title: Shortening delivery times of intensity modulated proton therapy by reducing proton energy layers during treatment plan optimization.
abstract: OBJECTIVE
  To shorten delivery times of intensity modulated proton therapy by reducing the number of energy layers in the treatment plan.
  
  
  METHODS
  We have developed an energy layer reduction method, which was implemented into our in-house-developed multicriteria treatment planning system "Erasmus-iCycle." The method consisted of 2 components: (1) minimizing the logarithm of the total spot weight per energy layer; and (2) iteratively excluding low-weighted energy layers. The method was benchmarked by comparing a robust "time-efficient plan" (with energy layer reduction) with a robust "standard clinical plan" (without energy layer reduction) for 5 oropharyngeal cases and 5 prostate cases. Both plans of each patient had equal robust plan quality, because the worst-case dose parameters of the standard clinical plan were used as dose constraints for the time-efficient plan. Worst-case robust optimization was performed, accounting for setup errors of 3 mm and range errors of 3% + 1 mm. We evaluated the number of energy layers and the expected delivery time per fraction, assuming 30 seconds per beam direction, 10 ms per spot, and 400 Giga-protons per minute. The energy switching time was varied from 0.1 to 5 seconds.
  
  
  RESULTS
  The number of energy layers was on average reduced by 45% (range, 30%-56%) for the oropharyngeal cases and by 28% (range, 25%-32%) for the prostate cases. When assuming 1, 2, or 5 seconds energy switching time, the average delivery time was shortened from 3.9 to 3.0 minutes (25%), 6.0 to 4.2 minutes (32%), or 12.3 to 7.7 minutes (38%) for the oropharyngeal cases, and from 3.4 to 2.9 minutes (16%), 5.2 to 4.2 minutes (20%), or 10.6 to 8.0 minutes (24%) for the prostate cases.
  
  
  CONCLUSIONS
  Delivery times of intensity modulated proton therapy can be reduced substantially without compromising robust plan quality. Shorter delivery times are likely to reduce treatment uncertainties and costs.
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, Rotterdam, The Netherlands. Electronic address: s.vandewater@erasmusmc.nl.
authors.forename: Steven
authors.initials: S
authors.lastname: van de Water
authors.affiliation: F. H. Burr Proton Therapy Center, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
authors.forename: Hanne M
authors.initials: HM
authors.lastname: Kooy
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Mischa S
authors.initials: MS
authors.lastname: Hoogeman
mesh_terms: D000465:Algorithms; D019985:Benchmarking; D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D009959:Oropharyngeal Neoplasms / Q000532:radiotherapy*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D061766:Proton Therapy / Q000379:methods* / Q000592:standards; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards; D061106:Radiotherapy Setup Errors; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods* / Q000592:standards; D013997:Time Factors; D035501:Uncertainty