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A male 10-year-old, 14.5 kg mixed-breed dog named Rusty initially presented to the primary care veterinarian after the client had observed the dog to be persistently licking his perineal area.
Expression of the anal glands revealed bloody discharge, prompting antibiotic therapy. As antibiotic therapy was unsuccessful, a cytology sample was taken of the affected gland revealing neoplastic disease. Subsequent removal of the anal sac revealed a right-sided anal sac adenocarcinoma (ASAC). The patient recovered well from surgery, maintaining a normal appetite and defecation pattern, although the owner noted occasional panting.
Rusty was then referred to Melbourne Animal Referral Services (MARS) in Hawthorn East, for further management and treatment. Histopathology of the primary tumour revealed an adenocarcinoma of apocrine gland origin with incomplete surgical margins and evidence of vascular invasion, consistent with high metastatic potential.
At MARS, the patient’s diagnostic workup included CT imaging of the chest and abdomen which demonstrated moderate sacral lymphadenopathy (highly suspicious for metastasis based on imaging alone), mild splenic nodular changes (favoured benign lesions here based on attenuation), and a clear thoracic CT (with no pulmonary metastases). These findings were most consistent with locoregional metastatic disease. Cytological analysis of the enlarged lymph nodes confirmed metastatic disease.
Intraoperative near-infrared fluorescence (NIRF) imaging
Given the incomplete excision and presence of regional metastasis, surgical removal of the identified metastatic lymph nodes was recommended as well as surgical staging of the remaining sub-lumbar lymph nodes. In this context, NIRF imaging can be considered as an advanced technique for intraoperative identification of lymphatic structures.
NIRF imaging involves the use of fluorescent dyes, most commonly indocyanine green (ICG), which emit fluorescence when excited by near-infrared light (typically 700–900 nm). When administered intravenously or peritumorally, the dye travels through lymphatic vessels and accumulates in sentinel or regional lymph nodes. Specialised cameras capture the emitted fluorescence, enabling real-time visualisation of lymphatic drainage patterns during surgery.
This technique allows surgeons to identify sentinel lymph nodes (SLNs) and assess their involvement, improve completeness of nodal dissection, and reduce morbidity by avoiding unnecessary tissue removal.
Application in anal sac adenocarcinoma
In canine ASAC, metastatic spread typically follows predictable routes—most commonly to the medial iliac, sacral, and hypogastric nodes, occasionally extending to the sciatic chain. These nodes can be challenging to localise due to their deep pelvic position and surrounding vasculature.
NIRF imaging, by mapping functional lymphatic drainage, enables accurate identification of affected nodes not evident on preoperative imaging, intraoperative confirmation of complete nodal excision, and reduced risk of leaving microscopic metastatic tissue.
In this case, NIRF was able to identify all highlighted sacral lymph nodes identified on CT imaging but also additional nodes in the region. In current practice, NIRF imaging is increasingly utilised in mammary carcinomas, mast cell tumours, and oral malignancies, and its application in anal sac tumours is emerging as a promising adjunct.
Advantages and limitations of NIRF
Advantages:
• Minimally invasive and rapid mapping (<10 minutes)
• No radiation exposure (unlike nuclear scintigraphy)
• Real-time intraoperative feedback
• Can be combined with standard surgical lighting systems
Limitations:
• Requires specialised imaging equipment
• Limited tissue penetration depth (~1 cm)
• False negatives possible in cases of lymphatic obstruction or prior surgery.
Nevertheless, early studies in both human and veterinary oncology demonstrate high accuracy for sentinel node localisation and improved surgical precision when NIRF is integrated into oncologic resections.
Beyond lymphatic mapping, NIRF has expanding roles in other applications in veterinary surgery, including:
• Tumour margin assessment (visualising residual tumour fluorescence post-resection)
• Vascular perfusion evaluation (ensuring adequate blood flow to anastomoses or flaps)
• Detection of biliary or ureteral leaks
• Fluorescence-guided resection of soft tissue sarcomas and other solid tumours.
Such versatility positions NIRF as a valuable adjunct for enhancing oncologic surgical outcomes and reducing recurrence risk.
Postoperative management and adjuvant therapy
Postop discussion with the dog’s owners outlined three main management pathways:
1. Monitoring
Regular rectal examination and imaging (CT or ultrasound every six months) to detect local recurrence or distant metastasis.
2. Local Control Options:
Radiotherapy: Provides superior local control when combined with surgery. Median survival of up to 2.5 years reported in dogs undergoing multimodal therapy.
Electrochemotherapy (ECT): Combines bleomycin with pulsed electric fields to enhance intracellular drug uptake. Effective for local recurrence and tumour bed control with minimal systemic toxicity.
3. Systemic Therapy:
Toceranib phosphate (Palladia): A tyrosine kinase inhibitor with anti-angiogenic and anti-proliferative activity. Administered orally over 6 months, with regular CBC, biochemistry, blood pressure, and UPC monitoring. Common side effects include mild gastrointestinal upset or lethargy.
Rusty’s owners are currently considering systemic therapy with or without local adjuvant treatment. Histopathology confirmed all removed lymph nodes to be metastatic clearly indicating NIRF’s superiority in identifying additional metastatic lymph nodes not necessarily noted on CT imaging.
Conclusion
This case illustrates the challenges of managing anal sac adenocarcinoma with nodal metastasis. NIRF imaging offers a practical and increasingly accessible solution to improve intraoperative lymph node identification, enabling more thorough staging and potentially reducing recurrence rates. As this technology becomes integrated into advanced veterinary surgical oncology, its applications—from sentinel node mapping to perfusion assessment—are likely to redefine precision surgery in companion animals.
Dr Ben Mielke BVSc. (Hons), MVetMed, MANZCVS (Surgery), DipECVS, MRCVS. ECVS and EBVS
Registered specialist in small animal surgery, MARS
Dr Mielke completed an undergraduate degree in veterinary science at the University of Melbourne before completing a master’s in veterinary medicine at the Royal Veterinary College. He worked as a mixed animal vet in rural Australia for two years prior to moving to the UK where he completed his residency training at the Royal Veterinary College and worked as a lecturer in small animal surgery. Dr Mielke’s is passionate about soft tissue surgery as well as brachycephalic airway syndrome, minimally invasive surgery, oncological and thoracic surgery.
Dr Irina Gramer (DVM) DipECVIM-CA (Oncology) MANZCVS (Oncology) PGCert VetEd FHEA MRCVS CVLM
Veterinary oncologist, MARS
Dr Gramer graduated from Justus-Liebig University in Giessen, Germany, in 2010, completing her doctoral thesis on molecular genetics in canine cancer patients. She then completed an oncology internship at a veterinary referral and critical care centre in the UK, followed by an oncology residency at the University of Liverpool. She achieved her Diploma status in 2017 becoming a European specialist. Dr Gramer moved to Australia in 2022. She is the founder of Oncology Camp, the co-founder of Melbourne VETWorks; and she chairs the ECVIM education committee.
