  Radiotherapy with heavy ions, in particular, 12C beams, is one of the most
advanced forms of cancer treatment. Sharp dose gradients and high biological
effectiveness in the target region make them an ideal tool to treat deep-seated
and radioresistant tumors, however, at the same time, sensitive to small errors
in the range prediction. Safety margins are added to the tumor volume to
mitigate these uncertainties and ensure its uniform coverage, but during the
irradiation they lead to unavoidable damage to the surrounding healthy tissue.
To fully exploit the benefits of a sharp Bragg peak, a large effort is put into
establishing precise range verification methods for the so-called image-guided
radiotherapy. Despite positron emission tomography being widely in use for this
purpose in 12C ion therapy, the low count rates, biological washout, and broad
shape of the activity distribution still limit its precision to a few
millimeters. Instead, radioactive beams used directly for treatment would yield
an improved signal and a closer match with the dose fall-off, potentially
enabling precise in vivo beam range monitoring. We have performed a treatment
planning study to estimate the possible impact of the reduced range
uncertainties, enabled by radioactive 11C beams treatments, on sparing critical
organs in the tumor proximity. We demonstrate that (i) annihilation maps for
11C ions can in principle reflect even millimeter shifts in dose distributions
in the patient, (ii) outcomes of treatment planning with 11C beams are
significantly improved in terms of meeting the constraints for the organs at
risk compared to 12C plans, and (iii) less severe toxicities for serial and
parallel critical organs can be expected following 11C treatment with reduced
range uncertainties, compared to 12C treatments.
