Comparison of x ray computed tomography number to proton relative linear stopping power conversion functions using a standard phantom

• Published in: Medical physics (Lancaster) Vol. 41; no. 6Part1; pp. 061705 - n/a
• Main Author: Moyers, M. F.
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01-06-2014
• Subjects: Adipose Tissue - radiation effects; Air; Algorithms; Aluminium; Aluminum; ANIMAL TISSUES; ATOMIC NUMBER; Biological material, e.g. blood, urine; Haemocytometers; biological tissues; Breast - radiation effects; Calibration; CLINICAL TRIALS; Collimation; COMPARATIVE EVALUATIONS; Computed tomography; Computerised tomographs; computerised tomography; COMPUTERIZED TOMOGRAPHY; Foams; Humans; Image scanners; LIGHT IONS; lung; Lung - radiation effects; LUNGS; Medical imaging; Models, Biological; NEOPLASMS; phantom; PHANTOMS; Phantoms, Imaging; PLANNING; proton; PROTON BEAMS; Protons; Protons - therapeutic use; radiation therapy; RADIOLOGY AND NUCLEAR MEDICINE; Radiotherapy Planning, Computer-Assisted - instrumentation; Radiotherapy Planning, Computer-Assisted - methods; Radiotherapy Planning, Computer-Assisted - standards; Signal generators; STOPPING POWER; Therapeutic applications, including brachytherapy; Tissues; Tomography, X-Ray Computed - methods; Uncertainty; Water; Water vapor; X RADIATION
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4870956

Purpose:
Adequate evaluation of the results from multi‐institutional trials involving light ion beam treatments requires consideration of the planning margins applied to both targets and organs at risk. A major uncertainty that affects the size of these margins is the conversion of x ray computed tomography numbers (XCTNs) to relative linear stopping powers (RLSPs). Various facilities engaged in multi‐institutional clinical trials involving proton beams have been applying significantly different margins in their patient planning. This study was performed to determine the variance in the conversion functions used at proton facilities in the U.S.A. wishing to participate in National Cancer Institute sponsored clinical trials.
Methods:
A simplified method of determining the conversion function was developed using a standard phantom containing only water and aluminum. The new method was based on the premise that all scanners have their XCTNs for air and water calibrated daily to constant values but that the XCTNs for high density/high atomic number materials are variable with different scanning conditions. The standard phantom was taken to 10 different proton facilities and scanned with the local protocols resulting in 14 derived conversion functions which were compared to the conversion functions used at the local facilities.
Results:
For tissues within ±300 XCTN of water, all facility functions produced converted RLSP values within ±6% of the values produced by the standard function and within 8% of the values from any other facility's function. For XCTNs corresponding to lung tissue, converted RLSP values differed by as great as ±8% from the standard and up to 16% from the values of other facilities. For XCTNs corresponding to low‐density immobilization foam, the maximum to minimum values differed by as much as 40%.
Conclusions:
The new method greatly simplifies determination of the conversion function, reduces ambiguity, and in the future could promote standardization between facilities. Although it was not possible from these experiments to determine which conversion function is most appropriate, the variation between facilities suggests that the margins used in some facilities to account for the uncertainty in converting XCTNs to RLSPs may be too small.