Diameter discrepancy and treatment accuracy in external beam radiotherapy

One of the most problematic elements of radiation therapy is the determination of contour data or treatment depth which may vary due to various parameters. The provision of this data is crucial for treatment calculations and setup. The present study is devoted to the assessment of discrepancies between the water equivalent (effective) diameter and patient diameter of the dose delivered to the target. Combined entrance and exit dose measurements were carried out on patients treated for thorax, abdomen, and pelvic cancers by 60Co gamma rays, using silicon diodes. The effective diameter and target dose were evaluated on the basis of dose transmission data. Our study reveals that the most influential parameter leading to discrepancies in target dose delivery is the difference between effective depth and patient depth. A difference of more than 5% in the target dose is bound to happen when the difference between the effective and contour diameters is greater than 10%. Therefore, using the effective diameter for treatment calculations provides a more realistic value of the target dose, since it incorporates the impact of all contributing factors.