Reproducibility of tangential breast fields using online electronic portal images.

Summary: Background: Treatment verification and reproducibility plays a major role in radiotherapy to achieve better tumour control. Small uncertainties in daily repositioning of the patients and internal organ motion can lead to discrepancies between the planned and delivered radiation treatments. A factor that influences dose homogeneity and treatment volume is the accuracy of treatment setup. Small deviations in positioning the patient with regard to the beam setup could have a relatively significant impact on the treatment volume and it is imperative to control the setup error during radiotherapy. This study focuses on the importance of inter- and intra-fraction error in tangential breast radiotherapy with an electronic portal imaging device. Aim: To study the variation in treatment setup due to intra-fraction and inter-fraction during tangential field breast irradiation. Materials/Methods: Twelve patients of carcinoma breast were selected for this study and CT based planning was performed with simple tangential fields. The patients were treated on a 6MV linear accelerator equipped with an electronic portal imaging device (EPID). Portal images were acquired for both medial and lateral tangential fields for 10 fractions and intra- and inter-fraction studies were performed for all the patients. Parameters such as central lung distance (CLD), central beam edge to skin distance (CBESD), central irradiated width (CIW) and cranio-caudal distances (CCD) were measured on the acquired portal image. Results: The average systematic differences observed for CLD, CBESD, CCD and CIW were 1.2mm, 2.8mm, 2.07mm and 3.30mm. For intra-fraction motion, the observed standard deviations for CLD, CBESD and CCD were 0.7mm, 0.73mm, and 1.36mm. Similarly the CLD, CBESD, CIW and CCD were analyzed for inter-fraction variation. Conclusions: The online portal imaging device is an important tool for ensuring the proper delivery of planned dose. Our results suggest that intra-fraction motion of the breast has less impact on the treatment volume. Regular treatment verification between treatment fractions will help in reducing the normal tissue toxicity and ensures proper dose delivery to the tumour volume. [Copyright &y& Elsevier]