Journal Article Response to “Comment on ‘Objective assessment in digital images of skin erythema

hiroaki, Matsubara  ,  Karasawa, Kumiko  ,  Matsufuji, Naruhiro  ,  Tsuji, Hiroshi  ,  Yamamoto, Naoyoshi  ,  Nakajima, Mio  ,  Karube, Masataka  ,  Takahashi, Wataru

43 ( 5 )  , p.2689 , 2016-05 , American Association of Physicists in Medicine
We appreciate the interest of Carrara et al.1 in our recent articleon the assessment method in digital images of skin erythemacaused by radiotherapy.2 Matsubara et al. presented a linearrelationship between skin blood flow and skin dose, whileCarrara et al. suggest that their clinical data do not reproduceit even though their sample number of patients treated is tentimes larger than that of Matsubara et al. We would like tosuggest that the disagreement between the data by Carraraet al. and Matsubara et al. is qualitatively reasonable becauseit arises from neglecting the recovery process of damagedskin as is explained below.Skin erythema is physiologically caused by expansion ofcapillaries due to increased blood flow, which is a part of thebiological skin repair response. As long as the skin erythemais an acute toxicity, the skin recovers and the symptom usuallydisappears in several months. This indicates that skin bloodflowincreases owing to the repair process just after irradiation,but decreases owing to the recovery process from the damagedskin a few tens of days later.Matsubara et al. treated six patients who had the carbonbeam therapy with a total prescription dose of 50 Gy (RBE).2It should be noted that the prescription dose was delivered bya single fraction from four ports, indicating that the treatmentwas completed in one day. On the other hand, Carrara et al.treated 61 patients who had the 6/15 MV photon therapy witha total prescription dose of 50 Gy but with a 2 Gy fraction,indicating that the treatment was completed in 25 days. Therecovery process for skin erythema is assumed to work duringthe treatment. This would result in the reduction of skin bloodflow per accumulated skin dose, causing nonlinearity betweenskin blood flow and skin dose. The disagreement between thedata by Carrara et al. and Matsubara et al. can be qualitativelyunderstood by taking the recovery process into account. Thatis why Matsubara et al. selected only the cases of a singlefraction in order to avoid the recovery effect.As Carrara et al. suggest, however, we agree that thelinearity observed by Matsubara et al. is not based on physicalevidence. What Matsubara et al. discussed was just that thegross trend of the obtained data could be expressed by thelinear relationship with moderate accuracy, where severalvariations due to individuality of patient or time dependenceof skin erythema were assumed to be negligible as discussedin Ref. 2. There is no known numerical relationship betweenaccumulated skin dose and time dependence of skin erythema.The new data suggested by Carrara et al., therefore, areassumed to be important for further investigation of skinerythema.

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