Background: The precision in carbon ion radiotherapy depends on the calibration of Hounsfield units (HU) as measured with computed tomography (CT) to water equivalence. This calibration can cause relevant differences between treatment planning and treatment delivery.
Methods: Calibration data for several soft tissues were measured repeatedly to assess the accuracy of range calibration. Samples of fresh animal tissues including fat, brain, kidney, liver, and several muscle tissues were used. First, samples were CT scanned. Then carbon ion radiographic measurements were performed at several positions. Residual ranges behind the samples were compared to ranges in water.
Results: Based on the measured data the accuracy of the current Hounsfield look-up table for range calibration of soft tissues is 0.2 +/- 1.2%. Accuracy in range calibration of 1% corresponds to approximately 1 mm carbon ion range control in 10 cm water equivalent depth which is comparable to typical treatment depths for head and neck tumors.
Conclusion: Carbon ion ranges can be controlled within approximately 1 mm in soft tissue for typical depths of head and neck treatments.