Effects of fast neutrons on the eye

Ocular, Orbital, and Adnexal Toxicity With High-dose Volumetric Modulated Arc Radiation Therapy for Orbital Malignancies

PURPOSE

Conventional modalities of external-beam radiation therapy (EBRT) are associated with high incidences of severe vision-threatening ocular and orbital toxicities when used to treat orbital malignancies. We investigate toxicities associated with high-dose volumetric modulated arc therapy (VMAT), a commonly used contemporary treatment modality for these tumors.

METHODS

Retrospective analysis of malignant orbital tumors managed with adjuvant high-dose VMAT preceded by globe-salvaging surgical therapy (GST) or exenteration. Dosimetric quantitation of target volumes and critical structures was performed. Incidence and severity of ocular, orbital, and adnexal toxicities were evaluated and assessed with regard to conventional EBRT toxicities for orbital malignancies described in the literature.

RESULTS

Eighty-four subjects (mean age = 65.9 ± 9.7 years) were included (N = 48 and N = 36 in GST and exenteration subgroups, respectively). Mean dose was 64.8 ± 2.1 Gy to the planning target volume. Dosing to critical structures typically did not surpass known tissue tolerance limits. Median follow up was 18.3 months. Visual acuity in the GST subgroup was not significantly different after VMAT (0.25 ± 0.06) compared with baseline (0.23 ± 0.02; P = 0.302). Whereas severe toxicities reported by major systematic analyses in the literature with older EBRT modalities were relatively common-for example, retinopathy (16-40%), optic neuropathy (16%), and corneal perforation (13%)-toxicities with VMAT were typically mild and less common. The most common toxicities with VMAT were mild dry eye (81.3%; 39/48), cataract (21.1%; 8/38 phakic eyes), and periocular dermatitis (15.5%; 13/84). Vision-threatening toxicities, including severe corneal pathologies, retinopathy, or optic neuropathy, were rare. There were no contralateral ocular or adnexal toxicities.

CONCLUSIONS

High-dose VMAT for orbital malignancies demonstrated low incidence and severity of eye-related toxicity, in contradistinction to adverse events reported from conventional forms of radiotherapy.

Ophthalmic and adnexal complications of radiotherapy

The role of radiotherapy in ophthalmic practice continues to grow. This growth has seen an expansion of indications for radiotherapy, a refinement of the modalities that can be used and a reduction in the ocular and adnexal complications that result from this form of therapy. The compendium of indications for radiotherapy in ophthalmology continues to grow and now includes many conditions such as the treatment of lid and adnexal disease, ocular surface disorders and both benign and malignant disease of the posterior segment and optic pathways. The radiotherapeutic modalities employed to manage these conditions are numerous and include both radioactive plaques (brachytherapy) and external beam radiation techniques. New techniques such as stereotactic radiosurgery are delivering benefits in the management of conditions such as optic nerve sheath meningioma, where the treatment of this blinding and occasionally life-threatening intracranial neoplasm now results in fewer adverse affects. The purpose of this review is to give a brief overview of the indications and treatment modalities, and a more in-depth discussion of the potential side-effects when radiotherapy is used for ocular and periorbital disease.

Radiotherapy-Induced Ocular Surface Disease

Today radiation is routinely used as a therapeutic modality for select tumors of the orbit, adnexa, paranasal sinus, and nasopharynx. Despite significant improvements in mechanisms of delivery and protective shielding, acute and chronic complications of radiation can affect different segments of the eye. In this report, we provide an overview of ocular damage secondary to radiotherapy. We identify the characteristic clinical changes and underlying pathophysiologic mechanisms involving the ocular surface and provide a rational approach to their prevention and treatment.

Relative biological effectiveness (RBE), quality factor (Q), and radiation weighting factor (w(R)). A report of the International Commission on Radiological Protection

The effect of ionising radiation is influenced by the dose, the dose rate, and the quality of the radiation. Before 1990, dose-equivalent quantities were defined in terms of a quality factor, Q(L), that was applied to the absorbed dose at a point in order to take into account the differences in the effects of different types of radiation. In its 1990 recommendations, the ICRP introduced a modified concept. For radiological protection purposes, the absorbed dose is averaged over an organ or tissue, T, and this absorbed dose average is weighted for the radiation quality in terms of the radiation weighting factor, w(R), for the type and energy of radiation incident on the body. The resulting weighted dose is designated as the organ- or tissue-equivalent dose, H(T). The sum of the organ-equivalent doses weighted by the ICRP organ-weighting factors, w(T), is termed the effective dose, E. Measurements can be performed in terms of the operational quantities, ambient dose equivalent, and personal dose equivalent. These quantities continue to be defined in terms of the absorbed dose at the reference point weighted by Q(L). The values for w(R) and Q(L) in the 1990 recommendations were based on a review of the biological and other information available, but the underlying relative biological effectiveness (RBE) values and the choice of w(R) values were not elaborated in detail. Since 1990, there have been substantial developments in biological and dosimetric knowledge that justify a re-appraisal of w(R) values and how they may be derived. This re-appraisal is the principal objective of the present report. The report discusses in some detail the values of RBE with regard to stochastic effects, which are central to the selection of w(R) and Q(L). Those factors and the dose-equivalent quantities are restricted to the dose range of interest to radiation protection, i.e. to the general magnitude of the dose limits. In special circumstances where one deals with higher doses that can cause deterministic effects, the relevant RBE values are applied to obtain a weighted dose. The question of RBE values for deterministic effects and how they should be used is also treated in the report, but it is an issue that will demand further investigations. This report is one of a set of documents being developed by ICRP Committees in order to advise the ICRP on the formulation of its next Recommendations for Radiological Protection. Thus, while the report suggests some future modifications, the w(R) values given in the 1990 recommendations are still valid at this time. The report provides a scientific background and suggests how the ICRP might proceed with the derivation of w(R) values ahead of its forthcoming recommendations.

Meibomian gland dysfunction

Blepharitis is probably the most common disease entity seen in the general ophthalmologist's office. A significant proportion of these cases are secondary to meibomian gland disease. This review outlines our knowledge of the histopathology, lipid abnormalities and role of microorganisms in meibomian gland dysfunction. We will also review the physiology of meibomian gland secretion and present models of meibomian gland dysfunction which have enhanced our knowledge of this condition. The importance of diagnosing associated conditions such as aqueous tear deficiency, contact lens intolerance, rosacea, and seborrheic dermatitis is emphasized. Although this condition causes significant morbidity in the population, there are effective treatments available and these will be discussed.

Comparative human cellular radiosensitivity: III. Gamma-radiation survival of cultured skin fibroblasts and resting T-lymphocytes from the peripheral blood of the same individual

Skin and blood samples were obtained from 34 donors, for whom there was no indication of abnormal radiosensitivity. From these, in 33 cases both fibroblast and T-lymphocyte cultures were obtained and in 26 cases at least three fibroblast and at least two G0 (resting) T-lymphocyte survival assays were possible. Within this set of results, differences in radiosensitivity between donors were significant for fibroblasts but not T-lymphocytes, although the range of radiosensitivity was similar for the two cell types (D 0.90-1.68 Gy for fibroblasts; 1.26-2.15 Gy for T-lymphocytes). Furthermore, there was little evidence for a correlation in radiosensitivity between the two cell types. These results suggest limitations in the predictive value of conventional measurement of cell survival.

Relationship of cataract to radiation sensitivity

Considerable exposure to radiation always causes posterior subcapsular cataract (PSC). This investigation was conducted to ascertain whether cellular hypersensitivity to radiation may be identified as a possible cause of cataract in persons exposed to low levels of radiation. Patients were studied in whom PSC had followed probable exposure to low levels of radiation or in whom PSC had developed before the age of 60 without known exposure. The patients with cataract were compared with age and sex matched controls without cataract. Radiation sensitivity was estimated by measuring clonal growth of skin fibroblasts and peripheral blood lymphocytes after exposure to graded doses of radiation and by measuring postirradiation reconstruction of separated nuclear material from lymphocytes. The results show variations in the level of radiation sensitivity between the patients, without significant differences from the controls. It is concluded that radiation hypersensitivity, as tested by the methods used in this study, is not normally associated with the development of posterior subcapsular cataract.

Ocular morbidity after radiotherapy of orbital and conjunctival lymphoma

One hundred and fifteen patients were treated for lymphoma of the conjunctiva and orbit between 1970 and 1984. One hundred and twelve patients received radiotherapy to the orbit, of whom 73 (65 per cent) had at least one year of follow-up by an ophthalmic surgeon. Patients with low-grade lymphomas (97) mostly received 30 Gy in 15 fractions to the orbit over a period of 3 weeks. Those with high-grade lymphomas (18) mostly received 40 Gy in 20 fractions over 4 weeks. Most were treated using anterior and lateral radiation fields to the orbit with shielding of the cornea and lens. This technique delivered a mean lens dose of 15 Gy. The early and late ocular and orbital morbidity in these patients was low. Eight radiation-induced cataracts developed of which only six interfered with vision. None has needed cataract surgery. No patients developed radiation retinopathy and only 5 had disorders of ocular lubrication.

Cataract analysis and the assessment of radiation risk in space

Radiation cataract, a non-stochastic effect on the lens, is readily amenable to non-invasive analysis. Thus, it provides the means to assess radiation risk in space and for long-term monitoring of those who frequent that environment. The importance of such evaluations are underscored by the uncertainties associated with the assignment of quality factors for the effects of heavy charged particles constituting cosmic and solar radiation. Experimental studies were conducted using albino rats to evaluate the cataractogenic potential of 570 MeV/amu Argon ions administered as both single and protracted doses. The cataract studies and investigations of quantitative cytopathological changes associated with them indicate that as the dose of heavy particles decreases, the relative biological effectiveness, compared to X rays, increases. Fractionating the exposures not only failed to reduce the cataractogenic effect but caused a dose-dependent enhancement in the time of onset of opacification. Cytopathologically, the damage caused by heavy particles, when compared to low-LET radiation was found to be quantitatively dissimilar but qualitatively identical. In addition, damage which might be consistent with microlesions was not evident. The data indicates that as regards the cataractogenic potential of heavy particles at low doses an assignment of a Quality Factor (QF) of at least 40 may be in order.

Approaches to radiation guidelines for space travel

There are obvious risks in space travel that have loomed larger than any risk from radiation. Nevertheless, NASA has maintained a radiation program that has involved maintenance of records of radiation exposure, and planning so that the astronauts' exposures are kept as low as possible, and not just within the current guidelines. These guidelines are being reexamined currently by NCRP Committee 75 because new information is available, for example, risk estimates for radiation-induced cancer and about the effects of HZE particles. Furthermore, no estimates of risk or recommendations were made for women in 1970 and must now be considered. The current career limit is 400 rem to the blood forming organs. The appropriateness of this limit and its basis are being examined as well as the limits for specific organs. There is now considerably more information about age-dependency for radiation effects and this will be taken into account. In 1973 a committee of the National Research Council made a separate study of HZE particle effects and it was concluded that the attendant risks did not pose a hazard for low inclination near-earth orbit missions. Since that time work has been carried out on the so-called microlesions caused by HZE particles and on the relative carcinogenic effect of heavy ions, including iron. A remaining question is whether the fluence of HZE particles could reach levels of concern in missions under consideration. Finally, it is the intention of the committee to indicate clearly the areas requiring further research.