The etiology of chronic myeloid leukemia (CML) remains essentially unknown with exposure to high doses of ionizing radiation being the only well-established risk factor.1 We have recently published two large population-based studies showing an increased prevalence of other malignancies in patients with CML, both preceding and subsequent to their diagnosis, as compared with age- and gender-matched controls.2, 3 One may therefore speculate whether CML patients have a congenital or acquired susceptibility to develop cancer. In the former case, one would expect an increased prevalence of malignancies among first-degree relatives (FDR) of CML patients.

In a previous report based on information in the Swedish Cancer Register, no increased aggregation of malignancies was detected among family members of CML patients diagnosed between 1958 and 2004.4 However, a more strict definition of CML (requiring, for example, the presence of a Philadelphia chromosome or the BCR/ABL fusion gene) was introduced with the updated WHO classification in 2002, making subsequent CML cohorts more stringently diagnosed.5

Aiming to examine the prevalence of malignancies among FDR of a large and well-defined contemporary CML cohort in Sweden compared with randomly selected population controls, we used information retrieved from four Swedish population-based registers.

To identify patients with CML diagnosed between 2002 and 2013, we used the Swedish CML Register to which virtually all Swedish CML patients diagnosed January 1st 2002 and later are reported. The diagnosis of CML was generally based on a cytogenetic finding of the Philadelphia chromosome by karyotyping or by fluorescence in situ hybridization, and/or by detection of the BCR-ABL1 fusion transcript by reverse transcription-polymerase chain reaction, as well as a blood and bone marrow picture typical for this disease, morphologically verified by hematopathologists (that is, granulocytosis, hypercellular marrow with a left-shifted granulocytopoiesis and a varying degree of increase in percentage of immature cells). In a minute portion (3%) of patients included in the CML Register, cytogenetics had not been performed but were included based on their clinico-pathological picture compatible with typical CML.

Each CML patient was matched with five, age-, sex- and county of residence-matched controls, randomly selected from the Swedish Total Population Register. All controls had to be alive and free of CML at the time of diagnosis for the matching CML patient.

FDR of both CML patients and matched controls were identified by use of the Swedish Multi-Generation Register, which includes information about parent–sibling–offspring relationships of persons born later than 1932 and registered in Sweden at some time since 1961. By means of record linkage to the Swedish Cancer Register, we retrieved information about malignancies diagnosed later than 1958.

To calculate odds ratio (OR) and 95% confidence intervals (CI), conditional logistic regression was used.

Using the Swedish CML Register, we identified 984 patients diagnosed as CML in chronic, accelerated or blastic phase. Among them, 184 patients were born before 1932 and were therefore excluded. For the 800 remaining CML patients, 4287 FDR were identified and included in the analysis (parents: 1346, siblings: 1497 and children: 1444). Correspondingly, 20 930 matched controls were included in the analysis.

In total, 611 malignancies were identified among the FDR of CML patients compared with 2844 in the control group, yielding an OR of 1.06 (95% CI: 0.96–1.16).

Neither hematological malignancies nor solid tumors were increased in the CML–FDR group (Table 1). Notably, none of the FDRs in the CML–FDR group had a CML diagnosis. Results remained unchanged following exclusion of nine patients in the CML register with only a clinico-pathological picture typical of CML and their matched controls.

Table 1 Odds ratio for malignancies among first-degree relatives of CML patients
Full size table

Thus, using population-based data based on the fate of more than 4000 FDR of 800 CML patients diagnosed in the modern era of cytogenetics and molecular assays, we found no evidence of familial aggregation of malignancies (including CML) of patients with CML. Taken together, our findings suggest that a hereditary predisposition to develop cancer is unlikely to be a part of the pathogenesis of CML.