Clinical beam quality specification in kilovoltage x-ray dosimetry
Date
2009-04-09T08:24:57Z
Authors
Mdziniso, Nhlakanipho Wisdom
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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.