Clinical beam quality specification in kilovoltage x-ray dosimetry

Abstract

Aim: Four clinically measurable dose descriptors: the ratio of absorbed doses at depths 1 cm and 2 cm (D1/D2), the ratio of absorbed doses at depths 2 cm and 5 cm (D2/D5), the tissue phantom ratio of depths 1 cm to 2 cm (TPR1,2), and the tissue phantom ratio of depths 2 cm to 5 cm (TPR2,5) were investigated in relation to the quality of superficial (low energy) and orthovoltage (medium energy) x-ray beams. Methods and Materials: D1/D2 and TPR1,2 were measured on a Gulmay D3300 unit. D2/D5 and TPR2, 5 were measured on a Gulmay D3300 unit and a Pantak Therapax DXT 300 unit. Different field sizes, half-value layers (HVLs), and distances from the source were investigated in a 30 cm × 30 cm × 30 cm water phantom and a 20 cm × 20 cm × 10 cm solid acrylic phantom. A PTW M30001 0.6 cc cylindrical ionization chamber and a T10008 electrometer system were utilized for all measurements. Results: D1/D2 reflected the changes expected in the penetration of superficial x-rays due to beam hardening and D2/D5 was found to vary appreciably with field size, source-to-surface distance (SSD) and HVL. The most practical conditions for the measurement of TPR1,2 as a potential beam quality specifier could not be established in this work and TPR2,5 varied with field size and HVL, irrespective of the distance from the source and the energy. Conclusions: D1/D2 and D2/D5 could be a practical quality index in field sizes of at least 11.28 cm diameter defined at an SSD of 50 cm. Measurements at different HVLs and source-to-chamber distances (SCDs) are needed to establish the most practical measurement conditions of TPR1,2. TPR2,5 measurements were more accurate than D2/D5, and should be investigated further as the beam quality index for orthovoltage x-ray beams.