Independent Interfraction Motion and Deformation of Pelvic Nodes and Prostate in Image Guided Concurrent Irradiation of Prostate and Pelvic Nodes

Reviewer: Nathan Jones DO
Abramson Cancer Center of the University of Pennsylvania
Last Modified: September 23, 2008

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Presenter: Adam Currey
Presenter's Affiliation: Medical College of Wisconsin
Type of Session: Scientific

Background

  • Patients with high risk prostate cancer are known to be at increased risk for occult nodal involvement
  • In approximately 15% of cases, interfraction motion of the prostate exceeds 1cm
  • Daily target localization is now commonly used based on the location of the prostate, as determined by direct imaging such as cone-beam CT or indirect imaging with implanted fiducials  or RF transponders
  • Interfraction motion of the pelvic lymph nodes is presumed to be independent of prostate motion, with the resulting impact on nodal dose unclear when daily localization is based on the prostate

Materials and Methods

  • Retrospective review of daily CT imaging of 6 high-risk prostate cancer patients treated with IMRT to the prostate and pelvic lymph nodes
  • 9-10 scans were randomly selected for each patient
  • Pelvic lymph node volumes were contoured by a single radiation oncologist on each of these scans according to the RTOG guidelines
  • PTV expansion was 1cm
  • Variation was characterized by distance between the centers of mass and Dice’s coefficient, which is a measure of deformation
  • D90 (dose to 90% of the volume) and V100 (percentage of the volume receiving 100% of the prescribed dose) were calculated for the pelvic lymph node volume before and after the daily shift

Results

  • The mean distance between the center of mass was 4.5 ± 2.2mm for the prostate and 7.8 ± 3.5mm for the pelvic lymph nodes, with Dice's coefficient of 65.4 ± 9.2% and 72.0 ± 8.8%
  • The D90 decreased by 5% pre-shift and 4% post-shift (p=0.09)
  • The DVH reveals virtually identical coverage of the pelvic lymph nodes pre and post shift
  • Pelvic lymph nodes were underdosed (D90 < 95% of the prescription dose) in 35% of cases after the shift
  • There was more underdosing of pelvic lymph nodes after the shift vs. before, suggesting that the motion of the prostate exceeds that of the pelvic nodes

Author's Conclusions

  • Pelvic lymph node motion and deformation does not correlate with prostate motion
  • Daily repositioning based on the prostate can negatively impact the pelvic lymph node coverage approximately 35% of the time, but the difference is small if PTV with 1cm expansion is used
  • Real-time adaptive treatment planning is necessary to further decrease PTV size without compromising dose coverage

Clinical/Scientific Implications

  • Daily IGRT for prostate and pelvic lymph node irradiation can compromise coverage to the pelvic nodes when only prostate location is taken into account
  • Adequate PTV margins should be used on the lymph node volumes if daily adjustments are to be made
  • Innovative techniques or adaptive treatment planning are needed if treatment volumes are to be additionally decreased
  • This discussion is based on treatment volumes as per the RTOG guidelines, and does not address discussion of what constitutes appropriate nodal volume
  • Furthermore, the routine treatment of clinically uninvolved nodes in high risk patients remains controversial in the wake of the updated RTOG 9413 results, with clinician variability in selection of treatment volumes.



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