Moderate application of topical therapies directly prior to radiotherapy (RT) treatment sessions may be safe and might exhibit minimal effects on delivered treatment dose, investigators report.
The researchers found that although avoiding topical agents prior to radiation treatments was widespread, preclinical data indicate that there is no difference in the radiation dose on the skin with or without a 1- to 2-mm-thick layer of topical agent.
In an online survey, which queried 133 patients and 108 clinicians in relation to existing practices surrounding topical agent use, 83.4% of patients were informed to discontinue application of topical therapies directly before RT treatment sessions. In addition, 54.1% were told to clean and remove any enduring topical agents before treatment. Among clinicians, 91.4% reported to have received or given advice to stop application of topicals before obtaining RT treatment, Brian C. Baumann, MD, and his associates reported in.
However, in a preclinical study using a mouse- and tissue-equivalent phantom model to determine the dosimetric effects of concomitant topical agent use with daily RT treatments, Dr. Baumann, of Washington University in St. Louis, and his colleagues determined that when a topical agent was given prior to RT at a thickness below 2 mm, no changes were seen in radiation treatment dose, irrespective of depth, photon and electron energy level, or beam angle.
However, a proportionally thicker covering (3 mm or more) caused a bolus effect at the surface, which resulted in electron dose increases of 2%-5%, and photon increases of 15%-35%, when compared with controls.
Investigators measured radiation dose and photon beam intensity at various surface depths after applying two commonly used topical therapies, a healing ointment of 41% petrolatum or silver sulfadiazine cream 1%. The agents were administered using a thick (1-2 mm) application and proportionally thicker (3 mm or more) covering.
“Thin or moderately applied topical agents, even if applied just before RT, may have minimal influence on skin dose regardless of beam energy or beam incidence,” the investigators wrote. However, the findings do suggest that “applying very thick amounts of a topical agent before RT may increase the surface dose and should be avoided,” they said.
The authors reported that the study was funded by development funds from the University of Pennsylvania, Philadelphia. One of the coauthors, James M. Metz, MD, disclosed service on advisory boards for Ion Beam Applications and Varian Medical Systems for proton therapy; however, these roles were not relevant to this study.
SOURCE: Baumann BC et al. JAMA Oncol. 2018 Oct 18. doi: 10.1001/jamaoncol.2018.4292.