Technical note: TROG 15.01 SPARK trial multi-institutional imaging dose measurement.

PURPOSE: The Trans-Tasman Radiation Oncology Group (TROG) 15.01 Stereotactic Prostate Adaptive Radiotherapy utilizing Kilovoltage intrafraction monitoring (SPARK) trial is a multicenter trial using Kilovoltage Intrafraction Monitoring (KIM) to monitor prostate position during the delivery of prostate radiation therapy. KIM increases the accuracy of prostate radiation therapy treatments and allows for hypofractionation. However, an additional imaging dose is delivered to the patient. A standardized procedure to determine the imaging dose per frame delivered using KIM was developed and applied at four radiation therapy centers on three different types of linear accelerator. METHODS: Dose per frame for kilovoltage imaging in fluoroscopy mode was measured in air at isocenter using an ion chamber. Beam quality and dose were determined for a Varian Clinac iX linear accelerator, a Varian Trilogy, four Varian Truebeams and one Elekta Synergy at four different radiation therapy centers. The imaging parameters used on the Varian machines were 125 kV, 80 mA, and 13 ms. The Elekta machine was measured at 120 kV, 80 mA, and 12 ms. Absorbed doses to the skin and the prostate for a typical SBRT prostate treatment length were estimated according to the IPEMB protocol. RESULTS: The average dose per kV frame to the skin was 0.24 ± 0.03 mGy. The average estimated absorbed dose to the prostate for all five treatment fractions across all machines measured was 39.9 ± 2.6 mGy for 1 Hz imaging, 199.7 ± 13.2 mGy for 5 Hz imaging and 439.3 ± 29.0 mGy for 11 Hz imaging. CONCLUSIONS: All machines measured agreed to within 20%. Additional dose to the prostate from using KIM is at most 1.3% of the prescribed dose of 36.25 Gy in five fractions delivered during the trial.

Theoretical evaluation of a novel method for producing fluorine-18 for Positron-emission-tomography (PET) applications utilizing the 3He(d,p)4He reaction.

A novel method of producing fluorine-18 for Positron emission tomography (PET) scan applications is evaluated theoretically. The method is based upon the 3He(d,p)4He nuclear reaction from which the protons produced are used to produce fluorine-18 via the 18O(p,n)18F reaction. The potential advantage of such a system over cyclotron-based production is of lower input beam energy, which may lower the cost of the system and potentially allow for onsite production. Two theoretical designs were investigated. The first utilizes a helium-3 beam incident on a deuterated plastic target such as Mylar which is backed with an oxygen-18 heavy-water (H2O18) target. The second design utilizes a super-heavy-water (D2O18) target effectively combining both targets into one. Theoretical yield calculations were performed for both designs and the practicalities, primarily those of thermal and target degradation effects were assessed. To produce sufficient fluorine-18 yield a 1 MeV helium-3 beam at 100 mA was simulated. For this beam it was calculated that 310 MBq of fluorine-18 activity, as required to scan a 74 kg patient would be generated in a 69 min or an 18 min production run for the Mylar and super-heavy-water systems respectively. The simulated beam is at 100 kW power and without significant cooling would vaporize the target materials with the melting point of Mylar and boiling point of water calculated to be breached within 0.41 µs and 0.45 µs from beam-on. To achieve sufficient fluorine-18 yield the helium-3 beam power had to be increased to impractical levels making the systems technically infeasible.