Degradation of target coverage due to inter-fraction motion during intensity-modulated proton therapy of prostate and elective targets

Acta Oncol. 2013 Apr;52(3):521-7. doi: 10.3109/0284186X.2012.752860. Epub 2013 Feb 14.

Abstract

Internal target and organ motion during treatment is a challenge in radiotherapy (RT) of the prostate and the involved elective targets, with residual motion being present also following image-guidance strategies. The aim of this study was to investigate organ motion-induced dose degradations for the prostate, seminal vesicle and the pelvic lymph node when treating these targets with proton therapy, using different image-guidance and delivery strategies.

Material and methods: Four patients were selected from a larger series as they displayed large inter-fractional variation in bladder and rectum volume. Intensity-modulated proton therapy plans were generated using both simultaneous integrated and sequential boost delivery. For each technique, three isotropic margin expansions (in the range of 4-10 mm) were evaluated for the clinical target volume of prostate (CTV-p), seminal vesicles (CTV-sv) and lymph nodes (CTV-ln). Simulation of the dose degradations for all treatment plans were based on dose re-calculations for the 8-9 repeat CTs available for each patient, after applying rigid registrations to reproduce set-up based on either intra-prostatic fiducials or bony anatomy.

Results: The simulated dose received by 99% of the target volume (D(99)) and generalized equivalent dose (gEUD) showed substantial inter-patient variations. For 40% of the investigated scenarios, the patient average simulated D(99) for all targets were within 2 GyE from the planned dose. The largest difference between simulated and planned dose was seen for the CTV-sv when using SIB delivery, with an average relative reduction in D(99) of 13% and 15% for the largest margin expansion, when positioned using fiducials and bony anatomy, respectively.

Conclusions: The most severe dose degradations were found for CTV-sv, but they were also evident for CTV-ln. The degradations could not be completely resolved, neither by using the largest margin expansion nor with the choice of set-up. With fiducial set-up CTV-p was robust against the inter-fraction changes.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Computer Simulation
  • Dose Fractionation, Radiation*
  • Dose-Response Relationship, Radiation
  • Humans
  • Male
  • Motion
  • Organ Size
  • Patient Positioning
  • Pelvis / diagnostic imaging
  • Pelvis / pathology
  • Prostate / pathology*
  • Prostate / radiation effects
  • Prostatic Neoplasms / radiotherapy*
  • Proton Therapy / adverse effects
  • Proton Therapy / methods*
  • Radiotherapy Planning, Computer-Assisted / methods*
  • Radiotherapy Setup Errors / prevention & control
  • Radiotherapy, Intensity-Modulated / adverse effects
  • Radiotherapy, Intensity-Modulated / methods*
  • Rectum / diagnostic imaging
  • Rectum / pathology
  • Tomography, X-Ray Computed / methods
  • Urinary Bladder / diagnostic imaging
  • Urinary Bladder / pathology