The use of proton computed tomography (CT) instead of X-ray to plan proton therapy can save patients from excessive radiation dose during multiple scans and improve the accuracy of treatment.
Proton therapy equipment manufacturer ProtonVDA and researchers from Loyola University Stritch School of Medicine, Northern Illinois University and Loma Linda University, USA, have found that proton CT reduces the uncertainty in the proton range, which may allow oncologists-radiologists to reduce the radiation zone around tumors and make it possible to more accurately deliver radiation doses to lesions.
The fact is that when using X-ray CT, the conversion of Hounsfield CT units into relative stopping power (RSP) is required to calculate the range of protons in the patient's body and draw up a treatment plan. This leads to uncertainty, which requires the use of wider radiation edges around the tumor. Proton CT directly measures the RSP, which reduces the level of uncertainty and can reduce the irradiation zone.
According to the senior author of the study, James Welsh, professor of radiation oncology at Loyola University's Stritch School of Medicine, if a small dose reduction with a single scan is unlikely to cause any harm, a 10-100-fold dose reduction with proton CT may allow the scan to be repeated many times. "Thus, if there is a clinical benefit from proton radiography and proton CT, the studies can be repeated as many times as necessary to maximize this benefit without fear that the benefits will be negated by the harmful effects of the radiation dose," he told Physics World.
For the analysis, the researchers used the ProtonVDA pRAD setup, a prototype of a clinical proton imaging system, and X-ray computed tomography, and then compared different RSP values. To confirm the accuracy of the proton CT, they visualized a cylindrical phantom with eight inserts equivalent to different tissues. When comparing the data obtained, the measured RSP index with known values had an accuracy of 1% or higher for various inserts, with the exception of inserts for sinus tissues.
They then applied ProtonVDA pRAD and a clinical vertical X-ray computed tomograph to scan the pig's head and a sample of the thoracic girdle and ribs. For the belt and ribs, the differences in RSP were no more than 0.6%; in rib-trabecular bones - 1.9% and 6.9% in dense bones. With horizontal head scanning using X-ray CT, the greatest difference in RSP was up to 41% in the tympanic bladders. They also found discrepancies of up to 4.3% in the skull and up to 4.4% in the brainstem. In eight other tissues, RSP differences ranged from -2.5% to +2.1% with an average value of -0.4%.
The results show that if the measured and calculated RSP values are similar for soft tissues, then X-ray CT is less accurate for planning treatment in dense bones or areas with cavities. Proton CT also used a much lower dose to visualize the pig's head than X-ray CT. In the first case, the dose was0.2-0.7 mGr, in the second - 3.9 and 39 mGr for scanning with low and high doses.
The researchers plan to conduct more quantitative dose comparisons, as well as explore the possibility of automatic data collection, optical rotation tracking, integration with a vertical treatment system, and testing of therapeutic beams and multilayer film sensors to better understand the potential of proton CT for low-dose radiation treatment planning.
"In principle, reducing the uncertainty in the proton range should give us the opportunity to use narrower edges around our clinical goals and thereby reduce the level of an undesirably high dose on some normal tissues," Welsh said. "In the future, we are going to conduct additional research to understand exactly what benefits can be derived from this."
The results were published in the journal Medical Physics.
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