Radiotherapy and oncofertility: From physiological foundations to radiological perspectives

To the Editor: Regrettably, the oncological discourse has historically sidelined fertility considerations. Patient–practitioner dialogs rarely traverse the domain of fertility conservation, a situation worsened by a general lack of specialized knowledge among healthcare providers. Research endeavors in the field of radiation and oncofertility are nascent, with a lack of detailed explanation on the radiation doses for different tissues and insufficient elaboration on reproductive physio-endocrinology, leaving vast territories uncharted. Therefore, it is imperative for practitioners in the oncological radiotherapy field to fathom the intricate dynamics between radiotherapy and reproductive health. Our review delves into the effects of ionizing radiation on fertility from both reproductive physiological and endocrine perspectives, and proposes fertility preservation strategies for cancer patients. The methods to select and synthesize data are detailed in the Supplementary File, https://links.lww.com/CM9/C307. Fundamentals of reproductive physiology and endocrinology. The ovary and testis, serving as both primary sexual and endocrine organs, are regulated by the hypothalamic–pituitary–gonadal (HPG) axis. The study of radiation’s effects on fertility must integrate reproductive and endocrine impacts. Female reproductive physiology and endocrinology. The elemental unit of female reproductive system is the follicle, known as the “ovarian reserve”. Each follicle consists of an immature oocyte and granulosa cells, and is divided into three stages: primordial follicles (PFs), primary and secondary follicles, and mature follicles. In the ovaries, the number of PFs is limited and declines until menopause. At birth, all primitive germ cells developed into primary oocytes which remain dormant for years. Before ovulation, they complete their first meiotic division to form secondary oocytes. If unfertilized, the oocytes degenerate and are absorbed; if fertilized, they further divide into mature ova. During puberty, the HPG axis governs follicle differentiation. The hypothalamus releases gonadotropin-releasing hormone (GnRH), stimulating the pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH), thereby facilitating follicle maturation. Concurrently, the ovaries fulfill an endocrine role. Estrogen and inhibin, secreted by the follicle’s granulosa cells, along with progestin produced by luteal cells, collectively regulate the menstrual cycle. Male reproductive physiology and endocrinology. The testes, pivotal in male fertility, compromise three main cell types: spermatogenic cells for sperm production, sustentacular cell (Sertoli cells) for support and inhibin synthesis, and interstitial cells (Leydig cells) for androgen secretion. Spermatogenic cells progress through several stages: spermatogonia, spermatocytes, spermatids, and spermatozoa. Clermont’s theory posits spermatogonial stem cells (SSCs), specifically A-type spermatogonia, including two subtypes: A-dark (Ad) cells and A-pale (Ap) cells. Ad cells serve as reserve stem cells that remain dormant until stimulated to divide. In contrast, Ap cells are highly active in mitosis, leading to B-type spermatogonia formation. Type B cells then undergo rapid mitosis to generate primary spermatocytes. Generally, cells with high proliferative capacity and undergoing active mitosis are vulnerable to ionizing radiation. Radiation sensitivity decreases from spermatogonia B > spermatogonia Ap > spermatocytes > spermatids > spermatozoa, and spermatogonia Ad showing relative radio-resistance. Spermatogenesis is closely regulated by hormones. Activation of the HPG axis prompts pituitary secreting FSH and LH. FSH promotes sperm development via Sertoli cells, while LH induces Leydig cells to produce testosterone, crucial for spermatogenesis and male traits. And FSH and LH are governed through negative feedback by both cells. Impact of radiotherapy on fertility.Impact on female fertility. Successful conception hinges on three essentials: sufficient follicular reserve, a well-functioning HPG axis, and optimal cooperation of accessory organs, including the uterus, vagina, and cardiovascular system. The adverse effect of radiotherapy on any of these factors can potentially reduce fertility. Germ cells are particularly vulnerable to radiation, with sensitivity varying by age, dose, fractionation, and dosage rate. As age increases, ovarian follicles reserves diminishes and radiation sensitivity heightens. Doses as low as 1–2 Gy in girls while 0.4–0.6 Gy in adults induce ovarian dysfunction. The effective sterilization dose, at which 97.5% of females display ovarian dysfunction post-radiotherapy, decreases with age: 20.3 Gy in newborns, 18.4 Gy at age of 20 years, 14.3 Gy at age of 30 years, and 6.0 Gy for those over 30 years old. A single dose of 1.7–6.4 Gy can induce temporary infertility, while 3.2–10.0 Gy can cause permanent infertility. Fractionated radiation usually lessens damage; higher doses such as 60 Gy in 2–4 fractions produce equivalent damage. Additionally, radiation also damages ovarian stroma and leads to fibrosis, although the corpus luteum shows more radio-resistant. Even the HPG axis is relatively resistant to cranial irradiation, low radiation (2.2–2.7 Gy) targeting the pituitary can affect fertility. Exposure to 35–40 Gy links to hypopituitarism, with similar effects observed at even lower doses above 30 Gy. High-dose exposure disrupts GnRH, FSH, and LH pathways, leading to mis-secretion of estrogen, progesterone, and prolactin, consequently, resulting in gonadal anomalies and infertility. Radiotherapy also compromises accessory organs, including uterus, vagina, and cardiovascular system. Doses above 5 Gy to the uterus are associated with fetal growth restriction and premature birth. Exposure to 14–30 Gy irradiation can severely damage the uterine muscle or vascular supply, potentially hindering conception or inducing miscarriage. At dose over 30 Gy, this damage becomes irreversible. Pelvic radiation also leads to loss of lubrication and vaginal stenosis, potentially reducing fertility. Pregnant women exposed to above 30 Gy of cardiac radiotherapy or combined anthracyclines have higher risk and should undergo thorough cardiac assessments. Overall, the irradiation impact on fertility varies with age, radiation dose, and fractionation, as detailed in Supplementary Table 1, https://links.lww.com/CM9/C307. The female radiotherapy-associated risk stratification by the American Society of Clinical Oncology is shown in Supplementary Table 2, https://links.lww.com/CM9/C307.[1] Impact on male fertility. Radiation sensitivity among spermatogenic cells varies with stages, generally decreasing through differentiation: spermatogonia B > spermatogonia Ap > spermatocytes > spermatids > spermatozoa. Spermatogonia Ad are radio-resistance, serving as reserve stem cells that remain dormant until stimulated to divide. Even low exposures of 0.1 Gy can cause a transient spermatogenic arrest. A dose of 0.65 Gy might induce azoospermia, with recovery time depending on doses: below 1 Gy results in azoospermia lasting for 9–18 months, 2–3 Gy for 30 months, 4–6 Gy for 5 years, and 6 Gy for permanent azoospermia. Fractionated irradiation is more harmful to testis than a single dose, with 2.5 Gy abdominopelvic radiotherapy in 18–35 fractions causing irreversible azoospermia. This effect contrasts with females, as in males, the relatively radio-resistant Ad spermatogonia differentiate into the more vulnerable type B spermatogonia under fractionated irradiation. Furthermore, low-dose radiation is more damaging than high doses, likely due to the “inverse fractionation effect,” leading to unrepairable mutations accumulation and escalated cell death in spermatogonia Ad. Children’s germ cells exhibit higher resistance to radiation than those of adults. Testicular interstitial cell function in boys may be compromised at doses exceeding 20 Gy. Non-reproductive cells in testes, such as Leydig and Sertoli cells, are relatively resistant to radiation. Under low-dose radiation, Sertoli cells can maintain or slightly decrease inhibin secretion, leading to elevated FSH levels. While testosterone secretion by Leydig cells remains stable, some patients may exhibit early increased LH levels. After adjuvant radiotherapy (14–20 Gy) for testicular cancer, 18 out of 42 individuals required hormonal supplementation one year later. Even with normal androgen levels post-radiation under 20 Gy, elevated serum LH levels hint subclinical interstitial Leydig cell damage. Radiotherapy to the hypothalamus and pituitary disrupts gonadotropin secretion and testicular function. Pubertal development and sexual function are impaired when exposed to irradiation 12 Gy. Cranial exposure to 35–40 Gy can compromise gonadotropin levels, potentially causing pubertal delay. The effects of radiotherapy on male fertility and risk stratification are shown in Supplementary Table 3, https://links.lww.com/CM9/C307.[1] Biological mechanisms of radiation-induced infertility. Radiation damages cells directly by deoxyribonucleic acid (DNA) double-stranded breaks, and indirectly by producing reactive oxygen species (ROS). ROS disrupt cellular functions by activating p53, triggering cytochrome C secretion and caspase activation, induing mitochondrial apoptosis, hampering DNA repair, and ultimately leading to cell death. Moreover, radiation affects interactions between tumor cells and tumor microenvironments, such as immune cells, fibroblasts, macrophages, and the extracellular matrix.[2] Additionally, radiation impacts female fertility primarily by damaging oocytes, through DNA ionization, triggering the TAp63 protein, and leading to PFs loss. Chk2-dependent apoptotic death mechanism is also involved in radiation-reduced follicles loss. Subsequent vascular damage and tissue fibrosis can further contribute to ovarian dysfunctions. In males, radiation damages spermatogonia and primary spermatocytes, causing increased apoptosis, even at low doses (0.25–2 Gy), and severely affecting spermatogenesis. The heightened radio-sensitivity of type Ap and B spermatogonia results in pronounced apoptosis, rendering them particularly susceptible to such damage. Abnormal sperm exhibit elevated Fas levels, and γ-H2AX and p53 interactions further promote spermatogonial apoptosis. DNA repairs are facilitated by the DNA-dependent protein kinase catalytic subunit pathway, underscoring alternative mechanisms in radiation response. Preservation approaches for fertility. Advanced radiotherapy techniques aim to protect fertility. Conventional gonadal shielding applies external shields to protect ovaries, but accurate positioning is challenging due to variable locations and scatter radiation. Microbeam radiotherapy, a form of spatially fractionated radiation therapy, utilizes parallel 50–150 keV X-ray microbeams to protecting SSCs during spermatogenesis. Proton beam therapy delivers precise radiation to tumors while sparing ovaries located beyond the spread-out Bragg peak.[3] Additionally, ultra-high dose rate (over 40 Gy/s) radiotherapy, delivering electron or proton beams, can reduce gonadal toxicity via FLASH effect.[4] Reproductive medicine offers successful techniques like embryo and oocyte cryopreservation. Ovarian tissue cryopreservation is vital for pre-pubescent girls, and ovarian transposition is crucial for abdominopelvic radiotherapy patients. GnRH agonists application remains debated. For males, fertility preservation options include sperm or testicular tissue cryopreservation, alongside hormone strategies such as GnRH or testosterone adjustments. The review provides a comprehensive analysis of fertility’s physiological and endocrine basis, extending beyond the traditional focus on reproductive cells and organs. Our review highlights the importance of whole-body interactions, incorporating both non-reproductive cells (supportive and stromal cells) and organs (HPG axis, uterus, and heart). Additionally, we explore the multifaceted radiotherapy parameters on fertility, including dosage, fields, and fractionation. Further, we shed light on radiobiology mechanisms and preservation approaches within oncofertility. Notably, microbeam, proton beam, and FLASH therapy show promise in fertility preservation. In conclusion, the effect of radiotherapy on fertility is multifaceted, influenced by dosimetry and fractionation, and patient-specific factors such as age, sex, and genetic traits. Promoting awareness and integrating individual fertility goals into radiotherapy planning is critically urgent. Acknowledgements We extend our gratitude to the colleagues in the Gynecologic Oncology Ward of the Cancer Center, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, as well as the physicists involved in the interpretation of radiotherapy dosages in this study. Funding This study was supported by the National Natural Science Foundation of China (No. 81974463). Conflicts of interest None.