Reirradiation of glioblastoma through the use of a reduced dose rate on a tomotherapy unit

Pulsed Reduced Dose Rate (PRDR) is a method of irradiation designed to minimize radiation-related toxicities in patients undergoing reirradiation for loco-regional reoccurrence of glioblastoma. PRDR delivers a standard 2 Gy fraction delivered on a conventional medical linear accelerator using conventional 3D conformal beam arrangements. To reduce the likelihood of normal tissue complications, radiation is delivered over ten 0.2 Gy sub-fractions with a 3 minute time interval between subfractions to give a maximal time averaged dose rate of 4 Gy/hr. However, a TomoTherapy unit has a fixed output rate of 8 Gy/min. If the dose per fraction is conventionally planned at less than 0.6 Gy/fraction, the result is a clinically unacceptable treatment plan. The method described in this paper involves a virtual grid style blocking scheme, where half of the beam angles are directionally blocked using 15 equally spaced segments surrounding the center of the image set. Ten patients treated using conventional PRDR with an average PTV volume of 353.3 ml were retrospectively re-planned using five techniques (standard 2 Gy fraction, 2 Gy in ten 0.2 Gy fractions without grid blocking, two grid patterns, and a combination plan incorporating both grids) and analyzed with conformation numbers (CN), homogeneity indexes (HI), and dose volumes to normal tissues. Plans were optimized using equal constraints and machine parameters. The grid method allowed for clinically acceptable treatment plans at 0.2 Gy with a treatment time ≤ 3min per subfraction. The average HI was slightly poorer for the combination plan versus the standard 2 Gy fraction plan (0.064 versus 0.027) and the CN was similar over all techniques (0.72 - 0.73) employed. Normal tissue dose volumes for each patient were also similar for each technique. Initial ion chamber measurements agree with predicted values for a 0.2 Gy subfraction. PRDR is deliverable on a TomoTherapy system using our virtual directional blocking method. Results can be slightly improved through the use of two grids alternated on a daily basis. The dose to normal structures for individual patients was similar for all methods.