Measurement of stopping power ratio of chemo-ports using energy spectrum extracted from integral depth dose

Purpose

In proton radiotherapy, the stopping power ratio (SPR) of non-biological materials must be independently measured with proton beams for accurate dose calculation. Small-size chemo-ports challenge the measurement. The purpose of this work is to measure the SPR of chemo-ports by using the energy spectra of the proton pencil beams.

Methods and materials

Chemo-ports used in this study were irradiated in both lateral and vertical directions by 100-, 160-, and 200-MeV monoenergetic proton pencil beamlets. The integrated depth doses (IDDs) were acquired using a multi-layer ion chamber (MLIC), with and without the chemo port in front of the MLIC. The energy spectrum (ES) of the IDD was extracted. The water equivalent thickness (WET) of the chemo-port was determined from the shift in corresponding peaks in the spectra. To reduce the effect of spot size and its Gaussian distribution on the measurement, the measurements were repeated with a lead collimator (5 mm circular opening) in front of the chemo-ports. Additionally, the WET values were also obtained by a conventional approach that calculated the shift of the peaks in the IDDs rather than in the energy spectra.

Results

The complex internal structure of the chemo-port was reflected in multiple peaks in the ES. The measured WET values from different energy beamlets agreed within 0.5 mm (4.6%) of each other using the ES method, while agreement up to 1 mm was observed from the traditional approach. When the collimator was used, the agreement was decreased to within 1.1 and 8 mm from the ES method and conventional approach, respectively.

Conclusion

Proton SPRs of chemo ports can be successfully measured using the ES method. Better agreement of the measured WET values from different energy pencil beams was obtained from the ES method than from a conventional approach. The use of a collimator can decrease accuracy.