Volume-based dose optimization in brachytherapy.

PURPOSE: We address the question of how to optimize the dwell time distribution in brachytherapy with a stepping source if a minimal tumor dose is prescribed within the planning target volume (PTV). METHODS AND MATERIALS: For a given PTV, reference points inside and at the surface of the PTV are generated and dose constraints are prescribed. The dose at these reference points can be calculated if the positions of the sources are known. We determine a set of dwell times such that the dose constraints are fulfilled, and at the same time, the total irradiation time is minimized. The simplex algorithm allows us to find a solution (if any exists) for this problem. RESULTS: The performance of this method has been tested for a geometrically simple PTV. This method gives better results than conventionally used algorithms for dwell time optimization. CONCLUSION: The method described in this paper allows a volume-oriented optimization for brachytherapy dose distribution. The algorithm guarantees finding a dwell time distribution which fulfills the prescribed dose constraints, if any solution exists.

[Dose-volume histograms and 3-D planning in breast-saving therapy of breast carcinoma]. / Dosis-Volumen-Histogramme und 3D-Planung bei der brusterhaltenden Therapie des Mammakarzinoms.

AIM: Improvement of the dose homogeneity in radiation treatment of the intact breast using 3D-planning and dose volume histograms. PATIENTS AND METHOD: 3D-planning, including the calculation of dose volume histograms of the planning target volume, was performed on 15 patients, who underwent radiation therapy with tangential photon beams. A standard plan and 2 modified or optimized plans were evaluated. Different dosimetric parameters like maximum dose, mean dose, standard deviation and the fractional volume which receives doses from 95 to 105% of the reference dose were compared and correlated with breast size. RESULTS: With increasing breast size standard planning leads to increased overdosage, both in magnitude and volume. Individual optimization by modifying weights and wedges gives no improvement in dose homogeneity, whereas a photon energy of 10 MV results in a more homogeneous dose distribution. The drawback of the higher energy is the increased underdosage of the skin. CONCLUSION: Using the standard geometry of tangential fields the dose homogeneity cannot be improved significantly by 3D-planning, compared to our standard technique.