Conventional MR sequencesare not sufficiently reliable in distinguishing between residual tumor and post-CRT tissue fibrosis.
However,
it has been shown that the qualitative assessment of DWI sequencessignificantly improves the diagnostic performance of conventional MRI in the evaluation of tumor response to CRT: particularly,
in distinguishing between CR and n-CR patients,
DWI has shown a higher sensitivity (52-64% vs.
0-40%) and an almost comparable specificity (89-97% vs.
92-98% ) vs.
standard MR sequences.
However,
DW images have limitations,
as complete tumor regression is not always accompanied by absence of SI,
since diffuse fibrosis associated with chronic inflammation,
the presence of mucin pools,
the air-rectal wall interface or the collapsed rectal wall may be visualized as high SI,
making difficult the identification of CR: this also occurred in 1/7 CR patients of our series (Fig.
3).
Therefore,
the MR quantitative evaluations have been proposed by calculating the mean ADC values,
the conventional volumetry on T2 weighted images (VT2) and,
the volume measured on the DW images (VDWI),
as well as the ratio between the values before and after CRT (Δ%).
In our series,
considering the entire group of lesions (CR + n-CR),
the post-CRT ADC value,
while being significantly higher than the pre-CRT ADC value,
was not able to distinguish CR patients from n-CR; moreover the ΔADC% was not able to make such discrimination.
Similar conclusions on the limited usefulness of the various measurements of the ADC- including the ΔADC% - for the assessment of CR also emerged from other studies.
Concerning the volumetric evaluation with MR T2-weighted imaging,
some studies associate with a volume reduction; other studies did not find any significant difference in order to identify the CR (TRG 4) as well as the "good responders" (TRG 3-4).
Other studies,
published between 2011 and 2015 have argued that the tumor volume measured in the DW imageswas more accurate than that obtained in the conventional T2 MR sequences.
The results of our series reveal a good accuracy of post-CRT VT2(AUC = 0.91) and the ΔVT2% (AUC = 0.84) (Table 2; Fig.
2); the post-CRT VDWIand ΔVDWI% resulted more accurate (AUC = 1) compared to the corresponding post-CRT VT2and ΔVT2%; however,
the differences were not statistically significant.
Therefore,
our experience confirms that volumetry on DW images is more accurate than that on T2 weighted images: in particular, post-CRT VDWI≤ 0.5 cm³ and ΔVDWI% ≥ 83%(our values of optimal cut-off)could indicate a pathologic complete response.
However, it still remains difficult to differentiate between patients with a CR (TRG 4) and patients with small microscopic clusters of residual neoplasm (TRG 3); further studies are required to address this issue.
Nevertheless at present,
although the tumor volumes determined on the basis of the presence (or absence) of high-signal intensity areas on DW-MRI better represent the existence of residual viable tumor,
we can hypothesize - in agreement with Curvo-Semedo - that a visual evaluation of a high-signal intensity area suggestive of residual tumor is sufficient,
and volumetric measurements are not even required.
Moreover,
the combination of MRI with clinical assessment (digital rectal examination and endoscopy) is recommended as the optional strategy for a safe and accurate selection of CRs after CRT.
There were some limitationsto our study: the small number of selected patients; the small size of the pre-CRT lesions with pathologic complete response; histopathological evaluation of tumor regression to therapy was performed on biopsy in 3/7 CR; the lack of direct correlation between volumetric data obtained by MR images and the volumetric data provided by the surgical specimens; the possible errors in the positioning and size of the ROIs drawn on the tumor margins; the inter-observer reproducibility of the method was not evaluated because of the long time required for measurements of volumes and ADC values; finally,
post-CRT N parameter,
so far considered in a single study,
was not assessed. However,
the prevalence of a positive lymph node status in case of CR of the primary tumor after CRT is very low (8%); moreover,
standard MRI is rather accurate in lymph node staging after CRT,
so the addition of functional imaging,
such as DWI ,
may not even be necessary.
In conclusion,
DW images improve the results of standard follow-up MR protocols in order to identify CR patients after neoadjuvant CRT in patients affected by LARC.
The functional volumetry is better than the conventional volume,
although no statistically significant differences were detected in this study.
In particular,
both post-CRT VDWI and ΔVDWI% results are very accurate; however standardized cut-off values are not available.
Conversely,
the pre- and post-CRT ADC valuesand ΔADC% are not sufficiently reliable to distinguish the CR patients from the total group of n-CR patients.