Clinical implementation of IMRT step and shoot with simultaneous integrated boost for breast cancer: A dosimetric comparison of planning techniques

Abstract

Purpose: Radiotherapy post-lumpectomy with two coplanar tangent beams is the standard treatment for women with early stage breast cancer. Despite the use of wedges as tissue compensators, the resultant plans often contains a significant dose gradient and 'hot spots' in excess of 15% or more of prescribed dose. In recent years a field-in-field (FIF) dose-compensation technique, which use two standard tangent fields and one or two (rarely three) small beams within these, was developed. It allows to obtain a more uniform dose throughout the target volume in the majority of cases but not in all. This study presents our experience to develop optimal intensity modulated radiation therapy (IMRT) techniques to be applied clinically in those cases where the traditional technique with two tangent fields or its variant field in field (FIF) are unable to achieve a satisfactory planning target volumes (PTVs) coverage and dose objectives to the organs at risk (OARs). Methods: We investigated two pure IMRT plans (named 3F-IMRT and 4F-IMRT) and a hybrid one (H-IMRT). Treatment plans were performed for 7 left-sided and 4 right-sided breasts using simultaneously integrated boost (SIB) planned technique with inverse optimization. Results were compared with those obtained with FIF technique. Dose prescribed was 45 Gy/20 fractions to the breast and 50 Gy /20 fractions to the lumpectomy cavity delivered in 5 fr/week. Dose–volume histograms were generated and parameters as target dose coverage, conformity and homogeneity as well as OARs dose distribution were analyzed. Finally the secondary cancer risk to contralateral breast due to radiation was evaluated as a further parameter for the choice of the optimal plan. Results: Compared to the FIF, the three IMRT plans provided the same target coverage and a better dose conformation, but a worst dose homogeneity of the boost target. The volume of the OARs, receiving higher doses than 15 Gy was reduced but was increased the volume receiving low doses. This causes the increase of the risk of radiation induced cancer, especially for the contralateral breast. For this organ, the highest value of the excess absolute risk (EAR) was associated to the 4F-IMRT, while the lower, to the FIF. Conclusion: The intensity-modulated radiation therapy techniques 5F-IMRT and 4F-IMRT were the best to be applied clinically in those cases, where the traditional technique of irradiation of the breast is unable to achieve the PTVs coverage and dose objectives to the OARs. However, all the IMRT techniques showed an increased volume of healthy tissues receiving low doses, so they should not be used in extensive manner and in particular should be avoided in the cases of young women due to the excess of risk to develop a secondary cancer.