Using field size factors to characterize the in-air fluence of a proton machine with a range shifter

Jiajian Shen, Jarrod M. Lentz, Yanle Hu, Wei Liu, Danairis Hernandez Morales, Joshua B. Stoker, Martin Bues

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


Introduction: The range shifter (RS) is used to treat shallow tumors for a proton pencil beam scanning system (PBS). Adding RS certainly complicates the commissioning of the treatment planning system (TPS) because the spot sizes are significantly enlarged with RS. In this work, we present an efficient method to configure a commercial TPS for a PBS system with a fixed RS. Methods: By combining a spiral delivery with customized control points, we were able to significantly improve measurement efficiency and obtain 250 field size factors (FSF) within three hours. The measured FSFs were used to characterize the proton fluence and fit the parameters for the double-Gaussian fluence model used in the TPS. Extensive validation was performed using FSFs measured in air and in water, absolute doses of spread-out Bragg peak (SOBP) fields, and the dose measurements carried out for patient-specific quality assurance (QA). Results: The measured in-air FSFs agreed with the model's prediction within 3% for all 250 FSFs, and within 2 for 94% of the FSFs. The agreement between model's prediction and measurement was within 2% for the in-air and in-water FSFs and the absolute doses for SOBP beams. The patient-specific QA of 113 fields showed an excellent gamma passing rates (96.95 ± 2.51%) for the absolute dose comparisons with gamma criteria of 2 mm and 2%. Conclusion: The excellent agreement between the model's prediction and measurements proved the efficiency and accuracy of the proposed method of using FSFs to characterize the proton fluence and configure the TPS for a PBS system with fixed RS.

Original languageEnglish (US)
Article number52
JournalRadiation Oncology
Issue number1
StatePublished - Mar 14 2017


  • Double Gaussians fluence
  • Field size factor
  • Proton pencil beam scanning
  • Range shifter
  • TPS commissioning

ASJC Scopus subject areas

  • Oncology
  • Radiology Nuclear Medicine and imaging


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