Small_Fields Flashcards
What defines a small field in radiation dosimetry?
A small field is defined by the following conditions: (a) loss of lateral charged particle equilibrium (LCPE), (b) partial occlusion of the primary photon source by the collimators, and (c) the size of the detector is similar or larger compared to the beam dimensions.
What causes the loss of lateral charged particle equilibrium (LCPE) in small fields?
LCPE occurs when the beam half-width or radius is smaller than the maximum range of secondary electrons contributing to the absorbed dose.
How can Monte Carlo simulations be used in small field dosimetry?
Monte Carlo simulations help study the situations where LCPE occurs by comparing dose-to-water (Dw) and water-collision-kerma (Kcol) on the central axis of high-energy photon beams.
What is the effect of partial occlusion of the primary photon source by collimators?
Partial occlusion reduces the beam output on the central axis and affects the spectrum of the source, which can significantly impact dosimetry due to the energy dependence of detectors.
What is volume averaging in the context of small field dosimetry?
Volume averaging refers to the effect where a real detector senses a dose distribution by averaging the signal over its sensitive volume, potentially causing an underestimation of the dose in the center and broadening of the penumbra.
How does volume averaging affect measured dose profiles in small fields?
Volume averaging can lead to an underestimation of the dose at the center of the field and cause the penumbra to appear broader, which can result in an overestimation of the field size.
What is the difference between absolute and relative dosimetry in small fields?
Absolute dosimetry refers to measuring the dose in absolute units (Gy), while relative dosimetry involves normalizing dose profiles to a value on the central axis, usually leading to uncertainties in dose profiles if the central axis dose is underestimated.
What is the Machine Specific Reference (MSR) Field in small field dosimetry?
MSR fields are reference fields specific to a treatment machine when a conventional 10x10 cm² field cannot be achieved. The dosimetry formalism for MSR fields accounts for differences between the MSR and the classical reference field.
How is the factor kQmsr,Q related to MSR fields in small field dosimetry?
The factor kQmsr,Q corrects for differences in beam quality between the MSR field and the classical reference field, taking into account the detector response and Monte Carlo simulations.
Why is it challenging to measure dose profiles in fields smaller than 2 cm?
In fields smaller than 2 cm, significant deviations in dose profiles can occur due to LCPE, occlusion effects, and the difficulty of accurate detector placement, leading to potential errors of up to 15% for different detectors.
