Validation of In vivo Diode Characteristics at Extended Source to Surface Distance for Total Body Irradiation

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Total body irradiation (TBI) requires precise dosimetry, especially for extended source-to-surface distance (SSD) treatments.
• The application of ISORAD diode detectors for in vivo dosimetry in TBI, particularly at extended SSDs, needs thorough investigation.
• Manufacturer specifications for ISORAD diode detectors have a 250 cm SSD limit, necessitating evaluation beyond this range.

Purpose of the Study:

• To evaluate the feasibility of using ISORAD diode detectors for in vivo dosimetry in TBI treatments at extended SSDs (up to 400 cm).
• To assess the operational characteristics of ISORAD diode detectors, including reproducibility, linearity, and dose rate dependence, up to 400 cm SSD.
• To compare the performance of ISORAD diode detectors with a Farmer chamber and validate their use in clinical TBI settings.

Main Methods:

• Four ISORAD-1163 n-type cylindrical diode detectors were characterized and compared with a 0.6cc Farmer chamber.
• Performance metrics including reproducibility, linearity, field size dependence, and dose rate dependence were assessed from 100 cm to 400 cm SSD.
• Post-irradiation signal drift, short-term reproducibility, signal stability, and directional dependence were monitored; a diode was used for in vivo TBI patient measurements.

Main Results:

• ISORAD diode detectors demonstrated acceptable dose rate effects (<0.91%), linearity (<0.99%), and reproducibility (<0.19%) up to 400 cm SSD.
• Minimal deviations in post-irradiation signal drift (<0.09%) and signal consistency (<0.09%) were observed.
• In vivo TBI patient measurements showed a maximum dose difference of 8.5% between calculated and actual diode readings.

Conclusions:

• ISORAD diode detectors exhibit consistent operational characteristics up to 400 cm SSD, exceeding manufacturer specifications.
• The study supports the applicability of these diodes for reliable in vivo dosimetry in total body irradiation treatments.
• Further investigation into directional dependence, particularly in the transverse plane beyond ± 30°, is warranted.