Factors for converting dose measured in polystyrene phantoms to dose reported in water phantoms for incident proton beams

• Published in: Medical physics (Lancaster) Vol. 38; no. 10; pp. 5799 - 5806
• Main Authors: Moyers, M. F., Vatnitsky, A. S., Vatnitsky, S. M.
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01-10-2011
• Subjects: ACCURACY; Algorithms; CALIBRATION; COMPUTERIZED SIMULATION; CONVERSION; DOSIMETRY; Dosimetry/exposure assessment; Error analysis; geometry; Geometry, differential geometry, and topology; Humans; IAEA; ionisation chambers; Ionization; IONIZATION CHAMBERS; Ions; MODULATION; MONTE CARLO METHOD; Monte Carlo methods; Monte Carlo simulations; MUSCLES; Nuclear interactions; PATIENTS; phantom; PHANTOMS; Phantoms, Imaging; polymers; POLYSTYRENE; Polystyrenes - chemistry; proton; PROTON BEAMS; PROTONS; Radiation Dosage; RADIATION DOSES; RADIATION PROTECTION AND DOSIMETRY; Radiometry - methods; Radiotherapy Dosage; Reproducibility of Results; Standards and calibration; Temperature; Therapeutics; WATER; Water - chemistry; Water vapor
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3639119

Purpose:
Previous dosimetry protocols allowed calibrations of proton beamline dose monitors to be performed in plastic phantoms. Nevertheless, dose determinations were referenced to absorbed dose-to-muscle or absorbed dose-to-water. The IAEA Code of Practice TRS 398 recommended that dose calibrations be performed with ionization chambers only in water phantoms because plastic-to-water dose conversion factors were not available with sufficient accuracy at the time of its writing. These factors are necessary, however, to evaluate the difference in doses delivered to patients if switching from calibration in plastic to a protocol that only allows calibration in water.
Methods:
This work measured polystyrene-to-water dose conversion factors for this purpose. Uncertainties in the results due to temperature, geometry, and chamber effects were minimized by using special experimental set-up procedures. The measurements were validated by Monte Carlo simulations.
Results:
At the peak of non-range-modulated beams, measured polystyrene-to-water factors ranged from 1.015 to 1.024 for beams with ranges from 36 to 315 mm. For beams with the same ranges and medium sized modulations, the factors ranged from 1.005 to 1.019. The measured results were used to generate tables of polystyrene-to-water dose conversion factors.
Conclusions:
The dose conversion factors can be used at clinical proton facilities to support beamline and patient specific dose per monitor unit calibrations performed in polystyrene phantoms.