Determining clinical photon beam spectra from measured depth dose with the Cimmino algorithm

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Accurate characterization of clinical photon beam spectra is essential for precise radiation dose calculations in radiotherapy.
• Existing methods for spectral determination can be complex and time-consuming.

Purpose of the Study:

• To develop and validate a method for determining clinical photon beam spectra from measured depth-dose data.
• To enable rapid and interactive spectral analysis for radiotherapy applications.

Main Methods:

• Utilized measured depth-dose data from small field sizes (6x6 cm2) to minimize electron contamination.
• Represented measured depth dose as a linear combination of monoenergetic photon beam depth doses (basis functions).
• Employed the Cimmino feasibility algorithm to derive spectral weights by iteratively minimizing discrepancies between predicted and measured depth doses.

Main Results:

• The derived depth dose, using spectral weights, agreed within approximately 1% with the measured depth dose across all depths for 6 and 15 MV photon beams.
• Physically realistic and smooth spectra were obtained by applying a small margin (+/- 1%) to the measured depth dose, addressing ill-posed nature of the problem.
• The maximum energy of the derived spectra closely matched the measured electron beam energy (within 1 MeV).

Conclusions:

• The described method provides a fast and interactive way to obtain realistic photon beam spectra from depth-dose measurements.
• This approach enhances the accuracy of dose calculations in clinical radiotherapy.
• The feasibility method with minimally relaxed constraints offers a practical solution for spectral determination.