Dosimetric effects within target and organs at risk of interfractional patient mispositioning in left breast cancer radiotherapy

To investigate the effects of interfraction setup uncertainties on the dose distribution within the clinical target volume (CTV) and the organs at risk (OAR) of left-sided breast cancer patients undergoing external radiotherapy. Interfractional setup errors were assessed by measuring surface control points displacements during 89 irradiation sessions in 4 patients, by means of opto-electronic localization. The measured position deviations were fed back to the treatment planning system for the evaluation of the corresponding dosimetric effects within CTV and OARs (lung, heart). Results revealed errors above 5 mm on some of the control points, but corresponding volumetric variations were on average below 2% for both the CTV within the 95–105% dose range and the OARs receiving more than 50% and 90% of the prescribed dose. A specific sensitivity to the setup errors was found as a function of the treatment plan design, leading to isolated cases exhibiting volumetric variations of CTV and OARs exceeding 2%. This study confirms the potential increase of treatment quality provided by the systematic patient position verification and highlights the role of opto-electronic position detection systems for the real-time check of patient setup errors and the evaluation of the corresponding dosimetric consequences, as a way to achieve consistent dose delivery.