Dosimetric Study of the Small Field Technique in Electron Beam Therapy (SFEBT)

Abstract

Purpose: Electron Beam Therapy (EBT) with small field techniques is primarily used for treating superficial and small lesions. Due to the limited penetration depth of electron beams, they are well-suited for treating skin and shallow tissue tumors. Small fields for electron beams require modifying standard large electron cones with lead cut-outs to achieve appropriate small fields for superficial lesions. This study thoroughly investigates absolute dose changes, the impact of reduced field size, and changes in PDD, dmax, and isodose curve constriction when using shorter SSDs instead of the standard 100 cm SSD.

Materials and Methods: Focusing on superficial lesions, we used a cone 10 × 10 cm² with 6 MeV energy, reducing the field size to cone 6 × 6 cm², cone 4 × 6 cm², and cone 2 × 2 cm²using lead cut-outs. We irradiated films and measured absolute dose with a PTW 34045 (Advanced Markus) parallel plate ionization chamber and a Farmer chamber at various SSDs (50 cm, 80 cm, 100 cm, 110 cm, and 120 cm). We compared the absolute output at different dmax points and determined changes in field size and isodose curve constriction/expansion to provide correction factors for absolute output dose for patient treatments at different SSDs.

Results: For a 6 MeV electron cone at SSD 100 cm with a cone 10 × 10 cm² field size, the maximum dose depth (dmax) on the percentage depth dose (PDD) curve is 1.43 cm. We calibrated the absolute dose at this point to 1 cGy/MU. When the field size was reduced from cone 10 × 10 cm² to cone 2 × 2 cm², the output dropped from 1 cGy/MU to 0.75 cGy/MU, and the dmax shifted from 1.43 cm to 1.1 cm. To achieve a higher dose rate, reducing the SSD from 100 cm to 50 cm increased the output from 1 cGy/MU to 2.56 cGy/MU. When reducing both the field size (cone 10 × 10 cm² to cone 2 × 2 cm²) and the SSD (100 cm to 50 cm), the output increased from 1 cGy/MU to 1.92 cGy/MU. Additionally, at SSD 100 cm, reducing the field size from cone 10 × 10 cm² to cone 2 × 2 cm² caused the 90% isodose curve to constrict while the low-dose isodose curve expanded. These parameters are crucial for calculating the patient’s treatment monitor units.

Conclusion: For treating superficial and small tumors with electron beams, the dose calculation capability of computerized treatment planning systems (TPS) is limited, especially when using large cone lead cut-outs and shorter SSDs than the standard 100 cm. Such specialized treatments require empirical measurements to determine the patient’s treatment monitor units. This study provides an accurate and effective solution for electron beam therapy with small fields and short SSDs.