Dosimetric Effect of Biozorb Markers for Accelerated Partial Breast Irradiation in Proton Therapy

Purpose To investigate dosimetric implications of biodegradable Biozorb (BZ) markers for proton accelerated partial breast irradiation (APBI) plans. Materials and Methods Six different BZs were placed within in-house breast phantoms to acquire computed tomography (CT) images. A contour correction method with proper mass density overriding for BZ titanium clip and surrounding tissue was applied to minimize inaccuracies found in the CT images in the RayStation planning system. Each breast phantom was irradiated by a monoenergetic proton beam (103.23 MeV and 8×8 cm2) using a pencil-beam scanning proton therapy system. For a range perturbation study, doses were measured at 5 depths below the breast phantoms by using an ionization chamber and compared to the RayStation calculations with 3 scenarios for the clip density: the density correction method (S1: 1.6 g/cm3), raw CT (S2), and titanium density (S3: 4.54 g/cm3). For the local dose perturbation study, the radiographic EDR2 film was placed at 0 and 2 cm below the phantoms and compared to the RayStation calculations. Clinical effects of the perturbations were retrospectively examined with 10 APBI plans for the 3 scenarios (approved by our institutional review board). Results In the range perturbation study, the S1 simulation showed a good agreement with the chamber measurements, while excess pullbacks of 1∼2 mm were found in the S2 and S3 simulations. The film study showed local dose shadowing and perturbation by the clips that RayStation could not predict. In the plan study, no significant differences in the plan quality were found among the 3 scenarios. However, substantial range pullbacks were observed for S3. Conclusion The density correction method could minimize the dose and range difference between measurement and RayStation prediction. It should be avoided to simply override the known physical density of the BZ clips for treatment planning owing to overestimation of the range pullback.

Evaluation of glandular dose in mammography in presence of breast cysts using Monte Carlo simulation

Background: Normalized glandular dose (DgN) is an important dosimetric quantity in mammography. Aim: In this study, the effect of the presence of breast cysts and their size, number and location on DgN is evaluated. Materials and methods: The effect of the presence of cysts in breast was examined using MCNPX code. This was performed by taking homogeneous breast phantoms containing spheroid breast cysts into account. The radius of the cysts, numbers of the cysts, and depth of the cysts, and their location were variable. Various electron energies were also considered. Finally, these results were compared with the results of a cyst-less breast phantom. Results: The results show that the effect of the presence of cysts in the breast depends on the size, number and location of cysts. The presence of cysts at lower depths leads to a decrease in the DgN values, compared to the breast phantom without cysts. The presence of cysts in the breast phantom has an effect of -7 to +14 percent on the DgN values under the conditions considered in this modeling. This effect is independent of the X-ray tube voltage, the breast phantom thickness, and glandular ratio, and depends only on the number and size and location of the cysts. The bigger radius and number of cysts, the greater effect on DgN value.