent of Pelvic Lymph Node Dissection and the Impact of Standard Template Dissection on Nomogram Prediction of Lymph Node Involvement☆
Introduction
Radical prostatectomy (RP) is an effective method of treating men with clinically localized prostate cancer. Lymph node dissection (LND) is an important oncologic component of the procedure, and its role in staging and as a therapeutic intervention has been the focus of renewed interest. The presence of lymph node metastasis is associated with an elevated risk of systemic dissemination of disease and death [1], [2]. Additional diagnostic and therapeutic benefits have been recognized when performing LND adjunctively during RP [3], [4], [5], [6].
Stage migration has resulted in a greater proportion of cases detected at early stages with a lower incidence of positive lymph nodes. These factors, allied with the potential risk of morbidity from LND, may be contributing to a decrease in the number or extent of LND procedures performed in the United States [7], [8], [9], [10]. To better estimate the likelihood of occult nodal disease and guide clinical decision making, we previously created a nomogram to predict the risk of lymph node involvement (LNI) at the time of the RP [11]. Of critical importance is the accuracy of such a clinical predictive tool, which depends on the data used to impute its functions. In the case of pelvic LNI, the extent of LND and tissue processing are of importance. As reported in several studies [3], [4], [5], [12], we have established the boundaries of a standard LND template in our center that includes the bilateral dissection of the external iliac, obturator, and hypogastric nodes.
We hypothesized that the current nomogram would demonstrate good discrimination but poor calibration when applied to a contemporary series constituted exclusively of standard LND cases. Specifically, we expected it to underestimate the real incidence of LNI during RP. Discrimination refers to the ability of a nomogram to rank patients by their risk. It is typically described in terms of the probability that in a given pair of patients, one with and one without LNI, the nomogram will give the patient with affected nodes a higher probability. Calibration summarizes how well the predicted incidence of LNI matches the observed incidence, that is, whether in 100 men given x% probability of LNI, close to x do indeed have positive nodes. We undertook this study with the objective of rebuilding the nomogram coefficients, including only patients who have undergone the standard LND in our institution, either to validate the accuracy of the current nomogram or to update the nomogram prediction formula to reflect contemporary LND procedures and results as performed during RP.
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Patients
We identified 3097 consecutive patients who underwent primary open RP between October 2000 and October 2008, and 1079 consecutive patients who underwent primary laparoscopic RP between February 2005 and October 2008. Robot-assisted laparoscopic prostatectomy cases were included in this series, with standard LND confirmed by chart review in all cases included in the analysis. We excluded 127 patients (3.0%) with limited LND (57 robotic patients and 70 others). We excluded patients with
Results
Table 1 lists the patient characteristics. The median number of nodes removed was 11, with ∼60% of patients having at least 10 nodes removed (n = 2224). Overall, 5.2% of patients (n = 194) had positive lymph nodes. The mean predicted probability of LNI was 3.2% from the previously published three-variable nomogram and 5.7% from the previously published four-variable nomogram. Both previously published nomograms had very high discriminative accuracy (area under the curve [AUC] of 0.861 for the
Discussion
Since 2002 we have adopted several nomograms for the prediction of clinical and pathologic outcomes after RP. One of these nomograms was developed to predict the preoperative risk of LNI during RP [11]. However, it was developed based on a cohort of patients that underwent both standard and limited LND templates across different centers. It was previously shown that more extended LND templates yield significantly higher number of total and positive lymph nodes, resulting in a better staging
Conclusions
Our updated nomogram demonstrates high discriminative accuracy and improved calibration relative to our prior nomograms when predicting LNI in a contemporary cohort of patients with prostate cancer managed with standard template pelvic LND during RP. This nomogram will be used as the preferred predictive model for counseling patients and developing studies at our institution.
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