Toxicity of chemotherapy and radiotherapy

Cancer in childhood is rare, with approximately 1400 new cases per year, and a cumulative risk of around one in 500 by the age of 15 years in resource-rich countries.  With long-term survival rates approaching 73%, it has been estimated that, by the year 2010, about one in 715 of the adult population will be a long-term survivor of childhood cancer (1). Cancer is more common after puberty during the reproductive lifespan of men and women (2,3), but many of these patients are cured by different treatment modalities. Long-term survivors are nevertheless at risk of developing a number of late sequelae (4), including impaired fertility, adverse pregnancy outcomes and health problems in offspring (5,6). Loss of fertility is one of the most devastating consequences of cytotoxic therapy for these young patients who, having overcome their disease, have expectations of a normal reproductive life.

Normal ovarian function

In the human ovary, oogonia that arise from primordial germ cells in the yolk sac reach a maximal complement of 6-7 million by the sixth month of gestation.  This represents the total fixed number of germ cells available.  Primordial follicles consist of a primary oocyte surrounded by a single layer of spindle-shaped cells.  Germ cell numbers decline in utero, so that there are approximately two million primordial follicles present at birth.  By a process of ongoing apoptosis and attrition, approximately 400,000 primordial follicles are present at the time of menarche.  This number continues to decline steadily throughout life.  The ‘fertile window’ in females is characterized by roughly 400 monthly ovulations of a mature oocyte. Continuous follicle recruitment, resulting in atresia or ovulation, leads to exhaustion of the follicle pool and menopause at a median age of 51 years (5).

Radiation and the hypothalamic-pituitary-ovarian axis         

The ovaries may be damaged following total body, abdominal or pelvic irradiation and the extent of the damage is related to the radiation dose, fractionation schedule and age at the time of treatment (7). The human oocyte is sensitive to radiation, with an estimated LD50 of less than 2 Gy (8). The number of primordial follicles present at the time of treatment, together with the dose received by the ovaries, will determine the ‘fertile window’ and influence the age of premature ovarian failure (POF).  Ovarian failure has been reported in 90% of patients followed up in the long term after total body irradiation (TBI) (10-15.75 Gy) and in 97% of females treated with total abdominal irradiation (20-30 Gy) during childhood (9,10). It is now possible to predict the age at which ovarian failure will occur, and the estimated sterilizing dose following any given dose of radiotherapy at any given age, based upon the application of a mathematical solution to the Faddy-Gosden model for natural oocyte decline (Figure 1) (11). This can help clinicians provide accurate information when counseling women about fertility following treatment for childhood cancer.

Surgical transposition of the ovaries outside the irradiation field prior to initiation of pelvic radiation is the most commonly used fertility preservation measure today. This approach may be considered for patients not receiving high-dose systemic chemotherapy. Surgery is effective at protecting the ovaries from irradiation damage, but fertility may be affected by scatter radiation, damage to the ovarian vasculature during surgery, or torsion of the transposed ovary (12).

Gonadotropin deficiency following high-dose cranial irradiation (>24 Gy in the treatment of brain tumors) manifests as delayed puberty or absent menses and can be treated by hormone replacement therapy. Interestingly, early puberty is often reported in females with cranial radiation doses of <24 Gy (13). However, we have shown a subtle decline in hypothalamic-pituitary-ovarian function following low-dose cranial radiotherapy (18-24 Gy). This is characterized by decreased LH secretion, an attenuated LH surge, and shorter luteal phases, which are likely to herald incipient ovarian failure or be associated with early pregnancy loss (14).

The ovary and follicles

The stockpile of primordial follicles found in the cortex of the ovaries represents the ovarian reserve. Histological studies of human ovaries have shown chemotherapy to cause ovarian atrophy and global loss of primordial follicles (15,16). However, these studies of human ovarian biopsies do not provide any information on the mechanism of injury. The effect of chemotherapy on the ovary is not an “all or nothing” phenomenon, and the number of surviving primordial follicles following exposure to chemotherapy correlates inversely with the dose of chemotherapy (17).

The mechanism involved in the loss of primordial follicles in response to anticancer therapy is not well understood. A few human and animal studies have demonstrated that chemotherapy induces damage to ovarian pregranulosa cells (18), and that apoptosis occurs during oocyte and follicle loss (19). In addition, injury to blood vessels and focal fibrosis of the ovarian cortex are further patterns of ovarian damage caused by chemotherapy, evidenced in ovaries of patients previously exposed to nonsterilizing chemotherapy (20). Fibrosis and vascular changes were also reported by Nicosia et al (21) and Marcello, who examined ovarian tissue from girls treated for acute lymphoblastic leukemia (ALL) (18).

Ovarian function

The ovary is susceptible to chemotherapy-induced damage, particularly following treatment with alkylating agents, such as cyclophosphamide (22,23). Ovarian damage is drug- and dose-dependent and is related to age at the time of treatment, with progressively smaller doses required to produce ovarian failure with increasing age (24,25).

A reduced follicular reserve may result in POF and menopause many years posttreatment, even in patients undergoing chemotherapy at a very young age (23). Significant depletion of the primordial follicle stockpile postchemotherapy in a normally ovulating female has been demonstrated in an animal model (17).

A variety of laboratory tests are used to estimate ovarian reserve and fertility preservation options before cancer treatment, and the risk of premature menopause and fertility treatments after cancer treatment (26) (Table 1). The risk of ovarian failure in several commonly encountered malignancies and other disorders requiring chemo- and/or radiotherapy is presented in Table 2.

Cyclophosphamide is widely used in combination chemotherapy regimens, and high-dose cyclophosphamide (200mg/kg) is frequently utilized as conditioning therapy before bone marrow transplantation (BMT), either alone, where recovery of ovarian function is more likely, or in combination with other chemotherapeutic agents or TBI (9) (Table 2).

Treatment of Hodgkin’s lymphoma with MOPP (mechlorethamine, vincristine, procarbazine and prednisolone) or ChlVPP (chlorambucil, vinblastine, procarbazine and prednisolone) has previously been associated with ovarian dysfunction in 19-63% of cases (27). Amenorrhea is more commonly observed in older women, but long-term follow-up is necessary, as a number of young women also develop premature menopause. Treatment with an ABVD regimen (adriamycin, bleomycin, vinblastine and dacarbazine), which contains no alkylating agents or procarbazine, results in significantly less gonadotoxicity, especially in patients under 25 years of age (28).  In a recent cohort study (29) of 518 female five-year survivors of Hodgkin’s lymphoma aged 14 to 40 years (median age: 25 years) at treatment, the Amsterdam group explored the incidence of POF before age 40.  Alkylating agents, especially procarbazine (HR: 8.1) and cyclophosphamide (HR: 3.5), showed the strongest associations. Ten years after treatment, the actuarial risk of premature menopause was 64% after high cumulative doses (> 8.4 g/m(2)) and 15% after low doses (<or= 4.2 g/m(2)) of procarbazine (29).

The risk of POF in Hodgkin’s disease and breast cancer is summarized in Table 2.

In case of germ cell tumors, fertility-sparing surgery is possible in a large proportion of patients. For patients with advanced-stage disease, maximum cytoreductive surgery appears to be beneficial. For patients who require postoperative chemotherapy, standard therapy involves a combination of bleomycin, etoposide, and cisplatin. Although POF may occur in a small proportion of patients, 80-99% of those who undergo fertility-sparing surgery and chemotherapy can expect to maintain reproductive function (30).  In a group of young women (median age: 25.5) who were treated with the VAC protocol (vincristine, dactinomycin, cyclophosphamide) for germ cell tumors, 13% were found to have irregular menses, 15% oligomenorrhea or amenorrhea, and 8% persistent amenorrhea (31).

Regarding chemotherapy in patients with autoimmune diseases, pulse cyclophosphamide therapy is frequently used for active lupus nephritis or neuropsychiatric lupus. The major determinants for the development of ovarian failure in patients with systemic lupus erythematosus (SLE) are age at the start of therapy and the cumulative cyclophosphamide dose (number of cycles and doses) (Table 2).

A recent publication assessed the risk of POF according to treatment protocol and patient age (Table 3).

Radiation and the uterus

The uterus is at significant risk of damage following abdominal, pelvic or total body irradiation, in a dose- and age-dependent manner (32). Uterine function may be impaired following radiation doses of 14-30 Gy as a consequence of disruption to uterine vasculature and musculature elasticity (33). Even lower doses of irradiation, as in TBI, have been reported to cause impaired growth and blood flow (34).

Efforts have been made to improve uterine function, but with limited success. In young adult women previously treated with TBI, physiological sex steroid replacement therapy improves uterine function (blood flow and endometrial thickness) and may potentially allow them to benefit from assisted reproductive technologies (34). Larsen et al studied uterine volume in 100 childhood cancer survivors and assessed uterine response to high-dose estrogen replacement therapy in three patients with ovarian failure and reduced uterine volume following abdominal and/or pelvic irradiation (35). There was no significant difference in uterine volume, endometrial thickness or uterine artery blood flow following steroid treatment, suggesting that higher doses of pelvic radiation cause greater damage than lower doses (as in TBI), and this damage may be irreversible. 

Testicular function

In males, testicular damage can involve the somatic cells of the testis (Sertoli and Leydig cells), or the germ cells.  Sertoli cells are responsible for nurturing developing germ cells, and Leydig cells produce testosterone.  Gonadal damage in males treated for cancer can result from either systemic chemotherapy or radiotherapy to a field that includes the testes.  Cytotoxic treatment targets rapidly dividing cells and it is therefore not surprising that spermatogenesis is impaired after treatment for cancer.  The exact mechanism of this damage is uncertain, but it appears to be linked to depletion of the proliferating germ cell pool.  Although the prepubertal testis does not complete spermatogenesis and produce mature spermatozoa, cytotoxic treatment given to prepubertal boys may impair future fertility. Importantly, the prepubertal testis is susceptible to cytotoxic damage. 

Radiotherapy to the testis

In males, radiation doses as low as 0.1-1.2 Gy can impair spermatogenesis, with doses over 4 Gy causing permanent azoospermia.  The somatic cells of the testis are more resistant than the germ cells, and Leydig cell dysfunction is not observed until 20 Gy in prepubertal boys and 30 Gy in sexually mature males (36).

Within the pediatric and adolescent age group, testicular damage occurs with direct irradiation to the testes, for example in the management of leukemia (37).  In patients with leukemic infiltration of their testes, radiation doses of 24 Gy are used, and this results in permanent azoospermia (38).  TBI applied as conditioning treatment before BMT also irradiates the testes. Although the effects of this can be difficult to elucidate as it is usually given concurrently with alkylating agents, doses of 9-10 Gy have produced gonadal dysfunction (39).

Chemotherapy and the testis

As with radiotherapy, the germinal epithelium of the testis is very sensitive to the detrimental effects of chemotherapy, irrespective of pubertal status at the time of treatment.  Therefore, after receiving gonadotoxic agents, patients may be rendered oligospermic or azoospermic. Testosterone production by Leydig cells is usually unaffected, however, and thus secondary sexual characteristics develop normally (40).  Following higher cumulative doses of gonadotoxic chemotherapy, Leydig cell dysfunction may also become apparent (41).

Treatment of Hodgkin’s lymphoma has involved the use of procarbazine, together with alkylating agents such as chlorambucil, mustine and cyclophosphamide.  While these drug combinations have yielded excellent survival rates, the majority of male patients subsequently developed permanent azoospermia (42,43).  Mackie et al (27) studied patients with a mean age of 12.2 years at diagnosis who were treated with ChlVPP, a regimen containing both chlorambucil and procarbazine.  On follow-up, 89% of these patients showed severe damage to the seminiferous epithelium up to ten years following therapy.  Consequently, the treatment of Hodgkin’s disease was modified in an attempt to reduce the gonadotoxicity, whilst maintaining long-term survival (44).  Treatment with the ABVD regimen, which contains no alkylating agents or procarbazine, results in significantly less gonadotoxicity, with no patients demonstrating permanent azoospermia (42).  However, anthracycline exposure in this regimen renders it potentially cardiotoxic in the long term.

Summary

Whilst many children and adults diagnosed with cancer can now realistically hope for long-term survival, they must often live with the consequences of their treatment.  Infertility is one of the most devastating adverse effects of cancer treatment in this patient group. Both chemotherapy and radiotherapy can impair future fertility, and treatments for certain cancers can be sterilizing (45).  Although predicting individual fertility following treatment is extremely difficult, further epidemiological studies and investigation of markers indicating gonadal damage may be of use to our patients.

 

 

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