Dosimetry of large electron beam fields and treatment protocol for mycosis fungoides using the total skin irradiation technique

Abstract

Introduction: Total Skin Electron Beam Therapy (TSEBT) is a technique used in the treatment of superficial skin diseases, such as cutaneous lymphomas. Although it has demonstrated its effectiveness through several clinical studies of the advanced stages of mycosis fungoides, it is not available in most cancer centers in Mexico, because it requires large electron fields in order to cover the entire body. Objective: In order to obtain large electron fields for TSEBT, conventional treatment distances are not sufficient. This has led to the need for an electron beam with special features and calibration at a distance of 500 cm or more, as well as the need to create a treatment protocol to develop the TSEBT programs in Mexico. Materials and methods: The ELEKTA SYNERGY Linear Accelerator was selected with a 6 MeV energy at a Source Skin Distance (SSD) of 500 cm, in high dose rate mode (1,000 MU=100 Gy), using a cone of 40×40 cm2 and the gantry angled to 90◦, obtaining a large electron field of 200×100 cm2 useful area. Routine calibration was performed at a Zref=1.40 cm and an SSD of 100 cm with a 40×40 cm2 cone using a plane-parallel ionization chamber and electrometer (Scandotronix Wellhofer model PPC05 FOCUS) and a water phantom. The same procedure for the absolute dose rate determination was also performed in treatment conditions (500 cm). For the beam characterization in terms of Percentage Depth Dose (PDD) and beam profiles, Radiochromic Gafchromic® EBT2 film (RCF) was used, after being calibrated for electrons in a solid water phantom (Scanditronix Wellhöfer) at a 1.4 cm depth and Source Axis Distance (SAD) of 100 cm and 500 cm, in order to acquire the equation relating the dose response with optical density. PDD was obtained at 0, 1, 1.2, 1.4, 1.5, 2, 3, 4, 5, 6, 7, 8 and 9 cm depths in the phantom. Spatial dose distribution was obtained by placing 3×3 cm2 samples of RCF on an acrylic screen situated in front of the patient (for the purpose of obtaining maximum absorbed dose on the skin). Lastly, a treatment protocol was proposed. Results: An effective maximum dose depth (Zref) for electrons of 1.4±0.05 cm was obtained according to the relative spatial dose distribution and the percentage depth dose for a SSD of 500±0.5 cm, over an area of 200×100 cm2. Horizontal and vertical beam profiles were plotted, showing a horizontal symmetry of ±0.35%, horizontal flatness of ±3.62%, vertical symmetry of ±2.1%, and vertical flatness of ±14.2%. Conclusions: The resulting horizontal and vertical profiles enabled the electron beam symmetry and flatness to be assessed. PDD was analyzed up to a 9±0.05 cm, establishing the electron depth penetration to ensure treatment of the skin surface.