Open Veterinary Journal, (2026), Vol. 16(4): 2378-2384

Case Report

10.5455/OVJ.2026.v16.i4.39

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Evaluation of intercostal thoracotomy as a surgical approach for feline thymoma: A case series of seven cats

Yuta Ikeda^* and Asaka Kawana

Animal Cancer Clinic Tokyo, Tokyo, Japan

*Corresponding Author: Yuta Ikeda. Animal Cancer Clinic Tokyo, Tokyo, Japan.
Email: yutaikeda12 [at] gmail.com

Submitted: 08/10/2025 Revised: 25/02/2026 Accepted: 13/03/2026 Published: 30/04/2026

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ABSTRACT

Background: Thymoma is the second most common mediastinal tumour in cats, and surgical excision is considered the treatment of choice. The median sternotomy is the most frequently used surgical approach. In contrast, although intercostal thoracotomy is widely employed in thoracic surgery (e.g., cardiac surgery or lung lobectomy), it has not been systematically reported in the context of feline thymoma. Herein, we report seven cases of feline thymoma treated via intercostal thoracotomy. We describe the tumour location (based on preoperative imaging), tumour diameter, choice of intercostal approach site, perioperative complications, and outcomes. To the best of our knowledge, this is the first case series to evaluate the feasibility and utility of intercostal thoracotomy for thymoma resection in cats.

Case Description: Seven client-owned cats underwent intercostal thoracotomy for thymoma. Preoperative evaluation with computed tomography or thoracic ultrasonography revealed that the mass was located on the left side of the thoracic cavity when referenced to the midline defined by the sternum in all cases. The median tumour diameter was 5.2 cm (range, 4.0–6.4 cm), indicating relatively small masses. On the basis of these findings, a left fourth intercostal thoracotomy was performed in all cats. Postoperative complications included anemia that did not require transfusion in two cats and subcutaneous oedema in three cats. The duration of hospitalisation ranged from 3 to 4 days. One cat died 105 days postoperatively from a cause unrelated to thymoma, whereas the remaining six cats were alive at the last follow-up (105–850 days). No recurrence or metastasis was detected in any patient during the observation period.

Conclusion: In this case series of seven cats with thymoma, intercostal thoracotomy-associated complications were minor, and the postoperative course was favourable. These findings suggest that intercostal thoracotomy is an effective surgical approach for feline thymoma, particularly when the tumour is relatively small and predominantly located on one side of the thoracic cavity. In selected cases, intercostal thoracotomy may represent a viable alternative to median sternotomy.

Keywords: Case series, Feline thymoma, Intercostal thoracotomy, Postoperative outcome, Surgical approach.

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Introduction

Thymoma is the second most common mediastinal tumour in cats, and histologically, cystic changes are observed in approximately 50%–60% of cases (Carpenter and Holzworth, 1982; Gores et al., 1994; Day, 1997; Patnaik et al., 2003). Although the metastatic rate is generally low, it has been reported to reach as high as 21% in some studies (Patnaik et al., 2003; Marks et al., 2024). Thymoma typically occurs in middle-aged to older cats without sex predisposition (Withrow and MacEwen, 2019). Clinical signs most frequently include dyspnoea or tachypnoea, cough, lethargy, anorexia, weight loss, and vomiting (Gores et al., 1994; Marks et al., 2024), although some cases are incidentally discovered without clinical signs (Marks et al., 2024). Paraneoplastic syndromes, including lymphocytosis, myasthenia gravis, megaesophagus, exfoliative dermatitis, polymyositis, and hypercalcemia, have been reported in up to 22% of feline thymomas. These conditions may resolve following tumor excision, whereas they develop postoperatively in some cats (Rottenberg et al., 2004; Zitz et al., 2008; Garneau et al., 2015; Hague et al., 2015; Marks et al., 2024). Surgical excision is considered the treatment of choice for thymoma, with a reported median survival time of 630–1,825 days following complete resection (Gores et al., 1994; Zitz et al., 2008; Garneau et al., 2015; Marks et al., 2024). Median sternotomy is the most common surgical approach for feline thymoma (Gores et al., 1994; Marks et al., 2024). However, intercostal thoracotomy is widely used in various thoracic procedures, including cardiac and lung lobectomy surgeries. Although sporadic case reports have described its use for thymoma resection, no comprehensive study has systematically evaluated this approach (Gores et al., 1994; Cavalcanti et al., 2014; Garneau et al., 2015). The present case series investigated seven cats with thymoma that underwent intercostal thoracotomy. The tumour location (based on preoperative imaging), tumor diameter, choice of intercostal approach site, perioperative complications, and clinical outcomes were evaluated to assess the feasibility and potential utility of intercostal thoracotomy as a surgical approach for feline thymoma.

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Case Details

The medical records of seven client-owned cats that underwent intercostal thoracotomy for thymoma at the Animal Cancer Clinic Tokyo between January 2022 and September 2025 were retrospectively reviewed. Four cats were Domestic Shorthairs, and the remaining three were American Shorthair, Scottish Fold, and British Shorthair. The mean age of cats was 9.1 years (median, 8.2 years; range, 6.5–13.9 years). The mean body weight was 4.6 kg (median, 4.6 kg; range, 2.9–5.8 kg). The most commonly observed clinical signs were anorexia (5/7; 71%), dyspnoea (3/7; 42%), cough (2/7; 28%), and exfoliative dermatitis (1/7; 14%).

Complete blood count and serum biochemistry analyses were performed for each cat. The observed abnormalities included anaemia (1/7; 14%) and elevated Alanine aminotransferase (1/7; 14%). Feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV) in-house enzyme-linked immunosorbent assay tests (SNAP^® FeLV/FIV Combo, IDEXX Laboratories, Westbrook, ME) were performed in all cats, and all tested negative for FeLV/FIV. Thoracic radiography and ultrasonography were performed in all cases, and computed tomography (CT) was performed in five cats. A cranial mediastinal mass was identified in all cases, with a median tumour diameter of 5.2 cm (range, 4.0–6.4 cm) on thoracic imaging. In all cases, the mass was predominantly located on the left side of the thoracic cavity relative to the midline defined by the sternum (Figs. 1 and 2). A cystic appearance within the mass was observed in three cats; pleural effusion was present in three cats; and no evidence of cranial vena cava invasion was detected. Ultrasound-guided fine-needle aspiration of the mediastinal mass was performed in five patients. The aspirates contained a mixed population of cell types, including small lymphocytes (5/5 cases), mast cells (1/5), epithelial cells (2/5), and erythrocytes (5/5). A presumptive diagnosis of thymoma was made based on the characteristic imaging and cytological findings. All cats received oral prednisolone before surgery, with a median dosage of 1 mg/kg (range, 0.5–2 mg/kg) for 10–62 days. In 2 of the 7 cats, the mass decreased in size by more than 30% during treatment. Following anesthesia induction, tumour excision was performed via a left fourth intercostal thoracotomy in all cats. Thoracotomy was combined with a sixth intercostal approach in one case and with a transverse rib incision in another (Fig. 3). Five tumors were well encapsulated and easily excised, whereas two were adherent to the pericardium. The sternal lymph node was excised in one cat. The thoracotomy was closed with interrupted circumcostal sutures, and a thoracostomy tube was placed in all cats during surgery. The tubes were aseptically managed and drained every 2–6 hours and were removed on postoperative day 3. The duration of hospitalisation ranged from 3 to 4 days. Postoperative complications included anemia that did not require transfusion in two cats and subcutaneous oedema in three cats; no other complications were observed. Histological evaluation confirmed thymoma in all patients. All tumours were predominantly composed of round or spindle-shaped thymic epithelial cells. Six tumours were richly infiltrated with mature lymphocytes, and all contained various sizes of cystic lesions. A capsular invasion was observed in two cats. Surgical margins were complete in five cats and incomplete in two. Histopathological examination of the excised sternal lymph node revealed no evidence of regional metastasis. The follow-up period ranged from 105 to 850 days (median, 257 days). The postoperative follow-up consisted of scheduled reexaminations, including physical examination and thoracic imaging (radiography and ultrasonography). One cat with exfoliative dermatitis died 105 days after surgery, whereas the other six cats remained alive at the time of the last follow-up (Table 1). Based on clinical and imaging findings, no recurrence or metastasis was detected in any case during the observation period. Survival analysis was not performed because of the limited number of events.

Table 1. Signalment, surgical findings, complications, and outcomes in seven cats with thymoma undergoing intercostal thoracotomy.

Fig. 1. Transverse contrast-enhanced CT image of case 4 showing a cranial mediastinal mass (white arrow) predominantly located on the left side of the thoracic cavity relative to the sternum (red arrow). R, right; L, left.

Fig. 2. Dorsal contrast-enhanced CT image of case 4 showing a cranial mediastinal mass (white arrow) predominantly located on the left side of the thoracic cavity. R, right; L, left.

Fig. 3. Intraoperative photograph of case 2 showing tumour excision performed via a left fourth intercostal thoracotomy. Cr, cranial; Cd, caudal.

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Discussion

Median sternotomy is generally performed for thymoma excision in cats (Gores et al., 1994; Marks et al., 2024). In contrast, reports describing intercostal thoracotomy for feline thymoma are scarce, consisting mainly of isolated case reports or small case series of up to four cats, in which tumour size, location, intercostal incision site, and complications were not evaluated in detail (Gores et al., 1994; Cavalcanti et al., 2014; Garneau et al., 2015). To the best of the authors’ knowledge, this is the first case series to assess the clinical usefulness of intercostal thoracotomy for thymoma resection in cats. Unlike previous isolated case reports or small descriptive series, the present study provides a structured evaluation of tumour characteristics, surgical approach selection, intraoperative modification, perioperative morbidity, and follow-up outcomes in a consecutive cohort of feline thymoma cases. This case series aims to contribute practical clinical information that may support evidence-based surgical decision-making when considering intercostal thoracotomy as an alternative to median sternotomy by systematically documenting these parameters. In this context, although the feasibility of a lateral approach for small unilateral masses may appear intuitive, objective clinical documentation of surgical selection criteria and perioperative outcomes in feline thymoma remains limited.

Preoperative radiography and CT demonstrated that all tumours were predominantly located on the left side of the thoracic cavity and were relatively small. Consequently, a left fourth intercostal thoracotomy was performed in all cats. In a previous study of 54 cats undergoing median sternotomy, the median tumour diameter was 6 cm (range, 2–15 cm) (Marks et al., 2024). In comparison, the tumours in this study were smaller, with a median diameter of 5.2 cm (range, 4.0–6.4 cm). These findings suggest that thymomas smaller than approximately 5 cm in diameter may be amenable to removal via intercostal thoracotomy. Although detailed descriptions of tumour position are lacking in previous literature, thymomas that occupy the cranial mediastinum and extend to contact both hemithoraces may be better approached via median sternotomy, which allows bilateral exploration (Yoon et al., 2004).

Case 3 had a relatively large mass (6.4 cm) requiring an additional sixth intercostal incision for adequate exposure, and a transverse incision of the third rib was needed to facilitate dissection of the cranial aspect of the mass in case 5. These cases indicate that intercostal thoracotomy allows flexible modification, such as additional intercostal or transverse incisions, depending on tumour size and location. Notably, two cats exhibited capsular invasion histologically, but both tumours were successfully excised through intercostal thoracotomy.

Perioperative mortality rates of 11%–22% have been reported in cats undergoing median sternotomy, with causes including intraoperative haemorrhage, pyogranulomatous pleuritis, cardiac arrest, aspiration pneumonia, and presumed cerebrovascular infarction (Gores et al., 1994; Zitz et al., 2008; Garneau et al., 2015; Marks et al., 2024). In contrast, no perioperative deaths or life-threatening complications occurred in this study. Although this difference may reflect selection bias related to tumour size, invasiveness, or limited number of cases, intercostal thoracotomy may be associated with lower perioperative mortality in feline thymoma.

Postoperative complications following median sternotomy in cats include inappetence, haemorrhage requiring transfusion, laryngeal paralysis, postoperative myasthenia gravis, megaoesophagus, Horner’s syndrome, infection associated with thoracic drains, and sternal dehiscence or fracture (Gores et al., 1994; Zitz et al., 2008; Hennet et al., 2022; Marks et al., 2024). Marks et al. (2024)documented postoperative complications in 20% of 54 cats undergoing median sternotomy. In comparison, Moores et al. (2007)reported complications, including seroma formation, wound inflammation, lameness, and wound dehiscence, in 47% of dogs and cats that underwent intercostal thoracotomy for various diseases. However, Bleakley et al. (2018)found that intercostal thoracotomy resulted in less postoperative pain, better oxygenation, and fewer complications than median sternotomy in dogs undergoing pulmonary lobectomy. In this study, postoperative complications consisted of mild, non-transfusion-requiring anaemia in two cats and transient subcutaneous oedema in three cats. These were considered clinically acceptable and self-limiting.

Thoracoscopic resection of cranial mediastinal masses has recently been reported in cats (Renier et al., 2025). In that study of 17 cats, 12 were diagnosed with thymoma. The median tumour diameter was 6.0 cm (range, 2.2–8.1 cm), and conversion to median sternotomy was required in 35% of the cases; however, no perioperative deaths occurred. These findings indicate that thoracoscopic resection should also be considered as a potential surgical option for the treatment of feline thymoma.

Previous studies have reported median survival times of 630–1,825 days following thymoma excision in cats (Gores et al., 1994; Zitz et al., 2008; Garneau et al., 2015; Marks et al., 2024). In the present study, the follow-up period ranged from 105 to 850 days (median, 257 days), with six of seven cats remaining alive at the end of the observation period. No local recurrence or metastasis was observed, and the overall survival was comparable to that previously reported.

This retrospective study has several limitations. Because no cats underwent median sternotomy for thymoma at our institution during the study period, a direct comparison between the two surgical approaches could not be performed, and evaluation relied on literature review. Future studies should include larger, prospectively collected cohorts managed by a single surgeon to minimise inter-operator variability and allow comparison of outcomes between intercostal thoracotomy and median sternotomy. Additionally, not all cats underwent preoperative CT, limiting detailed presurgical assessment of tumour size and location, which are key determinants of approach selection. Future investigations should focus on correlating CT findings with surgical accessibility and optimal incision site selection. Finally, postoperative pain assessment was not consistently documented in the medical records, preventing the analysis of perioperative pain—a critical parameter in evaluating thoracic approaches. Further studies are warranted that directly compare intra- and postoperative pain between median sternotomy and intercostal thoracotomy.

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Conclusion

Based on the findings of this case series, intercostal thoracotomy appears to be a feasible and effective surgical approach for feline thymoma, particularly when the tumour is relatively small and predominantly located on one side of the thoracic cavity. In selected cases, it may represent a viable alternative to median sternotomy. Further accumulation of cases is required to establish clearer selection criteria between intercostal thoracotomy and median sternotomy for feline thymoma resection.

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Acknowledgments

The authors sincerely thank the licensed veterinary nurses (Japan) for their dedicated daily care and compassionate surgical assistance.

Conflict of interest

The authors declare no conflict of interest.

Funding

This study received no specific grant.

Authors' contributions

Conceptualization: Y.I. (Yuta Ikeda), Surgery/investigation: Y.I. and A.S. (Asaka Kawana) Data curation, Formal analysis: Y.I. Writing—original draft; Y.I. Writing—review & editing, Y.I. All authors read and approved the final manuscript.

Data availability

The data that support the findings of this study are not openly available due to sensitivity reasons and are available from the corresponding author upon reasonable request.

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