The initial search yielded a total of 1,086 articles, of which 692 were eliminated due to duplication. After reviewing the title and abstract, a total of 117 articles were selected for full review; 28 articles were ultimately selected in this analysis (Fig. S1). Furthermore, only proven or probable cases were included; possible cases were excluded from the count of patients with IFI.

Counts are reported as follows: (a) ‘studies reporting’ = number of included publications mentioning a variable (n = 28); and (b) ‘cases/isolates’ = the number reported within studies (MM patients, IFI episodes, or microbiological isolates). Percentages use the stated denominator in each instance.

The geographic analysis derived from the systematic review shows a higher concentration of studies on invasive fungal infections (IFIs) in patients with multiple myeloma (MM) in regions such as Asia. Of the studies (a) included in this systematic review [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34], it is notable that most were conducted in developed countries such as the United States (14.3%, n = 4) [31,32,33,34], Japan (10.7%, n = 3) [19,20,21], and China (10.7%, n = 3) [10,11,12]. Notably, no studies from Latin American countries were identified.

Regarding the geographic distribution of reported cases (b), the highest number of patients was recorded in Sweden (49.7%, n = 813) [26], followed by South Korea (18.9%, n = 309) [22, 23], and the United States (11.6%, n = 190) [31, 32]. Other countries with significant figures were Australia (3.8%, n = 62) [7, 8], Japan (2.5%, n = 41) [19], Germany (2.4%, n = 40) [17, 18], Turkey (1.8%, n = 29) [29], China (1.7%, n = 28) [10,11,12], Finland (1.5%, n = 25) [14, 15], and Taiwan (1.5%, n = 24) [27, 28]. Smaller case numbers were reported in Poland (n = 16) [24], the United Kingdom (n = 16) [30], France (n = 7) [16], Belgium (n = 3) [9], and Spain (n = 3) [25]. Finally, multicenter studies in Europe accounted for 1.9% (n = 31) [13] of the patients included in this review.

This distribution suggests a geographic bias in data availability, possibly associated with hospital infrastructure, epidemiological surveillance systems, and scientific publication in these countries, which should be considered when interpreting the global burden of IFI in patients with MM (Fig. 1).

World map estimating the regions with the highest reporting of patients with multiple myeloma and IFI. Based on literature reviews. The map was created with Mapchart.net

Of the studies analyzed (a), 75.0% (n = 21) [7, 8, 10, 12, 14,15,16,17,18, 20,21,22,23,24,25,26,27,28,29,30, 33] were retrospective cohorts, and 10.7% (n = 3) [11, 19, 32] were prospective cohorts. Case–control studies, case series (14.3%, n = 4) [31,32,33,34], and retrospective observational studies were less frequent (Table 1). Only 11 of the 28 studies reported demographic data analyzed by sex [8, 13, 14, 18, 21, 25, 28, 31,32,33,34], including a total of 288 patients. In most of the 1,637 patients (82.4%, n = 1,349), sex was not specified, while 9.2% (n = 151) were men and 8.4% (n = 137) were women. Consequently, it was not possible to determine which sex was more frequently affected by IFIs across the studies. Regarding patient age, considerable variability was evident among the patients analyzed, with means ranging from 43 to 72 years. Interestingly, the highest mean age (over 60 years) was observed in MM patients from Sweden [26], Taiwan [27, 28], the United States [31,32,33,34], and Germany, while the lowest age (under 50 years) was reported in Japan [19, 20], Turkey [29], Finland [14, 15], and the United Kingdom [30]. The estimated overall mean age was 55.5 years, calculated from the available patient data (Fig. S2).

Regarding the type or site of infection (b), pulmonary involvement was the most frequent, representing 48.2% of cases (n = 27) [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,