If chemotherapeutic agents do not kill stem spermatogonia, spermatogenesis recovers within 12 weeks after the cessation of therapy^7,8). However radiation or chemotherapy drugs (e.g., alkylating agents) that kill stem cells can induce azoospermia that lasts much longer than 12 weeks⁵⁾. Spermatogenesis can be impaired by agents that alkylate DNA (e.g., cyclophosphamide, mechlorethamine, ifosfamide, procarbazine, busulfan, melphalan, and the nitrosoureas BCNU [carmustine] and CCNU [lomustine]) that cross-link DNA (e.g., cisplatin). The CCSS reported that survivors aged 15-44 years who were not surgically sterile were less likely to sire a pregnancy than siblings. For example, treatment with cyclophosphamide (parenteral >9.2 g/m², oral >10.637 g/m²) produced HR of 0.42 with a 95% confidence interval (CI) of -0.31 to 0.57; procarbazine treatment (4.2-7.0 g/m²) produced HR of 0.48 with 95% CI of -0.26 to 0.87⁶⁾.

The cumulative dose of a cytotoxic agent determines the duration and magnitude of impaired spermatogenesis. Prolonged azoospermia is associated with the total cumulative dose of cyclophosphamide (single agent⁹⁾, ≥19 g/m²; combination with other drugs¹⁰⁾, >7.5 g/m²), procarbazine¹¹⁾ (≥4 g/m²), melphalan¹²⁾ (≥140 mg/m²), cisplatin¹³⁾ (≥500 mg/m²), busulfan¹⁴⁾ (≥600 mg/m²), and chlorambucil¹⁵⁾ (≥1.4 g/m²). In addition, treatments with adriamycin, vinblastin, or cytosine arabinoside have additive effects with the above agents in causing prolonged azoospermia, but cause only temporary reductions in sperm count when not so combined⁵⁾. Whether the prepubertal testis is less sensitive than the postpubertal testis to damage by chemotherapy is not known. The CCSS reported that only very young boys (<4 years of age at diagnosis) were more likely to sire a pregnancy later in life than those who were 15-20 years of age at diagnosis⁶⁾. Although the prepubertal testis does not show complete spermatogenesis, there is evidence that cytotoxic treatment given to prepubertal boys affects their later fertility^10,16). The Children's Oncology Group suggests that having a prepubertal status at diagnosis is not protective against germ cell toxicity of alkylating agents¹⁷⁾.

Testicular germ cells are very sensitive to radiation and germinal cell depletion is dose dependent. Thus, a direct radiation dose as low as 0.15 Gy can cause a significant depression in the sperm count and temporary azoospermia can occur after a dose of 0.3 Gy^18,19). Higher doses cause a more rapid onset of oligospermia and azoospermia by a direct effect on the later stages as well as earlier stages of spermatogenesis¹⁹⁾. Differentiating cells are damaged by doses as low as 1 Gy, which reduce the numbers of spermatogonia and preleptotene spermatocytes¹⁸⁾. Irradiation with much higher doses (2-3 Gy) kills spermatocytes with a resultant decrease in spermatid numbers. Spermatids show no overt damage, but after 4-6 Gy irradiation the resultant spermatozoa are significantly decreased in number, signifying that some spermatid damage must have occurred¹⁹⁾. During the first 50-60 days after lowdose irradiation (1.5-2 Gy), sperm production remains above 50% of control values and then drops dramatically with resultant temporary oligo/azoospermia¹⁹⁾. A single testicular dose of radiation exceeding 4-6 Gy can result in permanent azoospermia^16,18,20). Fractionated irradiation of the testes can be more harmful; thus, fractionated doses greater than 0.35 Gy cause aspermia with the time taken for recovery increasing with dose, and with doses of more than 2 Gy, aspermia may be permanent²¹⁾.

By appropriately expressing chemotherapy doses on a per meter squared basis and calculating radiation doses to the gonad, the doses of a variety of chemotherapy and radiotherapy regimens that can produce permanent azoospermia in survivors of childhood and adolescent cancers after they reach puberty appear to be the same as those for adults²²⁾. Radiosensitivity is much lower in Leydig cells than in spermatogenic cells. Radiosensitivity is higher in prepubertal boys than adult men, so the testicular irradiation dose inducing Leydig cell dysfunction is >20 Gy in prepubertal subjects [2493061] and this dose is known to be higher after puberty²³⁾. The CCSS showed that the likelihood of survivors siring a pregnancy decreased after radiation administered to the testes exceeded 7.5 Gy (HR, 0.12; 95% CI, -0.02 to 0.64)⁶⁾.

Gonadotropin deficiency can develop when the hypothalamus and pituitary gland are damaged by surgery, tumors, or cranial radiation. Because radiation-induced gonadotropin deficiency is dependent on irradiation dose and target tumor location, conventional fractionated cranial irradiation (30-50 Gy) induces gonadotropin deficiency in 60% of pituitary tumor survivors after 10 years and in >20% of patients with nonpituitary brain tumors⁴⁾. The severity of gonadotropin deficiency varies from subclinical to severe, when it can diminish the levels of circulating sex hormones. Radiation-induced gonadotropin deficiency also has a wide clinical spectrum from subclinical to severe forms. Clinically significant gonadotropin deficiency is usually a late complication with a cumulative incidence of 20-50% on long-term follow-up, regardless of whether radiation was administered in childhood or during adulthood⁴⁾.

Semen analysis is the initial investigation of fertility potential in cancer survivors. This test provides information about the functional status of the germinal epithelium, epididymis, and accessory sex glands. The World Health Organization (WHO) manual has established new reference values for semen characteristics in its 5th edition, which are lower than those reported previously^24,25). Although semen analysis provides useful descriptive data including sperm count, motility, vitality, and morphology, parameters above the lower reference limits do not guarantee fertility, nor do values outside those limits necessarily imply male infertility or pathology²⁶⁾. An abnormal semen analysis is suggestive of testicular germ cell damage, but azoospermia may also be secondary to ejaculatory dysfunction or hormone deficiencies in survivors at risk for those complications¹⁷⁾. When the ejaculate volume is less than 1.0 mL, the physician should perform a postejaculatory urinalysis to rule out retrograde ejaculation²⁷⁾.

The initial hormonal evaluation should consist of measurements of serum FSH, luteinizing hormone (LH), and testosterone levels as indicated clinically. Men with primary testicular failure typically show low circulating testosterone levels. When the hypothalamus-pituitary axis is intact, serum FSH and LH levels are elevated after puberty. However, if this axis is damaged by radiation, FSH and LH levels do not increase and remain within normal or low normal range.

Although serum inhibin B is known to be a marker of germ cell function, the levels of inhibin B alone or inhibin B in combination with FSH do not reflect normal spermatogenesis in patients who have undergone cancer treatment in childhood²⁸⁾. Therefore, the COG-LTFU guidelines do not recommend screening for inhibin B routinely¹⁷⁾.

Approximately 85% of the testicular mass consists of germinal tissue, so a reduced germinal cell mass is associated with reduced testicular size and a soft consistency²⁹⁾. Azoospermic and severely oligoasthenozoospermic survivors of cancer had significantly smaller mean testicular volume and higher basal FSH levels than other survivors, but small testicles (sum of both testicular volume ≤20 mL) and/or abnormally high basal FSH levels (>10 mIU/mL) were present in only half of the azoospermic survivors³⁰⁾.