Modelling of a double-scattering proton therapy nozzle using the FLUKA Monte Carlo code and analysis of linear energy transfer in patients treated for prostate cancer.

BACKGROUND

The dose-averaged linear energy transfer (LET) in proton therapy (PT) has in pre-clinical studies been linked to the relative biological effectiveness (RBE) of protons. Until recently, the most common PT delivery method in prostate cancer has been double-scattered PT, with LET only available through dedicated Monte Carlo (MC) simulations. However, as most studies of the relationship between LET and RBE in double scattered PT have been focused on the head and neck region, existing MC implementations have not been capable of calculating LET for the longer field ranges used, for example, in the pelvic region.

PURPOSE

The initial aim of this study was to implement a MC code allowing for LET calculations in double-scattered PT of prostate cancer. Additionally, we explored LET profiles and LET as a function of field configuration, by performing MC calculations for a large prostate cancer cohort treated with double-scattered PT.

METHODS

The components of a passive scattered clinical treatment nozzle used for delivery of extended field ranges, with two associated modulation wheels, were implemented into an existing FLUKA MC framework for LET calculations. The code was validated to spread out Bragg peak (SOBP) measurements conducted using the treatment nozzle with 11 different range and modulation width configurations. After validation, LET distributions were calculated on the planning computed tomographies of 582 prostate cancer patients treated with two-field double-scattered PT. All patients had symmetric field configurations with respect to the sagittal plane, with one pair of posterior oblique, lateral opposing, or anterior oblique fields. Dose and LET volume parameters and the mean LET ratio between the bladder and rectum were compared across the three groups.

RESULTS

The range differences were below 1 mm for all SOBP scenarios used for calibration. For 9 of 11 SOBP scenarios, the modulation width differences were below 2 mm. For the patient simulations, the mean gamma pass rates (3 mm/3%) were at least 98% in the PTV, bladder, and rectum. Comparing anterior to posterior field configurations, the mean LET in the bladder increased within both the 10 and 70 Gy iso-dose regions, and conversely, the mean LET decreased for the rectum. There was a marked difference in the mean bladder-to-rectum LET ratios between anterior oblique, lateral opposing and posterior oblique field configurations.

CONCLUSION

A MC code allowing for accurate calculations of dose and LET in double-scattered PT of prostate cancer was implemented and validated. The LET distributions in the rectum and bladder showed a systematic dependence on the field configuration.