A5-year-old 75-kg (165-lb) male alpaca was evaluated at the Kansas State University Veterinary Medical Teaching Hospital because of a recurrent, right-sided maxillary mass. The mass had been surgically debulked 3 times at 53, 34, and 25 weeks prior to referral. Surgical notes from the second and third debulking surgeries were available for review. Both indicated that electrocautery was used to excise all of the abnormal soft tissue and bone rongeurs were used to remove associated bone fragments. Histologic evaluation after the first debulking surgery described the mass as a fibroma with osseous metaplasia. Distinct normal tissue borders were not evident, and it was believed that excision was likely incomplete. Histologic evaluation was not performed after the subsequent debulking procedures.
Physical examination at the first referral visit revealed a right-sided, ulcerated, approximately 1.5-cm-diameter soft tissue mass at the level of the third premolar tooth. No other abnormalities were found on physical examination. Examination of radiographs of the skull revealed ill-defined lysis in the right maxilla surrounding the third and fourth premolar teeth and the first molar tooth roots. Ill-defined sclerosis was observed surrounding and superimposed over the lytic region and margins of affected tooth roots, and surrounding periodontal ligaments were ill-defined (Figure 1). Radiographic interpretation was most consistent with soft tissue neoplasia invading bone, although osteomyelitis could not be excluded. Segmental maxillectomy was recommended on the basis of the recurrent nature of the soft tissue mass and the radiographic characteristics of bone lysis. Results of a preanesthetic CBC and serum biochemical analysis showed no clinically important abnormalities. Because the lesion appeared to be a recurrence of the previously resected oral mass that was diagnosed as a fibroma, thoracic radiography was not performed.
The owner consented to additional diagnostic testing and treatment, and surgery was scheduled 2 weeks after the initial referral examination. Immediately prior to surgery, a CT was performed with the alpaca in sternal recumbency under general anesthesia. Pre- and postcontrast transverse, 5-mm, helical images were obtained from the nasal planum to the level of C2; additional postcontrast 1-mm axial images were obtained of the region of the cheek teeth. Expansile osteolysis of the right maxillary alveolar bone beginning at the level of the third premolar tooth was observed (Figure 2). Osteolysis extended caudally to involve the maxillary alveolar bone to the level of the rostral roots of the second molar tooth and was most severe at the level of the fourth premolar and first molar teeth causing displacement and separation of those teeth. Associated with the lysis was a moderately contrast-enhancing soft tissue mass. The osteolysis appeared to be contained within the right maxillary alveolar bone. Lysis did encroach on, but did not involve, the nasal cavity. Additionally thinning of the bony components of the right infraorbital canal was evident, as was mild heterogenous mineralization of the right retropharyngeal and mandibular lymph nodes (Figure 3). However, these lymph nodes were of similar size to the corresponding contralateral lymph nodes. Because of the location of the retropharyngeal lymph nodes and symmetric size both on physical examination and CT imaging, fine-needle aspiration and cytologic evaluation were not performed.
Surgery was performed with the intent of complete surgical resection of the mass. However, because of the extensive nature of the mass and the history of multiple recurrences, adjuvant radiation therapy in the likely event that complete surgical margins could not be obtained was discussed with the owner prior to surgery. The animal was placed in left lateral recumbency for surgery. A 7-cm buccotomy incision was made, and a partial right maxillectomy was performed with a chisel and mallet to enable removal of the mass including an approximately 0.5-cm border of grossly normal tissue in all planes. The mass was submitted for histologic evaluation. A sample of nearby bone was obtained and submitted for bacteriologic culture and antimicrobial susceptibility testing. Gel foam, containing approximately 600 mg of lincomycin hydrochloride, was inserted at the defect site in an attempt to span the defect in the maxilla and minimize postoperative bacterial colonization. The incision was closed in a routine manner. Biopsy of regional lymph nodes was not performed. The patient recovered from anesthesia without incident.
Bacteriologic aerobic culture yielded growth of nonhemolytic Streptococcus spp (slight growth; unable to speciate), α-hemolytic Streptococcus spp (enrichment broth only), and Lactobacillus spp (slight growth). Anaerobic culture yielded growth of Bacteroides capillosus (moderate growth), Fusobacterium necrophorum (moderate growth), and Tissierella praeacuta (moderate growth). On the basis of antimicrobial susceptibility testing, all organisms were susceptible to ceftiofur sodium or cephalothin; therefore, the patient received ceftiofur sodium (2.2 mg/kg [1.0 mg/lb], SC, q 24 h) for 10 days. Histologic evaluation of the mass revealed a poorly delineated, unencapsulated, invasive neoplasm composed of stellate-shaped cells embedded in abundant fibrovascular stroma. Mild anisocytosis and anisokaryosis were present, and no mitotic figures were detected in ten 400× fields. The neoplasm also contained spicules of mature woven bone with rare foci of mineralization. Although the cellular features of the neoplasm were relatively benign, the invasive and destructive qualities were most consistent with an incompletely excised fibrosarcoma with osseous metaplasia.
On the basis of histopathologic results and because of the recurrent nature of the mass, adjuvant treatment with external beam radiation therapy was recommended. Additional surgical resection was discussed; however, because the aggressive surgical resection already performed had resulted in incomplete surgical margins, and considering the history of multiple recurrences, additional surgery was not deemed likely to result in complete tumor control. Definitive radiation therapy was elected, and CT was performed for radiation treatment planning 25 days after the partial maxillectomy. With the anesthetized patient in left lateral recumbency, pre- and postcontrast, 3-mm, helical transverse images were obtained from the nasal planum through the level of C2. An extruded polystyrene foam bead-style cushiona was placed under the alpaca's head prior to CT and used to assist with repositioning to simulate head positioning for radiation treatments. No evidence of tumor regrowth was detected on treatment-planning CT images (not shown).
Radiation therapy commenced 33 days after surgery, and the dose was delivered in 11 fractions on a Monday, Wednesday, and Friday basis until complete. Short-term anesthesia, with a solution of butorphanol (0.046 mg/kg [0.021 mg/lb]), ketamine (4.62 mg/kg [2.1 mg/lb]), and xylazine (0.46 mg/kg [0.21 mg/lb]) administered IM, was used for each radiation treatment.
Radiation therapy was delivered as determined by means of computerized planning.b The clinical target volume was established by determining the region of contrast enhancement observed on CT within the tissues adjacent to the surgical scar. This was considered to include potential residual gross disease and the entire surgical bed. Electron field shaping blocks were used to extend the treatment field proximally 2 cm beyond the margins of the clinical target volume, which thus defined the planning target volume. Radiation was delivered by a linear acceleratorc with 3 electron beams and a source-to-skin distance of 105 cm. Because the planning software indicated hot spots within soft tissues generated by electron beams, a source-to-skin distance of 105 cm for electrons was found to result in a more homogenous and smooth dose distribution within the tissues than a 100-cm distance. The prescribed radiation dose of 48 Gy was delivered in eleven 4.4-Gy fractions to 95% of the planning target volume, normalized at the isocenter. The mean dose was 50.55 Gy to the clinical target volume with a maximum dose of 54.82 Gy to the clinical target volume. Critical structures involved in the radiation field were the right eye and the tooth alveoli and surrounding bone. The maximum dose to the eye was 18.5 Gy with minimum and mean doses of 0 and 0.93 Gy, respectively. The maximum dose to the bone surrounding the dental structures was 55.1 Gy with minimum and mean doses of 0 and 4.29 Gy, respectively (Figure 4). The isocenter was placed 5 cm above the skin surface as required for electron treatments at a source-to-skin distance of 105 cm. The central axis beam entry point was chosen at the geographic center of the planning target volume. The depth of the prescription was chosen to place the treatment dose at the deep margin of the clinical target volume, and the dose for each electron beam was prescribed at the depth of maximum dose. Tissue inhomogeneity correction was used with the planning software. All 3 electron beams were set up with the same beam entry point. Beam 1 used 6 MeV and beam 2 used 9 MeV with the same gantry angle (10°), collimator angle (0°), field-shaping aperture, and 0.5-cm bolus. The eye was shielded by exclusion, with the field-shaping aperture placed in the electron cone. Beam 3 used 12 MeV, but there was a 10° shift in collimator angle and no field-shaping aperture was used. Beam 3 was used at a higher energy to improve the depth of penetration over the area of the clinical target volume and represented < 10% of the delivered dose (Figure 5).
The alpaca was hospitalized and monitored for signs of toxicosis throughout the duration of treatment and was treated with esomeprazole magnesium (0.5 mg/kg [0.23 mg/lb], SC, q 24 h, for 5 days; then 0.25 mg/kg [0.114 mg/lb], SC, q 24 h, for 5 days; then 0.25 mg/kg, SC, q 48 h, for 14 days) while hospitalized to decrease the risk of gastric ulceration. No evidence of oral mucositis was noted, and only grade I cutaneous toxicosis,1 in the form of alopecia, developed in the radiation field. Body weight was maintained throughout the treatment duration.
Follow-up examinations with CT scanning were performed at 6 and 58 weeks (Figure 6) after radiation therapy was completed. No evidence of tumor regrowth was noted on visual or CT examination at these times. Additionally, the animal maintained normal prehension and mastication abilities and had no gross evidence of tumor recurrence on examination at 110 weeks after the completion of radiation therapy. In the opinion of the owner, the animal had an excellent quality of life.
Vac-Lok cushions, Civco, Orange City, Iowa.
Teletherapy Planning Software XiO, version 4.33.02, Elekta, Stockholm, Sweden.
Clinac2100 Teletherapy Unit, Varian, Palo Alto, Calif.
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