Improving dose distribution in the junction area in external radiotherapy of breast cancer

Abstract

In this paper we studied and evaluated four techniques (T₁, T₂, T₃, T₄) to expand the secondary collimators by 1mm in order to improve dose distribution in the junction area of the treatment fields of the chest wall and supraclavicular lymph nodes when breast radiotherapy.

In technique T₁ or conventional technique a single isocenter with half beam was used, in T₂ only the lateral tangential field was expanded, in T₃ only the medial tangential field was expanded and in T₄ the medial and lateral tangential fields were expanded together. The expansion techniques (T₂, T₃, T₄) were evaluated in comparison with the T₁ by studying the dose volume histograms (DVHs) and calculating dose homogeneity index HI and the dose of organs at risk (OARs). To verify the position of the secondary collimators after expansion, the calculated dose using the treatment planning system (TPS) was compared with the measured dose at three different depths.

The results indicated that there was a gradual increase in the minimum dose D_min (p < 0.05) when using expansion techniques at rates of 5%, 6%, and 9%, and that there was an increase in the volume of the tumor that received 95% of the prescribed dose PD (V95%) at rates of 50%, 51% and 63%, and thus an improvement in the HI by 9%, 19% and 30%. These techniques did not affect the OAR (p > 0.05). the mean uncertainty between the measured and calculated values was 2%.

we can be concluded the importance of using T₄ in planning radiotherapy for breast tumors because it has a significant role in achieving a more homogeneous dose distribution in the junction area JA, with the necessity of experimental and frequent verification of the position of the beam collimators