Masters Thesis Defense - Dosimetric Implications of Breast Composition During X-Ray and Electron Beam Radiotherapy

Who: Karmen Li

Time: 11am, Tuesday, 13th November, 2007

Place: P-250

Abstract: Literature has shown that breast tissues are composed of about 70% fat and 30% parenchyma, and therefore exhibit lower density that water. During computerized treatment planning for external beam radiotherapy, then software accounts for breast density as derived from the CT data. Manual non-CT dosimetry does not however account for this effect. The purpose of our study was to determine breast tissue density of our patients and analyze its impact on photon and electron beam dosimetry.

We analyzed 100 data sets among 48 patients with breast cancer. All patients underwent CT simulation on a Philips AcQSim scanner (3 mm slices). An electron density phantom was scanned for verifying the proper density conversion at the treatment planning computers (ADAC Pinnacle3 and BrainSCAN). The CT data were analyzed to determine the average physical density of breast tissues. Tangential 6 MV wedge pair dosimetry for breast was carried out, and heterogenous and homogeneous density plans were compared in terms of Dose Volume Histogram (DVH) for breast and lung. Similarly, electron beam dose distributions were compared between actual breast and unit density.

Breast tissue density from 48 patients was found to be 0.892 ± 0.026 g/cc on ADAC Pinnacle3 and 0.92 ± 0.05 g/cc on the BrainSCAN system. As expected, mean tissue density decreased with age; 0.915 g/cc at 36 years to 0.865 g/cc at 86 years. Tangential 6 MV beam dosimetry showed that historic contour-based planning with assumed density of 1 g/cc delivered higher doses to breast than indicated by plan. Electron dosimetry showed a marked increase in depth of penetration resulting from the density correction.

We conclude that dosimetric implications of tissue inhomogeneneity within breast are significant.

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