Prediction of response to neoadjuvant radiotherapy in patients with locally advanced rectal cancer by means of sequential 18FDG-PET.
OBJECTIVE
Morphologic imaging techniques perform poorly in assessing the response to preoperative radiotherapy (RT), mainly because of desmoplastic reactions. The aim of this study was to investigate the potential of sequential 18-fluoro-2-deoxy-d-glucose (18FDG-PET) in assessing the response of rectal cancer to neoadjuvant RT and to determine which parameters can be used as surrogate markers for histopathologic response.


METHODS
18FDG-PET scans were acquired before and during the 5th week after the end of RT. Tracer uptake was assessed semiquantitatively using standardized uptake values (SUV). The percentage differences (%Δ) between pre- and post-RT scans in SUV(max), SUV(mean), metabolic volume (MV), and total glycolytic volume (tGV) were calculated.


RESULTS
Forty-five consecutive patients with histologically confirmed rectal adenocarcinoma were enrolled. After neoadjuvant RT, 20 of the 45 patients were classified as histopathologic responders and 25 as non-responders. Intense 18F-FDG uptake was seen in all tumors before neoadjuvant RT (average SUV(max) 12.9 ± 6.0). When patients were classified as histologic responders and nonresponders, significant differences in %ΔSUV(max) (55.8% vs. 37.4%, p = 0.023) and %ΔSUV(mean) (40.1% vs. 21.0%, p = 0.001) were observed between the two groups. For %ΔMV and %ΔtGV, decreases were more prominent in responders but were not significantly different from those in nonresponders. As demonstrated by receiver operating characteristic analysis, %ΔSUV(mean) was a more powerful discriminator than was %ΔSUV(max). The sensitivity, specificity, accuracy, positive predictive value, and negative predictive value for optimal threshold of %ΔSUV(mean) (24.5%) were 80%, 72%, 76%, 70%, and 82% respectively.


CONCLUSIONS
Sequential 18FDG-PET allows assessment of the response to preoperative RT. Both %ΔSUV(mean) and %ΔSUV(max) correlate with histopathologic response and can be used to evaluate and compare the effectiveness of different neoadjuvant treatment strategies. The maximum accuracy figures and the positive predictive value figures for both Δ%SUV(mean) and Δ%SUV(max) are, however, too low to justify modification of the standard treatment protocol of an individual patient.
Everaert
Hendrik
H
Department of Nuclear Medicine, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium.
Hoorens
Anne
A
Vanhove
Christian
C
Sermeus
Alexandra
A
Ceulemans
Gaetane
G
Engels
Benedikt
B
Vermeersch
Marieke
M
Verellen
Dirk
D
Urbain
Daniel
D
Storme
Guy
G
De Ridder
Mark
M
D000230:Adenocarcinoma / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy / Q000601:surgery; D000368:Aged; D005260:Female; D019788:Fluorodeoxyglucose F18 / Q000493:pharmacokinetics; D006801:Humans; D008297:Male; D020360:Neoadjuvant Therapy; D049268:Positron-Emission Tomography / Q000379:methods; D057234:Preoperative Period; D012372:ROC Curve; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D012004:Rectal Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy / Q000601:surgery; D012074:Remission Induction / Q000379:methods; D012680:Sensitivity and Specificity; D016896:Treatment Outcome
D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18