Enhanced radiobiological effects at the distal end of a clinical proton beam: in vitro study.
In the clinic, the relative biological effectiveness (RBE) value of 1.1 has usually been used in relation to the whole depth of the spread-out Bragg-peak (SOBP) of proton beams. The aim of this study was to confirm the actual biological effect in the SOBP at the very distal end of clinical proton beams using an in vitro cell system. A human salivary gland tumor cell line, HSG, was irradiated with clinical proton beams (accelerated by 190 MeV/u) and examined at different depths in the distal part and the center of the SOBP. Surviving fractions were analyzed with the colony formation assay. Cell survival curves and the survival parameters were obtained by fitting with the linear-quadratic (LQ) model. The RBE at each depth of the proton SOBP compared with that for X-rays was calculated by the biological equivalent dose, and the biological dose distribution was calculated from the RBE and the absorbed dose at each position. Although the physical dose distribution was flat in the SOBP, the RBE values calculated by the equivalent dose were significantly higher (up to 1.56 times) at the distal end than at the center of the SOBP. Additionally, the range of the isoeffective dose was extended beyond the range of the SOBP (up to 4.1 mm). From a clinical point of view, this may cause unexpected side effects to normal tissues at the distal position of the beam. It is important that the beam design and treatment planning take into consideration the biological dose distribution.
Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan y_matsu@nirs.go.jp.
Yoshitaka
Y
Matsumoto
Advanced Medical Sciences, Graduate School of Medicine, Hokkaido University, 15-7 Kita, Kita-ku, Sapporo, Hokkaido, 060-8638, Japan Research Center for Innovative Oncology, National Cancer Center Hospital East, 6-5-1 Kashiwanoha, Kashiwa, Chiba 277-8577, Japan.
Taeko
T
Matsuura
Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.
Mami
M
Wada
Research Center for Innovative Oncology, National Cancer Center Hospital East, 6-5-1 Kashiwanoha, Kashiwa, Chiba 277-8577, Japan.
Yusuke
Y
Egashira
Research Center for Innovative Oncology, National Cancer Center Hospital East, 6-5-1 Kashiwanoha, Kashiwa, Chiba 277-8577, Japan.
Teiji
T
Nishio
Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.
Yoshiya
Y
Furusawa
D045744:Cell Line, Tumor; D002470:Cell Survival / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D061766:Proton Therapy; D011880:Radiotherapy Planning, Computer-Assisted; D012062:Relative Biological Effectiveness; D012468:Salivary Gland Neoplasms / Q000473:pathology / Q000532:radiotherapy; D014410:Tumor Stem Cell Assay
biological effectiveness; cell survival; distal end; proton beam; spread-out Bragg-peak