Abstract
Treatment planning systems for proton therapy require a CT calibration curve relating Hounsfield units to proton stopping powers. An understanding of the accuracy of this curve, together with its limitations, is of utmost importance because the calibration underpins the calculated dose distribution of every patient preparing to undergo proton therapy, independent of delivery technique. The most common approach to the calibration is the stoichiometric method, which is well‐defined and, in principle, straightforward to perform. Nevertheless, care must be taken when implementing it in the clinic in order to avoid introducing proton range uncertainties into treatment plans that are larger than the 3.5% that target margins are typically designed to account for. This work presents a variety of aspects related to the user‐specific implementation of the stoichiometric calibration, from both a measurement setup and a data‐handling point of view, and evaluates the potential impact of each for treatment planning purposes. We demonstrate that two alternative commercial vendors' tissue phantoms yield consistent results, that variable CT slice thickness is unimportant, and that, for a given cross‐sectional size, all phantom data can, with today's state‐of‐the‐art beam hardening‐related artifact reduction software, be acquired quickly and easily with a single scan, such that the resulting curve describes the calibration well at different positions across the imaging plane. We also show that one should be cautious of using metals in the calibration procedure and of using a single curve for anatomical sites differing widely in size. Further, we suggest that the quality of the parametric fit to the measurement data can be improved by performing a constrained, weighted linear regression. These observations, based on the 40 separate curves that were calculated, should help the medical physicist at any new proton therapy facility in deciding which considerations are worth particular attention.PACS numbers: 87.53.Bn, 87.55.‐x, 87.57.Q‐, 87.59.bd
Highlights
Protons of therapeutic energy passing through tissue slow down continuously, primarily via inelastic collisions with atomic electrons, until eventually reaching a halt
While this could be derived directly from computed tomography (CT) images acquired in a proton beam, such proton CT scanners have yet to become commercially viable, technical developments are ongoing.[2,3,4,5,6,7,8,9,10,11] this map must currently be constructed from X-ray CT images, and the information contained therein transformed in some indirect manner to relative proton stopping powers
Many of the uncertainties associated with its application have been studied, and there is consensus in the community that in most practical situations this translates into a relative uncertainty in the proton range of ~ 3.5% at the 95% confidence level.[14,15,16] Margins along the beam direction are added routinely to the clinical target volume in treatment planning to allow for this
Summary
Protons of therapeutic energy passing through tissue slow down continuously, primarily via inelastic collisions with atomic electrons, until eventually reaching a halt. The rate of energy deposition increases with depth and is most dramatic in the “Bragg peak” near the end of the protons’ range.[1] The relationship between range and incident beam energy is well-defined, which means that this range can be matched to the targeted depth by appropriate selection of beam energy Such characteristics provide proton therapy with a distinct physical advantage over conventional X-ray therapy; the gradual buildup of dose prior to the dramatic increase within the target, coupled with a lack of exit dose, leads to overall less ionization energy being deposited in healthy tissues for the same prescribed dose to the target. The calculation of proton therapy dose distributions typically relies on knowledge of the three-dimensional map of relative (to water) proton stopping powers throughout the parts of the patient anatomy through which the beam passes. Many of the uncertainties associated with its application have been studied, and there is consensus in the community that in most practical situations this translates into a relative uncertainty in the proton range of ~ 3.5% at the 95% confidence level.[14,15,16] Margins along the beam direction are added routinely to the clinical target volume in treatment planning to allow for this
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