Low-Dose Total Skin Electron Beam Therapy Using the Modified Stanford Technique: An In-Vivo Dosimetry Confirmation Study

Purpose: Low-dose total skin electron beam therapy (LD-TSEBT) has gained popularity in the treatment of Mycosis Fungoides (MF) due to reduced toxicity and similar response rates. The combination of LD-TSEBT with the modified Stanford technique (mST), a condensed TSEBT cycling approach [1], is an attractive and convenient approach for MF patients. Thorough in-vivo data confirming dosimetry with these modifications is limited. We retrospectively analyzed optically stimulated luminescence dosimeter (OSLD) data for patients receiving LD-TSEBT using the mST.

Methodology: Patients treated with LD-TSEBT (12Gy in 6 fractions, 3 fractions/week) using the mST (6 dual-fields with 6 positions treated each cycle) at Thomas Jefferson University Hospital (TJUH) were identified. Patient characteristics, Modified Severity-Weighted Tool (mSWAT), and Body Surface Area (BSA) involvement were recorded. NanoDot™ (Landauer Inc, Glenwood IL) OSLDs were used at ten anatomical sites: vertex, larynx, right shoulder, right forearm, right hip, umbilicus, left dorsal hand, left medial hand, right knee, and left dorsal foot. OSLD read out variability for TSEBT was estimated at 10% per TG-191 [2]. Measurements were aggregated and compared to the prescription dose, and per patient measurements compared to EORTC homogeneity tolerance criteria: +/-10% prescription [3].

Results: 36 patients treated at TJUH with LD-TSEBT using the mST between 2015 – 2021 were identified. Median patient age, height, weight and BMI were 67 years (range 29 - 89), 1.7 meters (range 1.5 – 2.0), 88 kg (range 48 - 114) and 24.5 kg/m2 (range 17 - 45), respectively. Median mSWAT score was 31.2 (range 0 - 130) and BSA involvement was 19 (range 2-95). 720 OSLD measurements in aggregate were recorded. The median of all OSLD measurements relative to prescription dose was 98.7% (range 42% - 146%, std 15%). Highest median doses were recorded at the umbilicus with 105% (range 88% - 113%, std 7%), and lowest median doses at the left dorsal hand with 79% (range 64% - 97%, std 9%) due to high setup variability. Median OSLD measurement per patient was 99%, (range 83% - 114%, std 8%). Among all patients when accounting for left dorsal hand and OSLD read out variability (+/- 10%), 60% received prescription dose at all measurement sites. When evaluating for EORTC homogeneity tolerance criteria (+/-10%), 89% of patients were within tolerance at all measurement sites.

Conclusions: LD-TSEBT using the mST delivers accurate dose with most patients meeting EORTC tolerance criteria. Certain measurement points are more susceptible to variability due to patient and OSLD setup factors, and need to be considered. OSLDs should be utilized in patients undergoing LD-TSEBT with the mST to ensure adequate dosing given the truncated cycling and fractionation regimen.

References: 1. Song, J Am Acad Dermatol, 2020.
2. Kry, Med Phys, 2020.
3. Jones, J Am Acad Dermatol, 2002.