Uterine leiomyomas are one of most prevalent benign gynecology diseases existing in more than 60% of women (1). Leiomyomas might cause symptoms including hypermenorrhea, resultant anemia, and bulky mass led to compression effects, such as abnormal defecation, dysfunctional voiding, abdominal protruding fullness, or even low back soreness (2). Hormonal treatment, like gonadotropin-releasing hormone agonist (GnRHa), ulipristal, and letrozole, had proved efficacy of volume-reduction (3). Previous literature also found the benefit and potential efficacy for Vitamin D and Myo-inositol for uterine fibroid management (4–7). Laparotomic or minimally-invasive myomectomy or hysterectomy are common operations currently (8).

Ablation of uterine leiomyomas with high-intensity focused ultrasound (HIFU) had been developed for years and has become an option of non-surgical treatment (9). Unlike medical diagnostic ultrasounds with acoustic intensity < 0.1 W/cm², HIFU produces high intensity acoustic energy over 100 W/cm² which can be focused into the targeted tumor to provide local heat over 60°C and generate local tissue ablation by accompanied cavitation and mechanic physical effects (10, 11). The leiomyoma after hyperthermia ablation may triggered coagulation necrosis in early phase and further cell apoptosis in late phase (12, 13). The treated leiomyoma cells may be engulfed or digested by immune or inflammatory cells, such as macrophage, and then the ablated leiomyomas shrink (14–16).

Combination of HIFU and hormonal treatment can be also an available therapy for uterine leiomyoma and adenomyosis. Hormonal treatment can be used before or after HIFU treatment to control the related menstrual symptoms of leiomyoma or adenomyosis, make HIFU treatment much more efficiently, or generate better tumor ablation or more volume reduction of targeted tumor after HIFU treatment (17). From previous literatures, GnRHa used before HIFU of leiomyomas or adenomyosis can make more efficient HIFU treatment with less consumption of energy (18, 19). Besides, GnRHa alone or combined with levonorgestrel intrauterine device can also be used after HIFU to make better volume reduction, prolonged symptoms relief, and improved quality of life (20, 21).

Letrozole is one of the third-generation aromatase inhibitors commonly used for hormonal treatment of breast cancer with expression of estrogen receptors (22). Uterine myometrium had abundant aromatase that can respectively converts androstenedione and testosterone to estrone and estradiol that furtherly trigger the development of leiomyoma stem cells (23–25). Letrozole decreases the circulating estrogen by inhibiting transformation of androgen into estrogen, and therefore have efficacy on leiomyoma treatment (26, 27). Previous research reported letrozole treatment before laparoscopic myomectomy can shorten the operation time and decrease the intraoperative blood loss (28). However, combining letrozole with HIFU had not been reported before. The current retrospective study was aimed at evaluating the clinical feasibility and efficacy of letrozole treatment before HIFU ablation of uterine leiomyoma.

HIFU had already been used in treatment of leiomyoma with non-invasive methods. While compatible with minimal invasive surgery for myomectomy or hysterectomy, HIFU had been already showed the better improvement of quality of life with significance (34). Besides, a prospective “IDEAL” study by Chen et al. (35) also disclosed more rapid improvement of life quality and less morbidity via HIFU ablation than surgery. Plenty of articles had shared the experience of size reduction rates of leiomyoma after HIFU ablation. Lee et al. (36) demonstrated reduction rates as 54.7, 62.5, and 73.8% at 3, 6, and 12 months follow-ups after HIFU. The results were consistent between different articles by different groups (37), and also similar with current present study including 50–60%, 60–70%, 70–75%, and 75–80% in 3, 6, 12, and 24 months of surveillance respectively that were demonstrated. Besides, for those fibroids with initial symptoms such as hypermenorrhea, voiding frequency, or abdominal fullness, our study demonstrated there were improved symptoms in all patients after HIFU treatment while the fibroid size was decreased, and no differences were found among three groups.

The mechanism of HIFU ablation had been already well-known. HIFU can eject and collect many ultrasonic beams focusing on the target lesion site, and produce high thermal effects with cavitation effects or mechanical effects leading to coagulation necrosis in the target leiomyomas tissue (38, 39). Besides, the focused hyperthermia produced by HIFU can cause damage of cellular membranes along with protein denaturation, cell energy metabolism disruption after mitochondria dysfunction, and further decreased oxygenation as well as cellular necrosis after vascular destroy (12, 13, 40). In addition, delayed cell apoptosis also appeared after vascular damage and accompanied inflammatory response caused by hyperthermia (12, 41). The necrotic and apoptotic cells after HIFU can be phagocytosed and digested by the inflammatory cells including macrophage (42, 43). Size reduction of leiomyomas after HIFU ablation can be achieved by accumulation of death cell digestion by inflammatory cells as time went on.

Previous literatures had already demonstrated the parameters of HIFU ablation efficiency. Retrospective study by Fan et al. (37) disclosed several factors with significance for energy efficiency of single uterine leiomyoma receiving ultrasound-guided HIFU, and they reported better efficiency in patients with hypodense signal on T2WI of pre-HIFU MRI, large fibroid size, mild enhancement on T1WI of pre-HIFU MRI, and anteverted uterus. Patients with huge size of leiomyoma have higher incidence of tumor-lysis syndrome with hyperuricemia and hyperkalemia after HIFU treatment (44). Therefore, adjuvant medical treatment before HIFU treatment to shrink leiomyoma size and make reinforcement of HIFU efficacy is often used.

Other than reducing size of leiomyoma before HIFU, methods to enhance the treatment efficacy is of interest. Smart et al. (45) reported that a higher energy efficiency by NPV per energy (0.06 vs. 0.03 J/cm³, p < 0.05) was observed in GnRHa users compared with patients accepting HIFU alone. Yang et al. (18) reported pre-HIFU GnRHa treatment can shorten HIFU treatment duration (102 vs. 49 min, p = 0.046), reduce sonication time (25.4 vs. 38.9 min, p = 0.048), enhance treatment efficiency (9.9 vs. 23.8 KJ/cm³), and increase NPV ratio (69.2 vs. 50.2 %). However, GnRHa frequently causes estrogen-deprived symptoms including hot flush, insomnia vaginal dryness, and loss of bone mineral density (46). Park et al. (47) reported single dose of short-acting GnRHa before HIFU could also enhance energy efficiency (NPV per energy: 0.046 vs. 0.031 cm³/J, p = 0.041) with less GnRHa-related hypoestrogenic symptoms.

Letrozole is used initially for medical treatment of breast cancer, and the importance was increased since the issues of onco-fertility and fertility assistance after cancer treatment get more and more attention (48). The long-term risky evaluation to neonates and children after the artificial reproductive technologies was needed (49–52), and the consultation from multi-disciplinary collaboration was therefore necessary (53). Comparing with GnRHa, Letrozole also had inferior size reduction effect of leiomyomas. In a randomized controlled clinical trial, the volume reduction of leiomyoma before laparoscopic myomectomy was 45.6% in patients receiving letrozole for 12 weeks and 33.2% in those with short-acting GnRHa for 3 doses (28). The rapid onset of action and avoidance of initial gonadotropin flare with letrozole is an advantage to the patients with symptomatic leiomyoma who are mostly pre-menopausal. Letrozole can inhibit estrogen formation by suppression of aromatase and furtherly decreased the activity of estrogen receptor. From previous research, estrogen receptor had several signaling pathways including epidermal growth factor receptor (EGRF), human epidermal growth factor receptor 2 (HER2, ERBB2), and insulin-like growth factor receptor (IGFR) (54). Letrozole caused regression of tumors of estrogen-dependent human breast cancer MCF-7Ca cells grown in nude mice and increased the number of cells undergoing apoptosis (55). Meresman et al. reported that letrozole enhanced the apoptosis in the endometrial culture of patients with endometriosis (56). Animal study demonstrated apoptosis and cell proliferation in surrounding areas of coagulated necrotic tissues after ablation could be the mechanism of HIFU treatment (12). The study showed that apoptosis index detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) reached a peak value at 72 h after ablation, and the highest proliferating cell nuclear antigen (PCNA)-positive index was found at 144 h after ablation. Apoptotic bodies and oncotic mitochondria in surrounding areas were observed under the electron microscope (12). HIFU ablation can also lead to cell apoptosis and furtherly modulate tumor microenvironment of immunity and the expression of growth factors that can induce antitumor effects (57–59). Therefore, letrozole might enhance the treatment efficacy of HIFU on ablation of uterine leiomyoma.

Although the NPV ratio in our study was no significant differences among groups (p = 0.231), patients using letrozole had significantly higher treatment efficiency than those without premedication and those with GnRHa with higher NPV per min (4.52 vs. 2.39 vs. 2.34 cm³/min, p = 0.050). Comparing with GnRHa patients, letrozole group also had higher NPV per energy (0.074 vs. 0.046 cm³/J, p = 0.101) and higher NPV per sonication (29.52 vs. 18.09 cm³/sonication, p = 0.098). The degree of treatment efficacy is ever larger in patients with leiomyoma larger than 10 cm in diameter. Less energy consumption and treatment duration might also result into post-HIFU discomfort and complications. Additionally, only one (9.1%) of our patients receiving pre-HIFU GnRHa had conventional 3 months dosage, but 71.4% of letrozole group cohorts used 3 months regimen, and this may also lead to the difference of efficacy. However, a study of larger sample size should be done in the future to validate our findings.

To the best of our knowledge, the present study is the first one to combine letrozole with HIFU ablation for the treatment of uterine leiomyoma. The limitation of our study was small sample size. Besides, the regimen of GnRHa was also not unified in the medications, dosage, or using duration since it was a retrospective article. Further validation with more patients or a prospective study is needed in the future.

In summary, Letrozole as a pretreatment medication before HIFU therapy of leiomyoma can increase energy efficiency and treatment efficiency of HIFU treatment with less hypoestrogenic symptoms comparing with GnRHa, especially for leiomyoma larger than 10 cm in diameter.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors.

The studies involving human participants were reviewed and approved by Institutional Review Board of Chang Gung Memorial Hospital (IRB No. 202200674B0, on 9 May 2022). Written informed consent for participation was not required for this study in accordance with the national legislation and the institutional requirements.

Conceptualization: W-CC, H-HC, and T-CC. Methodology, validation, and data curation: W-CC and C-CH. Software, formal analysis, project administration, funding acquisition, and writing—original draft preparation: W-CC. Investigation: W-CC, C-CH, and T-CC. Resources: H-JH, W-JC, and T-CC. Writing—review and editing: W-CC and H-HC. Visualization: W-CC, C-CH, and H-HC. Supervision: H-HC and T-CC. All authors have read and agreed to the published version of the manuscript.

This study was supported by grants from Taiwan's Ministry of Science and Technology (MOST 109-2222-E-182A-002) and Chang Gung Medical Foundation (CORPG3J0441, CRRPG2L0011, CMRPG2L0261).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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