RBE-weighted dose conversions for carbon ionradiotherapy between microdosimetric kinetic model and local effect model for the targets and organs at risk in prostate carcinoma

BACKGROUND AND PURPOSE

The aim of this study was to establish curves for the conversion of RBE-weighted doses for targets and organs at risk (OARs) from the microdosimetric kinetic model (MKM) calculation to that of the local effect model I (LEM) for carbon ion radiotherapy (CIRT) for prostate carcinoma (PCA).

MATERIALS AND METHODS

This study was performed in the experimental treatment planning system (eTPS, V8A, Raystation, Sweden), which incorporates both MKM and LEM. CIRT plans from 10 PCA patients were collected. There were 5 steps to establish the curves: (1) design MKM plans in eTPS; (2) recalculate the physical doses from MKM to LEM and create a LEM plan in eTPS; (3) plot the RBE-weighted MKM to LEM conversion curves; (4) convert the MKM rectum constraint dose volume histogram (DVH) from NIRS to a LEM DVH; and (5) compare patients' rectum DVHs and follow-up with the converted constraint DVH.

RESULTS

The conversion factors for MKM doses of 0.18 Gy (RBE) to 4.55 Gy (RBE) per fraction to LEM doses were 2.72-1.06. For fraction sizes of >1 Gy (RBE), the conversion factors matched Fossati's curve and for fraction sizes of <1.00 Gy (RBE) the values were on the extrapolated Fossati's curve. A LEM rectum constraint DVH was established. Ten patients' rectum DVHs were all lower than LEM constraint DVHs. No complications were reported clinically.

CONCLUSION

For PCA receiving CIRT, the RBE-weighted doses using MKM for targets and OARs could be converted to LEM doses using conversion curves.