Monte Carlo modeling of radiation dose from radiation therapy with superficial x-rays.

Abstract

Introduction: Superficial x-rays (50-100 kVp) are used for treating non-melanoma skin cancer and intraoperative radiation therapy (IORT). At these energies, the photoelectric effect significantly increases absorbed dose to bone compared to soft tissue.

Methods: We used EGSnrc MC simulations to investigate bone dose enhancement during radiotherapy with the Sensus SRT-100 machine. Simulated beams were validated against laboratory measurements and compared to a commercial treatment planning system (SmART-ATP). Transmission simulations indirectly predicted bone dosage. Simulated beams were utilized as a mock treatment plan from a human cone-beam computed tomography (CBCT) image.

Results: EGSnrc accurately modeled the Sensus SRT-100 beams (100, 70, and 50 kVp) with a root mean square error (RMSE) of percentage depth dose ratios between Monte Carlo predictions and lab measurements of 1.66, 0.47, and 0.99, respectively. PDDs from simulations of a water phantom with bone slabs showed peak doses at water-bone interfaces relative to surface doses. At 0.3 cm depth bone slab, doses reached 410%, 490%, and 510% for 50, 70, and 100 kVp, respectively. At 1.5 cm depth, doses were 140%, 215%, and 270%. At 2.5 cm depth, peak doses were 74%, 130%, and 170% for 50, 70, and 100 kVp, respectively. A simulated treatment plan (4 Gy surface dose) using a CBCT of a human head predicted the dose to the skull to be around 20, 19, and 15 Gy for the 100, 70, and 50 kVp beams, respectively.

Conclusions: The study demonstrated EGSnrc's efficiency in conjunction with SmART-ATP for treatment planning. MC simulations effectively quantified bone dose enhancement during superficial x-ray radiotherapy, highlighting its importance in treatment planning and dose calculations. Clinicians should consider measuring bone depth prior to treatment to avoid excessive bone dose.