Purpose: To retrospectively compare dose prescription for MammoSite RTS using single versus multiple reference points through DVH evaluation. Method and Materials: CT images of four women treated with MammoSite RTS were selected for a retrospective study of dose prescription. The balloon was identified and contoured on the CT images using Plato BPS. A PTV was created by volumetrically expanding the balloon volume by 1cm. The final volume, PTV_EVAL consisted of the PTV minus the balloon volume. Six reference points were added along the outside edge of PTV_EVAL. Four points were added at locations lateral to the axis of the catheter, and two points were added along the axis of the catheter. Dose was prescribed to the six points individually, and DVHs of PTV_EVAL were calculated. Then dose was prescribed to 4, 5, and 6 reference points simultaneously, and a new DVH was created for each of those three cases. Results: For each prescription, the V150, V100, V90 and D95 from the DVH for PTV_EVAL were recorded and averaged across the four patients. When dose was prescribed individually to the two points along the catheter axis, the tissue volume receiving excess dose was unacceptable. When dose was prescribed individually to the 4 points lateral to the catheter axis, the coverage became inadequate. Plans in which dose was prescribed to multiple points simultaneously displayed sufficient coverage with lower volumes receiving excess dose. Conclusion: Due to the anisotropy of the source, prescriptions to individual points resulted in unacceptable coverage of the target volume or excess dose to tissue, while prescribing to multiple points simultaneously resulted in both more acceptable coverage and lower volumes of tissue receiving excess dose. Using four lateral points and one axial point resulted in the most target coverage while still limiting excess dose to tissue.

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Purpose: To create a 3D model to simulate IMRT treatment delivery. The model includes dose distribution resolution degradation from the MLC leaf width and dose deposition. The resulting deliverable distribution may be evaluated to determine conformity and the achievable spatial and dosimetric resolution. Method and Materials: Phantom image sets were created to represent head and neck anatomy with critical structures. Image sets contained 128 slices, each 128×128 pixels, with 1mm pixel size. Target volumes with radioresistant subvolumes of varying size and shape were added to the image sets. A nine‐field treatment technique was simulated. Slice‐by‐slice along the beam central axis, the image set was sampled in the direction perpendicular to leaf motion at each leaf width center. Each slice was convolved with a rect function representing MLC leaf width and convolved with a dose spread kernel appropriate for the depth along the central axis of the beam. The resulting 3D dose distributions for each field were summed to give the deliverable dose distribution for the nine‐field treatment. Results: The result of the model is a deliverable dose distribution in 3D. This deliverable dose distribution may be analyzed by comparison with the desired dose distribution. Information on the distribution's conformity may be attained by calculation of a distance‐to‐agreement map to provide information on spatial resolution, or by calculation of a map containing percent differences in dose to give insight into the dosimetric resolution. Dose intensity profiles may be extracted from the deliverable dose distribution as a means to determine achievable dosimetric gradients. Conclusion: This 3D model simulates the degradation of an ideal dose distribution due to the IMRT treatment delivery process. The resulting deliverable dose distribution may be used for evaluating the achievable spatial and dosimetric resolution for the current IMRT technique.