Technique for commissioning lying-on-the-floor total skin electron therapy for emergency palliation.
Dziemianowicz E, Laugeman E, and Wen N. Technique for commissioning lying-on-the-floor total skin electron therapy for emergency palliation. Med Phys 2018; 45(6):e169.
Purpose: A technique for total skin electron therapy (TSET) with the patient lying on the floor (LOTF) at extended source to skin distance (SSD) is presented. Methods: The LOTF TSET technique employs two parallel 40 x 40 cm2 AP 6 MeV electron beams separated by a gap. SSD was maximized by lowering a stretcher to its lowest position, and subtracting the anticipated patient separation. Beam quality at extended (175 cm) SSD was measured using a parallel plate chamber at various depths in solid water. Output was measured using the same set-up, and verified with OSLDs. The beam profile was measured in air using Gafchromic film. The gap between the two parallel AP fields was calculated to maximize uniformity. Dose uniformity near the gap was measured using film wrapped around a cylindrical phantom. The same set-up was used to evaluate dose to lateral (oblique) surfaces. Results: There was less than 1 mm difference in beam quality parameters (dmax, R90, R50 and Rp) between LOTF TSET geometry (6 MeV, 40 x 40 cm2 field size, at 175 cm SSD) and standard geometry (6 MeV, 10 x 10 cm2 field size, at 100 cm SSD). Using LOTF TSET geometry, the dose rates at dmax and the surface were 0.286 and 0.215 cGy/MU respectively. These results agreed within 2% of OSLD measurements. At 175 cm SSD, the in-air dose profile FWHM was 78 cm, while the light field width was 70 cm. Dose at the light field edges was approximately 67% of the dose at central axis (CAX). A 9 cm gap (at 175 cm SSD) was determined to maximize uniformity across the match line. Phantom film measurements verified dose across the gap region to be within 8% of dose at CAX. Lateral surface dose enhancement greater than 8% was observed. Conclusion: This technique yields acceptable uniformity and can be commissioned within an emergent timeframe to treat patients unable to stand.