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Radiation Oncology

Imaging Features of Anal Carcinoma after Chemoradiation

Published Online:Mar 13 2025https://doi.org/10.1148/rg.240119
Figure 1.

Figure 1. Axial CT image for radiation therapy planning, with marked dose contouring, in a 57-year-old woman. The clinical target volume (CTV) for the primary tumor (magenta outline) is marked by drawing around the gross tumor volume (orange outline) with an additional 1.0-cm radius margin. The nodal CTV (light purple outline) is marked by drawing around the metastatic mesorectal node (red outline) with an additional 0.5-cm radius margin. Other pelvic nodal stations, such as the inguinal nodes or pelvic side wall nodes are targeted with an elective nodal CTV (green outline). The right femoral head (yellow outline) and left femoral head (blue-green outline) are delineated. The radiation treatment arc is outlined in dark blue.

Figure 2.

Figure 2. Chart illustrates the categorization of anal MRI protocols according to techniques that were almost always used (“Dos”) or never used (“Don'ts”) in published literature studies and those with which there was some institutional variation (“Maybes”). ADC = apparent diffusion coefficient, DWI = DW imaging, FOV = field of view, FSE = fast spin echo, SE = spin echo, STIR = short τ inversion recovery, TSE = turbo spin echo.

Figure 3.

Figure 3. Anorectal SCC in a 66-year-old woman who had a complete response at pelvic MRI. (A) Axial pretreatment MR image shows an intermediate-signal-intensity tumor in the anal canal between the 6- and 2-o'clock positions (solid arrow), with extramural extension of the tumor at the 2-o’clock position (dashed arrow). (B) On the axial posttreatment T2-weighted MR image, the previously visualized intermediate-signal-intensity tumor between the 6- and 2-o'clock positions has been completely replaced by thick, markedly hypointense fibrosis (solid arrow). The focus of extramural tumor extension at the 2-o’clock position is also replaced by fibrotic signal intensity (dashed arrow). (C) Posttreatment DW image (b = 1000 sec/mm2) shows a hypointense tumor bed between the 6- and 2-o'clock positions (arrowhead), which is suggestive of facilitated diffusion, a marker of excellent treatment response.

Figure 4.

Figure 4. Residual disease after CRT for ASCC in a 58-year-old man. (A) Axial T2W pretreatment MR image shows an annular intermediate-signal-intensity tumor (arrow) in the anal canal. (B) Axial T2W posttreatment MR image shows a residual intermediate-signal-intensity tumor (arrow), which is reduced in size compared with its pretreatment size. (C, D) Posttreatment 1000-sec/mm2 DW image (C) shows increased signal intensity (black arrowhead), which is matched by low signal intensity (white arrowhead) on the ADC map (D), consistent with diffusion restriction and suggestive of residual tumor. (E) Axial posttreatment fused FDG PET/CT image shows residual focal high-grade radiotracer uptake (arrow) that is greater than twice the hepatic uptake, suggestive of residual tumor.

Figure 5.

Figure 5. Anorectal SCC in a 62-year-old woman who had a good local-regional disease response initially but a new distant metastasis at follow-up. (A) Sagittal T2W posttreatment MR image shows an excellent treatment response at the primary tumor bed, with dense hypointense fibrosis (solid arrows). There is associated anorectal mucosal inflammation, seen as T2 hyperintensity (dashed arrows), in the central canal. At the periphery of the image, there is subtle intermediate signal intensity in the sacrum (arrowhead), which is suspicious for new bone metastasis. (B) Posttreatment maximum intensity projection FDG PET image shows an excellent response of the primary anal tumor, with no residual metabolic activity at the tumor bed (solid arrow). However, there is focal high-grade uptake in the sacrum (arrowhead), corresponding to the sacral abnormality seen at MRI and consistent with a sacral metastatic deposit. Another metastasis was found in the liver (dashed arrow).

Figure 6.

Figure 6. Nodal involvement after CRT for ASCC in a 62-year-old woman who presented with bleeding, a history of constant pain, and an enlarging lump around the anus and on both sides of the groin. Clinical examination revealed bilateral palpable inguinal nodes and a fungating anal lesion. She had undergone end-colostomy formation for symptom control in addition to radical CRT. (A) Axial T2W pelvic MR image at baseline shows abnormally enlarged bilateral inguinal lymph nodes with intermediate signal intensity and an irregular contour (arrows), suggestive of nodal involvement. (B) Axial fused FDG PET/CT image at baseline shows focal high-grade radiotracer activity in the abnormally enlarged bilateral inguinal lymph nodes (arrows), suggestive of nodal involvement. (C) Axial T2W pelvic MR image 3 months after CRT shows a marked interval reduction in the size of the inguinal lymph nodes, with residual T2-hypointense fibrotic changes (arrows), suggestive of a complete regional nodal response. (D) Axial fused FDG PET/CT image 3 months after CRT shows no high-grade metabolic uptake (arrows) suggestive of a complete regional nodal response. (E) However, a second corresponding 3-month post-CRT FDG PET/CT image shows a new hypermetabolic left common iliac node (arrowhead) at the edge of the radiation therapy field, suggestive of progressive disease. The iliac node was treated with stereotactic ablative radiation therapy, with a sustained treatment response seen on further follow-up images.

Figure 7.

Figure 7. ASCC in a 29-year-old man who is HIV positive whose posttreatment images showed features of mucositis and inflammatory pseudotumor. (A) Axial T2W pretreatment MR image shows an intermediate-signal-intensity tumor (arrow) between the 9- and 6-o'clock positions. (B) Coronal T2W pretreatment MR image shows an intermediate-signal-intensity tumor (arrow) primarily on the left side of the anal canal and anorectal junction. (C) Axial T2W MR image 3 months after CRT completion shows a high-signal-intensity lobulated focus protruding toward the anal lumen between the 9- and 1-o'clock positions (solid arrow), away from the epicenter of the previous tumor, suggestive of a pseudotumor. Note how the primary tumor shows a good response to treatment, with dense low-signal-intensity fibrosis in the tumor bed (dashed arrow) from the 1-o’clock to 4-o’clock position. (D) Coronal T2W MR image 3 months after CRT completion shows dense low-signal-intensity fibrosis on the left side of the anal canal and anorectal junction at the site of the tumor bed (dashed arrow) but increased signal intensity in the central canal (solid arrow), suggestive of mucosal inflammation. (E, F) Axial posttreatment DW (b = 1000 sec/mm2) (E) and ADC (F) images show no restricted diffusion. The tumor bed is dark on the DW (dashed arrow in E) and ADC (solid arrow in F) images, representing a T2 dark-through effect caused by dense fibrosis, a marker of excellent treatment response. The pseudotumor appears bright on the DW (solid arrow in E) and ADC (dashed arrow in F) images, representing a T2 shine-through effect. (G) Coronal pretreatment fused FDG PET/CT image shows a bulky anal canal tumor (arrow) with avid FDG uptake conforming to the shape of the mass seen at pelvic MRI. (H) Coronal posttreatment fused FDG PET/CT image shows stereotypical linear FDG uptake along the mucosa (arrow), suggestive of mucositis. This linear uptake corresponds to the central T2 hyperintensity seen at pelvic MRI. Note how the metabolic activity of mucositis is markedly diminished compared with that of the primary tumor.

Figure 8.

Figure 8. Recurrent perianal ulceration, fistulas, sinus tracts, and abscesses in a 45-year-old woman. ASCC was incidentally found in a tissue sample obtained during surgical drainage of the perianal abscess. The patient received CRT as the mainstay of treatment, and a seton was inserted to aid drainage of the sinus tracts. (A) Axial T2W highly–fluid-sensitive fat-suppressed pretreatment MR image clearly shows the perianal fistulas and sinus tracts (all arrows). The left limb of a complex perianal fistula (dashed arrow) contains a locule of gas. (B) Coronal post-CRT fused FDG PET/CT image shows the seton (arrow), which appears as a curvilinear high-attenuating structure on the CT component of the PET/CT image. As shown in this case, a seton may have moderately increased FDG uptake around it due to inflammatory changes in the fistula.

Figure 9.

Figure 9. Post-CRT ischioanal abscess in a 41-year-old woman. (A) Axial pretreatment MR image shows a bulky intermediate-signal-intensity anal tumor extending into the left ischioanal fossa (arrow). (B) Axial posttreatment MR image 3 months after the completion of CRT shows a reduction in tumor volume. However, there is residual intermediate-signal-intensity tumor (dashed arrow), and the patient has developed a fluid-signal-intensity collection in the left ischioanal fossa (solid arrow), which appears to be contiguous with the tumor.

Figure 10.

Figure 10. Tram-track fibrosis after CRT. (A) Coronal (top) and axial (bottom) drawings illustrate how the tram-track sign manifests between the pretreatment and posttreatment periods, whereby the cancer lesion transitions from tumor (T) to fibrosis (F). Note how parallel fibrotic lines form on either side of the internal sphincter following successful CRT. (B, C) Axial (B) and coronal (C) T2W pretreatment MR images in a 75-year-old patient with ASCC show an intermediate-signal-intensity anal tumor extending between the 7- and 1-o'clock positions (arrow). (D, E) Axial (D) and coronal (E) T2W 3-month posttreatment MR images in the same patient show that the previously seen intermediate-signal-intensity tumor has been replaced by parallel hypointense lines of fibrosis (arrowheads) that have the appearance of a tram track and are present on either side of the internal sphincter. This finding is representative of an excellent treatment response.

Figure 11.

Figure 11. Anorectal SCC in a 67-year-old man. This case is a good example of how the presence of viable residual tumor was clarified by using FDG PET/CT. (A) Axial T2W pretreatment pelvic MR image shows an intermediate-signal-intensity tumor (arrow) between the 6- and 10-o'clock positions. (B) Axial T2W posttreatment pelvic MR image shows a significant interval reduction in the size of the tumor. However, there is a low- to intermediate-signal-intensity focus (arrowhead) at the site of the previous extramural tumor. (C, D) The suspicious focus (arrow) is not convincingly hyperintense on the DW (b = 1000 sec/mm2) image (C) and mildly hypointense on the ADC map (D). (E) Axial posttreatment fused FDG PET/CT image shows avid FDG uptake (arrowhead) that corresponds to the area of suspicion seen at pelvic MRI, suggesting viable metabolically active residual tumor. The patient was not a suitable candidate for salvage surgery due to extensive cardiovascular disease and was found to have further disease progression with an enlarging residual tumor and new metastases at subsequent imaging.

Figure 12.

Figure 12. Anorectal junction SCC in a 59-year-old woman. This case is a good example of how FDG PET/CT findings can help reassure the reporting radiologist that the diagnosis is correct. (A) Sagittal T2W pretreatment MR image shows an intermediate-signal-intensity tumor (arrow) at the anorectal junction. (B) Sagittal T2W posttreatment MR image shows T2-hyperintense mucosal and submucosal rectal edema (solid arrow); however, the tumor bed still appears to have somewhat intermediate signal intensity (dashed arrow). (C, D) Posttreatment DW image (b = 800 sec/mm2) (C) shows an area of mild central hyperintensity (solid arrow), which appears mildly hypointense (dashed arrow) on the posttreatment ADC map (D). This area corresponded to an area of mucosal edema rather than a tumor bed when these findings were cross-referenced with multiplanar MRI findings. Furthermore, the image quality of the DW and ADC images was degraded by gas in the rectal lumen. (E) Sagittal posttreatment fused FDG PET/CT image shows no abnormal FDG uptake at the tumor bed (arrowhead). Given the reassuring appearance of this area at posttreatment FDG PET/CT, this patient underwent routine surveillance, with no disease recurrence seen at subsequent scans.

Figure 13.

Figure 13. SCC of the anorectal junction in a 67-year-old man. (A) Coronal pretreatment fused FDG PET/CT image shows avid FDG uptake at the site of the anorectal tumor (black arrow). Inferior to the tumor, there is prominent sphincteric activity with avid but stereotypical bilinear FDG uptake by the smooth muscles of the internal sphincters (white arrows). (B) Sagittal T2W pretreatment MR image shows an intermediate-signal-intensity tumor in the anorectal junction (black arrow) but no tumor signal intensity at the level of the internal sphincters (white arrows). (C) Coronal FDG PET/CT image 3 months after CRT shows no residual metabolic uptake in the tumor bed (arrowhead) but persistent prominent but stereotypical bilinear metabolic activity in the sphincters (arrows). Such findings could be mistaken for residual disease if they are not correlated with the pretreatment scan findings to identify the site of the tumor bed. (D) Sagittal T2W posttreatment MR image shows only low-signal-intensity fibrosis at the tumor bed (arrowhead) and hyperintense mucosal and submucosal edema in the lower rectum (dashed arrow). There is no tumor signal intensity in the internal sphincters (solid arrows). (E) Posttreatment DW image shows no increased signal intensity at the tumor bed (arrowhead). (F) Posttreatment ADC map shows a hypointense signal at the tumor bed (dashed arrow), suggestive of T2 dark-through effect, and some T2 shine-through effect in the central mucosa (solid arrow).