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An Example of Regression on Time-Dependent Covariates

Kathleen M. Egan, ScD; Louise M. Ryan, ScD; Evangelos S. Gragoudas, MD

Arch Ophthalmol. 1998;116:366-370.

Objective  To evaluate whether the removal of the eye after radiotherapy alters the rates of metastatic death in patients with melanoma of the choroid.

Patients and Methods  Using an extension of the Cox model, we based our analysis on a cohort of 1541 consecutive patients with unilateral choroidal or ciliary body melanoma treated with protons (70 cobalt-gray equivalent in 5 to 7 fractions) at the Harvard University (Boston, Mass) cyclotron between July 1, 1975, through December 31, 1993, and who were observed prospectively up to September 30, 1995. Patient survival and the status of the treated eye were updated annually.

Results  By September 1995 (median follow-up among survivors, 8 years), 137 patients underwent enucleation after radiotherapy for complications (n=103) or tumor regrowth (n=34). The overall 10-year rate of eye retention was 89% (95% confidence interval, 87%-91%). Of the 1541 patients, 300 died of tumor metastasis, 38 following enucleation of the affected eye (mean interval from enucleation to death, 25 months). The multivariate rate ratio for metastatic death associated with enucleation (modeled as a time-dependent covariate) was 0.9 (95% confidence interval, 0.6-1.4) for enucleation due to complications and 3.8 (95% confidence interval, 2.3-6.3) for enucleation associated with tumor regrowth.

Conclusions  In the absence of tumor viability, enucleation after primary irradiation for choroidal melanoma has no deleterious effect on patients' survival. Enucleation concurrent with tumor regrowth is associated with high death rates; growth of the tumor in the eye may presage systemic recurrence and death from metastasis.

From the Retina Service, Massachusetts Eye and Ear Infirmary, Department of Ophthalmology, Harvard University School of Medicine (Drs Egan and Gragoudas), and the Departments of Epidemiology (Dr Egan) and Biostatistics (Dr Ryan), Harvard University School of Public Health, Boston.

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