Using proton CT instead of X-ray CT for proton therapy planning could spare patients from overexposure to radiation when undergoing multiple scans and may improve the accuracy of the treatment.

Proton therapy manufacturer ProtonVDA and researchers at Loyola University Stritch School of Medicine, Northern Illinois University and Loma Linda University found that proton CT reduced range uncertainties, which could allow radiation oncologists to potentially use smaller margins around tumors and more precisely deliver proton radiation to cancer sites.

The reason for this is because using X-ray CT requires the CT Hounsfield units to be converted into proton relative stopping power (RSP) to calculate proton range in the patient and generate a plan. This leads to uncertainties, which necessitates the need for wider margins. Proton CT directly measures RSP, which decreases uncertainties and may allow for smaller margins.
While small reductions in dose associated with a single scan are not likely to cause harm, the 10- to 100-fold decrease brought on by proton CT could enable scans to be repeated regularly, according to senior author James Welsh, a professor of radiation oncology at Loyola University Stritch School of Medicine. “Thus, if there is a clinical benefit to proton radiography and proton CT, the studies can be repeated as often as necessary to maximize such benefit — without the fear that such benefits will be negated by any detrimental effects from radiation dose,” he told Physics World.

For their analysis, the researchers used ProtonVDA pRAD, a prototype clinical proton imaging system, and X-ray CT and then compared the different RSP values. To validate the accuracy of proton CT, they imaged a cylindrical phantom with eight tissue-equivalent inserts. When compared, the measured RSP with the known values had an accuracy of 1% or better for the different tissue-equivalent inserts, except the sinus insert.

They then applied the ProtonVDA pRAD and a clinical vertical X-ray CT scanner to a pig’s head and a sample of porcine pectoral girdle and ribs. For the girdle and ribs, the RSP differences were 0.6% or less for all soft tissues; 1.9% in the rib trabecular bones; and 6.9% in the compact bone. For the head, which they also scanned horizontally with X-ray CT, the largest RSP difference was up to 41% in the tympanic bullae. They also found discrepancies of up to 4.3% in the skull and up to 4.4% in the brain stem. In the eight other tissues, RSP differences ranged from –2.5% to +2.1%, with a mean of –0.4%.

The findings show that while for soft tissue, measured and calculated RSP values are similar, X-ray CT is less accurate for treatment planning in dense bone or cavitated regions. Proton CT also had a far lower imaging dose of 0.2–0.7 mGy for the pig’s head than X-ray CT (3.9 and 39 mGy, for low- and high-dose scans).

The researchers plan to conduct more quantitative dose comparisons, as well as evaluate the automation of data acquisition, optical tracking of the rotation, integration with an upright treatment system and testing of treatment beams and film stacks to better understand the potential of proton CT for low-dose treatment planning.

“In principle, reduction of range uncertainty should allow us to use tighter margins around our clinical targets and thereby reduce unwanted high dose to some normal tissues,” said Welsh. “Just how much benefit this provides will be the subject of upcoming investigations.”

The findings were published in Medical Physics.

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