Investigating the effect of different photon energy levels on field in field technique in whole breast radiotherapy

Objective: The aim of this study was to investigate the effect of different photon energy levels on field-in-field (FIF) technique in whole breast radiotherapy. Method: Treatment plans were created using tangentially field-in-field (tFIF) technique with 4 and 6 MV photon energy levels on computed tomography (CT) images of human-like phantom with different breast volume. Treatment plans, Dose-volume histogram (DVH) s and isodose curves of planned target volume (PTV) and organ at risk (OAR) were evaluated. In addition, homogeneity index (HI) and conformity index (CI) parameters of treatment plans were calculated. In addition, whole breast radiotherapy was planned in our clinic with tFIF technique and alternative tFIF plans were designed for 10 patients who are still on treatment. DVH data, calculated HI and CI parameters of PTV and OARs of routine approved treatment plans and alternative plans designed were statistically evaluated. The dose of the contralateral breast was measured by in-vivo dosimetry using semiconductor MOSFET dosimeter in human-like phantom and in patients who continue routine treatment. Measurement results were compared with TPS results. Results: The HI and CI parameters of the phantom designed plans with different photon energy levels gave close results. Higher contralateral breast doses were measured in the 6 MV-tFIF plans of the phantom compared to the 4 MV-tFIF plans. There was no statistically significant difference in HI and CI parameters between 4 MV-tFIF and 6 MV-tFIF plans of breast patients. The contralateral breast doses of breast patients measured on the 6 MV-tFIF and 4 MV-tFIF plans were found close. It was determined that TPS calculated the dose inadequate Radiation dose in the out of field regions. Conclusion: When the energy selection in breast radiotherapy is performed, the contralateral breast dose should be considered. Also, in selected energy the evaluating of plans should be made by considering that TPS data gives inadequate results. Key words: Field-in-field (FIF), In-vivo dosimetry, Treatment planning system (TPS), Whole breast radiotherapy