CT of the sublumbar lymph nodes for anal sac adenocarcinoma staging

Why the iliosacral lymph nodes decide the plan

Anal sac adenocarcinoma spreads early to the regional lymph nodes, and a small primary tumour does not rule this out:

• In 116 dogs with abdominal staging and cytological or histological confirmation (Jones and Wustefeld-Janssens, Veterinary and Comparative Oncology, 2023), 53 (46%) had lymph node metastasis at presentation. The rate was 20% (9 of 46) for primary tumours under 2 cm and 63% (44 of 70) for tumours of 2 cm or more.
• In 70 dogs staged with CT of the thorax, abdomen and pelvis (Sutton et al., Journal of Small Animal Practice, 2022), iliosacral lymphadenomegaly was present in 71% and pulmonary metastases in 11%. Anal sac mass size was not associated with lymph node enlargement. No dog had pulmonary metastases without locoregional lymphadenomegaly.
• Calcium is not a substitute for imaging. In 58 dogs (Toth et al., Veterinary Medicine and Science, 2024), hypercalcaemia was associated with confirmed or suspected iliosacral metastasis, but 11 of 24 dogs (46%) with nodal disease were normocalcaemic.

The number of metastatic nodes also matters. In 57 dogs treated with sacculectomy and lymphadenectomy (Tanis et al., Veterinary and Comparative Oncology, 2022), more than four metastatic nodes was associated with shorter progression-free interval and disease-specific survival. The size of the largest node was not associated with outcome, and its measurement on imaging was not consistently reproducible.

The anatomy that makes staging difficult

The iliosacral lymph centre consists of the medial iliac, internal iliac (formerly hypogastric) and sacral lymph nodes. The sacral nodes lie inside the pelvis, ventral to the sacrum and dorsal to the pubis. According to a comparison of CT and ultrasound in 20 dogs (Palladino et al., Journal of Veterinary Internal Medicine, 2016), this makes them hard to image with ultrasound, and they can be missed at exploratory surgery. In a lymphoscintigraphy study in eight healthy dogs (Linden et al., Veterinary Radiology & Ultrasound, 2019), a sacral node was the sentinel node in three (37.5%), and the injection technique changed which node was identified.

Ultrasound, CT and MRI compared

Study	What it found
CT vs ultrasound, 12 dogs, prospective (Pollard et al., Veterinary and Comparative Oncology, 2017)	CT identified more iliosacral nodes (61 vs 30), more medial iliac nodes (33 vs 19) and more sacral nodes (15 vs 0). Ultrasound identified slightly more abnormal nodes (7 vs 5).
CT vs ultrasound, 20 dogs, retrospective (Palladino et al.)	CT found lymphadenomegaly in 13 dogs. Ultrasound found at least one enlarged node in all 13, but found every enlarged node in only 4 (30.8%). It detected 7 of 8 enlarged medial iliac, 4 of 11 internal iliac and 4 of 9 sacral nodes.
MRI vs ultrasound, 6 dogs with confirmed nodal metastasis (Anderson et al., Veterinary and Comparative Oncology, 2015)	MRI showed lymphadenopathy in all 6 dogs; ultrasound in 2. Lymphadenopathy was mainly sacral.

The authors of the larger CT study concluded that abdominal ultrasound is an effective screening test for iliosacral lymphadenomegaly, but CT should be considered when an additional metastatic site would change the treatment plan. They note that identifying which specific nodes are enlarged may matter for surgical and radiotherapy planning.

Sublumbar CT protocol at a glance

Staging CT that covers the thorax, abdomen and pelvis, as in the 70-dog staging study, includes the lungs and every iliosacral node group, including the intrapelvic sacral nodes. Sacral nodes tend to be the smallest in this group, and thinner slices may improve measurement accuracy.

What the radiologist looks at

• Each node group on each side. Medial iliac, internal iliac and sacral nodes, and which of them are enlarged.
• Size against a reference. There are no pathognomonic imaging signs of nodal metastasis, so size is the usual criterion. The 20-dog study normalised node diameter to the aortic diameter at L5–L6. Using 30 presumed normal dogs as a reference, it classed a node as enlarged at a node-to-aorta ratio of at least 1.3 for medial iliac nodes and at least 1.0 for internal iliac and sacral nodes.
• The number of abnormal nodes, because more than four metastatic nodes was associated with shorter survival.
• The lungs, when the thorax is included.

What your report will include

Every Veheri case is read by a radiologist or neurologist, and when you order you can choose Diplomate-only reporting or the full specialist panel. You can chat directly with the specialist assigned to your case, and a simplified pet owner report helps you explain the findings to the owner.

When you take the report forward:

• Enlarged does not prove metastatic. In the 20-dog study only 5 dogs had nodes sampled, and all 5 were metastatic. The authors recommend sampling mildly to moderately enlarged nodes by ultrasound-guided aspiration or core biopsy.
• Surgical and radiotherapy planning may depend on which specific nodes are enlarged, not only on whether any are.
• Normal calcium does not rule out nodal disease: 11 of 24 dogs (46%) with nodal disease were normocalcaemic.

What to include when you request a CT report

• signalment: age, breed and sex
• side and measured size of the anal sac mass, and rectal examination findings
• cytology or histology of the primary tumour and any nodes sampled
• calcium results
• previous abdominal ultrasound findings, so the two studies can be compared
• the planned treatment: sacculectomy, lymphadenectomy, radiotherapy or medical therapy

How Veheri can help

Veheri’s radiologists report veterinary CT studies remotely for clinics, and offer expert support with CT protocols, positioning and acquisition. For lung staging, see thoracic CT for pulmonary metastases in dogs, and for the wider picture, our oncology imaging guide.