Fifty patients with MRI diagnosis of LARC between December 2009 and January 2014 were considered for inclusion in our retrospective study based on the following criteria: endoscopic diagnosis and histopathologic (biopsy) proved rectal carcinoma; conventional MR pre-CRT completed with DWI; combined neoadjuvant therapy: the treatment protocol included external beam radiotherapy for a total of 45 to 50.4 Gy (1.8 Gy/fraction) and chemotherapy with 5-fluorouracil (continuous infusion of 225/mg/m²/day for 7 days for the duration of radiation therapy) or Capecitabine per os (825 mg/m² 2 times/day from Monday to Friday for the duration of the radiation treatment); conventional MR completed with DWI after neoadjuvant treatment; histopathological examination of the surgical specimen or,
alternatively,
biopsy performed during follow-up endoscopy in patients with a strong evidence of complete response to therapy based on clinical and instrumental investigations,
in which it was considered preferable an attitude of surveillance to surgical approach.
Of the 50 patients initially enrolled,
18 were excluded: 2 patients for metastatic disease and comorbidities; 1 patient for the poor quality of DWI due to artifacts caused by metallic hip implants; 4 patients lost at follow-up (FU) after performing post-CRT MR; 11 patients underwent surgery after staging MRI.
The final population eligible for our study encompassed 32 patients (33 lesions in 32 patients: one patient had two synchronous lesions,
one in the rectum and one in the anal canal): 18 males and 14 females – mean age 65.9-years (range: 35-85 years). All MR images were retrospectively evaluated in consensus by two radiologists; the observers were blinded to the clinical patient data and pathology reports.
Twenty-nine of 32 patients underwent TME; 3/32 patients did not undergo surgery,
due to strong clinical evidence of a complete response (repeated negative colonoscopy and biopsies after CRT).
Tumour response after CRT was determined in all the 33 lesions according to the pathologic classification suggested by "Dworak's tumor response grading system"
All patients provided written informed consent and were investigated by MRI with a magnetic field of 1.5 Tesla (Magnetom Avanto,
Siemens Medical Solutions,
Erlangen,
Germany; Philips Achieva,
Best,
Nederland).Patients did not receive bowel preparation; however,
in 57/64 MR examinations rectal distension was performed with 50-120 cc of ultrasound gel; in 7/64 examinations no rectal distension was performed due to lack of cooperation of the patients (4/7cases)or to low rectal tumors (the lesion was in the lumen of the anal canal in 3/7 cases).
The imaging protocol consisted of the following:
· sagittal TSE T2 weighted (TR: 3.200 ms; TE: 100 ms;; FOV 280x280; matrix 348x280; two signal averages slice thickness: 3 mm);
· paraxial (section perpendicular to the longitudinal tumor axis) TSE T2 weighted to accurately evaluate the tumor thickness (TR 3000 ms; TE 100 ms; matrix 348 x 278;three signal averages; FOV 210x228 mm; slice thickness: 3 mm);
· para coronal (section parallel to the longitudinal tumor axis) TSE T2 weighted (TR: 3.200 ms; TE: 100 ms; matrix 348x280; two signal averages; FOV 280x280 mm; slice thickness: 3 mm);
· paraxial DWI (TR: 5.400 ms; TE: 53 ms; matrix: 250x200; four signal averages; FOV: 350x306 mm; slice thickness: 4 mm; using 2 b-value: 0,
800 s/mm²) [5,8,25].
On the T2-weighted images,
tumors were defined as areas of intermediate signal compared with the hypointense signal of the normal adjacent muscular rectal wall (Fig.
1a).
On post-CRT T2-weighted MR images,
areas of markedly low signal intensity (SI) at the location of the primary tumor bed were interpreted as fibrosis.
As the risk for residual tumor in these fibrotic areas is known to be about 50%,
they were also included in the volumetric measurements (Fig.
1d).
On the pre and post-CRT DW images,
measurements were performed on high b–value (800 sec/mm2) images (Fig.
1b,e).
During the DWI analysis session,
T2-weighted images were used as the reference for tumour location.
On DW images tumors were identified as areas of high SI; on the post-CRT acquisition,
complete response was defined as complete absence of SI in the previous tumour site,
using normal rectal wall as internal reference (Fig.
1e).
Volumetric assessment of the tumor was performed for each lesion,
in both paraxial sections on T2-weighted and DW images on high b-value (b= 800 sec / mm ²) with identical angled planes.
Freehand regions of interest (ROI) were manually drawn at the edges of the tumor for each section containing the lesion.
Whole tumor volume was calculated by multiplying each cross-sectional area by the section thickness and then summing all the partial volumes (Fig.
1a,b,d,e).
For both data sets (T2 weighted and DWI),
the pre- and post-CRT tumor volumes (VT2and VDWI) were determined; moreover,
the tumor volume reduction ratios for both T2-weighted and DW images (ΔVT2% and ΔVDWI%) were calculated as follows: (Vpre - Vpost )/Vpre x 100.
ADC maps were automatically generated by using a monoexponential decay model including the two b values (0 and 800 sec/mm2),
on which freehand ROIs were drawn at the edges of the tumor for each axial section containing the lesion (Fig.
1c).
The tumor margins on ADC maps were defined referring to the paraxial T2-weighted and DW images on the high b-value (b = 800 sec / mm²); mean ADC value was extrapolated by ADC values obtained in the axial sections and the relative standard deviations with the goal of reducing the structural differences induced by the inherent tumor heterogeneity.
When no remaining high SI was visualized on the post-CRT DW images (Fig.
1e),
three ROIs were drawn at the former location of the primary tumor with reference to the post-CRT paraxial T2-weighted images (Fig.
1f).
Mean ADC values of the tumor lesions (pre and post-CRT) as well as the percentage of ADC change (ΔADC%) were calculated.
ΔADC% was determined as follows: (ADCpost – ADCpre)/ADCpre ×100.