Small photon beam dosimetry is a critical aspect of modern radiation therapy (RT) techniques, such as stereotactic radiosurgery and stereotactic body radiation therapy. Traditional RT dosimetry methods, designed for standard beam sizes of the order of 10×10 cm2, cannot be applied to beams smaller than 3×3 cm2.

This study investigates a novel small cavity point ionization chamber and introduces the dose-area product ratio (DAPR20,10) as an innovative parameter for beam quality characterization in small fields. It also addresses challenges like detector perturbation and positioning. Comprehensive experimental and Monte Carlo calculation studies validate the feasibility and accuracy of these methodologies, particularly using large-area plane-parallel ionization chambers and the small-cavity ionization chamber.

The DAPR20,10 is found to be largely independent of field size and shape, with certain exceptions, and dependent on beam energy. The results demonstrate that DAPR20,10 could be an effective replacement for the traditional beam quality specifier tissue-phantom ratio (TPR20,10) in small beam quality specification, offering reduced uncertainties in clinical applications.

The study evaluates field output factors and correction factors for a small-cavity ionization chamber, emphasizing their dependence on photon beam size, energy and detector design. For small photon fields of e.g. 5×5 mm2, uncertainties due to detector positioning and polarity effects are highlighted, with recommendations for improved measurement accuracy through repeated setups and polarity corrections.

The findings contribute to the advancement of small beam dosimetry, paving the way for standardized methodologies in RT. Future research should focus on further clinical validation, the effect of uncertainties in all steps from the primary standard to dose delivery in a patient, optimizing detector design, and exploring the DAPR-based dosimetry protocol to enhance the precision and reliability of small photon beam delivery