A study of dose delivery in small segments

Abstract

Purpose: Use of conformal therapy techniques increases the number of beams used in order to obtain better conformation of the treated volume to the planning target volume. As the number of beams increases, the number of monitor units (MU) for each beam decreases. In this work we have studied, the influence of low MU on dose and homogeneity. Methods and Materials: To study the field symmetry and flatness, films were irradiated. The “dose” for each field was always 60 MU; but it was divided into different segment sizes: 2 segments of 30 MU, 3 segments of 20 MU, and so on up to 12 segments of 5 MU. After being developed, films were scanned and analyzed using a densitometer. The measurements were carried out for three X-ray energies: 6 MV, 10 MV, and 18 MV. Results: Each measurement was repeated twice for each energy, and the results were equal. The means of the symmetry and flatness values obtained for each energy are lower than the commonly accepted limits. Conclusion: The dose delivered by adding small segments is equivalent to the dose delivered by a conventional segment with our Philips Linacs SL15 and SL20.