Department of Radiation Oncology, University of Iowa, 200 Hawkins Drive, Iowa City, Iowa 52242, USA.
Med Phys. 2010 Jun;37(6):2761-9. doi: 10.1118/1.3426307.
Increased use of cone beam CT guidance in image guided radiotherapy has prompted the inclusion of the imaging dose in treatment plans, thus using imaging beams to treat tumors. Sublethal radiation damage repair during tau(d), the time between imaging and treatment, could reduce the effectiveness of the imaging dose, resulting in tumor underdosage. The theoretical magnitude of this effect was quantified using radiobiological modeling.
The therapeutic effective dose (TED), which, if delivered using only therapeutic beams, would result in the same tumor cell survival as for both the imaging and therapeutic beams, was derived using the generalized linear-quadratic model. The correction factor P(d) by which therapeutic dose can be scaled to compensate for sublethal damage repair was also derived. TED and P(d) are dependent on alpha/beta, sublethal damage repair half-time (T(r)), imaging dose (D(I)) and dose rate (D(I)), therapeutic dose (D(T)) and dose rate (D(T)), and tau(d). TED and P(d) were calculated as a function of tau(d), and each parameter was varied independently while holding the remaining parameters at their reference values. The reference values were based on prostate cancer cells and were D(p) = D(I)+D(T) = 1.8 Gy, D(I)/D(p) = 5%, D(I) = 0.33 Gy/min, D(T) = 1.0 Gy/min, alpha/beta = 3.1 Gy, T(r) = 16 min, and tau(d) = 0 min. Estimates of the expected values of TED and P(d), (TED) and (P(d)), were calculated using tau(d) and D(T) distributions from a few thousand prostate treatment fractions.
For a typical tau(d) value of 5.0 min and all other parameters set to their reference values, TED was 0.5% lower than the prescription dose D(p). For tau(d) = 20 min and all other parameters at reference conditions, TED dropped by 5% relative to D(p) when D(I)/D(p) was 20% and by 2% relative to D(p) when alpha/beta = 1 Gy or T(r) = 5 min. TED/D(p) varied more with D(T) than D(I) when tau(d) < or = 20 min, varying by up to 1% over 0.05 < or = D(T) < or = 10 Gy/min and by less than 0.1% over 0.05 < or = D(I) < or = 2.0 Gy/min. Under the reference conditions, (TED) was lower than D(p) by 0.5%. For the extreme D(I)/D(p) = 20% and all other parameters at their reference values, setting alpha/beta = 1 Gy resulted in (TED) dropping below D(p) by 2.5% and setting T(r) = 5 min resulted in (TED) dropping below D(p) by 4%. For tumors with a T(r) of 16 min or greater and alpha/beta of 11 Gy, (TED) dropped below D(p) by 0.2% or less.
For prostate tumors receiving a reasonable percentage of 5% of their total dose from imaging beams, the theoretical drop in (TED) relative to D(p) was 0.5%. This loss could be accounted for during treatment planning by scaling the therapeutic dose by the expected sublethal damage repair factor (P(d)). For nonprostate tumors with alpha/beta values of 11 Gy, the theoretical drop in (TED) relative to the reference TED was low at 0.2%.
随着锥形束 CT 在图像引导放疗中的应用日益增多，治疗计划中纳入了成像剂量，从而利用成像束治疗肿瘤。在成像和治疗之间的 tau(d)时间内，亚致死辐射损伤的修复可能会降低成像剂量的效果，导致肿瘤剂量不足。使用放射生物学建模来量化这种效应的理论幅度。
通过广义线性二次模型，得出如果仅使用治疗束进行治疗，将导致与治疗和成像束相同的肿瘤细胞存活的治疗有效剂量（TED）。还推导了治疗剂量可以缩放以补偿亚致死损伤修复的校正因子 P(d)。TED 和 P(d)取决于 alpha/beta、亚致死损伤修复半衰期（T(r)）、成像剂量（D(I)）和剂量率（D(I)）、治疗剂量（D(T)）和剂量率（D(T)）以及 tau(d)。TED 和 P(d)作为 tau(d)的函数进行计算，在保持其余参数为参考值的情况下，独立改变每个参数。参考值基于前列腺癌细胞，D(p) = D(I)+D(T) = 1.8 Gy，D(I)/D(p) = 5%，D(I) = 0.33 Gy/min，D(T) = 1.0 Gy/min，alpha/beta = 3.1 Gy，T(r) = 16 min，tau(d) = 0 min。使用来自数千个前列腺治疗分数的 tau(d)和 D(T)分布，计算 TED 和 P(d)的预期值（TED）和（P(d)）的估计值。
对于典型的 tau(d)值为 5.0 min 且所有其他参数均设置为参考值的情况，TED 比处方剂量 D(p)低 0.5%。当 tau(d) = 20 min 且所有其他参数均处于参考条件时，当 D(I)/D(p)为 20%且当 alpha/beta = 1 Gy 或 T(r) = 5 min 时，TED 相对于 D(p)下降了 5%。当 tau(d) <或= 20 min 时，TED 相对于 D(I)的变化更多地取决于 D(T)，在 0.05 <或= D(T) <或= 10 Gy/min 时变化高达 1%，在 0.05 <或= D(I) <或= 2.0 Gy/min 时变化小于 0.1%。在参考条件下，（TED）比 D(p)低 0.5%。对于极端 D(I)/D(p) = 20%和所有其他参数均处于参考值的情况，将 alpha/beta 设置为 1 Gy 会导致（TED）比 D(p)低 2.5%，将 T(r)设置为 5 min 会导致（TED）比 D(p)低 4%。对于 T(r)为 16 min 或更大且 alpha/beta 为 11 Gy 的肿瘤，（TED）比 D(p)低 0.2%或更低。
对于接受总剂量的合理百分比为 5%的成像束的前列腺肿瘤，TED 相对于 D(p)的理论下降幅度为 0.5%。在治疗计划中，可以通过将治疗剂量乘以预期的亚致死损伤修复因子（P(d)）来弥补这一损失。对于 alpha/beta 值为 11 Gy 的非前列腺肿瘤，TED 相对于参考 TED 的理论下降幅度很小，为 0.2%。
