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While applying thick amounts of a topical agent just before radiation therapy (RT) increased radiation exposure to the skin, thin or moderate amounts had minimal influence, a new study found.

But a survey conducted as part of the study found that the vast majority of patients (83.4%) and physicians (91.4%) reported either receiving or giving advice to avoid topical agents such as petroleum-based jelly before undergoing RT, according to researchers led by Brian Baumann, MD, of Washington University in St. Louis.

"The results of this multidisciplinary study suggest that the current practice of advising patients to avoid applying topical agents for several hours before RT does not rest on sound science, with the possible exception of patients who apply very copious amounts of topical agents immediately before treatment," the authors wrote in . "We suggest that patients be advised that applying thin or moderate amounts of a topical agent, even right before RT, is acceptable and unlikely to increase toxic effects to the skin."

Using either en face 6- or 15-megavoltage (MV) photons, the researchers found no difference in the delivered dose of radiation at the skin surface compared with the 2-cm depth with a 1- to 2-mm application of petroleum-based jelly or silver sulfadiazine cream. There was also no difference at the skin or at 2-cm depth when using various beam angles, except for a 6% increase at an incident angle of 60 degrees with the use of the silver sulfadiazine cream.

When topical agents were applied at a thickness of 3 mm or more there was an increased dose at the surface skin. At en face 6-MV dose, the surface dose of radiation was 1.05 Gy with petroleum and 1.02 Gy with the sulfadiazine cream, compared with 0.88 Gy without a topical.

Similarly, at en face 15-MV, the thicker application of ointment resulted in surface doses of 0.70 Gy for petroleum-based agents and 0.60 Gy for silver sulfadiazine cream, compared with 0.52 Gy without an agent. With this thicker application (≥3 mm), the use of 6- and 9-MeV electrons also resulted in small increases of between 2% to 5% in surface dose.

However, application in these higher amounts is unlikely to occur, the authors noted. The thicknesses studied were derived from measurements (1-2 mm) of petroleum-based jelly used by 20 cancer patients with severe dermatitis (≥grade 2), with none using more than 2 mm. When shown a 3-mm thickness of a topical agent, all denied having ever used such an amount.

"It is somewhat surprising that there have been no studies assessing dose alterations resulting from cream or ointment use because this is an issue that affects not only patients with breast cancer but also most patients undergoing radiotherapy," Baumann and colleagues wrote.

, Simon Brown, MD, of Oregon Health & Science University in Portland, and Chelsea Pinnix, MD, PhD, of MD Anderson Cancer Center in Houston, wrote that these results reflect the evolution of RT since its earliest use, but are not the first to evaluate the effect of topical agents on surface dose.

A also showed that surface doses of radiation were only increased when topical agents were applied at an above-average thickness.

Taken together, these studies suggest "that the common recommendation to avoid using topical agents immediately before RT is not applicable in many situations," they wrote.

"This study is a welcome reminder to challenge norms despite their popularity and to be aware of the evidence (or lack thereof) when recommending interventions," said Brown and Pinnix, adding that they applaud the efforts of Baumann's group "to debunk a prevalent myth in radiation oncology."

The current study was conducted in three parts. First, the researchers conducted a 24-question online survey to 133 patients who received RT for cancer and to 105 clinicians who were involved in the management of patient skin care during RT. Most of the patients had breast cancer (n=92).

Next, at the surface and at 2-cm depth in a tissue-equivalent phantom, the researchers measured the dosimetric effect of delivering 200 monitor units both without topical agents and with either a non-metallic (petroleum) or metallic (silver sulfadiazine) topical ointment.

Finally, the researchers used 24 C57BL/6 mice to evaluate if petroleum-based topical agents altered skin radiation dose. They measured the effect of 2-Gy or 15-Gy dose on DNA damage with phosphorylated histone (γ-H2AX) immunofluorescent staining and on apoptosis with terminal deoxynucleotidyl transferase neck end labeling (TUNEL) assay, but found no differences in γ-H2AX-positive foci or in TUNEL staining regardless of topical agent type or thickness.