On-line cone beam CT image guidance for vocal cord tumor targeting.
OBJECTIVE
We are developing a technique for highly focused vocal cord irradiation in early glottic carcinoma to optimally treat a target volume confined to a single cord. This technique, in contrast with the conventional methods, aims at sparing the healthy vocal cord. As such a technique requires sub-mm daily targeting accuracy to be effective, we investigate the accuracy achievable with on-line kV-cone beam CT (CBCT) corrections.


METHODS
CBCT scans were obtained in 10 early glottic cancer patients in each treatment fraction. The grey value registration available in X-ray volume imaging (XVI) software (Elekta, Synergy) was applied to a volume of interest encompassing the thyroid cartilage. After application of the thus derived corrections, residue displacements with respect to the planning CT scan were measured at clearly identifiable relevant landmarks. The intra- and inter-observer variations were also measured.


RESULTS
While before correction the systematic displacements of the vocal cords were as large as 2.4+/-3.3mm (cranial-caudal population mean+/-SD Sigma), daily CBCT registration and correction reduced these values to less than 0.2+/-0.5mm in all directions. Random positioning errors (SD sigma) were reduced to less than 1mm. Correcting only for translations and not for rotations did not appreciably affect this accuracy. The residue random displacements partly stem from intra-observer variations (SD=0.2-0.6mm).


CONCLUSIONS
The use of CBCT for daily image guidance in combination with standard mask fixation reduced systematic and random set-up errors of the vocal cords to <1mm prior to the delivery of each fraction dose. Thus, this facilitates the high targeting precision required for a single vocal cord irradiation.
Osman
Sarah O S
SO
Radiation Oncology, Division of Medical Physics, Erasmus MC-Daniel den Hoed Cancer Center, Rotterdam, The Netherlands. s.osman@erasmusmc.nl
de Boer
Hans C J
HC
Astreinidou
Eleftheria
E
Gangsaas
Anne
A
Heijmen
Ben J M
BJ
Levendag
Peter C
PC
D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D005260:Female; D006801:Humans; D007822:Laryngeal Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D008297:Male; D015588:Observer Variation; D009862:Online Systems; D011832:Radiation Injuries / Q000517:prevention & control; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated; D012494:Sampling Studies; D014827:Vocal Cords / Q000000981:diagnostic imaging / Q000528:radiation effects*