Equilibrium temperature distributions in uniform phantoms for superficial microwave applicators: Implications for temperature-based standards of applicator adequacy

Abstract

Equilibrium temperature distributions are computed using measured SAR distributions for five different superficial microwave (915 MHz) applicators. We assume a model with uniform conduction and blood flow. A Green's function approach is used to calculate equilibrium solutions which identically obey boundary conditions at the surface of the phantom and at infinite depth. The equilibrium solutions are categorized by surface temperature (T_S), maximum allowed temperature (T_M), and by a parameter (referred to as the diffusion length, λ) which characterizes the contributions of thermal conduction relative to blood flow. The computed equilibrium temperature distribution at depths of 2 and 3 cm is strongly dependent on λ and on T_M. It is not strongly dependent on surface temperature for T_S below 35°C. In previous work we compared the SAR distribution with local control of 53 superficial tumours with over 1 year of follow-up. As an alternative to an SAR-based description of applicator adequacy we consider a temperature-based standard. Tumours are categorized by the minimum value of λ that would allow full coverage of the tumour volume by the 42°C contour, assuming a T_M of 47.5°C and a T_S of 35°C. Eighteen of 27 lesions (67%) were locally controlled for λ > 1 cm. The local control in 26 lesions with λ ° 1 cm was 31 % (p = 0.016). The lesions with the best results were those with both good coverage in theory (λ >1 cm) and with all monitored catheter tracks achieving at least one session with 30 min at or above 43°C. We found that the temperature-based standard of applicator adequacy was not independent of an SAR-based standard, and in this cohort of patients either a minimum SAR criterion or a maximum diffusion length criterion would serve equally well as a screen for inappropriate applicators.

Key Words:

• External hyperthermia
• microwave applicators
• external radiation therapy
• SAR measurements