The dosimetry of small, megavoltage photon fields: correction factors, dose area products and detector designs

Thesis

In recent years, small fields have come to play a key role in advanced radiotherapy, yet protocols to perform dosimetry under small field conditions are still in their infancy. In 2008, the IAEA and AAPM published a formalism [Med. Phys. 35, 5179-5186] recommending the use of point-dose correction factors. This thesis uses Monte Carlo simulations to demonstrate that the values of these correction factors depend strongly on both detector design and field size, as well as other variables such as detector off-axis position and detector azimuthal angle.

Mass density is found to be the principal determinant of detector water non-equivalence. Furthermore, it is shown that it is possible to compensate for the mass-density of a detector cavity by incorporating additional components of contrasting mass-density into that detector’s design. For small cavities, such design modifications enable the detector’s small- to large- field response ratio to be matched to that of a “point-like” water-structure: ideal detector performance can be achieved across a variety of irradiation conditions.

For existing commercial detectors, a Dose Area Product (DAP) formalism is also developed and shown to be much more robust than the point-dose correction factor approach.

In conclusion, correction factor values for existing detector designs depend on a host of variables and their calculation typically relies on the use of time-intensive Monte Carlo methods. This thesis indicates that future moves towards density-compensated detector designs or DAP-based protocols can simplify the methodology of small field dosimetry.

Authors: Underwood, T

• Publication date: 2013
• DOI: 10.5287/ora-yrkewyw5x
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: small photon fields; megavoltage; radiation; dosimetry; detector designs; medical physics
• Subjects: Radiation; Oncology; Biomedical engineering