Portal films for relative exit dose determinations

Area of Science:

• Medical Physics
• Radiotherapy Imaging
• Radiation Dosimetry

Background:

• Portal films are standard in radiotherapy for verifying patient positioning.
• These films also contain quantitative dose information, but achieving high accuracy for absolute dosimetry is challenging.
• Relative dosimetry from portal films could enhance treatment accuracy if dose response is linearized.

Purpose of the Study:

• To investigate a rapid dose response linearization technique for portal films.
• To evaluate the technique's ability to yield relative exit dose distributions.
• To assess the potential for combining portal film data with diode measurements for 2D transmission information.

Main Methods:

• Simulated treatment situations were used to develop and test the linearization technique.
• Portal films were analyzed to extract relative exit dose distributions.
• Results were compared against a treatment planning system using anthropomorphic phantoms.

Main Results:

• The developed technique successfully generated relative exit dose distributions from portal films.
• Distributions obtained from phantoms showed consistency with treatment planning system data, generally within +/- 5% and often within +/- 2%.
• The combination of linearized portal films and diode measurements can provide comprehensive 2D transmission data.

Conclusions:

• Extraction of exit dose information from portal films is valuable for radiotherapy.
• This technique is particularly useful for complex treatment scenarios like mantle or compensated fields.
• Improved relative dosimetry from portal films enhances the verification and accuracy of radiation delivery.