3.2. Description of Studies and Study Characteristics
The 15 studies that passed all eligibility criteria came out between 1900 and 2020 (Table 1); 12 of the studies were published in English12,30,42,43,34–41, two of them published in Chinese10,44, and one published in Spanish1, including 5 studies in KTP group10,30,34,35,44 and 10 articles in CO2 group1,12,36–43. These studies totally involved 612 patients with LP and 2120 surgeries treated with CO2 laser/KTP laser, including 102 KTP cases and 510 CO2 cases separately. There are 8 retrospective studies1,12,36–38,40,42,43 with only 2 prospective works39,41 in CO2 group, while KTP group contains 2 retrospective10,44 and 3 prospective studies30,34,35. Besides, the patient population selected in studies is different and most studies recruited patients with recurrent laryngeal/respiratory papillomatosis, while others included inverted papilloma30, laryngeal papilloma1,38,42,44, juvenile-onset RRP39 (Table 1). All studies included males and females with a higher percentage of males (Mele: 65.61% and 66.25% for KTP and CO2 respectively), and there was no differences between the gender percentages of the two groups (p>0.05) (Table 2). However, significant difference existed between the average age of KTP and CO2 group (p<0.001) before surgery (Table 2). Besides, sample sizes ranged between 3 and 244 (median 39). Specifically, the sample sizes for the CO2 group and KTP group were 3-222 and 9-39, respectively. The evidence levels of all the studies were classified as level D. All studies accessed samples from outpatient departments.