SIRs for cancers that developed after primary BC were calculated using KCCR data. Analysis of the data revealed that, in the present cohort, BC survivors had a 6% lower risk of developing a new malignancy compared with the general population. Cancers of the tongue, tonsils, digestive system (e.g., stomach, colon, and liver) and non-Hodgkin lymphoma were less likely to occur as SPCs in patients with BC. However, these findings were incongruent with those reported previously [4, 5, 11]. While the reasons for reduced SPC risk in the BC patients are unclear, they might be related to smoking cessation and lifestyle modification after a BC diagnosis. Additionally, these results might, in part, be due to shared etiologies (genetic background and environment) and treatment-related factors .
A previous study that evaluated Korean patients with prostate cancer and kidney cancer using similar methods revealed SPC SIRs of 0.75 and 1.13, respectively [6, 7]. In the present study, the incidences of prostate cancer and kidney cancer were greater. This increased incidence might be due to shared etiological, environmental, and genetic factors between the first and second malignancies . Moreover, a surveillance effect might contribute to increased risk immediately after diagnosis and might explain the elevated prostate cancer and kidney cancer risk after primary BC.
In a study examining associations between urinary tract cancers, Kinoshita et al. demonstrated that BC and prostate cancer share similar traits such as DNA repair and N-acetyl transferase polymorphism . Kellen et al. reported that prostate cancer risk increases in patients < 70 years old within one year of BC diagnosis . Lococo et al. also reported a significant increase in the relative risk of kidney cancer following BC .
In this study, we interestingly found that the risk for tongue and tonsil cancer significantly decreased in patients with BC, and the result for tongue cancer is significantly lower in those over 60 compared to those aged 40–59. Chemical factors like tobacco and alcohol, biological factors like human papillomavirus (HPV), syphilis, oro-dental factors, dietary deficiencies, chronic candidiasis and viruses have been known to be significantly associated with oral cancer . The mechanism of the declined risk of tongue and tonsil cancer is still unclear. However, we speculated that life style modification (smoking cessation, diet, and so on) may reduce chance of developing tongue and tonsil cancer.
Smoking is a well-known risk factor for BC, kidney, lung, mouth, and pharynx cancers  and has been estimated to cause half of all BC cases in Western countries . In contrast to our hypothesis, the present study did not show an increase in the number of subsequent respiratory system malignancies. Therefore, a sub analysis according to histological lung cancer type was performed, showing a significant increase in the incidence of lung squamous cell carcinoma and adenocarcinoma as SPCs. Possible reasons include potential etiological or genetic background differences between the Western and Asian patients, decreased smoking contribution compared with the West, and the possibility that the other cancers were smoking-related and occurred before the BC diagnosis.
The risk of cancer caused by radiation follows the individual exposed to radiation and continues to increase throughout the individual’s lifetime . Studies evaluating the risk of secondary cancers after radiation therapy for prostate cancer have shown mixed results [21,22,23]. Recent meta-analyses have shown that patients who received prostate cancer radiotherapy are more likely to have a second malignancy of the bladder, colon and rectum than patients who have not received radiotherapy . To our knowledge, although no studies have reported the risk of secondary cancer after radiation therapy for bladder cancer, our study showed that secondary cancers were more common in the digestive organs, such as the small intestine, colon, rectum, and female/male genital systems than patients who do not received radiotherapy. We assume that this result is related to the radiation field and is similar to the results of the meta-analysis mentioned above . However, the lack of information such as the specific type of radiation treatment and dose of radiation is another limitation of this study.
To our knowledge, this is the first study to evaluate the histological subtypes of lung cancers as an SPC. Cigarette smoking is an established risk factor for lung cancer, but the severity of its association with other histologic types is unclear. Khuder [25, 26] reported that all histologic lung cancer types were significantly associated with cigarette smoking, and the association was stronger for squamous cell and small cell carcinomas compared with large cell cancer and adenocarcinoma. In the present study, squamous cell carcinoma and adenocarcinoma exhibited a significantly elevated risk of occurring as an SPC. Although not significant, small cell lung cancer risk also increased over an extended follow-up period. If smoking is a shared risk factor for BC and secondary lung cancer, the influence of smoking on each histologic type of secondary lung cancer in BC patients might be presumed to be different to that for primary lung cancer.
Cumulative survival curves of patients with or without SPC were estimated to investigate whether SPC affects the survival rate of patients who have BC. In particular, for the first 8 years, the SPC group had superior survival rates compared with the non-SPC group. Overall, this study demonstrated that patients in the non-SPC group had significantly more advanced BC at the time of diagnosis. Therefore, the survival rate of the non-SPC group was lower than that of the SPC group for the first 2.5 years after diagnosis of BC. Conversely, after 2.5 years the reverse was noted with survival in the SPC group being inferior to that in the non-SPC group. These findings suggested that the SPC group would require more attentive and systemic surveillance after 2.5 years of follow-up.
The present study has several limitations. First, information concerning several potential confounding variables including smoking, alcohol consumption, obesity, and familial cancer history were not available. Second, there was limited data available concerning genetic factors and specific cancer stages among the patients, making it impossible to evaluate the correlation between disease severity and SPC incidence. Third, the higher incidence of SPC might be associated with close surveillance or misclassifications because BC, prostate cancer, and kidney cancer often develop synchronously [15, 16]. Fourth, the median follow-up period was 4.13 years, which was not relatively long duration. Further studies with long follow-up periods will be needed to estimate the precise risk of developing SPC and to overcome surveillance bias. Fifth, it was impossible to divide into non-muscle invasive cancer and muscle invasive cancer in our study. The survival of patients with bladder cancer may be affected by degree of muscle invasion.