two cats with a marginally resected eyelid malignant peripheral nerve sheath tumour were treated with adjuvant strontium plesiotherapy a few weeks after surgery . the dose applied in both cases was 200 gy to the surface , in five fractions , on a monday  wednesday  friday basis . the treatment aimed to achieve a clinical margin of approximately 1 cm around the surgical scar and multiple application fields were required to cover such an area . local recurrence was not seen in either case after 1330 and 645 days , respectively . the majority of periocular malignant peripheral nerve sheath tumours reported in the literature recur after surgery . in the two cases described in this report the combination of surgery and adjuvant plesiotherapy has been able to provide good local control with minimal toxicity . this multimodal approach could be considered as an alternative to aggressive surgery such as enucleation or exenteration . in people , peripheral nerve sheath tumours ( pnsts ) are a group of neoplasms that arise from schwann cells , perineurial cells and intraneuronal fibroblasts . depending on their cell of origin , pnsts can be further divided into schwannoma , neurofibroma and malignant pnsts , respectively . according to the most recent edition of the world health organization s international classification of tumors of domestic animals , pnsts are , in veterinary medicine , divided into only benign and malignant variants because of their uncertain histogenenesis . pnsts can be difficult to differentiate histologically from other spindle - cell tumours such as perivascular wall tumours and fibrosarcomas , and immunohistochemistry is often required to substantiate the diagnosis . the most common immunohistochemical marker used to support the morphological diagnosis of pnsts is s100 , which is expressed in up to 100% of cases . a recent retrospective study described pathological and clinical characteristics of 59 pnsts in 53 cats . the recurrence rate for benign pnsts was 15% ( 6/41 ) ; none of the tumours removed with clear margins recurred ( 19/19 ) , while recurrence was seen in 27% ( 6/22 ) of the incompletely removed tumours . between benign pnsts , only one involved the lower eyelid and the tumour recurred after incomplete excision ; another three were completely removed from the periocular skin , and none of them recurred . the recurrence rate of malignant pnsts was 44% ( 7/16 ) ; none of the tumours removed with clear margins recurred ( 6/6 ) , while recurrence was seen in 70% ( 7/10 ) of the tumours removed with incomplete margin . of the malignant pnsts there was one tumour involving the upper eyelid and the tumour recurred after incomplete excision . in another recent retrospective series describing 43 feline eyelid tumours submitted to a single pathology laboratory , there were three pnsts involving the eyelid . in this study pnsts a recent case series described presentation and outcome of cutaneous periocular pnsts in six cats . as in the previous study , pnsts were not divided into benign and malignant and margin of excision were not described . after multiple conservative surgeries , four of these cats underwent exenteration or enucleation as a curative - intent procedure ; none of the cats undergoing aggressive surgery presented tumour recurrence . radical or wide surgical resection or debulking surgery followed by adjuvant radiotherapy is considered the treatment of choice for the management of soft tissue sarcomas in cats and dogs . however , adjuvant irradiation of the periocular area with external beam radiotherapy using curative - intent doses ( > 45 gy ) is limited by the risk of incurring in severe toxicity . side effects reported in this location include dermatitis , conjunctivitis , keratitis , iritis or chronic complications such as chronic keratoconjunctivitis sicca , corneal ulceration , secondary uveitis , chronic conjunctivitis , cataract formation , radiation retinopathy and optic neuropathy . strontium plesiotherapy consists of the direct application of the radioactive source to the tumour surface . strontium 90 undergoes beta ( )decay emitting -particles , which are characterized by limited penetration ability into the irradiated tissue ( 23 mm ) . the shallow penetration is the main attraction of this treatment because it allows applying a large dose of radiation to the surface without affecting the deeper tissue . strontium plesiotherapy has been successfully used in the treatment of various periocular tumours in cats and dogs ; for example , feline mast cell tumours , canine limbal melanoma and canine limbal hemangiosarcomas . herein , we present the use of conservative surgery followed by adjuvant strontium plesiotherapy in the treatment of two cases of malignant feline pnsts involving the eyelid . histopathology and immunohistochemistry in both cases were reviewed by a board - certified pathologist ( rr ) . a 11-year - old , female , neutered , domestic longhair cat was presented to the ophthalmology unit with a 4 mm mass on the left lower eyelid ( figure 1a ) . cytology showed a spindle cell neoplastic population with severe nuclear atypia and occasional multinucleated cells . case 1 : malignant peripheral nerve sheath tumour of the left lower eyelid ( a ) before and ( b ) after surgery . case 2 : ( d ) plastic jaeger lid plate in place to protect the eye during strontium plesiotherapy . ( e ) cutaneous acute moist dermatitis 2 weeks after treatment , ( f ) followed by leukotrichia after a few months . treatment options , including aggressive surgery or more conservative surgery followed by strontium plesiotherapy , were discussed with the owners and they elected for the more conservative approach . a wedge resection of the mass with double layer closure was performed , leaving a relatively tight lower eyelid ( figure 1b ) , but no functional problems . histology revealed a poorly demarcated neoplastic proliferation of spindle cells expanding the eyelid dermis and arranged in bundles with occasional nuclear palisading . mitoses were 10 in 10 high - powered fields ( hpfs ) . a histological diagnosis of soft tissue sarcoma , most likely a malignant pnst , immunohistochemistry showed that the neoplastic cells were positive for s100 , confirming the diagnosis of malignant pnst . the treatment was delivered on a monday  wednesday  friday basis for five fractions to a total surface dose of 200 gy . each fraction consisted of five contiguous application fields to obtain a clinical margin of approximately 1 cm around the scar . no acute side effects developed during the treatment and the cat was discharged without any medication . at re - examination 5 weeks later the owner reported no side effect apart from radiation - induced localized alopecia ( figure 1c ) . alopecia was still present at the second re - examination 6 months later , but the remainder of the ophthalmological examination was unremarkable . after collapsing , the cat was presented to the referring vet after 1330 days and the owner elected for euthanasia without any further investigations . at the time of the consultation the owner did not allow a post - mortem examination . a 5-year - old , female , neutered , domestic shorthair cat was referred to the oncology unit after incomplete excision of a 7 mm mass from the lateral aspect of the left lower eyelid . a blepharoplasty had been performed to resect the mass , but information about the specific surgical technique used was not available upon referral . histology revealed a poorly demarcated spindle cell proliferation expanding the eyelid dermis with cells organized in bundles and admixed with oedematous fibrillar matrix . the tumour was initially diagnosed as incompletely excised soft tissue sarcoma and subsequent immunostaining for s-100 allowed a final diagnosis of malignant pnst . revision surgery using a more aggressive approach or adjuvant plesiotherapy was discussed with the owners , and they elected for the more conservative approach . the treatment was delivered on a monday  wednesday  friday basis for five fractions to a total surface dose of 200 gy . each fraction consisted of six contiguous application fields providing a clinical margin of approximately 1 cm around the scar . during the treatment the globe was protected using a plastic jaeger lid plate ( figure 1d ) . at the end of the radiation course a few days after the end of the radiotherapy moist desquamation developed ( veterinary radiation therapy oncology group grade 2 skin acute toxicity ; 
 figure 1e ) . at re - examination 3 weeks later , the dermatitis was completely resolved , but the irradiated area was completely alopecic . at re - examination after 6 months , the irradiated area presented diffuse leukotrichia ( figure 1f ) , but there was no evidence of chronic ocular toxicity . at the time of writing , 645 days after the end of the radiotherapy , the cat is still well and there is no evidence of tumour recurrence . a 11-year - old , female , neutered , domestic longhair cat was presented to the ophthalmology unit with a 4 mm mass on the left lower eyelid ( figure 1a ) . cytology showed a spindle cell neoplastic population with severe nuclear atypia and occasional multinucleated cells . case 1 : malignant peripheral nerve sheath tumour of the left lower eyelid ( a ) before and ( b ) after surgery . case 2 : ( d ) plastic jaeger lid plate in place to protect the eye during strontium plesiotherapy . ( e ) cutaneous acute moist dermatitis 2 weeks after treatment , ( f ) followed by leukotrichia after a few months . treatment options , including aggressive surgery or more conservative surgery followed by strontium plesiotherapy , were discussed with the owners and they elected for the more conservative approach . a wedge resection of the mass with double layer closure was performed , leaving a relatively tight lower eyelid ( figure 1b ) , but no functional problems . histology revealed a poorly demarcated neoplastic proliferation of spindle cells expanding the eyelid dermis and arranged in bundles with occasional nuclear palisading . mitoses were 10 in 10 high - powered fields ( hpfs ) . a histological diagnosis of soft tissue sarcoma , most likely a malignant pnst , immunohistochemistry showed that the neoplastic cells were positive for s100 , confirming the diagnosis of malignant pnst . the treatment was delivered on a monday  wednesday  friday basis for five fractions to a total surface dose of 200 gy . each fraction consisted of five contiguous application fields to obtain a clinical margin of approximately 1 cm around the scar . no acute side effects developed during the treatment and the cat was discharged without any medication . at re - examination 5 weeks later the owner reported no side effect apart from radiation - induced localized alopecia ( figure 1c ) . alopecia was still present at the second re - examination 6 months later , but the remainder of the ophthalmological examination was unremarkable . after collapsing , the cat was presented to the referring vet after 1330 days and the owner elected for euthanasia without any further investigations . at the time of the consultation a 5-year - old , female , neutered , domestic shorthair cat was referred to the oncology unit after incomplete excision of a 7 mm mass from the lateral aspect of the left lower eyelid . a blepharoplasty had been performed to resect the mass , but information about the specific surgical technique used was not available upon referral . histology revealed a poorly demarcated spindle cell proliferation expanding the eyelid dermis with cells organized in bundles and admixed with oedematous fibrillar matrix . the tumour was initially diagnosed as incompletely excised soft tissue sarcoma and subsequent immunostaining for s-100 allowed a final diagnosis of malignant pnst . revision surgery using a more aggressive approach or adjuvant plesiotherapy was discussed with the owners , and they elected for the more conservative approach . the treatment was delivered on a monday  wednesday  friday basis for five fractions to a total surface dose of 200 gy . each fraction consisted of six contiguous application fields providing a clinical margin of approximately 1 cm around the scar . during the treatment the globe was protected using a plastic jaeger lid plate ( figure 1d ) . at the end of the radiation course a few days after the end of the radiotherapy moist desquamation developed ( veterinary radiation therapy oncology group grade 2 skin acute toxicity ; 
 figure 1e ) . at re - examination 3 weeks later , the dermatitis was completely resolved , but the irradiated area was completely alopecic . at re - examination after 6 months , the irradiated area presented diffuse leukotrichia ( figure 1f ) , but there was no evidence of chronic ocular toxicity . at the time of writing , 645 days after the end of the radiotherapy , the cat is still well and there is no evidence of tumour recurrence . marginal resection followed by strontium plesiotherapy was able to provide good local control in the two cases presented herein without performing aggressive surgery such as enucleation or exenteration . this was an extremely promising result considering that 11/14 periocular pnsts reported in the literature recurred after surgical excision . the dose of radiation prescribed in these two cases was 200 gy to the surface or 50 gy at a depth of 2 mm , which is comparable with the total dose commonly prescribed as adjuvant treatment in cats with injection - site sarcoma treated with external beam radiotherapy . compared with external beam radiotherapy , strontium plesiotherapy can be used to treat small areas ( 28 cm ) because only a limited number of contiguous fields can be successfully applied to an area . both tumours presented in this report underwent surgery when their size was still small ( 4 and 7 mm , respectively ) , leaving a surgical scar smaller than 1 cm . a margin of approximately 1 cm around the scar was applied to define the treatment area , but this decision was driven by the practicality of the treatment , as specific data are not available in the literature . when planning external beam radiotherapy as adjuvant treatment of injection - site sarcomas a margin of 3 cm around the surgical scar is commonly applied because of their tendency to infiltrate along the fascial planes . in the two cases presented in this series , we applied a narrower margin because non - injection site sarcomas are less biologically aggressive and infiltrative than injection - site sarcomas . acute and long - term side effects associated with the treatment were limited to the skin and very mild . at re - examination at 6 months there was no evidence of ocular toxicity such as cataract . in this sense we think that protecting the eye with a plastic jaeger lid plate provided further attenuation to the already weak deep radiation dose . excisional globe - sparing surgery of small periocular pnsts followed by adjuvant plesiotherapy may be a valid alternative to enucleation or exenteration . a larger number of cases would be necessary to further support the positive outcome seen in these cases .
case series summarytwo cats with a marginally resected eyelid malignant peripheral nerve sheath tumour were treated with adjuvant strontium plesiotherapy a few weeks after surgery . 
 the dose applied in both cases was 200 gy to the surface , in five fractions , on a monday  wednesday  friday basis . 
 the treatment aimed to achieve a clinical margin of approximately 1 cm around the surgical scar and multiple application fields were required to cover such an area . 
 local recurrence was not seen in either case after 1330 and 645 days , respectively.relevance and novel informationthe majority of periocular malignant peripheral nerve sheath tumours reported in the literature recur after surgery . 
 in the two cases described in this report the combination of surgery and adjuvant plesiotherapy has been able to provide good local control with minimal toxicity . 
 this multimodal approach could be considered as an alternative to aggressive surgery such as enucleation or exenteration .