Dosimetric impact of radiofrequency tumor treating field arrays on dose on surface and at depth

Tumor Treating Fields (TTFields) have demonstrated a survival benefit in patients with glioblastoma and are increasingly being investigated for concurrent use with radiotherapy. However, published data regarding the dosimetric impact of TTFields transducer arrays during photon irradiation remain limited, particularly with respect to surface dose enhancement and attenuation at treatment depth. A clear, quantitative understanding of these perturbations is necessary to optimize clinical implementation.

This study aimed to directly quantify the physical dosimetric perturbations introduced by TTFields transducer arrays during megavoltage photon irradiation and to evaluate whether array repositioning mitigates surface dose enhancement and depth attenuation.

An experimental measurement was conducted using a solid-water slab phantom and a 6-MV Varian TrueBeam linear accelerator with both flattened and flattening-filter-free (FFF) beams. An Optune 3 × 3 ceramic transducer array was placed on the phantom surface. Dose at depths of 1.5, 5, and 10 cm was measured using a Farmer-type ionization chamber. Surface dose was assessed using silicon diodes, MOSFET detectors, and Gafchromic EBT3 film (referred to as radiochromic film in this paper). Measurements were performed with and without the array to establish baseline comparisons. To evaluate mitigation strategies, the array was shifted by one transducer diameter in four cardinal directions, and measurements were repeated. Dose ratios were calculated relative to baseline conditions without the array. Statistical significance was assessed where appropriate.

The presence of the TTFields array resulted in statistically significant attenuation at depth, with dose reductions of approximately 2%-4% up to 10 cm (p < 0.001). Surface dose increased substantially, with MOSFET and diode measurements demonstrating enhancement factors ranging from 1.81 to 1.87 relative to baseline. Radio film showed higher apparent enhancement, attributed to energy-dependent over-response in the high-gradient surface region. Repositioning the array reduced peak surface dose by up to 40%. Film-based dose profiles demonstrated a 24% reduction in localized hotspots following array shifts. Despite mitigation, residual surface dose remained elevated at approximately 1.6-2.3 times baseline values.

TTFields transducer arrays introduce clinically meaningful dosimetric perturbations, characterized by significant surface dose enhancement, and measurable attenuation at depth. While array repositioning reduces localized hotspots, it does not eliminate elevated scalp dose.These findings underscore the importance of treatment-specific dosimetric evaluation and careful surface dose assessment when delivering radiotherapy concurrently with TTFields.