RBE of kV CBCT radiation determined by Monte Carlo DNA damage simulations.

Due to the higher LET of kilovoltage (kV) radiation, there is potential for an increase in relative biological effectiveness (RBE) of absorbed doses of radiation from kV cone beam computed tomography (CBCT) sources in reference to megavoltage or Co-60 doses. This work develops a method for accurately coupling a Monte Carlo (MC) radiation transport code (PENELOPE) with the damage simulation (MCDS) to predict relative numbers of DNA double strand breaks (DSBs). The MCDS accounts for slowing down of electrons and delta ray production within the cell nucleus; however, determining the spectrum of electrons incident on the cell nucleus from photons interacting in a larger region of tissue is not trivial. PENELOPE simulations were conducted with a novel tally algorithm invoked where electrons incident on a detection material were tracked and both the incident energy and the final deposited dose were recorded. The DSB yield predicted by a set of MCDS runs of monoenergetic electrons was then looked up in a table and weighted by the specific energy of the incident electron. Our results indicate that the RBE for DSB induction is 1.1 for diagnostic x-rays with energies from 80 to 125 kVp. We found no significant change in RBE with depth or filtration. The predicted absolute DSB yields are about three times lower for cells irradiated under anoxic conditions than the yield in cells irradiated under normoxic (5%) or fully aerobic (100%) conditions. However, oxygen concentration has a negligible (± 0.02) effect on the RBE of kV CBCT x-rays.

Clinical consequences of changing the sliding window IMRT dose rate.

Changing pulse repetition frequency or dose rate used for IMRT treatments can alter the number of monitor units (MUs) and the time required to deliver a plan. This work was done to develop a practical picture of the magnitude of these changes. We used Varian's Eclipse Treatment Planning System to calculate the number of MUs and beam-on times for a total of 40 different treatment plans across an array of common IMRT sites including prostate/pelvis, prostate bed, head and neck, and central nervous system cancers using dose rates of 300, 400 and 600 MU/min. In general, we observed a 4%-7% increase in the number of MUs delivered and a 10-40 second decrease in the beam-on time for each 100 MU/min of dose rate increase. The increase in the number of MUs resulted in a reduction of the "beam-on time saved". The exact magnitude of the changes depended on treatment site and planning target volume. These changes can lead to minor, but not negligible, concerns with respect to radiation protection and treatment planning. Although the number of MUs increased more rapidly for more complex treatment plans, the absolute beam-on time savings was greater for these plans because of the higher total number of MUs required to deliver them. We estimate that increasing the IMRT dose rate from 300 to 600 MU/min has the potential to add up to two treatment slots per day for each IMRT linear accelerator. These results will be of value to anyone considering general changes to IMRT dose rates within their clinic.

Sci-Sat AM: Brachy - 06: Monte carlo DNA damage simulations of kV cbct radiation.

When performed daily, cone beam CT (CBCT) images can accumulate radiation dose to non-negligible levels. Because kV x-rays have a larger relative biological effectiveness (RBE) than its MV x-rays, the accumulated absorbed dose needs to be multiplied by an appropriate RBE to better evaluate the impact of CBCT dose in a treatment planning context. We investigated this question using PENLEOPE simulations to look in detail at the electron energy spectra produced by kV x-rays and Co-60 γ-rays in biologically motivated geometries. The electron spectra were input into the published Monte Carlo Damage Simulation (MCDS) and used to estimate the average number of double strand breaks (DSBs) per Gy per cell. Our results suggest an approximately 10% increase in the RBE for DSB induction. For the majority of treatment planning scenarios where imaging dose is only a small fraction of the total delivered dose to target volumes and organs at risk, the increase in RBE is not critical to be factored in, however for it may play a significant role in predicting the induction of secondary cancers.

Poster - Thur Eve - 55: Monte Carlo simulations of variations in planned dose distributions in a prostate patient population.

Clinical outcome studies with clear and objective endpoints are necessary to make informed radiotherapy treatment decisions. Commonly, clinical outcomes are established after lengthy and costly clinical trials are performed and the data are analyzed and published. One the challenges with obtaining meaningful data from clinical trials is that by the time the information gets to the medical profession the results may be less clinically relevant than when the trial began, An alternative approach is to estimate clinical outcomes through patient population modeling. We are developing a mathematical tool that uses Monte Carlo techniques to simulate variations in planned and delivered dose distributions of prostate patients receiving radiotherapy. Ultimately, our simulation will calculate a distribution of Tumor Control Probabilities (TCPs) for a population of patients treated under a given protocol. Such distributions can serve as a metric for comparing different treatment modalities, planning and setup approaches, and machine parameter settings or tolerances with respect to outcomes on broad patient populations. It may also help researchers understand differences one might expect to find before actually doing the clinical trial. As a first step and for the focus of this abstract we wanted to see if we could answer the question: "Can a population of dose distributions of prostate patients be accurately modeled by a set of randomly generated Gaussian functions?" Our results have demonstrated that using a set of randomly generated Gaussian functions can simulate a distribution of prostate patients.

Poster - Thurs Eve-03: Dose verification using a 2D diode array (Mapcheck) for electron beam modeling, QA and patient customized cutouts.

PURPOSE: To assess a diode detector array (MapCheck) for commissioning, quality assurance (QA); and patient specific QA for electrons. METHOD AND MATERIALS: 2D dose information was captured for various depths at several square fields ranging from 2×2 to 25×25cm2 , and 9 patient customized cutouts using both Mapcheck and a scanning water phantom. Beam energies of 6, 9, 12, 16 and 20 MeV produced by Varian linacs were used. The water tank, beam energies and fields were also modeled on the Pinnacle planning system obtaining dose information. Mapcheck, water phantom and Pinnacle results were compared. Relative output factors (ROF) acquired with Mapcheck were compared to an in-house algorithm (JeffIrreg). Inter- and intra-observer variability was also investigated Results: Profiles and %DD data for Mapcheck, water tank, and Pinnacle agree well. High-dose, low-dose-gradient comparisons agree to within 1% between Mapcheck and water phantom. Field size comparisons showed mostly sub-millimeter agreement. ROFs for Mapcheck and JeffIrreg agreed within 2.0% (mean=0.9%±0.6%). CONCLUSION: The current standard for electron commissioning and QA is the scanning water tank which may be inefficient. Our results demonstrate that MapCheck can potentially be an alternative. Also the dose distributions for patient specific electron treatment require verification. This procedure is particularly challenging when the minimum dimension across the central axis of the cutout is smaller than the range of the electrons in question. Mapcheck offers an easy and efficient way of determining patient dose distributions especially compared to using the alternatives, namely, ion chamber and film.