Dosimetric Effect of Prostate Motion During Helical Tomotherapy
Received 13 June 2008; received in revised form 3 September 2008; accepted 6 September 2008. published online 23 February 2009.
Purpose
To assess the dosimetric consequence of intrafraction prostate motion on helical tomotherapy plans.
Methods and Materials
An electromagnetic tracking device was used to measure real-time prostate motion for 515 fractions (16 patients). Motion tracks were used to retrospectively recalculate dose distributions using a four-dimensional calculation engine. The minimum dose (Dmin), maximum dose (Dmax), and dose to 95% of the volume (D95%) were calculated for target volumes and compared with respective values from the treatment plan. The dosimetric effect was evaluated for each fraction. For each patient, the running cumulative effect was assessed throughout the course of treatment. Calculations were repeated assuming a time delay between initial patient setup and start of treatment.
Results
Averaged over all fractions, the mean change in target D95% was <1% (SD, 3–4%). Reductions in target D95% of up to 20% were seen in individual fractions. Changes in prostate D95% were similar in frequency and magnitude to D95% changes in the planning target volume. The cumulative effect on target D95% was approximately 1% (SD, 1%). The average cumulative effect after five fractions was 1% (SD, 1.5%).
Conclusions
In general, the dosimetric effect of observed prostate motion on target D95%was small. Infrequently severe D95% degradations were observed for individual fractions, but their effect on the cumulative dose distribution was quickly reduced with minimal fractionation.
‡Department of Medical Physics, University of Wisconsin, Madison, WI
Reprint requests to: Katja M. Langen, Ph.D., Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, 1400 South Orange Avenue, Orlando FL 32806. Tel: (321) 841-7242; Fax: (407) 649-6895
Funding for this research was provided in part by TomoTherapy, Inc.
W.L. and G.O. are employees of and have a financial interest in TomoTherapy, Inc. K.M.L. has served as a consultant to Calypso Medical Technologies.
ABSTRACT