Watch & wait - Post neoadjuvant imaging for rectal cancer

Abstract

Rectal cancer management has evolved over the past decade with the emergence of total neoadjuvant therapy (TNT). For select patients who achieve a clinical complete response following TNT, organ preservation by means of the watch-and-wait (WW) strategy is an increasingly adopted alternative that preserves rectal function and quality of life without compromising oncologic outcomes. Recently, published 5-year results from the OPRA trial demonstrated that organ preservation can be achieved in approximately half of patients managed with the WW strategy, with most local regrowth events occurring within two years. Considering the potential for local regrowth, the implementation of the WW strategy mandates rigorous clinical and radiographic surveillance. Magnetic resonance imaging (MRI) serves as the conventional imaging modality for local staging and surveillance of rectal cancer given its excellent soft-tissue resolution. This review will discuss the current evidence for the WW strategy and the role of restaging rectal MRI in determining patient eligibility for this strategy. Restaging rectal MRI acquisition parameters and treatment response assessment, including important factors to assess, pitfalls, and classification systems, will be discussed in the context of the WW strategy.

Keywords: Magnetic resonance imaging; Neoadjuvant therapy; Rectal cancer; Watch and wait.

Figure 1.

57-year-old male with rectal cancer who underwent total neoadjuvant therapy. Baseline rectal MRI (1A–D) shows a tumor with T2 intermediate signal (1A, arrow, axial oblique T2-weighted imaging), high DWI signal (IB, arrow, b-value 1000), and low ADC signal (1C, arrow) in the lower rectum, as well as irregular and T2 heterogenous suspicious mesorectal lymph node measuring 0.5 cm in the short axis (ID, axial T2 T2-weighted imaging). A year after the completion of total neoadjuvant therapy and 1.5 years after baseline MRI, there is no evidence of tumor regrowth (2A–D), with a low T2 signal scar in the tumor bed (2A, arrow, T2 axial oblique) without associated diffusion restriction (2B–C, arrows). The previously suspicious lymph node has decreased in size and T2 signal intensity, measuring 0.2 cm in the short axis (2D, T2 axial).

Figure 2.

61-year-old male with rectal adenocarcinoma who underwent total neoadjuvant therapy. Baseline rectal MRI shows a circumferential mass with intermediate signal on T2-weighted imaging (A) and diffusion restriction on DWI/ADC (B, C). Restaging rectal MR demonstrates a decrease in the size of the mass, with few areas of fibrosis and areas of viable tumor with intermediate signal intensity on T2-weighted imaging that the rectal tumor has (D), and a focal area of diffusion restriction on DWI/ADC (E, F), consistent with incomplete response. Follow-up MRI 2 months later shows a scar on T2-weighted imaging (G) with persistent diffusion restriction on DWI/ADC (H, I), consistent with continued incomplete response. Subsequent pathologic analysis of the resection specimen showed residual adenocarcinoma exhibiting changes consistent with treatment effect.

Figure 3.

50-year-old male with rectal adenocarcinoma who underwent total neoadjuvant therapy. Baseline MRI shows a semicircumferential rectal mass (arrows) with intermediate signal on T2-weighted imaging (A) and diffusion restriction on DWI/ADC (B, C). Restaging rectal MRI depicts a marked decrease in the size of the mass with T2 hypointense fibrosis on T2-weighted imaging (D) and a focal area of diffusion restriction (arrows) on DWI/ADC (E, F), consistent with near complete response. Follow-up MRI 3 months later shows scar on T2-weighted imaging (G) with resolution of the diffusion restriction on DWI (FI) and ADC (I), consistent with complete response.

Figure 4.

Endoscopic appearance of complete/near complete and incomplete response. Endoscopic images prior to neoadjuvant treatment (left) and upon initial response assessment after the completion of neoadjuvant treatment (right). Yellow arrows highlight the significant findings at the site of the primary tumor.

Figure 5.

Role of endoscopy in watch-and-wait management. Top panel demonstrates a case of near complete response evolving to clinical complete response. Bottom panel demonstrates a case of clinical complete response which later demonstrated local regrowth at the site of the tumor scar.