The value of magnetic resonance imaging in target volume delineation of base of tongue tumours--a study using flexible surface coils.
BACKGROUND
Magnetic resonance imaging (MRI) provides superior diagnostic accuracy over computed tomography (CT) in oropharyngeal tumours. Precise delineation of the gross tumour volume (GTV) is mandatory in radiotherapy planning when a GTV boost is required. CT volume definition in this regard is poor. We studied the feasibility of using flexible surface (flex-L) coils to obtain MR images for MR-CT fusion to assess the benefit of MRI over CT alone in planning base of tongue tumours.


METHODS
Eight patients underwent CT and MRI radiotherapy planning scans with an immobilisation device. Distortion-corrected T1-weighted post-contrast MR scans were fused to contrast-enhanced planning CT scans. GTV, clinical target and planning target volumes (CTV, PTV) and organs at risk (OAR) were delineated on CT, then on MRI with blinding to the CT images. The volumetric and spatial differences between MRI and CT volumes for GTV, CTV, PTV and OAR were compared. MR image distortions due to field inhomogeneity and non-linear gradients were corrected and the need for such correction was evaluated.


RESULTS
The mean primary GTV was larger on MRI (22.2 vs. 9.5 cm(3), p=0.05) than CT. The mean primary and nodal GTV (i.e. BOT and macroscopic nodes) was significantly larger on MRI (27.2 vs. 14.4 cm(3), p=0.05). The volume overlap index (VOI) between MRI and CT for the primary was 0.34 suggesting that MRI depicts parts of the primary tumour not detected by CT. There was no significant difference in volume delineation between MR and CT for CTV, PTV, nodal CTV and nodal PTV. MRI volumes for brainstem and spinal cord were significantly smaller due to improved organ definition (p=0.002). Susceptibility and gradient-related distortions were not found to be clinically significant.


CONCLUSIONS
MRI improves the definition of tongue base tumours and neurological structures. The use of MRI is recommended for GTV dose-escalation techniques to provide precise depiction of GTV and improved sparing of spinal cord and brainstem.
Ahmed
Merina
M
Department of Radiotherapy, The Royal Marsden NHS Foundation Trust, London, UK. Merina.Ahmed@rmh.nhs.uk
Schmidt
Maria
M
Sohaib
Aslam
A
Kong
Christine
C
Burke
Kevin
K
Richardson
Cheryl
C
Usher
Marianne
M
Brennan
Sinead
S
Riddell
Angela
A
Davies
Mark
M
Newbold
Kate
K
Harrington
Kevin J
KJ
Nutting
Christopher M
CM
D002294:Carcinoma, Squamous Cell / Q000473:pathology* / Q000532:radiotherapy; D003287:Contrast Media; D005240:Feasibility Studies; D019786:Gadolinium DTPA; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008536:Meglumine; D009942:Organometallic Compounds; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D014057:Tomography, X-Ray Computed; D014062:Tongue Neoplasms / Q000473:pathology* / Q000532:radiotherapy
D003287:Contrast Media; D009942:Organometallic Compounds; D008536:Meglumine; D019786:Gadolinium DTPA; C072417:gadoterate meglumine