Investigation of Hot-Spots in TomoDirect 3DCRT Breast Treatment

Background: Conventional tomotherapy platforms only allow for the delivery of helical IMRT. However the use of IMRT and helical delivery in breast cancer treatment is non-standard. Newer tomotherapy units are equipped with a static-beam mode with 3DCRT capabilities. During the clinical use, we frequently observe hot-spots in the plan that renders the plan clinically unacceptable. The purpose of this study is to investigate the underlying cause of the hot-spots in tomotherapy static-beam breast treatment and possible solutions. Materials/Methods: Theories about the formation of the hot-spot were developed. Eight lumpectomy patients contoured according to RTOG-1005 specifications were also used to illustrate the magnitude of hot-spots under various planning strategies. Two tangential beams were used for the whole breast irradiation plan with prescription dose of 40 Gy in 15 fractions. Results: The hot-spot was identified as the behavior of the optimization engine when part of the target region was blocked. With the current design of tomotherapy’s 3DCRT planning where user adjustment was greatly limited, none of the planning strategies were able to reduce the hot-spots to acceptable levels in the eight patients studied. The best strategy still produced an average of 48.5 Gy (121% of prescription dose) hot-spot dose and 30.4 cc hot-spot volume (volume receiving > 110% prescription dose). It is also shown that the hot-spot was not a result of energy or other physical limitation of the radiation device. By manually adjusting the plan sinogram, the maximum hot-spot dose drops from 121% to 111% and the hot-spot volume drops from 30 cc to 6 cc on average. Conclusions: While TomoDirect 3DCRT showed great promise in breast treatment, treatment planning software improvements may be needed in order to improve the clinical acceptability by reducing hot-spots in normal tissue.