Post CRT (Chemoradiotherapy )Evaluation in Tumor

• The imaging findings at T2-weighted imaging after neo-adjuvant therapy include a decrease in tumor extent, interval development of T2 hypointense fibrosis with T2 hyperintense mucin production changes at the tumor bed, and complete normalization of the rectal wall layers without evidence of a tumor or fibrosis (scar).
• Persistent intermediate T2 signal intensity with or without  T2-hypointense  fibrotic changes at the tumor bed is indicative of residual tumor. 
• The presence of a dense and thick fibrotic scar at the tumor bed is considered a good response with minimal residual disease.
• Complete resolution of tumor signal intensity with the return of normal bowel wall layers and development of a thin T2-hypointense fibrotic scar at the tumor bed are two radiologic features closely associated with a pathologic complete response.
• Assessment with DWI is binary, in which diffusion restriction (high signal intensity at  DWI  with corresponding low signal intensity on apparent diffusion coefficient  [ADC]  maps)  is suggestive of residual malignancy, and its absence is suggestive of a good response.

Brown et al have proposed the most widely used mrTRG system, and it is based on T2-weighted  sequences without incorporating DWI, as outlined.

 mr-TRG Grade  - Radiologic Response Definition as shown in Figures 1, 2,3,4 and 5

• mrTRG 1(Complete response)-Return of normal bowel wall layers without evidence of the treated tumor, or thin curvilinear hypointense scar along the mucosa or sub-mucosa at the tumor site.
• mrTRG 2(Near complete response)-Thick dense hypointense fibrotic scar without obvious intermediate signal intensity at the tumor site, indicating minimal residual disease or no tumor.
• mrTRG 3(Moderate response)- >50% fibrosis or mucin and visible intermediate signal intensity tumor.
• mrTRG 4(Slight response)-Little areas of hypointense fibrosis or mucin, but mostly intermediate T2 residual tumor signal intensity.
• mrTRG 5(No response)-Intermediate T2 signal intensity, same appearance as the original tumor

Pitfalls

• Submucosal edema in uninvolved wall- appears as intermediate to high T2 signal intensity thickening (red arrow in A )and may mimic tumor as in baseline ( blue arrow in B) in figure 6.
• T2 shine-through effect- observed as a hyperintense signal on DWI and ADC images, indicates facilitated diffusion.
• T2 dark through effect- observed as a hypointense signal on DWI and ADC images, mostly in T2 hypointense fibrotic scar.
• Acellular mucin pools- No reliable imaging modality to distinguish cellular from acellular mucin as both are hyperintense on T2w images.

Post-CRT (Chemo-radiotherapy) Evaluation in lymph nodes (Figure 7)

• Chemoradiation therapy leads to a decrease in the size and number of malignant lymph nodes.
• Morphological changes in the form of T2w hypointense fibrosis without diffusion restriction are observed.
• Some nodes show mucinous degeneration (T2w hyperintense) is also a sign of treatment response.