Radiation cataract is a cataract caused by exposure to ionizing radiation such as X-rays and gamma rays. Posterior subcapsular cataract is characteristic, but there are also reports of cortical cataract. It is known that ocular radiation exposure causes cataracts, and it has become clear that even low-dose exposure increases the long-term risk of cataracts. Ionizing radiation, both low and high dose, is a proven cause of cortical cataract, posterior subcapsular cataract, and mixed cataract 1).
Low-dose exposure, such as exposure of emergency workers in nuclear power plant accidents, occupational exposure of medical workers, and medical exposure from CT scans, also poses a long-term risk of cataracts.
The following groups are at high risk for radiation cataract.
A systematic review and meta-analysis showed a significant increase in cataract risk among interventional cardiologists and catheterization laboratory staff 3). A large cohort study of US radiologic technologists also showed an increased risk of cataract incidence with relatively low-dose occupational exposure 5,6).
The threshold dose for radiation cataract has been significantly lowered due to recent epidemiological studies. The old and new standards are shown below.
| Standard | Exposure Condition | Cataract Causing Visual Impairment | Mild Cataract |
|---|---|---|---|
| Old standard (before 2007) | Single acute exposure | 5 Sv | 0.5–2 Sv |
| Old standard (before 2007) | Multiple or chronic exposure | 8 Sv | 5 Sv |
| ICRP 2012 revision | Unified regardless of acute, fractionated, protracted, or chronic exposure | 0.5 Gy | — |
In 2012, the International Commission on Radiological Protection (ICRP) defined the threshold dose as the dose that causes vision-impairing cataracts in 1% of the exposed population 20 years or more after exposure, setting it at 0.5 Gy. This threshold is unified at 0.5 Gy regardless of exposure type (acute, fractionated, protracted, or chronic), and it is considered that there is no relationship between the threshold dose and severity.
The occupational exposure limit for the eye was also revised. From the previous annual limit of 150 mSv, the ICRP 2011 recommendation lowered it to an average of 20 mSv per year over 5 years (not exceeding 50 mSv in any single year).
Even low-dose exposure increases the long-term risk of cataracts. In 2012, ICRP lowered the threshold dose to 0.5 Gy, significantly stricter than the previous standard (5 Sv for single acute exposure causing vision-impairing cataracts). The US radiologic technologist cohort study also showed an increased risk of cataract incidence with relatively low occupational exposure 6). Low-dose radiation exposure should be considered to accelerate age-related changes in the lens.
The main subjective symptoms caused by radiation cataracts are shown below.
Radiation-induced lens opacities begin as polychromatic fine punctate opacities and vacuoles in the central posterior subcapsular region, gradually enlarging and progressing as follows.
Early Stage (Micro-opacities)
Punctate opacities and vacuoles: Fine, polychromatic punctate opacities and vacuoles appear in the central posterior subcapsular region.
Water clefts: Water clefts due to dissociation of the Y-shaped suture. These may appear from this stage onward.
Progressive stage
Plaque-like and granular opacities: Punctate opacities enlarge and coalesce, spreading as plaque-like and granular opacities in the posterior subcapsular region.
Further progression
Donut-shaped posterior subcapsular opacity: A donut-shaped posterior subcapsular opacity with a relatively clear center is observed.
Advanced stage
Disc-shaped opacity: A disc-shaped opacity consisting of two layers of membranous opacity is formed. This causes significant visual impairment.
Typical radiation cataract in highly exposed eyes presents a morphology of opacity that is significantly different from age-related cataract or steroid cataract, making differentiation relatively easy.
On the other hand, radiation cataract due to low-dose exposure progresses very slowly. The lower the dose, the longer the time to onset (several decades), and age-related changes are superimposed, making judgment difficult. Since age-related cataract also produces vacuoles, posterior subcapsular opacity, water clefts, and superficial cortical opacity, it is not easy to determine whether the opacity seen in an aged lens is due to radiation exposure.
In typical cases after high-dose exposure, differentiation is relatively easy due to the characteristic progression pattern: polychromatic fine punctate opacities → doughnut-shaped → saucer-shaped posterior subcapsular opacities. In low-dose exposure, changes are gradual and overlap with age-related changes, so a detailed history of exposure (exposure dose, duration, and cause) is essential for differentiation.
Ionizing radiation is an established risk factor for all of the following cataract types1).
| Cataract Type | Association with Radiation |
|---|---|
| Cortical Cataract | Ionizing radiation (low and high dose) |
| Posterior subcapsular cataract | Ionizing radiation (low dose, high dose) |
| Mixed cataract | Ionizing radiation (low dose, high dose) |
The diagnosis of radiation cataract is made by combining characteristic opacification patterns with a history of exposure.
The ICRP (2012) defines radiation-induced lens opacities into the following two categories, which are used to set threshold doses.
No special examination is required; diagnosis is made by routine slit-lamp microscopy (especially retroillumination). A detailed history of radiation exposure is the most important information, and the dose, duration, and cause of exposure must always be confirmed. In cases where differentiation from age-related changes is difficult, the exposure history itself becomes the decisive factor for diagnosis.
Prevention is the most important measure for radiation cataract.
The use of lead glass or lead acrylic protective eyewear (eye shields) is the most reliable preventive measure. However, usage rates in clinical settings are low, and it is recommended to ensure thorough use by healthcare workers and to use them for patients undergoing examinations with high ocular radiation exposure. Radiation protection shields and lead glasses are considered effective 8). Adherence to occupational exposure limits based on the ICRP 2011 recommendations is also an important preventive measure for radiation dose management.
For radiation-induced cataracts that cause visual impairment, standard cataract surgery is performed.
For radiation-induced cataracts with visual impairment, standard phacoemulsification (PEA) with IOL implantation is effective, and postoperative prognosis is as good as that for age-related cataracts. Surgical indication is when the posterior subcapsular opacity exceeds 2 mm in diameter and causes visual function decline. In cases of low-dose exposure, it often takes a long time from onset to surgery, but surgical outcomes are no different from other posterior subcapsular cataracts.
The lens is a highly radiosensitive tissue. Its pathogenesis is understood as follows.
Radiation exposure generates free radicals in cells of the germinative zone and fiber cells, causing cell damage. Consequently, damaged lens equatorial cells migrate to the posterior capsule, leading to decreased transparency of posterior subcapsular lens fiber cells and aggregation of crystallins, resulting in posterior subcapsular cataract.
Genetic damage from radiation exposure to the germinal zone is essential for cataract development. Even if the lens is exposed to radiation while the germinal zone alone is protected by a shield, radiation cataract does not occur. This provides the theoretical basis for the effectiveness of shielding the eye with lead protective glasses as prevention.
A meta-regression analysis of hematopoietic stem cell transplantation regimens has confirmed a dose-response relationship for radiation cataract development 2). An updated review of the effects of ionizing radiation on the eye also reports the impact of low-dose exposure, and the elucidation of the relationship with dose is progressing 4).
Radiation cataract does not develop immediately after exposure. The lower the dose, the longer the latency period before onset, which can sometimes be several decades. Once it develops, it gradually progresses with age-related changes. Low-dose radiation exposure should be interpreted as accelerating age-related changes in the lens.
There is ongoing debate about whether a threshold dose exists for radiation cataract or whether the dose-response relationship is linear (no-threshold LNT model). Continuous research is being conducted, including on the validity of the ICRP value of 0.5 Gy 4). Ongoing reviews of the effects of ionizing radiation on the eye are leading to reassessment of the threshold dose.
The US Radiologic Technologists study is longitudinally tracking the relationship between occupational exposure and cataract risk. It has shown that even relatively low-dose occupational exposure increases the risk of cataract, and the findings are used to evaluate the appropriateness of current occupational exposure limits 5,6).
Although the effectiveness of leaded protective eyewear is established, its usage rate in clinical settings remains low. Educational and awareness programs to increase usage rates and the development of more comfortable protective equipment are challenges.
Research is underway to identify biomarkers and imaging diagnostic methods to differentiate radiation-induced cataracts from age-related cataracts after low-dose exposure. If specific biomarkers are identified, they are expected to be applied for differentiation in cases with unknown exposure history.
