Internal radiation dose estimation using multiple D-shuttle dosimeters for positron emission tomography (PET): A validation study using NEMA body phantom

Biological dosimetric impact of dose-delivery time for hypoxic tumour with modified microdosimetric kinetic model

Background

An improved microdosimetric kinetic model (MKM) can address radiobiological effects with prolonged delivery times. However, these do not consider the effects of oxygen. The current study aimed to evaluate the biological dosimetric effects associated with the dose delivery time in hypoxic tumours with improved MKM for photon radiation therapy.

Materials and methods

Cell survival was measured under anoxic, hypoxic, and oxic conditions using the Monte Carlo code PHITS. The effect of the dose rate of 0.5-24 Gy/min for the biological dose (Dbio) was estimated using the microdosimetric kinetic model. The dose per fraction and pressure of O2 (pO2) in the tumour varied from 2 to 20 Gy and from 0.01 to 5.0% pO2, respectively.

Results

The ratio of the Dbio at 1.0-24 Gy/min to that at 0.5 Gy/min (RDR) was higher at higher doses. The maximum RDR was 1.09 at 1.0 Gy/min, 1.12 at 12 Gy/min, and 1.13 at 24 Gy/min. The ratio of the Dbio at 0.01-2.0% of pO2 to that at 5.0% of pO2 (Roxy) was within 0.1 for 2-20 Gy of physical dose. The maximum Roxy was 0.42 at 0.01% pO2, 0.76 at 0.4% pO2, 0.89 at 1% pO2, and 0.96 at 2% pO2.

Conclusion

Our proposed model can estimate the cell killing and biological dose under hypoxia in a clinical and realistic patient. A shorter dose-delivery time with a higher oxygen distribution increased the radiobiological effect. It was more effective at higher doses per fraction than at lower doses.

Development of PHITS graphical user interface for simulation of positron emitting radioisotopes production in common biological materials during proton therapy

The Monte Carlo (MC) method is a powerful tool for modeling nuclear radiation interaction with matter. A variety of MC software packages has been developed, especially for applications in radiation therapy. Most widely used MC packages require users to write their own input scripts for their systems, which can be a time consuming and error prone process and requires extensive user experience. In the present work, we have developed a graphical user interface (GUI) bundled with a custom-made 3D OpenGL visualizer for PHITS MC package. The current version focuses on modeling proton induced positron emitting radioisotopes, which in turn can be used for verification of proton ranges in proton therapy. The developed GUI program does not require extensive user experience. The present open-source program is distributed under GPLv3 license that allows users to freely download, modify, recompile and redistribute the program.

A SIMPLE AND NOVEL APPROACH TO STUDY KINETICS AND ESTIMATE RADIATION DOSES FROM INTERNALLY ADMINISTERED RADIOPHARMACEUTICALS USING AN EXTERNAL DOSE MEASUREMENT SYSTEM

Various methods have been reported to study radiotracer kinetics and make internal dosimetry feasible in the routine clinical nuclear medicine practice. The aim of the present study was to quantify cumulative activity and organ doses using an indigenously designed and fabricated external dose measurement system. The measurement was demonstrated on patients undergoing whole-body (WB) 18F-FDG (Fluorine-18-fluorodeoxyglucose) direct positron emission tomography/computed tomography investigations. An external dose measurement system comprising of an ionisation chamber-survey meter and the movable focussing collimator was used to quantify the uptake of 18F-FDG in liver and brain. Cumulative activity and normalised cumulative activity in these organs were calculated. The results were validated by performing measurements on a phantom uniformly filled with known activity of 18F-FDG.The difference in the absorbed dose estimated with and without collimator was statistically significant (p < 0.05). The external dose measurement technique is relatively novel, convenient and reliable for the assessment of internal absorbed dose of organs.

Medical application of particle and heavy ion transport code system PHITS

The Particle and Heavy Ion Transport code System (PHITS) is a general-purpose Monte Carlo simulation code that has been applied in various areas of medical physics. These include application in different types of radiotherapy, shielding calculations, application to radiation biology, and research and development of medical tools. In this article, the useful features of PHITS are explained by referring to actual examples of various medical applications.

Error evaluation of the D-shuttle dosimeter technique in positron emission tomography study

The D-shuttle dosimeter technique is a convenient approach for estimating the radiation dosimetry in a positron emission tomography (PET) study that employs multiple D-shuttle dosimeters attached to the body surface of a patient. To bring this technique into clinical usage, it is very important to evaluate its performance by investigating the bias associated with D-shuttle dosimeter positioning and by comparing the estimates with those of the whole-body dynamic PET imaging technique. The torso cavity and six spheres of the NEMA body phantom were filled with ¹⁸F-FDG solution, and then, the phantom was imaged for 1 h. We assumed the mislocated positioning of the D-shuttle dosimeters by shifting them in the z-direction (upper) in a range of 1-5 cm from the original positions. The cumulative radioactivities, absorbed doses, and effective dose were estimated using accurate and mislocated positions of the D-shuttle dosimeters. For comparison, the cumulative radioactivities were also estimated from the PET images, and then, the absorbed doses and effective dose were computed. The maximum bias of the average estimated cumulated radioactivities and the effective doses was - 15.0% and - 19.7% for the 1 cm shifted positions, respectively. The ratios of absorbed doses obtained from D-shuttle and PET measurement against the actual values were between 0.9 and 1.3, and 0.7 and 1.0, respectively. The bias associated with the D-shuttle dosimeter positions was significant and probably consistent, and both dosimetric techniques exhibited good performance in this phantom study.