Lyndon Morley

SU-F-T-379: Dosimetric Impacts of Topical Agents and Dressings On Skin in Radiotherapy

PURPOSE This study investigated the superficial dose enhancement in the application of topical agents, clinical materials (thermal mask and bolus) and dressings in megavoltage photon beam radiotherapy. Different topical skin agents, clinical materials and dressings were evaluated and compared for their skin dosimetric impacts on the patients during radiation treatment. METHODS Superficial dose enhancements, or percentage doses with and without the studying materials, were measured using the 6 MV (Field size = 10×10 cm2 ) photon beams produced by a Varian TrueBeam linear accelerator. Twelve topical agents, five dressings (dry and wet conditions) and three clinical materials were studied. A solid water phantom was used with a MOSFET dose detector (TN-1002RD, Thomson and Nielsen Electronic, Ottawa, Ontario, Canada) located under a 1-mm PMMA slab to measure the skin dose. The distance between the radiation source and phantom surface was set to 100 cm in all measurements. The topical agents were distributed evenly with 1.5 mm thickness using our specific sample holder on the phantom surface. Extrapolations were made of 0.5 mm thickness for the agents to provide meaningful clinical value. RESULTS By comparing surface doses without studying materials, it is found that no topical agents had superficial dose enhancement higher than the clinical materials namely, thermoplastic mask (128%), 5-mm Superflab™ bolus (158%) and 10-mm Superflab™ bolus (171%) regarding the same thickness. Superficial dose enhancement of dry dressing did not exceed 110.5%, while wet dressings produced higher dose enhancements (133% for wet Mepilex lite and 141% for wet Mepilex Ag transfer). CONCLUSION It is concluded that the evaluated topical agents and dry dressings did not increase the superficial dose to a concerning level, even using excessive thickness in every fraction of radiation treatment. Wet dressings were found producing the bolus effect, but was still substantially less than applying a thin 5-mm bolus.

Technical challenges of sparing infrahyoid swallowing organs at risk in oropharynx squamous cell cancer treated with IMRT

This study reports clinical performance in the sparing of infrahyoid swallowing organs at risk (SWOARs) in oropharynx cancer intensity-modulated radiation therapy (IMRT) plans. Rates of meeting dose-volume planning goals are reported and compared with geometry-based estimates of what is achievable. This study also develops 3 measures of target-SWOAR geometry and tests their usefulness in providing geometry-based dose-volume planning goals. A total of 50 oropharynx cancer IMRT plans were reviewed. Success rates in meeting institutional dose-volume goals were determined for the glottic larynx (G), postcricoid pharynx (P), and esophagus (E). The following 3 measures of target-SWOAR geometry were investigated as methods of identifying geometry-based planning goals: presence of gross disease in neck levels 3 to 4, target-SWOAR overlap, and a 3-dimensional (3D) measure of target-SWOAR geometry. Locally advanced disease was predominant in this patient population with target volumes overlapping SWOARs in 68% to 98% of cases. Clinical rates of success in meeting dose-volume goals varied by SWOAR (16% to 82%) but compared well with estimated potentially achievable rates in most cases (14% average difference between clinical and potential). Cases grouped by the presence of levels 3 to 4 neck nodes or target-SWOAR overlap did not have significantly different SWOAR doses. Cases grouped using a 3D measure of target-SWOAR geometry differed significantly, providing useful geometry-based planning goals (e.g., mean Glottis dose <45Gy was achieved 19%, 44%, or 81% of the time in each of 3 groups). This study describes the technical challenge of sparing SWOARs and investigates several potential methods for grouping cases to assist with treatment plan evaluation. Quantifying the 3-D relationship between the targets and SWOARs is a promising way of approaching this complex problem. Data presented in this paper may be useful to evaluate treatment plans using objective geometry-based goals.