Abstract
This study explores the potential of cone‐beam computed tomography (CBCT) for monitoring relative beam range variations due to daily changes in patient anatomy for prostate treatment by anterior proton beams. CBCT was used to image an anthropomorphic pelvic phantom, in eight sessions on eight different days. In each session, the phantom was scanned twice, first at a standard position as determined by the room lasers, and then after it was shifted by 10 mm translation randomly along one of the X, Y, or Z directions. The filling of the phantom bladder with water was not refreshed from day to day, inducing gradual change of the water‐equivalent path length (WEPL) across the bladder. MIMvista (MIM) software was used to perform image registration and re‐alignment of all the scans with the scan from the first session. The XiO treatment planning system was used to perform data analysis. It was found that, although the Hounsfield unit numbers in CBCT have substantially larger fluctuations than those in diagnostic CT, CBCT datasets taken for daily patient positioning could potentially be used to monitor changes in patient anatomy. The reproducibility of the WEPL, computed using CBCT along anterior–posterior (AP) paths across and around the phantom prostate, over a volume of 360 cc, is sufficient for detecting daily WEPL variations that are equal to or larger than 3 mm. This result also applies to CBCT scans of the phantom after it is randomly shifted from the treatment position by 10 mm. limiting the interest to WEPL variation over a specific path within the same CBCT slice, one can detect WEPL variation smaller than 1 mm. That is the case when using CBCT for tracking daily change of the WEPL across the phantom bladder that was induced by spontaneous change in the bladder filling due to evaporation. In summary, the phantom study suggests that CBCT can be used for monitoring day to day WEPL variations in a patient. The method can detect WEPL variation equal to or greater than 3 mm. The study calls for further investigation using the CBCT data from real patients. If confirmed with real patients' data, CBCT could become, in addition to patient setup, a standard tool for proton therapy pretreatment beam range check.PACS number: 87.55.Tm
Highlights
473 Bentefour et al.: cone-beam computed tomography (CBCT) for pretreatment range check in proton therapy question is, whether CBCT can be used for tracking daily variations of the patient waterequivalent path length and, subsequently, could be used as tool for beam range check before proton therapy treatment?(3)Range uncertainty is one of the most difficult problems in particle therapy treatment.[4,5]Such uncertainty translates directly into potential undershooting or overshooting of the Bragg peak in the patient
Positron emission tomography (PET) imaging is a post-treatment that is based on treatment-activated isotopes.[7,8,9] While that method provides a verification of the treatment, the drawback is that it verifies the beam range only after the treatment, but not before, since the imaging depends on isotope activities produced by the treatment itself
The two distributions represented by solid squares and open squares correspond, respectively, to the water Hounsfield unit (HU) distributions with the lowest and highest HU mean value from the CBCT scans of the phantom after it was randomly shifted by 10 mm from the room isocenter
Summary
473 Bentefour et al.: CBCT for pretreatment range check in proton therapy question is, whether CBCT can be used for tracking daily variations of the patient waterequivalent path length and, subsequently, could be used as tool for beam range check before proton therapy treatment?(3)Range uncertainty is one of the most difficult problems in particle therapy treatment.[4,5]Such uncertainty translates directly into potential undershooting or overshooting of the Bragg peak in the patient. A new method has been suggested, prostate treatment by anterior or anterior oblique field, which may significantly improve rectal sparing by using the much sharper distal dose falloff rather than the currently employed parallel opposed field approach.[11] Such treatment would require range verification prior to each beam delivery session to guarantee that the beam is covering the full prostate but stopping before the rectum This range verification can be done by the method newly suggested by Lu[12] and further developed by Gottschalk et al[13] This method uses a simple yet robust relationship between the timing of the treatment beam energy modulation and the WEPL of the beam in the patient. A clinical version of an in vivo range verification system using this method with a 12-detector array of 1 mm diodes mounted on a rectal balloon is under development.[15,16]
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