As mentioned earlier, human testes are relatively more sensitive to radiations compared to rodents. This means even subtle exposure to low-dose radiation can result in impairment of testicular function. For example, treatment with 1 Gy radiation showed significant reduction in the number of spermatocytes just after 14 day of treatment. The reduction in spermatocyte count was even more drastic at 25 days after treatment (Rowley et al. 1974). It was noted that reduction in spermatocytes count was not abrupt. Although radiation kills the cells immediately by necrosis and/or apoptosis or during their proliferation, the decline in spermatogonia number to their lowest level does not happen at once but instead occurs in a progressive manner. It may take several weeks to reach their lowest level depending upon the dose of irradiation (Rowley et al. 1974; Clifton and Bremner 1983). However, azoospermia is not achieved until 18 weeks of irradiation (Paulsen 1973). The actual reason of this gradual decline is not very clear, but one can speculate that a small population of non-cycling A-spermatogonia escapes the potential effects of radiation perhaps when they are in dormant phase of cell cycle (Meistrich 2013). Recovery of the A-spermatogonia starts after 21 weeks of irradiation which indicates that the self-renewal exceeds cell depletion at this time. As seen in animal studies, as well as in humans, the damage to spermatogenesis or testicular functions depends upon the radiation doses. High doses of radiation can result in permanent azoospermia leading to the killing of all spermatogonial stem cells. When a single dose of 10 Gy was given to the patients undergoing bone marrow transplantation, only 15 % of them regained their fertility (Meistrich 2013; Sanders et al. 1996).

The recovery of spermatogenesis after cancer treatment has been found to be linked to many factors such as the time of diagnosis, sperm parameters before the start of treatment, the nature of the treatment, both sperm parameters, and the nature of treatment, or sometimes no relation has been found with any of the above factors (Hansen et al. 1990; Petersen et al. 1999; Ishikawa et al. 2004; Bahadur et al. 2005; Pectasides et al. 2004; Eberhard et al. 2004; Lampe et al. 1997; Gandini et al. 2006). Therefore, regular cancer screening is imperative in patients with family history of cancer. This will allow the early detection and treatment of the disease with less severe effects on their future fertility.

When mice were subjected to whole body irradiation of 2 Gy, a significant reduction in epididymal sperm count was observed in the irradiated group compared to the control, 28 days after treatment (Searle and Beechey 1974). This sperm concentration further reduced almost reaching zero at 42 and 49 days after treatment. Similar results in sperm count and morphology have been documented by others when testes of two different strains of mice (C₅₇ BL and B₆C₃F1) were exposed to wider range (0, 0.3, 1.0, and 3 Gy) of X-rays. The highest dose (3 Gy) caused a drastic decrease in sperm number obtained from cauda epididymis of both the strains at 42 and 49 days postirradiation. Interestingly, the rise in percentage of sperm abnormalities was notable as early as 21 days and was seen in both groups (Bruce et al. 1974). In a recent study, treatment of mice with 2 Gy dose has shown significant reduction in cauda epididymal sperm count as well as percentage sperm viability in irradiated group compared with control group even after 24 h of treatment (Li et al. 2013).

When rats were irradiated (whole body irradiation), with a wider range of irradiation doses (0.675, 1.350, 2.700, and 4.050 Gy), abnormal sperm morphology was observed compared to control values in almost all doses. These abnormalities became more severe in 2.700 and 4.050 Gy groups, where most of the sperm tails were eroded out (Chatterjee et al. 1994).

In monkeys (Macaca fascicularis), when testes were irradiated with a single dose of 2 Gy X-rays, mean sperm concentration per ejaculate significantly decreased to 9.2 ± 3.5 × 10⁶ at day 35 postirradiation compared to pretreatment value (60.3 ± 15.5 × 10⁶). The decrease in sperm count was more drastic at 60 days postirradiation, and some monkeys appeared with azoospermia beyond that period (Foppiani et al. 1999). The same study found reduction in percentage sperm morphology (71.4 ± 9.9) 42 days after irradiation compared to pretreatment values (89.8 ± 3.0).

A multicenter prospective study conducted at CECOS network of France has published interesting data on radiotherapy and sperm parameters. They recruited 129 testicular germ cell tumor (TGCT) patients. One semen sample was collected from each patient before starting the treatment to serve as his control, and then treatment was started. Subsequent semen samples were collected at 3, 6, 12, and 24 months postirradiation. The results showed significant reduction in sperm count and sperm motility at 3 months postirradiation, which remained significantly low until 12 months. The sperm count and motility reached control values only after 24 months postirradiation (Bujan et al. 2013; Di Bisceglie et al. 2013). In their study, sperm count decreased at 6 months after treatment and remained low compared to baseline values up to 12 months postirradiation. Eighteen months after treatment, almost all patients recovered normal sperm count, and their counts remained unchanged for the rest of study period (36 months).

Radiation effects are more severe if it involves total body irradiation (TBI) and/or combined with cyclophosphamide (CY). Out of 25 patients who underwent a combination of bone marrow transplantation and TBI/CY treatment, only one patient had recovered spermatogenesis levels to normal. The recovery time was much delayed (75 months) compared to those patients who had only radiation therapy (Jacob et al. 1998). The kinetics of recovery of spermatogenesis after radiation therapy is much more delayed compared to chemotherapy (Meistrich 2013; Bujan et al. 2013). In patients treated with hemi-pelvic or given pelvic radiotherapy, spermatogenic recovery did not start until 9 months postirradiation even with modest doses (0.5–0.8 Gy), and at higher dose (1.7 Gy), the delays were even more prolonged (Meistrich 2013).

Information on sperm DNA integrity after radiation treatment is scanty in animal models. Very few studies have been conducted on mouse. In one study, mice were subjected to acute testicular X-rays exposure with a maximum dose of 6 Gy. Spermatozoa were collected from epididymis 35 days postirradiation for DNA damage investigation. Analysis revealed that 30–40 % of spermatozoa had damaged DNA after the treatment. Some had chromosomal aneuploidy, and some others had DNA strand breaks (Pinkel et al. 1983). In an in vitro study, mouse spermatozoa recovered from epididymis were exposed to range of radiation doses (0–100 Gy). A linear dose-dependent DNA damage was observed in irradiated spermatozoa compared with controls (Haines et al. 1998). Further, in a more recent study on mice, Li et al. (2013) found a dose-dependent increase in sperm DNA fragmentation. They measured both sperm DNA fragmentation index (DFI) and high DNA stainability (HDS) through sperm chromatin structure assay (SCSA). The term DFI reflects the percentage of damaged DNA divided by the total sperm DNA which actually gives the loss of sperm DNA. HDS represents the immaturity and less condensed chromatin where sperm attain greater stainability. In these experiments, percentage of DFI was significantly higher in experimental group exposed to 2 Gy X-rays compared to control group (21.35 ± 0.78 vs 9.54 ± 0.31). Percentage HDS was also significantly higher in the experimental group compared to control group (12.03 ± 0.35 vs 3.90 ± 0.17). In another interesting study, sperm DNA damage was induced by exposing mice testicular region to different radiation doses (0.0, 2.5, 5.0, and 10.0 Gy). Comet assay was performed to quantify the sperm DNA damage. The percent tail DNA damage in spermatozoa exposed to 2.5 Gy was significantly higher compared to controls (7.98 ± 0.42 vs 5.44 ± 0.35). The difference became more significant at 5 Gy and 10 Gy (9.67 ± 0.44 and 12.14 ± 0.52) respectively (Kumar et al. 2013b).