The impact of dose delivery time on biological effectiveness in proton irradiation with various biological parameters

• Published in: Medical physics (Lancaster) 2020-09, Vol.47 (9), p.4644-4655
• Main Authors: Kasamatsu, Koki, Matsuura, Taeko, Tanaka, Sodai, Takao, Seishin, Miyamoto, Naoki, Nam, Jin‐Min, Shirato, Hiroki, Shimizu, Shinichi, Umegaki, Kikuo
• Format: Article
• Language: English
• Subjects: Dose-Response Relationship, Radiation; Linear Energy Transfer; LQ model; Male; Phantoms, Imaging; Proton Therapy; Protons; Radiation, Ionizing; RBE; Relative Biological Effectiveness; SLD repair
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1002/mp.14381

Purpose The purpose of this study is to evaluate the sublethal damage (SLD) repair effect in prolonged proton irradiation using the biophysical model with various cell‐specific parameters of (α/β)x and T1/2 (repair half time). At present, most of the model‐based studies on protons have focused on acute radiation, neglecting the reduction in biological effectiveness due to SLD repair during the delivery of radiation. Nevertheless, the dose‐rate dependency of biological effectiveness may become more important as advanced treatment techniques, such as hypofractionation and respiratory gating, come into clinical practice, as these techniques sometimes require long treatment times. Also, while previous research using the biophysical model revealed a large repair effect with a high physical dose, the dependence of the repair effect on cell‐specific parameters has not been evaluated systematically. Methods Biological dose [relative biological effectiveness (RBE) × physical dose] calculation with repair included was carried out using the linear energy transfer (LET)‐dependent linear‐quadratic (LQ) model combined with the theory of dual radiation action (TDRA). First, we extended the dose protraction factor in the LQ model for the arbitrary number of different LET proton irradiations delivered sequentially with arbitrary time lags, referring to the TDRA. Using the LQ model, the decrease in biological dose due to SLD repair was systematically evaluated for spread‐out Bragg peak (SOBP) irradiation in a water phantom with the possible ranges of both (α/β)x and repair parameters ((α/β)x = 1–15 Gy, T1/2 = 0–90 min). Then, to consider more realistic irradiation conditions, clinical cases of prostate, liver, and lung tumors were examined with the cell‐specific parameters for each tumor obtained from the literature. Biological D99% and biological dose homogeneity coefficient (HC) were calculated for the clinical target volumes (CTVs), assuming dose‐rate structures with a total irradiation time of 0–60 min. Results The differences in the cell‐specific parameters resulted in considerable variation in the repair effect. The biological dose reduction found at the center of the SOBP with 30 min of continuous irradiation varied from 1.13% to 14.4% with a T1/2 range of 1–90 min when (α/β)x is fixed as 10 Gy. It varied from 2.3% to 6.8% with an (α/β)x range of 1–15 Gy for a fixed value of T1/2 = 30 min. The decrease in biological D99% per 10 min was 2.6, 1.2, and 3.0% for the pros