title: Degradation of target coverage due to inter-fraction motion during intensity-modulated proton therapy of prostate and elective targets.
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.
  
  
  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.
authors: Department of Medical Physics, Aarhus University Hospital, Aarhus, Denmark.
authors: Sara
authors: S
authors: Thörnqvist
authors: Ludvig P
authors: LP
authors: Muren
authors: Lise
authors: L
authors: Bentzen
authors: Liv B
authors: LB
authors: Hysing
authors: Morten
authors: M
authors: Høyer
authors: Cai
authors: C
authors: Grau
authors: Jørgen B B
authors: JB
authors: Petersen
mesh_terms: D003198:Computer Simulation; D019583:Dose Fractionation, Radiation; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D008297:Male; D009038:Motion; D009929:Organ Size; D056888:Patient Positioning; D010388:Pelvis / Q000000981:diagnostic imaging / Q000473:pathology; D011467:Prostate / Q000473:pathology* / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D061766:Proton Therapy / Q000009:adverse effects / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control; D050397:Radiotherapy, Intensity-Modulated / Q000009:adverse effects / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging / Q000473:pathology; D014057:Tomography, X-Ray Computed / Q000379:methods; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000473:pathology