A Monte Carlo evaluation of beam characteristics for total body irradiation at extended treatment distances
The aim is to study beam characteristics at large distances when focusing on the electron component. In particular, to investigate the utility of spoilers with various thicknesses as an electron source, as well as the effect of different spoiler‐to‐surface distances (STSD) on the beam characteristics and, consequently, on the dose in the superficial region. A MC model of a 15 MV Varian accelerator, validated earlier by experimental data at isocenter and extended distances used in large‐field total body irradiation, is applied to evaluate beam characteristics at distances larger than 400 cm. Calculations are carried out using BEAMnrc/DOSXYZnrc code packages and phase space data are analyzed by the beam data processor BEAMdp. The electron component of the beam is analyzed at isocenter and extended distances, with and without spoilers as beam modifiers, assuming vacuum or air surrounding the accelerator head. Spoiler thickness of 1.6 cm is found to be optimal compared to thicknesses of 0.8 cm and 2.4 cm. The STSD variations should be taken into account when treating patients, in particular when the treatment protocols are based on a fixed distance to the patient central sagittal plane, and also, in order to maintain high dose in the superficial region. PACS numbers: 87.55.D‐, 87.55.de, 87.55.K‐
Analytical Anisotropic Algorithm Calculation in Total Body Irradiation: A Comparison with Monte Carlo Calculation and Dosimetry
Purpose: This study aimed to investigate the feasibility and accuracy of an analytical anisotropic algorithm calculation of the Varian Eclipse treatment planning system in the TBI treatment planning at an extended source-to-surface distance of 400 cm. Materials and Methods: The TBI treatment was planned with the Varian Eclipse TPS using the AAA for the dose calculation. Monte Carlo calculations were performed using the EGSnrc code package. Percentage depth dose curves, central axis dose profiles and absolute doses of the 6 MV photon beams from the Varian TrueBeam linear accelerator were measured at an extended SSD of 400 cm in water, solid water and anthropomorphic phantoms. Results: Calculated and measured PDD curves were in good agreement at the depths of 30.0 cm or less. Calculated J Cancer Sci Clin Ther 2021; 5 (4): 532-547 DOI: 10.26502/jcsct.5079136 Journal of Cancer Science and Clinical Therapeutics 533 and measured central axis dose profiles were clinically acceptable, observing the largest uncertainties close the field edges. Absolute dose measurements in water phantoms showed that the MC and AAA calculations overestimated doses by up to 2.2 % and 1.8 %, respectively. In the anthropomorphic phantom, the difference between measured and calculated mean doses in the whole body volume was 1.0 % and 6.2 % for MC and AAA, respectively. In the AAA calculation, the largest observed difference was 8.3 % in the head. Conclusions: The Eclipse’s AAA calculation can be safely used for the TBI treatment planning at the extended SSD of 400 cm. However, in order to achieve a higher level of accuracy in the TBI planning, one must carefully validate the TPS at extended SSDs.
A translational couch technique for total body irradiation
The authors have devised a computer controlled translational couch to administer total body irradiation accurately and safely. In this technique, patients comfortably rest on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient's dimensions, beam geometry and machine dose rate. In the authors' design, the couch velocity is continuously updated for possible machine dose rate fluctuations to eliminate dose non-uniformity within the patient. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents special challenges for dosimetry calculations. A dosimetry parameter is introduced that converts moving beam output to fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been implemented that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Comprehensive dose measurements within phantoms confirmed the validity of the calculations method. Further improvements in dose uniformity within the patient by modulating couch velocity with patient thickness and shielding the organs at risk are discussed.