PubMed · 5022723
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
1972-04-22. Measuring radiation.. https://pubmed.ncbi.nlm.nih.gov/5022723/
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
Consistent collimator overlaps in field matching with computer-controlled asymmetric x-ray collimators have been observed and investigated. The results indicate that, although this consistent overlapping is small and within manufacturer's +/- 1 mm discrepancy specification, it can produce more than 10% cold spots at matchlines if unheeded. This undesirable dose inhomogeneity, however, can be improved to within +/- 3% with a simple and practical solution of artificially offsetting the collimators by exactly the same amount (e.g., 1 mm) they overlap.
The trend towards dose escalation in radiation therapy creates the need for non-coplanar beam arrangements to help minimize healthy tissue morbidity in the proximity of treatment volumes. Treatment planning software packages are able to generate external beam non-coplanar isodose distributions and this must be validated before use in a clinical setting. Kodak XV film was used to measure the relative dose within various tissue equivalent phantoms for comparison with the distributions predicted by the G.E. Target ("Target") treatment planning software. The results confirm that Target is able to predict the isodose distribution for coplanar and non-coplanar beams incident on patient equivalent phantoms containing relatively large, semi-infinite inhomogeneities well enough to warrant its implementation into routine clinical use. However, we have found that Target may not be able to adequately predict dose distributions around smaller inhomogeneous inclusions. Further work will be required to investigate this potential problem.
The field uniformity of a superficial X-ray machine operating at 90 kV, 10 mA and filtered with a 1.1 mm aluminium beam hardening filter was investigated at a depth of 1 cm below the surface. Uniformity measurements were carried out using films and a densitometer to detect the relative absorbed dose across the field. Film dosimetry was assessed by comparison with ionization chamber dosimetry in a water tank. The original flat hardening aluminium filter was replaced by a combined, profiled filter to improve the uniformity across field sizes 20 cm, 5 cm and 2 cm diameter as well as hardening the beam. Flatness of the beam profile was improved for the 20 cm field size from +/-7.5% to +/-1.3% across the anode/cathode direction and from +/-7.9% to +/-4.7% in the anode/cathode direction. For the 5 cm field size the improvement was from +/-4% to +/-3% and from +/-5.3% to +/-3.6% and for 2 cm field size from +/-3.4% to +/-2.8% and from 10.5% to +/-9.7% in the same directions, respectively. Beam quality measurements were made and the original half-value-layer was reduced from 2.21 +/- 0.09 mm to 2.07 +/- 0.09 mm. The project demonstrated that it was possible to build a filter capable of flattening the beam profile for different sized applicators without significantly changing the penetrating ability of the beam.