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Intra-fractional uncertainties in image-guided intensity-modulated radiotherapy (IMRT) of prostate cancer

Intrafraktionelle Unsicherheiten bei der bildgeführten intensitätsmodulierten Strahlentherapie (IMRT) des Prostatakarzinoms

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Abstract

Purpose

To evaluate intra-fractional uncertainties during intensity-modulated radiotherapy (IMRT) of prostate cancer.

Patients and Methods

During IMRT of 21 consecutive patients, kilovolt (kV) cone-beam computed tomography (CBCT) images were acquired prior to and immediately after treatment: a total of 252 treatment fractions with 504 CBCT studies were basis of this analysis. The prostate position in anterior-posterior (AP) direction was determined using contour matching; patient set-up based on the pelvic bony anatomy was evaluated using automatic image registration. Internal variability of the prostate position was the difference between absolute prostate and patient position errors. Intra-fractional changes of prostate position, patient position, rectal distension in AP direction and bladder volume were analyzed.

Results

With a median treatment time of 16 min, intra-fractional drifts of the prostate were > 5 mm in 12% of all fractions and a margin of 6 mm was calculated for compensation of this uncertainty. Mobility of the prostate was independent from the bony anatomy with poor correlation between absolute prostate motion and motion of the bony anatomy (R2 = 0.24). A systematic increase of bladder filling by 41 ccm on average was observed; however, these changes did not influence the prostate position. Small variations of the prostate position occurred independently from intra-fractional changes of the rectal distension; a weak correlation between large internal prostate motion and changes of the rectal volume was observed (R2 = 0.55).

Conclusion

Clinically significant intra-fractional changes of the prostate position were observed and margins of 6 mm were calculated for this intra-fractional uncertainty. Repeated or continuous verification of the prostate position may allow further margin reduction.

Zusammenfassung

Ziel

Ziel der Arbeit war die Analyse von intrafraktionellen Fehlern in der intensitätsmodulierten Strahlentherapie (IMRT) des Prostatakarzinoms.

Patienten und Methodik

Bei 21 Patienten wurde vor und nach der IMRT-Behandlung ein Kilovolt-Cone-Beam-CT (kV CBCT) aufgenommen. Basierend auf 252 Behandlungsfraktionen und 504 CBCT-Studien wurde die Position der Prostata in anterior-posteriorer (AP) Richtung mittels manueller Konturregistrierung und die Lagerung des Patienten durch automatische Registrierung des Beckenskeletts ermittelt. Die interne Lagevariabilität der Prostata war definiert als die Differenz der Positionsfehler bezüglich Prostata und knöcherner Anatomie. Intrafraktionelle Veränderungen wurden für die Position der Prostata und des Patienten, die Füllung der Blase und die AP-Weite des Rektums bestimmt.

Ergebnisse

Bei einer medianen Behandlungszeit von 16 min waren intrafraktionelle Veränderungen der Prostataposition in 12% der Fraktionen > 5 mm; ein Sicherheitssaum von 6 mm wurde zur Kompensation berechnet. Die Beweglichkeit der Prostata war im Wesentlichen unabhängig von dem pelvinen Knochenskelett (R2 = 0,24). Eine systematische Zunahme des Blasenvolumens um durchschnittlich 41 ccm wurde während der Behandlung gemessen; dies hatte aber keinen Einfluss auf die AP-Position der Prostata. Während kleine Bewegungen der Prostata unabhängig von Änderungen der Rektumweite waren, wurde für interne Prostatabewegungen > 5 mm eine Korrelation (R2 = 0,55) mit intrafraktionellen Änderungen der Rektumweite beobachtet.

Schlussfolgerung

Intrafraktionelle Änderungen der Prostataposition waren von klinisch relevantem Ausmaß und benötigen einen Sicherheitssaum von 6 mm. Die kontinuierliche oder wiederholte Verifikation der Prostataposition könnte eine weitere Verkleinerung der Sicherheitssäume ermöglichen.

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Correspondence to Matthias Guckenberger.

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Polat, B., Guenther, I., Wilbert, J. et al. Intra-fractional uncertainties in image-guided intensity-modulated radiotherapy (IMRT) of prostate cancer. Strahlenther Onkol 184, 668–673 (2008). https://doi.org/10.1007/s00066-008-1875-6

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  • DOI: https://doi.org/10.1007/s00066-008-1875-6

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