Validation of the Stand-up Technique for Total Skin Irradiation by Monte Carlo Simulation

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2019

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Abstract

Purpose/Objective(s): The standard total skin irradiation (TSI) procedure for patients with Mycosis fungoides at our clinic is the Stanford technique where dual electron beams are directed toward patient standing at an extended source to skin distance (SSD) of 300 cm. Patients rotate along the cranial-caudal axis in 6 directions to get full coverage to skin surface. The purposes of this study are to validate the commissioning dosimetric data using Monte Carlo (MC) systems, and to investigate the effect of scattering filter on the standard stand-up technique with a single beam.

Materials/Methods: The first MC system is the EGSnrc environment with BEAMnrc and DOSXYZnrc packages, which has been the standard MC simulation system used in the radiation therapy field. In this study, extended SSD with electron beam was tested, which is not a common use of EGSnrc. The second system is the VirtuaLinac, a recently developed cloud-based application from Varian for research purpose based on GEANT4 platform. For both MC systems, the same phase space files which have been previously validated were used. At each direction, a dual-field electron beam with jaws opening of 36 × 36 cm2 and gantry angle at ±19° degrees from horizontal direction was used. The following quantities were studied and compared with the measurements during commissioning: for each field/direction at the treatment SSD, the percentage depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output on a flat solid water phantom; the composite dose distribution on a cylindrical phantom of 30 cm diameter. For the investigation part, the materials (Cu, Fe, Au, Zn, Ag) were chosen because of their stability and availability. The thickness ranges from 0.05 mm to 0.55 mm, depending on characteristics of materials. The extended source to skin distance (SSD) from 250 cm to 350 cm were studied. For each material, we vary the thickness and SSD, to evaluate following quantities: percent depth dose (PDD), profiles and output at dmax, and compared them with the standard dual beams at treatment SSD.

Results: For the dual-field electron beam from one direction, the average(maximum) difference in profiles between EGSnrc/VirtuaLinac and measurement were -5.5% (-8.7%) and 0.9% (2.0%). Both dmax (1.1 cm) and R50 (2.1 cm) in PDD of both MC systems agreed well with the measurements within 1 mm. The X-ray contamination at 15 cm depth was 0.5%/0.6% for EGSnrc/ VirtuaLinac, compared with the measured value of 0.8%. The output was -2.4%/-3.2% difference for EGSnrc/VirtuaLinac when compared with measurement. When radiation from all six directions are combined on a cylindrical phantom, the ratio of output at the surface from 6 directions to a single direction, defined as B-factor, is 3.1 from both MC systems and the measurement. The dmax also shifted toward the surface at 0.15 cm. The X-ray contamination of all fields was 1.2 % and 1.3% for EGSnrc and VirtuaLinac, compared with 2% in the measurements. For the investigation part, no material shows acceptable profile flatness (±10% within the central 160 cm) at 250 cm SSD. At 300 cm SSD, Au (0.1 mm), Ag (0.25 mm), and Cu (0.45 mm) are acceptable. Zn (0.45 mm) requires 325 cm SSD to meet the requirement. For these 4 configurations, the dmax is 0.87-0.99 cm, similar to dual beam (0.97 cm); R50 is 1.85-1.91 cm, compared with dual beam of 2.06 cm; the output ranges from 0.025-0.029, lower than the dual beam (0.080). The composite fields for 4 configurations, the dmax is 0.1 cm, compared with dual beam (0.16 cm). The surface dose is 97%, similar to dual beam (96%). B-factor is 3.3-3.4, compared with dual beam (3.1). The maximum x-ray contamination is 3%, slightly higher than dual beam (2%).

Conclusions: The results from both Monte Carlo systems in general agree well with the measurement for the validation part. Furthermore, MC results suggest the stand-up TSI technique can be implemented using a single beam if the customized filter is used. In addition to those measurable quantities, the Monte Carlo simulation can provide further information such as the full dose distribution of the patient phantom, thus become the foundation for investigations for future technique optimizations.

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Tseng, Wen-Chih (2019). Validation of the Stand-up Technique for Total Skin Irradiation by Monte Carlo Simulation. Master's thesis, Duke University. Retrieved from https://hdl.handle.net/10161/18854.

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