Anal Carcinoma

Todd M. Blodgett, MD

Alex Ryan, MD

Omar Almusa, MD

Key Facts

Terminology

Anal cancer, squamous cell carcinoma (SCCA) of the anus

Imaging Findings

Most lesions arise in anal canal
Consider PET/CT for staging
MR and endoluminal US commonly used to assist in determining depth of penetration and local spread
Post-treatment PET/CT results are more predictive of survival outcome than pre-treatment factors such as T-stage and nodal status
Patient with partial metabolic response, i.e., persistent FDG uptake in irradiated region, have 2 year progression-free survival rate of 22%
Best imaging features include focal intense FDG activity on a PET/CT scan with a correlative CT abnormality ± inguinal/iliac nodes
Focal intense FDG activity is usually identified within the primary lesion if large enough (> 6 mm)
Regional lymph nodes including inguinal, perirectal, and iliac nodes may be involved

Top Differential Diagnoses

Physiologic FDG Activity at the Anorectal Junction
Metastatic Disease
Distal Colonic Adenocarcinoma
Rectovaginal Fistula
Inflammation of the Anus

Diagnostic Checklist

FNA of inguinal adenopathy helpful to establish histologic proof of metastasis

Axial CECT shows massive diffuse thickening of the anal mucosa

in this patient with recently diagnosed squamous cell carcinoma of the anus.

Axial fused PET/CT shows correlative circumferential intense FDG activity corresponding to the anal mucosal thickening

TERMINOLOGY

Abbreviations and Synonyms

Squamous cell carcinoma (SCCA) of the anus
Anal carcinoma
Anal cancer

Definitions

Carcinoma arising from tissue of the anal canal or anal margin
- Subclassified as transitional and cloacogenic

IMAGING FINDINGS

General Features

Best diagnostic clue
- Usually diagnosed by physical exam
- Best imaging features include focal intense FDG activity on a PET/CT scan with a correlative CT abnormality ± inguinal/iliac nodes
Location
- Most lesions arise in anal canal
  - Anatomic area extends from anorectal ring to zone approximately halfway between pectinate (dentate) line and the anal verge
- Carcinomas arising proximal to pectinate line (transitional zone between glandular mucosa of rectum and squamous epithelium of distal anus)
  - Basaloid, cuboidal, or cloacogenic tumors
  - About 1/3 of anal cancers have this histology
- Malignancies distal to pectinate line are of squamous histology
  - Account for 55% of all anal cancers
  - Ulcerate more frequently
- Dentate line and extending approximately 1 cm proximally
  - Transitional zone of epithelium that connects squamous cell epithelium of anoderm with columnar epithelium of rectum
  - Transitional zone includes columnar, cuboidal, transitional, and squamous epithelial cells
  - Represents the source for a variety of malignancies that arise in the anal canal
- WHO divides anal canal into 2 regions for grading malignancies
  - Anal canal portion: Proximal to dentate line and including transitional zone
  - Anal margin: Anoderm distal to dentate line
- Anal canal malignancies metastasize to
  - Mesenteric lymph nodes and portal circulation
  - Regional inguinal nodes and via systemic circulation
- Nodal dissemination pathways commonly target perirectal, iliac, and inguinal basins
- Distant spread frequently involves liver and lung
  - Metastases to spine and musculoskeletal system are rare
Size: Variable, ranging from subcentimeter to several centimeters

Imaging Recommendations

Best imaging tool
- MR and endoluminal US are commonly used to assist in determining depth of penetration and local spread
- Consider PET/CT for regional and distant staging
Protocol advice
- Immediate voiding prior to PET is recommended to minimize FDG activity in the bladder
- Scan from bottom to top to minimize FDG accumulation in the bladder during the exam

Nuclear Medicine Findings

PET
- Focal intense FDG activity is usually identified within primary lesions > 6 mm
- Regional lymph nodes including inguinal, perirectal, and iliac nodes may be involved
  - Usually FDG avid unless small
FDG PET for prognosis
- Complete metabolic response was associated with significantly improved progression-free and cause-specific survival compared with partial response
  - Patients with complete metabolic response had 2 year progression-free rate of 95%
  - Patients with partial metabolic response, i.e., persistent FDG uptake in irradiated region, have 2 year progression-free survival rate of 22%, regardless of presenting T-stage
  - Post-treatment PET/CT results were more predictive of survival outcome than pre-treatment factors such as T-stage and nodal status