REPRODUCIBLE AND PREDICTABLE YEARLY PATTERN IN THE INCIDENCE OF UTERINE CERVICAL-CANCER

A yearly pattern in the occurrence of uterine cervical cancer (UCC), o btained from cytological examinations reported as type V (cases conclu ding a malignant alteration), has been previously shown for data obtai ned in the Monterrey Metropolitan Area (state of Nuevo Leon, Mexico) f or a span of 10 years (1978-1987), with a peak of relative incidence i n the month of February being highly stable for consecutive years. Wit h the aim of extending and validating those results, we analyzed the m onthly totals of positive detected cases of UCC in the states of Nuevo Leon, Chihuahua, Coahuila, and Tamaulipas (covering most of Northern Mexico) during the same period. To eliminate bias due to the seasonal variation in the number of screening smears, data were first expressed in relation to the number of cytological examinations done the same m onth. The least-squares fit of a 1-year cosine curve to the data of re lative incidence in the four states reveals a statistically significan t yearly pattern (p = .008), with a maximum of relative incidence in F ebruary almost double that during the rest of the year. Results indica te that the relative incidence of UCC is higher than the yearly averag e during the winter, with secondary peaks in May and October. In view of the nonsinusoidal waveform in the incidence of UCC, we undertook a multiple-component analysis, allowing several cosine functions to be s imultaneously fitted to the data. Results indicate that the yearly pat tern in the relative incidence of UCC can be represented by a model th at includes two components with periods of 12 and 4 months (p = .004). The same model can be documented as statistically significant indepen dently for each of the four states. These results, summarizing over 22 00 positive cases of UCC detected in more than 1,100,000 screening sme ars, are in full agreement with those found previously for part of the state of Nuevo Leon and reveal a highly stable and predictable yearly pattern of variation in the relative incidence of UCC in Northern Mex ico.