Nonrigid Patient Setup Errors in the Head-and-Neck Region

Purpose:To investigate the magnitude and clinical relevance of relative motion/nonrigid setup errors in the head-and-neck (H&N) region.Material and Methods:Eleven patients with tumors in the H&N region were immobilized in thermoplastic head masks. Patient positioning was verified using a kilovoltage cone-beam CT (kv CBCT) prior to 100 treatment fractions. Five different regions of interest (ROIs) were selected for automatic image registration of planning CT and verification CBCT: (1) the whole volume covering planning CT and CBCT, (2) the skull, (3) the mandible, (4) C1–C3, and (5) C4–C6. Differences were calculated describing relative motion between the ROIs.Results:The 3-D patient setup error was 3.2 mm ± 1.7 mm based on registration of the whole volume. No systematic relative motion (group mean errors < 0.5 mm and < 0.5°) between planning and treatment for any ROI was observed. Mobility was largest for the skull and the mandible relative to C4–C6 with 3-D displacements of 4.7 mm ± 2.5 mm and 4.4 mm ± 2.5 mm. Relative rotations were largest around the left-right axis (nodding) between C1–C3 and C4–C6 with maximum 11°. No time trend of relative motion was observed. Margins for compensation of relative motion ranged between 5 mm and 10 mm.Conclusion:The simplification of the patient as a rigid body was shown to result in significant errors due to relative motion in the H&N region. Margins for compensation of relative motion exceeded margins for compensation of patient positioning errors.Ziel:Untersucht wurden das Ausmaß und die klinische Relevanz nichtrigider Lagerungsfehler bei der Behandlung von Kopf-Hals-Tumoren.Material und Methodik:Elf Patienten waren für die Strahlenbehandlung von Kopf-Hals-Tumoren mittels thermoplastischer Masken immobilisiert. Bei 100 Fraktionen wurde die Patientenpositionierung mittels eines Kilovolt-Cone-Beam-CT (kv-CBCT) kontrolliert. Die automatische Registrierung von Planungs-CT und Verifikations-CBCT basierte auf fünf verschiedenen Regionen: 1. dem gesamten Volumen von Planungs-CT und Verifikations-CBCT, 2. Schädel, 3. Unterkiefer, 4. C1–C3 und 5. C4–C6. Die Unterschiede zwischen den Registrierungen beschrieben das Ausmaß der Relativbewegungen im Kopf-Hals-Bereich.Ergebnisse:Der dreidimensionale Lagerungsfehler, basierend auf Region 1 (gesamtes Volumen), betrug 3,2 mm ± 1,7 mm. Für keine der anatomischen Regionen wurde eine systematische Relativbewegung zwischen Planung und Behandlung festgestellt (< 0,5 mm und < 0,5°). Am größten war die Beweglichkeit des Schädels und des Unterkiefers relativ zur kaudalen Halswirbelsäule: 4,7 mm ± 2,5 mm und 4,4 mm ± 2,5 mm. Rotationsbewegungen waren zwischen kranialer und kaudaler Halswirbelsäule am größten: maximal 11° um die Links-rechts-Achse (Nickbewegung). Ein zeitlicher Trend der Relativbewegungen wurde nicht beobachtet. Sicherheitssäume von 5–10 mm waren zur Kompensation dieser Relativbewegungen notwendig.Schlussfolgerung:Substantielle Relativbewegungen im Kopf-Hals-Bereich widerlegen ein rigides Patientenmodell. Sicherheitssäume von 5–10 mm zur Kompensation dieser Relativbewegungen verdeutlichen die klinische Relevanz.

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