The main objective of this research was to establish the frequency of thyroid cancer in the Congolese population, secondarily aiming to identify the most frequent types of cancer. Our series included 6,106 cases, of which 68.2% were female and 31.8% were male. This predominance of the female gender was also found in the series of Mashinda who studied the prevalence of cancer in the urban setting of Kinshasa [16] and it was also the case for the Lukanu series, which included cases from a rural setting of Congo in Kimpese [17]. These are the first two epidemiological studies analyzing neoplastic diseases carried out in the DRC and which preceded our research.
Female predominance in overall cancer diagnosis was observed in the DRC, a similar trend was also observed in Brazil [18]. The cascade of sexual hormone activity, in particular estrogens and the aromatization of androgens via Mitogen-activated protein kinase (MAPK), causing the decline of immune cells and promoting the proliferation of cancer cells and the inhibition of apoptotic activity can explain the female predominance [19, 20]. However, some European series have found a male predominance [21–25].
The male gender was more represented in the age groups up to 30 years and over 60 years; on the other hand, the female gender was more represented in the age groups between 30 and 60 years. Most cancers in women occurred between the ages of 40 and 60 and in men over 50. Overall, 77% of cancer patients were over 40 years of age. Most series around the world, according to which the older the age the greater the probability of developing a neoplastic disease [26, 27].
In our series, breast cancer occupies the first position in terms of frequency of cancer in women and cervical cancer occupies the second position. However, in the series published by Mashinda et al., cervical cancer was the most frequent followed by breast cancer [16]. This difference can be explained by the methodology, the study period, the progress of the national policy on cancer screening in women and by the development of diagnostic means. Mashinda studied the records of two pathology laboratories, while we researched the records of 4 pathology laboratories. Mashinda analyzed results from 1965 to 2008, while we studied data between 2005 and 2019. We must consider that the means of raising awareness have evolved and the educational level of the population has increased over time. All these parameters can explain this difference. The findings of our study are similar to the Lukanu series; breast cancer was the most frequent followed by cervical cancer [17]. Our results also mirrored those found in several African and worldwide series [28–31].
When we consider both sexes, breast cancer was the most frequent type of cancer in our series, this result being similar to those in the literature [29].
Regarding cancer in male patients, prostate cancer was the most frequent in our series. This result was similar to the series of Lukanu [17]. On the other hand, Mashinda found lymphoid organ cancer as the most frequent followed by prostate cancer [16]. Our results are similar to those found in the literature [29]. There has been an improvement in the awareness of the Congolese population regarding prostate cancer over the past two decades.
Thyroid cancer is the most common type of cancer of the endocrine system [32]. Our series found a frequency of 1.7%. In women, the frequency is 2% and it is 1.1% in men, with a female/male ratio of 4. The series described by Mashinda et al found a thyroid cancer frequency of 0.5% in women [16]. The frequency of thyroid cancer in women in our series compared to that of Mashinda’s is multiplied by 4, a female predominance that is confirmed in the literature [33, 34]. Thyroid cancer occupied the sixth position of all listed cancers among women in our series, whereas it is the 5th most frequent cancer in women worldwide [35].
The increase in thyroid cancer frequency has also been observed in several studies around the world over the past three decades [10]. The mechanisms underlying this increase have not yet been elucidated. However, nutritional, hormonal, anthropometric, environmental, and other factors are suspected. Many authors also consider that excessive iodine intake, and the development and accessibility of diagnostic tools participate to the increase in diagnosis [36, 37].
Our study found that nearly 7% of thyroid cancers were anaplastic and 1% were medullary, while these cancers are rare in the literature [38]. This can be considered as a particularity of the DRC regarding thyroid cancer, especially since calcitonin is not generally measured in the assessment of thyroid nodules or preceding thyroidectomy.
It is known that 90% of thyroid cancers are differentiated and have good prognosis and that only 5 to 10% are undifferentiated and have therefore a bad prognosis [39, 40]. This high prevalence of undifferentiated cancers in our series constitutes a particularity of the Congo. This can be explained by the fact that, undifferentiated thyroid cancers, given their aggressive behavior, are more likely to warrant a surgical evaluation. Thyroid cancers in our study originated from surgical pathology reports. Nevertheless, this particularity requires more in-depth studies to better understand the causes and mechanisms.
Since iodine deficiency in the soil is considered a risk factor for anaplastic cancer [41], the question to be raised is whether iodine deficiency could be responsible for this higher frequency of undifferentiated cancers. knowing that iodine saturation in the Congo was only obtained in 1993 [42]. Another potential mechanism is that initially differentiated cancers have lost differentiation over time [43] due to late diagnosis.
This work has the limitations of retrospective studies. In addition, it is biased due to the fact that we only took into consideration the patients who had carried out the anatomopathology while those who had not carried out one, were not included in this study, this may have influenced a high frequency of cancers and certain histological types. Finally, limitation in diagnostic facilities in data reporting in a resource-poor health care facility are also potentially limiting.
Despite these limitations, this work gives a scoping vision of cancer in the DRC and in particular of thyroid cancer. It has established the frequencies of different forms of cancer in a country where cancer data are rare.