Use of Ir 192 interstitial brachytherapy for an equine malignant dermal schwannoma

INTRODUCTION Schwannomas, also termed neuromas, neurinomas, or neurolemmomas originate from the Schwann cells of nerve roots. In cases described in humans, schwannomas may occur either as a primary brain tumour or as a peripheral nerve tumour. Within the brain, cranial nerve (CN)-8 is most commonly affected and the tumour is termed an acoustic schwannoma, followed by CN-5. Schwannomas may originate from any cranial or spinal root; however, they do not originate from CN-1 and CN-2, since these are myelinated by oligodendroglia. Acoustic schwannomas typically present with progressive, unilateral hearing impairment and can compress neighbouring structures such as the pons and cerebrum, leading to neurological deficits. They are typically benign tumours that can be readily detected using MRI and respond to surgical removal. Focal neuropathies may be caused by schwannomas, also termed neurilemmomas, which tend to be solitary subcutaneous tumours that grow within nerve sheaths, are benign, and can be excised. Schwannomas may occur in adult horses aged 3 to 16 years with no breed or sex predisposition. No typical site exists, and lesions are reported as solitary, firm, <4 cm in diameter with no damage to the overlying skin and hair coat. Schwannomas may also appear as single or multiple firm, rounded, 2–3 mm diameter papules, which occur more commonly on the eyelids. Following further enlargement, these can attain a diameter of 2 cm before becoming multiloculated, alopecic and ulcerated. This report details the use of interstitial brachytherapy in a mare with a malignant, dermal schwannoma that recurred and was not amenable to surgical excision.


INTRODUCTION
Schwannomas, also termed neuromas, neurinomas, or neurolemmomas originate from the Schwann cells of nerve roots.In cases described in humans, schwannomas may occur either as a primary brain tumour or as a peripheral nerve tumour.Within the brain, cranial nerve (CN)-8 is most commonly affected and the tumour is termed an acoustic schwannoma, followed by CN-5 11 .Schwannomas may originate from any cranial or spinal root; however, they do not originate from CN-1 and CN-2, since these are myelinated by oligodendroglia 11 .Acoustic schwannomas typically present with progressive, unilateral hearing impairment and can compress neighbouring structures such as the pons and cerebrum, leading to neurologi-cal deficits.They are typically benign tumours that can be readily detected using MRI and respond to surgical removal.Focal neuropathies may be caused by schwannomas, also termed neurilemmomas, which tend to be solitary subcutaneous tumours that grow within nerve sheaths, are benign, and can be excised 11 .
Schwannomas may occur in adult horses aged 3 to 16 years with no breed or sex predisposition 1 .No typical site exists, and lesions are reported as solitary, firm, <4 cm in diameter with no damage to the overly-ing skin and hair coat 13 .Schwannomas may also appear as single or multiple firm, rounded, 2-3 mm diameter papules, which occur more commonly on the eyelids 6,15,19 .Following further enlargement, these can attain a diameter of 2 cm before becoming multiloculated, alopecic and ulcerated 13 .
This report details the use of interstitial brachytherapy in a mare with a malignant, dermal schwannoma that recurred and was not amenable to surgical excision.

CASE HISTORY
A 10-year-old Hanoverian mare was examined at the Onderstepoort Veterinary Academic Hospital (OVAH) for a right buccal schwannoma that recurred 3 months after initial surgical excision.At presentation, there was an externallyvisible, localised buccal swelling and a mucosal erosion inside the right cheek.The mare was reported not to be relaxed in the bridle and resisted the bit.Results of histological examination of the mass were suggestive of a schwannoma or peripheral nerve sheath tumour (PNST).

Clinical findings
At the time of examination at the OVAH, the mare was bright, alert, in good bodily condition, afebrile, with normal cardiovascular and respiratory parameters.Palpation of the right cheek revealed a subcutaneous swelling with poorly- defined margins present in the buccal region (Fig. 1).The horse was sedated with detomodine hydrochloride (Domosedan ® , Pfizer) (3 mg, IV) and oroscopy was performed, which revealed a mucosal erosion inside the right cheek (Fig. 2).
Ultrasonography of the right cheek, mandibular lymph node, the parotid and retropharyngeal region was performed.There was an inhomogeneous hyperechoic mass of 7 cm rostrocaudal length and 4 cm dorsoventral height with a lateral wide pedunculated mural base of at least 3.5 cm width at the inside of the right cheek at the level of 106-109 and 406-409 cheek teeth.The mass was well defined where it protruded into the buccal cavity but poorly defined at its base, thus appearing to infiltrate into the cheek musculature.A right oval to round mandibular lymph node (2 × 1.4 cm) was present medial to tooth 107 and was heterogeneously hypoechoic with minimal blood flow.No enlarged lymph nodes were seen in the parotid or retropharyngeal region.Ultrasonographic findings were consistent with a broadly pedunculated mass with reactive mandibular lymphadenopathy.A fine-needle aspirate (FNA) of the right mandibular lymph node revealed an elevated number of plasma cells and prominent numbers of immature lymphocytes consistent with reactive hyperplasia.A FNA of the parotid gland showed no evidence of inflammation and was consistent with a hyperplastic reaction.Ultrasonography of the abdomen revealed no abnormalities, while thoracic ultrasound showed mild dimpling of the visceral lung surface with associated ring down artefacts restricted to the cranioventral lung lobes.

Histopathology
Punch biopsies were taken from the buccal mass under local anaesthesia following admission to the OVAH.Examination of biopsy specimens at low magnification revealed a well-demarcated, non-encapsulated, mid-to deep-dermal neoplastic mass (Fig. 3).The mass consisted of uniform, interlacing whorls of fusiform cells.The epidermis was ulcerated and contaminated with opportunistic bacterial colonies.Small aggregates of lymphocytes were present along the deeper margins of the mass.No interaction between the neoplastic cells and the epidermis was evident, suggesting that the mass was unlikely to be a sarcoid.This was further supported by negative immunohistochemical staining for papilloma virus (rabbit polyclonal, N1547 Dako, Glostrup, Denmark).At higher magnification, individual neoplastic cells were characterised by long, slender, interconnecting, eosino-philic cytoplasmic processes, separated by clear slits (Fig. 4).The nuclei were cigar-shaped, elongated and heterochromatic.In longitudinal sectioned regions of the mass, nuclei were large and oval, suggesting a flattened, disc-shaped morphology.The mitotic rate was low (less than 1/hpf).Immunohistochemical staining for S-100 antigen (rabbit-polyclonal, Z0311 Dako, Glostrup, Denmark) yielded patchy positive staining of tumour cells (Fig. 5).All neoplastic cells were strongly positive for vimetin (mouse monoclonal, Dako, Glostrup, Denmark).Staining for glial fibrillary acidic protein (rabbit polyclonal, Z0334 Dako, Glostrup, Denmark) was negative.
Relevant areas of the tumour in the paraffin embedded tissue sample were processed for electron microscopy and revealed narrow spindle cells with elongated cytoplasmic extensions arranged in a whorling fashion in a collagenous matrix (Fig. 6a).The tumour cells exhibited elongated nuclei (Fig. 6a) with smooth or indented outlines and with prominent nucleoli (Fig. 7b).Dilated cisternae of the endoplasmic reticulum with electrondense and electron-lucent contents were seen (Fig. 6b).Intermediate filaments were present in the cytoplasm (Fig. 7a).Numerous proteoglygan particles were found in the extracellular matrix.Fibroblasts and a few histiocytes were present among the tumour cells.No external lamina surrounding the tumour cells or cytoplasmic filaments forming focal densities could be demonstrated.

Diagnosis
The histopathological features were consistent with a diagnosis of sarcoma (absence of nuclear palisading and dense cellular arrangement) and the immunohistochemical results were suggestive of a schwannoma or PNST 2 .The tumour cells that did not stain positively for S-100 were most likely poorly differentiated Schwann cells or perineural cells.

Interstitial tumour therapy
On re-admission to the OVAH, trimethoprim-sulfamethoxazole (Purbac ® , Aspen Pharmacare) (25 mg/kg, PO, q 12h), phenylbutazone (Phenylbutazone BP, Kyron Laboratories) (2 mg/kg, PO, q 12h) was started 72 hours prior to day of surgery.Results of a complete blood count and serum biochemistry were unremarkable.On the day of radiotherapeutic implant, romifidine hydrochloride (Sedivet ® , Boehringer Ingelheim) (0.04 mg/kg, IV) and butorphanol tartrate (Torbugesic ® , Fort Dodge Animal Health) (0.04 mg/kg, IV) were administered prior to induction of general anaesthesia using ketamine hydrochloride (Anaket-V, Centaur Laboratories) (2.2 mg/kg, IV) and GGE (GGE powder, Kyron Laboratories) (50 mg/kg, IV).The horse was transferred to the surgery suite and positioned in left lateral recumbency and maintained using halothane to effect and partial intravenous anesthesia (PIVA) using a combination of ketamine (1 mg/kg/hr, IV), lidocaine (Lignocaine Injection, Bayer (Pty) Ltd) (0.01 mg/kg/min, IV) and romifidine (0.03 mg/kg/hr, IV).Following surgical preparation of the right buccal region, 9 parallel-placed stainless steel pins (10 cm in length) were placed subcutaneously at 1.5 cm intervals, entering the schwannoma dorsally and exiting ventrally.Plastic guide tubes were then attached to each pin and threaded dorsoventrally by pulling each pin ventrally.Platinum-coated Ir 192 wires (AEC-Amersham) were then loaded into the guide tubes and the tubes sealed with studs (Fig. 8).Oroscopy was performed to ensure that implant penetration into the oral cavity was prevented during implantation.No attempt was made to surgically debulk the buccal tumour.Following recovery from general anaesthesia, the horse was isolated in a radiation-controlled area of the hospital until the precalculated radiation dose had been delivered, after which the guide tubes containing the Ir 192 wires were removed.During this period, the mare was monitored for any discomfort, swelling or discharge associated with the implantation.The implants were removed after

Radiation control
Radiation protection was ensured by discussing procedures with all relevant personnel and students.A microcom-puter-based instrument that measured Gamma, Beta and X-ray radiation (Smart-ION Chamber Survey Meter 2130, Thermo Scientific, Erlangen, Germany) and an electronic pocket dosimeter (EPD) ('MYDOSE-mini' pocket dosimeter, Aloka Co Ltd., Tokyo, Japan) was available for monitoring radiation exposure.Postimplant procedures also included providing a dedicated radiation stall with appropriate radiation warning signs, a notice of the isolation period, and daily monitoring charts for the mare and for isolation stall contamination.

Outcome
Short-term complications from the interstitial brachytherapy included loss of patency of the right nasolacrimal duct with increased tear flow of the right eye (fluorescein eye stain could not be visualised at the external puncta of the nasolacrimal duct), erythema and moist dermatitis at the entry and exit sites of the guide tubes, leukotrichia, localised soft tissue swelling and mild left-sided deviation of the muzzle.Follow-up consultation 3 months later revealed no visible tumour recurrence following digital palpation, slight muzzle deviation to the left most likely due to trauma of the N. facialis secondary to pin placement and the right nasolacrimal duct appeared patent.Although no further biopsy was performed, the mare continues to be in remission 12 months after the use of interstitial brachytherapy.

DISCUSSION
In published reports, benign schwannomas account for 2 to 5% of equine cutaneous neoplasms 9,10,12,15,19 .However, the prevalence may be falsely increased owing to sarcoids being misdiagnosed as schwannomas 13 .Often, these tumours may be mistaken for fibroma, fibrosarcoma or sarcoid and confirmation of  Schwann cell origin requires immunohistochemistry 16 .
Histopathological, immunohistochemical and electron microscopy results in this case were suggestive of the malignant form of schwannoma, also known as malignant PNST or neurofibrosarcoma.This was based on the absence of external lamina 5 and electronmicroscopic evidence of similar tissues that occasionally may be positive with the S-100 immunohistochemical stain such as myofibroblastic-, melanocytic and chondroblastic tissue.The malignant form of schwannoma is poorly described in horses and is considered extremely rare in large domestic animals 2 .To the authors' best knowledge, the use of brachytherapy for a malignant dermal schwannoma that recurred and could not be surgically excised owing to location or ill-defined borders has not been reported previously.
Despite the treatment of choice being wide surgical excision, schwannomas may recur.Tomographic mapping to determine areas of disease extension and vascularity was unavailable but may have assisted the procedure.Cutaneous tumours in horses tend to have poorlydefined margins, may be locally infiltrative and have a high recurrence rate.Fifty per cent of periocular schwannomas recurred following surgical removal within a 6-month period 6 .Apart from surgical resection, reported therapeutic interventions have included interstitial brachytherapy using Au 198 for periocular schwannomas 19 , intralesional cisplatin and intralesional BCG 7 .When recurrence takes place, and especially when wide excision may not be possible either due to anatomical location or ill-defined tumour margins for the reasons indicated above, an adjunctive therapy may be necessary.Although the efficacy is unknown at this stage, such therapy may include the use of a Nd:YAG or CO2 laser, intralesional chemotherapy using cisplatin or 5-fluorouracil, cryotherapy, brachytherapy or teletherapy.
Although surgical resection is the mainstay for primary treatment of all localised soft tissue sarcomas of the extremity and superficial trunk, brachytherapy is used as an adjunct to resection in human sarcomas.However, brachytherapy is not recommended for low-grade sarcomas because it did not influence local recurrence rates in a randomised prospective study of human patients 8 .The success of this treatment, although anecdotal, may suggest a difference between human and equine low-grade sarcomas.
Brachytherapy involves the use of radioactive sources such as Ir 192 , I 125 and Sr 90 to deliver radiation directly to affected tissue either as interstitial (through the use of Ir 192 as in this report) or surface therapy, also termed plesiotherapy 3 .The radioactive sources can be beads, wires, seeds or needles and its use is limited to soft tissue tumours only.Following sedation or general anesthesia, Ir 192 implant wires are inserted through guide tubes placed approximately 1 cm apart in the target tissue, providing a LDR of 0.3 to 0.5 Gy/h and the total radiation dose is typically 60-70 Gy over a 5-7-day period 3 .Alternatively, an automated remote loading technique 14 may be used to insert Ir 192 wires with a high activity into the guide wires for 5-10 minutes, which may be repeated 7 days later, allowing a total radiation dose of 25-30 Gy 3 .
Reported success rates using brachytherapy for the treatment of tumours have ranged from 74 to 100 % at 1 year 17,18,20 , while a 98 % success rate was reported using Ir 192 implantation for periocular sarcoids 4 .Since brachytherapy may be gaining popularity in the treatment of equine cutaneous tumours, certain precautions need to be followed.Radiation licensing restricts the use of Ir 192 to referral hospitals with radiation isolation facilities, personal safety must be adhered to, and the cost of treatment may be prohibitively high.
Personal radiation monitoring was performed using EPDs worn in the operator's left handed vest pocket.These instruments use semiconductor detectors to show the integrated radiation dose equivalent on a liquid crystal display.Other advantages include ease of operation and high sensitivity.Aldditionally, a larger advanced ion chamber meter with an audible alarm preset and large digital and analogue display was provided for radiation control of the isolation stall.
In conclusion, the use of interstitial brachytherapy allowed a localised radiation dose to be delivered to a malignant, dermal schwannoma that recurred 3 months after surgical excision while avoiding signs of systemic toxicity.Typical post-implantation symptoms consisting of localised tissue swelling, erythema, leukotrichia and moist dermatitis occurred.An excellent cosmetic result was achieved with minimal disfigurement and scarring (Fig. 9).

Fig. 2 :Fig. 3 :
Fig. 2: Oroscopic view of the right cheek of a 10-year-old Hanoverian mare with a right buccal schwannoma.Note the mucosal erosion present on the buccal aspect of cheek teeth 106-109 and 406-409 (demarcated by arrowheads), rostral is to the left.

Fig. 4 :Fig. 5 :Fig. 6 :
Fig. 4: Photomicrograph of a section of the buccal mass from the horse in Fig. 1.Neoplastic spindle cells are characterised by long, slender, interconnecting, eosinophilic cytoplasmic processes, separated by clear slits.The nuclei are cigar-shaped, elongated and heterochromatic.In longitudinal sectioned regions of the mass, nuclei are large and oval in shape, suggesting a flattened, disc-shaped morphology.The mitotic rate was low (less than 1/hpf).H&E stain; scale bar = 100 µm.
14 days without sedation by releasing the studs and sliding the plastic guide tubes containing the platinum-coated Ir 192 wires out of the subcutaneous tissues.The pattern of implantation was deter-mined by ultrasonographic evaluation of the right buccal region, digital palpation and the size and shape of the tumour.The standard planar implant protocol (Paris system) was used and the radiation calculation dose was determined for a treatment depth of 1.5 cm pre-implant to be 50 gray (Gy) at a low-dose radiation (LDR) of 0.5 mCi/cm.The actual implant region was 9 × 7 cm.The post-implant recalculated radiation dose was 65.0 Gy over a 14-day period.This allowed a LDR protocol of approximately 5 Gy per day.Radiation source of 50 cm Ir 192 wire with activity of 0.5 mCi/cm was used with a lead shielding of 2.8 cm (source strength = 500 µGy/h/m 2 ).The physical properties of Ir 192 are a half life of 74.1 days, a principle gamma mean energy of 0.38 MeV with a half value layer (HVL) in lead of 6 mm and an exposure rate constant of 4.8 Rh-m/Ci at ref air-kemma rate of 100 µGy/h.

Fig. 8 :
Fig. 8: Post-operative photograph of the horse after 9 platinum-coated Ir 192 wires were loaded and sealed in guide tubes.