KEY POINTS
- The experiment used radioresistant human SQ20B laryngeal squamous cell carcinoma cells irradiated at the proximal edge, plateau, and distal edge of a passive helium spread-out Bragg peak.
- A 16.9 MeV/u helium beam was shaped using a range-modulating wheel, producing a shallow spread-out Bragg peak with a distal depth of approximately 1.5 mm in water. Cells received doses from 1 to 4 Gy at a mean dose rate of 3 Gy/min.
- The beamline was reproduced in GATE Monte Carlo simulations, and biological response was calculated using the NanOx model and its Biodose Actor. Photon-reference linear-quadratic parameters were used to derive experimental and predicted RBE.
- At 4 Gy, NanOx predicted surviving fractions of 11%, 7%, and 4% at the proximal edge, plateau, and distal edge, respectively, implying increasing biological effectiveness toward the distal region.
- Experimentally fitted surviving fractions at 4 Gy were 10%, 12%, and 8%, respectively. Unlike the model, the measurements did not demonstrate a clear monotonic reduction in survival across the Bragg peak.
- Deviations between modeled and experimental survival curves were 20–37%, with NanOx tending to underestimate survival at higher doses and in the distal region. Experimental RBE ranged from 2.1 to 2.4, compared with predicted values of 2.1–2.9.
- Interpretation is limited by large interexperiment variability, one cell line, indirect helium-specific model calibration, very low beam energy, and lack of correction for possible LET dependence in film dosimetry. The results cannot be directly transferred to clinical helium treatment planning.
CLINICAL TAKEAWAY
The study shows why helium therapy cannot simply inherit biological models developed from sparse or indirect data. NanOx produced reasonable RBE magnitudes but did not reproduce the measured spatial response reliably enough for clinical biological optimization without broader validation.