Problems Affecting Accurate Dose Measurement in Small-Field for Linear Accelerator.

Background: The use of relatively narrow fields has become necessary with the advent of intricate and accurate radiation therapy delivery dose to patients; therefore, small-field dosimetry faces several difficulties. Both dose calculations and measurements require to be performed with extra care, due to the uncertainty that might be increased by using such small field sizes.

Material And Methods: In this study, we investigated the effect of detectors size on the dosimetry of small fields [starting with radiation fields from (1cm x 1cm), (2cm x 1cm), and (3cm x 1cm)...etc., up to (4cm x 5cm) and (5cm x 5cm)]. We used the linear accelerator and different types of ionization chambers i.e. [Farmer FC65-P, CC13, and CC01 (pinpoint)] an addition to semiconductors i.e. (IBA Razor diode)], and we investigated all detectors to read the absorbed dose in water under the reference conditions (field 10cm x10cm, SSD 100cm and depth 10cm).

Results: While measuring the absolute dose under reference conditions, all detectors had a non-significant difference of less than ±2%, except for the Razor diode, which showed a significant difference of ± 5%. On the other hand, when small fields were measured, we discovered a significant difference of 48%, compared to the Razor diode.

Conclusion: The Razor diode is more stable in small-field dosimetry than other detectors. Also, the Razor Diode is intended for relative dosimetry but, it shall not be used for absolute dose measurements.