Ocular and periocular radiation toxicity in dogs treated for sinonasal tumors: A critical review

Abstract

Visual impairment from radiation‐induced damage can be painful, disabling, and reduces the patient's quality of life. Ocular tissue damage can result from the proximity of ocular organs at risk to irradiated sinonasal target volumes. As toxicity depends on the radiation dose delivered to a certain volume, dose‐volume constraints for organs at risk should ideally be known during treatment planning in order to reduce toxicity. Herein, we summarize published ocular toxicity data of dogs irradiated for sinonasal tumors from 36 publications (1976‐2018). In particular, we tried to extract a dose guideline for a clinically acceptable rate of ocular toxicity. The side effects to ocular and periocular tissues were reported in 26/36 studies (72%) and graded according to scoring systems (10/26; 39%). With most scoring systems, however, toxicities of different ocular and periocular tissues are summed into one score. Further, the scores were mostly applied in retrospect and lack volume‐ and dose‐data. This incomplete information reflects the crux of the matter for radiation dose tolerance in canine ocular tissues: The published information of the last three decades does not allow formulating dose‐volume guidelines. As a start, we can only state that a mean dose of 39 Gy (given in 10 x 4.2 Gy fractions) will lead to loss of vision by one or both eyes, while mean doses of <30 Gy seem to preserve functionality. With a future goal to define tolerated doses and volumes of ocular and periocular tissues at risk, we propose the use of combined ocular toxicity scoring systems.

Estimation of planning organ at risk volumes for ocular structures in dogs undergoing three‐dimensional image‐guided periocular radiotherapy with rigid bite block immobilization

“…Thus, more careful sculpting of (high) radiation dose around nearby OARs or PRV is of even greater importance today. This is especially true for irradiation of sinonasal tumors to avoid possibly debilitating late toxicity 2 . As far as we know, only a PRV of the lens but no other ocular or periocular structures exists for dogs 6 …”

“…A PRV of the lens has already been established in dogs 6 . The lens is exclusively sensitive already to low doses of radiation and cataract formation can lead to loss of vision 2,35 . Cataract formation is, however, a non‐life‐threatening disease and can be treated with phacoemulsification surgery to restore vision 36 .…”

“…Cataract formation is, however, a non‐life‐threatening disease and can be treated with phacoemulsification surgery to restore vision 36 . Other ocular and periocular organs can be subject to radiation‐induced damage: keratitis, keratoconjunctivitis sicca, retinal hemorrhage or glaucoma can occur and impair quality of life or lead to vision impairment 2,37,38 . Establishing a PRV not only for the lens, but also for other periocular and ocular structures is an important step towards future research including establishment of tolerance doses.…”

“…Before the age of newer radiation techniques in veterinary medicine, moderate to severe damage to ocular structures was common, when treating sinonasal tumors in dogs, and could not be avoided technically. This resulted in painful toxicities, often leading to (unilateral) loss of vision, enucleation and other pathological changes 2 . Nowadays, with intensity‐modulated radiation therapy (IMRT), selective sparing of OARs is technically more feasible 3,4 .…”

Can volumetric modulated arc radiation therapy reduce organ at risk dose in stage 4 sinonasal tumors in dogs treated with boost irradiation?

“…Organ at risk tolerance or the respective dose-volume constraints, however, are not standardized in veterinary medicine. A dosevolume constraint of less than 15 Gy to 60% of the OAR volume (D60<15 Gy) for the ocular bulb has been recommended, when treating nasal tumors with a commonly used protocol of 10x4.2 Gy [5,12]. With the same protocol, a threshold dose to avoid keratoconjunctivitis sicca of a mean dose of less than 20 Gy (D mean <20 Gy) was proposed as dose constraint for lacrimal glands [13].…”