Nomogram to predict the benefit of radiation for older patients with breast cancer treated with conservative surgery

Abstract

Purpose: The role of radiation therapy (RT) after conservative surgery (CS) remains controversial for older patients with breast cancer. Guidelines based on recent clinical trials have suggested that RT may be omitted in selected patients with favorable disease. However, it is not known whether this recommendation should extend to other older women. Accordingly, we developed a nomogram to predict the likelihood of long-term breast preservation with and without RT.

Methods: We used Surveillance, Epidemiology, and End Results-Medicare data to identify 16,092 women age 66 to 79 years treated with CS between 1992 and 2002, using claims to identify receipt of RT and subsequent mastectomy. Time to mastectomy was estimated using the Kaplan-Meier method. Cox proportional hazards models determined the effect of covariates on mastectomy-free survival (MFS). A nomogram was developed to predict 5- and 10-year MFS, given associated risk factors, and bootstrap validation was performed.

Results: With a median follow-up of 7.2 years, the overall 5- and 10-year MFS rates were 98.1% (95% CI, 97.8% to 98.3%) and 95.4% (95% CI, 94.9% to 95.8%), respectively. In multivariate analysis, age, race, tumor size, estrogen receptor status, and receipt of RT were predictive of time to mastectomy and were incorporated into the nomogram. Nodal status was also included given a significant interaction with RT. The resulting nomogram demonstrated good accuracy in predicting MFS, with a bootstrap-corrected concordance index of 0.66.

Conclusion: This clinically useful tool predicts 5- and 10-year MFS among older women with early breast cancer using readily available clinicopathologic factors and can aid individualized clinical decision making by estimating predicted benefit from RT.

Fig 1.

Cumulative risk of mastectomy by receipt of radiation therapy in (A) the entire patient cohort (N = 16,092); (B) patients with clinical node-negative disease; (C) patients with pathologic node-negative disease; and (D) patients with node-positive disease. P value is from the log-rank test. RT, radiation therapy.

Fig 2.

Nomogram for predicting 5- and 10-year mastectomy-free survival (MFS) after conservative surgery. To calculate probability of MFS, first determine the value for each factor by drawing a vertical line from that factor to the points scale. Then sum all of the individual values and draw a vertical line from the total points scale to the 5- and 10-year MFS probability lines to obtain MFS estimates. For radiation therapy (yes/no), the points assigned should be chosen based on whether the patient's nodes were clinically negative (and pathologically not sampled), pathologically negative, or pathologically positive. One and only one of these three options should be selected. The “yes/no” adjacent to each of these three options refers to whether the patient received radiation therapy not to their nodal status. For example, if a patient received radiation therapy and was clinically node negative, then 0 points are given for radiation therapy (yes/no). In contrast, if a patient did not receive radiation and was pathologically node positive, then 100 points are given for radiation therapy (yes/no). ER, estrogen receptor.

Fig 3.

Nomogram model calibration curves: (A) 5-year and (B) 10-year nomogram calibration curves. The cohort was divided into seven equal groups according to predicted probability of mastectomy-free survival (MFS). The dashed line represents the ideal fit, where nomogram-predicted probability (x-axis) matches the observed probability from Kaplan-Meier estimates (y-axis). Blue circles represent nomogram-predicted probabilities for each group, and X's represent the bootstrap-corrected estimates. Error bars represent the 95% CIs of these estimates.