Residency · Residency · Radiation Oncology
Late Effects of Pediatric Radiation: Growth, Neurocognition, and Secondary Malignancies
Introduction
Advances in pediatric oncology have dramatically improved survival rates, with over 80% of children diagnosed with cancer now surviving at least five years. Despite these successes, the treatments that cure these cancers often lead to significant late effects, which have become the leading cause of morbidity and mortality among long-term survivors. Radiation therapy, a cornerstone of treatment for many pediatric cancers, poses unique risks due to the vulnerability of developing tissues. This lecture explores the major categories of late effects from pediatric radiation and discusses strategies to mitigate these complications.
Epidemiology of Late Effects
In the United States, there are over 500,000 survivors of childhood cancer. The Childhood Cancer Survivor Study (CCSS) is a landmark cohort that follows more than 35,000 of these survivors to characterize long-term outcomes. By age 50, approximately 53% of survivors have developed at least one severe or life-threatening chronic health condition. The cumulative incidence of severe chronic conditions at 30 years post-treatment is 42% among those who received radiation compared to 22% in those who did not. Radiation dose and the volume of tissue irradiated are the strongest predictors of these late effects.
Growth and Musculoskeletal Effects
Growth Hormone Deficiency
Cranial radiation doses exceeding 18 Gy significantly increase the risk of growth hormone (GH) deficiency, which is the most common endocrinopathy following cranial radiation therapy. There is a clear dose-response relationship, with nearly 100% of patients developing GH deficiency after receiving more than 30 Gy to the hypothalamic-pituitary axis. The onset of deficiency may be delayed, often manifesting 2 to 5 years after treatment. Growth hormone replacement therapy can partially restore growth velocity in affected children.
Skeletal Growth Disturbance
Radiation exposure to growth plates results in premature closure and growth arrest. When vertebrae are asymmetrically irradiated, scoliosis and kyphosis can develop. Craniospinal irradiation often leads to a shortened sitting height, while irradiation of the extremities may cause limb length discrepancies. The severity of these skeletal effects depends on the age at treatment, with younger children experiencing more profound impacts.
Soft Tissue Hypoplasia
Radiation also causes atrophy of muscle and fat within the treated field, leading to cosmetic asymmetry such as facial underdevelopment following head and neck radiation. Additionally, radiation to the oral cavity can impair dental and mandibular development.
Neurocognitive Effects
Cranial Radiation Impact
Neurocognitive effects are most pronounced after whole brain radiation or craniospinal irradiation. These effects are dose-dependent and include declines in IQ, processing speed, attention, and working memory. The primary pathological correlates are white matter damage and volume loss. Younger age at treatment, particularly under five years, is associated with greater cognitive decline. For example, young children receiving 24 Gy whole brain radiation therapy (WBRT) may experience an IQ decline of approximately 2 to 4 points per year.
Specific Deficits
Processing speed is the most consistently affected cognitive domain, along with impairments in executive function and attention. Academic skills such as reading and mathematics decline relative to peers, and social cognition and adaptive functioning may also be compromised. These deficits are progressive, accumulating over years after treatment.
Mitigation Strategies
Emerging strategies to reduce neurocognitive late effects include proton therapy, which reduces radiation dose to uninvolved brain tissue and shows promise in preserving cognitive function. Hippocampal avoidance techniques and conformal radiation methods aim to minimize the volume of normal brain irradiated. Cognitive rehabilitation programs and educational support services are important adjuncts. Pharmacologic interventions such as methylphenidate and modafinil have been used, although evidence supporting their efficacy is limited.
Endocrine Late Effects
Thyroid Dysfunction
Hypothyroidism occurs in 30-50% of patients who receive neck or mediastinal radiation, with risk increasing at doses above 20 Gy to the thyroid gland. Annual monitoring of thyroid-stimulating hormone (TSH) is recommended, and thyroid hormone replacement therapy should be initiated as needed. Radiation also elevates the risk of thyroid nodules and thyroid cancer.
Gonadal Dysfunction
In females, ovarian failure can result from pelvic or total body irradiation (TBI) doses exceeding 10-15 Gy, often leading to premature menopause. In males, Leydig cell dysfunction occurs at doses above 20 Gy to the testes, while spermatogenesis is affected at much lower doses, starting at 2-3 Gy. Fertility preservation counseling prior to treatment is essential, and oophoropexy may be performed to protect ovarian function in patients receiving pelvic radiation.
Central Precocious Puberty
Low-dose cranial radiation (18-24 Gy) can paradoxically trigger central precocious puberty, accelerating pubertal onset while simultaneously inhibiting growth hormone secretion. This results in compromised final adult height due to early epiphyseal fusion. Treatment with gonadotropin-releasing hormone (GnRH) agonists may be indicated to manage this condition.
Adrenal Insufficiency
High-dose cranial radiation exceeding 30 Gy that affects the hypothalamic-pituitary axis can cause ACTH deficiency, leading to adrenal insufficiency. Patients require education on cortisol stress dosing to manage this potentially life-threatening condition.
Secondary Malignancies
Radiation-Related Solid Tumors
Survivors of childhood cancer who received radiation have a 2- to 10-fold increased relative risk of developing secondary solid tumors compared to the general population. These malignancies typically arise 10 to 30 years after radiation exposure. Risk is higher with greater radiation doses and younger age at exposure. The most common secondary tumors include breast cancer, thyroid cancer, sarcomas, central nervous system tumors, and skin cancer.
Breast Cancer After Chest Radiation
Women treated with chest radiation before age 30 face a 20- to 30-fold increased risk of breast cancer, a risk comparable to that seen in BRCA mutation carriers. Screening guidelines recommend annual breast MRI starting at age 25 or eight years after radiation therapy, whichever occurs later, with mammography added at age 30. The risk increases with radiation doses exceeding 20 Gy to breast tissue.
Radiation-Induced Sarcomas
Sarcomas induced by radiation typically develop within or at the edges of prior radiation fields, with a latency period usually exceeding 10 years. The most common histologies are osteosarcoma, malignant fibrous histiocytoma, and angiosarcoma. These tumors often present as high-grade lesions and carry a poor prognosis.
Strategies to Reduce Risk
To minimize the risk of secondary malignancies, radiation dose and volume should be kept as low as possible. Proton therapy reduces the integral dose to normal tissues, and response-adapted protocols may safely omit radiation in select cases. Lifelong cancer surveillance following the Children’s Oncology Group (COG) Long-Term Follow-Up Guidelines is essential.
Cardiovascular Late Effects
Cardiac Disease
Radiation can cause a range of cardiac complications including pericarditis, cardiomyopathy, valvular disease, and coronary artery disease. The risk increases with a mean heart dose above 5 Gy and is compounded by concurrent anthracycline chemotherapy. The median latency period for cardiac disease is 15 to 20 years, and it remains a leading cause of non-cancer mortality among Hodgkin lymphoma survivors.
Screening Recommendations
Echocardiography is recommended every 2 to 5 years starting two years after treatment, with more frequent screening for patients exposed to higher radiation doses or anthracyclines. Management of cardiovascular risk factors such as dyslipidemia, hypertension, and diabetes is critical, alongside counseling on exercise and healthy lifestyle choices.
Pulmonary Late Effects
Radiation can cause acute radiation pneumonitis and chronic pulmonary fibrosis. These effects are exacerbated by exposure to pulmonary-toxic chemotherapy agents such as bleomycin, busulfan, or BCNU. Pulmonary function testing should be performed at baseline and during follow-up to monitor for restrictive lung disease and reduced diffusing capacity for carbon monoxide (DLCO). Patients should be advised against smoking and made aware of anesthetic risks related to compromised lung function.
Key Clinical Pearls
More than half of childhood cancer survivors will develop a significant chronic health condition by age 50, with radiation therapy being a major contributor. Neurocognitive decline following cranial radiation is progressive and dose-dependent, with younger children at greatest risk. Proton therapy offers a meaningful advantage by reducing integral radiation dose and potentially mitigating multiple categories of late effects. Breast cancer screening with MRI should begin at age 25 or eight years after chest radiation in female survivors. Lifelong survivorship care following the Children’s Oncology Group Long-Term Follow-Up Guidelines is mandatory for all childhood cancer survivors who have received radiation.
References
- Oeffinger KC, Mertens AC, Sklar CA, et al. Chronic health conditions in adult survivors of childhood cancer. N Engl J Med. 2006;355(15):1572-1582.
- Merchant TE, Kiehna EN, Li C, et al. Modeling radiation dosimetry to predict cognitive outcomes in pediatric patients with CNS embryonal tumors including medulloblastoma. Int J Radiat Oncol Biol Phys. 2006;65(1):210-221.
- Armstrong GT, Stovall M, Robison LL. Long-term effects of radiation exposure among adult survivors of childhood cancer: results from the Childhood Cancer Survivor Study. Radiat Res. 2010;174(6):840-850.
- Turcotte LM, Liu Q, Yasui Y, et al. Temporal trends in treatment and subsequent neoplasm risk among 5-year survivors of childhood cancer, 1970-2015. JAMA. 2017;317(8):814-824.