The Influence of Intrafraction Movement on Margins for Prostate Radiotherapy

Reviewer: Charles Wood, MD
Abramson Cancer Center of the University of Pennsylvania
Last Modified: October 18, 2005

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Presenter: D. W. Litzenberg
Presenter's Affiliation: University of Michigan
Type of Session: Plenary


As set-up and interfraction errors are being reduced by in-room imaging guidance, the importance of intrafraction errors increases. Unfortunately, little data exists documenting the significance of intrafraction target movement.  The Calypso® 4D Localization System utilizes electromagnetic tracking of implanted markers to provide continuous organ monitoring. Data was collected using this investigational system to assess the relative importance of intrafraction organ movement during radiation treatment.

Materials and Methods

  • 11 patients underwent ultrasound-guided placement of 3 transponders within the apex, right base, and left base of the prostate
  • An isocenter was chosen that corresponded to the geometric center of the transponders, and patients were thereafter positioned via the electromagnetic system
  • Following initial positioning and radiographic verification, the position of the implanted transponders was monitored for 8 minutes at a rate of 10 measurements per second
  • Transponder positions were compared with previously measured data on organ position relative to skin marks, and the combined data were considered using van Herk's formula to establish margins in the following situations
    • Skin marker-based positioning
    • Pretreatment transponder positioning
    • Intratreatment continuous tracking consisting of a 3 mm threshold-based beam cutoff with repositioning and resumption of treatment


  • Without consideration of intrafractional organ motion, necessary average prostate margin for skin marker-based positioning was 8.0 mm, 7.3 mm, and 10 mm in the left-right (LR), anterior-posterior (AP), and cranial-caudal (CC) directions, respectively
  • With consideration of intrafractional organ motion, necessary average prostate margin for skin marker-based positioning increased to 8.2 mm, 10.2 mm, and 12.5 mm in the LR, AP, and CC directions, respectively
  • With transponder positioning prior to each radiation fraction, necessary average prostate margin decreased to 1.8 mm, 5.8 mm, and 7.1 mm in the LR, AP, and CC directions, respectively
  • With transponder repositioning prior to treatment of each field, necessary average prostate margin again decreased to 0.4 mm, 2.3 mm, and 1.8 mm in the LR, AP, and CC directions, respectively
  • With intrafractional adjustment, necessary average prostate margin further decreased to 0.3 mm, 1.5 mm, and 1.5 mm in the LR, AP, and CC directions, respectively

Author's Conclusions

  • Interfractional, interbeam, and intrafractional adjustment allowed increasingly small margins to be employed for prostate radiation treatment
  • Significant margin reduction was accomplished by interbeam adjustment, though 2 of the 11 patients in the study with wide variation in organ motion benefited from continuous monitoring and intrafraction adjustment

Clinical/Scientific Implications

This investigational device allowed continuous non-ionizing tracking of organ motion via transponders that were amenable to placement in the path of the radiation beam without a compromise in dose distribution.  Although this technology is still early in its development, it could potentially be utilized in other treatment sites for documentation of organ motion.  Further study is needed to more fully characterize its utility and potential for clinical implementation.

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