AbstractID: 8579 Title: Helical Tomotherapy or IMRT to boost a "sub-optimal" prostate
brachytherapy implant: a feasiblity study
Ultrasound-guided permanent prostate brachytherapy alone using I-125 seeds is an effective treatment modality for early stage, low
risk prostate cancer. In our experience, the dose distribution conforms well to the prostate gland in the majority of cases as measured
by the dosimetric data from CT on day 30 (V90 and D90). However, we have identified occasional underdosing of the base of the
prostate. If this underdose is considered clinically significant, it is difficult to correct this by additional boost with brachytherapy or
external beam. We are investigating the feasibility of planning an external beam boost using helical tomotherapy and IMRT. We use
accepted parameters such as V90 and D90, as well as “regional” analysis to identify cases with potential clinically significant
underdose. Our initial approach to optimize a boost plan with tomotherapy or IMRT is to define the brachytherapy prescription “dose
surface” (V144Gy) as an organ-at-risk. The portion of the prostate GTV outside of the prescription dose is defined as the new target
volume (CTV) for inverse planning. The high (150%) and low (50%) brachytherapy “dose surfaces” (V216Gy and V72Gy) are used
as additional constraints as well as the rectal and urethral volumes. Our initial experience indicate that both tomotherapy and IMRT
yield plans that conform well to the complex shape of the underdose region of brachytherapy provided a small additional dose to
limited regions of the previously irradiated volume is acceptable.