Radiotherapy and Oncofertility: From Physiological Foundations to Radiological Perspectives

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Radiotherapy and Oncofertility: From Physiological Foundations to Radiological Perspectives

The intersection of radiotherapy and fertility preservation represents a critical yet historically overlooked area in oncology. As cancer survival rates improve, the imperative to address long-term quality-of-life issues, including fertility, has gained prominence. This article synthesizes the physiological, endocrine, and radiological dimensions of radiotherapy’s impact on fertility, offering a comprehensive framework for understanding and mitigating these effects.

Fundamentals of Reproductive Physiology and Endocrinology

Female Reproductive System

The ovary serves dual roles as a reproductive and endocrine organ, governed by the hypothalamic–pituitary–gonadal (HPG) axis. The functional unit of the ovary is the follicle, which comprises an oocyte surrounded by granulosa cells. Follicles exist in three developmental stages: primordial follicles (PFs), primary/secondary follicles, and mature follicles. At birth, females possess a finite reserve of PFs, which progressively decline until menopause. Primordial follicles remain dormant until puberty, when the HPG axis initiates cyclical follicle maturation.

The HPG axis regulates folliculogenesis through gonadotropin-releasing hormone (GnRH), which stimulates pituitary secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH promotes granulosa cell proliferation and estrogen production, while LH triggers ovulation and corpus luteum formation. Estrogen and progesterone from ovarian follicles and the corpus luteum regulate menstrual cyclicity, while inhibin (secreted by granulosa cells) modulates FSH release via negative feedback.

Male Reproductive System

The testes contain three key cell types: spermatogenic cells (producing sperm), Sertoli cells (providing structural and metabolic support), and Leydig cells (synthesizing testosterone). Spermatogenesis begins with spermatogonial stem cells (SSCs), classified as A-dark (Ad) or A-pale (Ap). Ad spermatogonia are reserve stem cells with low mitotic activity, whereas Ap spermatogonia proliferate actively to form type B spermatogonia, which differentiate into primary spermatocytes. Spermatocytes undergo meiosis to produce haploid spermatids, which mature into spermatozoa.

The HPG axis regulates spermatogenesis via FSH and LH. FSH acts on Sertoli cells to support germ cell development, while LH stimulates Leydig cells to produce testosterone. Testosterone is essential for spermatogenesis and secondary sexual characteristics. Feedback inhibition by inhibin (from Sertoli cells) and testosterone (from Leydig cells) maintains hormonal balance.

Impact of Radiotherapy on Fertility

Female Fertility

Radiotherapy compromises female fertility through three mechanisms: ovarian follicular depletion, HPG axis disruption, and damage to accessory reproductive organs.

  1. Ovarian Damage

    • Radiosensitivity: Oocytes and granulosa cells are highly radiosensitive. The effective sterilization dose (ESD)—the dose causing ovarian failure in 97.5% of patients—varies with age:
      • 20.3 Gy in newborns
      • 18.4 Gy at age 20
      • 14.3 Gy at age 30
      • 6.0 Gy in women over 30
    • Single doses of 1.7–6.4 Gy induce temporary infertility, while 3.2–10.0 Gy cause permanent sterility. Fractionated regimens (e.g., 60 Gy in 2–4 fractions) produce equivalent damage.

  2. HPG Axis Dysfunction

    • Cranial irradiation targeting the pituitary or hypothalamus disrupts GnRH, FSH, and LH secretion.
    • Doses >30 Gy risk hypopituitarism, leading to estrogen/progesterone deficiency and amenorrhea.

  3. Uterine and Vascular Damage

    • Uterine exposure to >5 Gy correlates with fetal growth restriction; 14–30 Gy causes irreversible myometrial fibrosis and vascular insufficiency.
    • Pelvic irradiation induces vaginal stenosis and reduced lubrication, further impairing fertility.

Male Fertility

Male germ cells exhibit stage-dependent radiosensitivity:

  • B spermatogonia > Ap spermatogonia > spermatocytes > spermatids > spermatozoa.
  • Ad spermatogonia are relatively radioresistant but may undergo apoptosis under fractionated regimens.

  1. Testicular Damage

    • 0.1 Gy: Transient spermatogenic arrest.
    • 0.65 Gy: Azoospermia lasting 9–18 months.
    • 2–3 Gy: Azoospermia for 30 months.
    • 4–6 Gy: Recovery after 5 years; >6 Gy causes permanent sterility.
    • Fractionated abdominopelvic irradiation (e.g., 2.5 Gy in 18–35 fractions) irreversibly damages spermatogonia.

  2. Leydig and Sertoli Cell Dysfunction

    • Leydig cells tolerate <20 Gy but may exhibit subclinical damage (elevated LH levels).
    • Sertoli cell dysfunction reduces inhibin secretion, elevating FSH.

  3. HPG Axis Effects

    • Cranial irradiation (>12 Gy) impairs GnRH/LH/FSH secretion, delaying puberty or causing hypogonadism.

Biological Mechanisms of Radiation-Induced Infertility

Ovarian Toxicity

  • Direct DNA Damage: Ionizing radiation induces double-strand breaks in oocyte DNA, activating p53 and TAp63 proteins, which trigger apoptosis.
  • Reactive Oxygen Species (ROS): Secondary ROS production exacerbates mitochondrial dysfunction and granulosa cell death.
  • Vascular and Stromal Damage: Post-radiation fibrosis reduces ovarian blood flow, accelerating follicular atresia.

Testicular Toxicity

  • Germ Cell Apoptosis: Radiation activates Fas-mediated apoptosis in spermatogonia. DNA repair via the DNA-PK pathway is inefficient in mitotically active cells.
  • Inverse Fractionation Effect: Low-dose fractionated regimens disproportionately damage Ad spermatogonia due to cumulative mutations.

Fertility Preservation Strategies

Advanced Radiotherapy Techniques

  1. Gonadal Shielding: External shields reduce ovarian/testicular dose but are limited by anatomical variability.
  2. Proton Beam Therapy: Spares ovaries/testicles distal to the tumor via the Bragg peak.
  3. FLASH Radiotherapy: Ultra-high dose rates (>40 Gy/s) minimize gonadal toxicity through the FLASH effect.
  4. Microbeam Radiotherapy: Spatially fractionated beams (50–150 keV) spare spermatogonial stem cells.

Reproductive Interventions

  1. Female Patients

    • Oocyte/Embryo Cryopreservation: Standard for post-pubertal women.
    • Ovarian Tissue Cryopreservation: Vital for pre-pubertal girls.
    • Ovarian Transposition: Surgical relocation of ovaries from radiation fields.
    • GnRH Agonists: Controversial; may suppress follicular apoptosis during treatment.

  2. Male Patients

    • Sperm Cryopreservation: Gold standard for post-pubertal males.
    • Testicular Tissue Freezing: Experimental for pre-pubertal boys.
    • Hormonal Protection: Testosterone or GnRH analogs to suppress spermatogenesis during irradiation.

Clinical Considerations and Future Directions

Radiotherapy’s impact on fertility is multifactorial, influenced by:

  • Age: Younger patients have greater ovarian/testicular reserves but higher sensitivity to HPG axis disruption.
  • Dose/Fractionation: Cumulative dose, fraction size, and treatment volume dictate gonadal toxicity.
  • Genetic Factors: Polymorphisms in DNA repair genes (e.g., BRCA) may modify radiosensitivity.

Emerging strategies like FLASH and proton therapy hold promise for minimizing off-target effects. However, integrating fertility preservation into radiotherapy planning requires multidisciplinary collaboration, patient education, and individualized risk stratification.

doi.org/10.1097/CM9.0000000000003478

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