2054 - Effect of Immobilization Mask and Beam Energy on Dose Coverage to Small Joints in Treating Osteoarthritis with Low-Dose Radiation Therapy

Purpose/Objective(s): Osteoarthritis (OA) is the most common form of arthritis, affecting over 32 million Americans. Low dose radiation therapy (LDRT) has been used to treat symptomatic OA. For treating small joints, treatment energies recommended include orthovoltage in the range of 100 to 200 kV or 6 MV photons. The primary purpose of this study is to determine if a commercial treatment planning system (TPS) can accurately calculate dose coverage of small joints when 6 MV energy is used. Additionally, the use of a 2.5 MV beam in small joints and the effect of an immobilization mask on target dose coverage are studied.

Materials/Methods: The Monte Carlo code, BEAMnrc, was used to generate 6 MV and a 2.5 MV beams that are currently utilized for imaging. The simulated beam was then used to calculate dose distributions based on a previously treated patient plan for OA of a left hand using 6 MV parallel opposed AP/PA with a field size of 20 x 18 cm². The CT based dose calculations with- and without an immobilization mask using 6 MV and 2.5 MV beams were compared. The target dose coverages were analyzed using a dose-volume-histogram (DVH). Aquaplast masks were included in the body contour while the targeted hand contour does not include the mask for target dose coverage analysis. In the dose calculation without mask, the density of mask in the CT images was set to air for accurate comparison. The comparison dose calculated by the Monte Carlo (MC) simulations is regarded as the Gold-Standard.

Results: The target dose coverage based on the DVH analysis is shown in Table 1. In all cases D₅₀ is kept at 100% in the comparison. The dose calculated by the treatment planning system significantly underestimates target coverage D₉₀ by up to 13% of prescribed dose in treating small joints. D₉₀ was 95.1% and 93.0% of prescribed dose with and without a custom hand mask based on MC calculations, respectively. The custom immobilization mask provides adequate buildup for 6 MV beam to treat small joints. If a 2.5 MV beam is used no buildup is necessary.

Conclusion: The model-based treatment planning system significantly underestimates dose in small joints. The immobilization mask has a buildup effect which can eliminate the need of additional bolus. The advantage of using a 2.5 MV beam especially in treating small joints is limited. Table 1. List of calculated minimums, mean, and maximum percentage of prescribed doses to the target and the minimum prescribed dose received by 100% (D₁₀₀), 90% (D₉₀), 50% (D₅₀), and 10% (D₁₀) of the target volume based on DVH from treatment planning system and Monte Carlo (MC) systems with and without hand mask.