End-to-end test and MOSFET in vivo skin dosimetry for 192Ir high-dose-rate brachytherapy of chronic psoriasis

Purpose
This study was performed using end-to-end testing and real-time in vivo skin dose measurements, using metal oxide semiconductor field effect transistor (MOSFET) dosimeters on our first chronic psoriasis patient treated with iridium-192 (¹⁹²Ir) high-dose-rate (HDR) brachytherapy (BT).

Material and methods
Treatment delivery was planned with the prescription dose of 1.8 Gy to a 3 mm depth for 12 fractions, using our custom-fabricated surface mold and Varian soft catheters. The optimal technique to provide an adequate and acceptable skin dose as well as its feasibility were evaluated by an end-to-end exercise using a perspex finger phantom. The accuracy and reliability of MOSFET dose measurement was explored with a thermoluminescence dosimetry (TLD) before being used in vivo to monitor skin doses during treatment delivery for each BT fraction.

Results
Using custom-made surface mold (2.4 mm Med-Tec thermoplastic mask for hand fixation and 5 applicators attached to each finger for dose delivery), the optimal skin dose on the phantom was obtained without the need for additional bolus to increase thickness of applicator. We acquired mean skin doses at different skin depths from various dose-volume parameters of no-bolus and 3 mm-added bolus plans. They were 125% and 110% (1 mm), 120% and 108% (2 mm), and 114% and 106% (3 mm), respectively. There was excellent agreement between MOSFET and TLD for ¹⁹²Ir HDR-BT within ±3% (mean 2.65%, SD = 2.05%). With no energy correction, MOSFET overestimated the Acuros BV surface doses by up to 7% in the phantom study and in the clinical case.

Conclusions
We demonstrated achievable HDR-BT for our first case of nail bed psoriasis. The end-to-end exercise was an efficient methodology to evaluate new feasibility for this technique. Real-time dose monitoring using MOSFET was an effective and reliable tool to ensure treatment quality and patient safety.