Effect of immobilization mask and beam energy on dose coverage to small joints in treating osteoarthritis with low-dose radiation therapy.

Purpose: Osteoarthritis (OA) is the most common form of arthritis, affecting over 32 million Americans. Low dose radiation therapy (LDRT) is being used to treat OA, including small joints. Treatment energies recommended include both orthovoltage and 6 MV photons. This study evaluates treatment plan accuracy of small joints using a commercial treatment planning system (TPS) when 6 MV is used. The effect of bolus and immobilization mask on target dose coverage and the use of 2.5 MV beams are also studied.

Methods: Monte Carlo calculated dose distributions were used to evaluate the dose calculation accuracy of small joints by the Varian Eclipse system (AAA V.16) for one patient. The CT based dose calculations with- and without an Aquaplast immobilization mask using 6 MV and 2.5 MV beams were compared. The target dose coverages were analyzed using a dose volume histogram (DVH). The effect of the Aquaplast mask on target dose coverage was evaluated. The doses calculated by Monte Carlo (MC) were regarded as the Gold Standard.

Results: The dose calculated by the Eclipse system significantly underestimated D target coverage by up to 21% of the prescribed dose. D was 92.9%, 91.7% and 89.6% of prescribed dose with 1 cm bolus, with a custom Aquaplast mask, and without a custom Aquaplast mask based on MC calculations, respectively, as compared to 86.8%, 83.2% and 73.9% when using Eclipse.

Conclusion: Eclipse calculations are less accurate, and underestimate D target dose by 7% even with bolus. When Monte Carlo is not available, prescribing to the D50 in Eclipse can lead to an actual D coverage of >90%. The immobilization mask provides adequate buildup for 6 MV beam. To obtain the full benefit of lower-energy beams the 2.5 MV-flattened beam provided the best dose coverage regardless of the use of a mask when treating small joints.