Spectral perturbations from silicon diode detector encapsulation and shielding in photon fields

Area of Science:

• Medical Physics
• Radiotherapy
• Detector Physics

Background:

• Silicon diodes are standard for relative dose measurements in radiotherapy.
• Encapsulation, often with tungsten shielding, protects diodes and modifies their response to photons.
• New models suggest unshielded diodes can be corrected for photon field use.

Purpose of the Study:

• To investigate spectral effects of typical diode encapsulations, including shielding, on clinical diodes.
• To determine if detector encapsulation perturbs the spectrum in a water phantom.
• To evaluate the suitability of unshielded diodes for photon dose measurements.

Main Methods:

• Monte Carlo simulations using PENELOPE-2005 to study spectral effects.
• Application of variance reduction via correlated sampling for efficiency.
• Comparison of simulated spectra with reference spectra in water.

Main Results:

• Correlated sampling proved efficient and unbiased.
• Unshielded diode encapsulation did not perturb the spectrum.
• Tungsten shielding increased primary electron fluence and did not fully eliminate field-size dependence.
• Corrected unshielded diodes yielded results comparable to or better than shielded diodes.

Conclusions:

• Photon field spectra in water are suitable for modeling unshielded silicon diode responses.
• Unshielded diodes, with corrections, can effectively replace shielded diodes for photon dose measurements in radiotherapy.