再增殖动力学对永久性植入物等效效应剂量的影响。

Klinik für Strahlentherapie und Radiologische Onkologie, Technische Universität München.

Strahlenther Onkol. 1994 Oct;170(10):602-7.

PURPOSE

Determination of isoeffect doses for permanent implants with different half-life times.

METHOD

To calculate the isoeffective doses a model is used which describes the number of clonogenic tumor cells as a function of time. The effective irradiation time for permanent implants depends on the half-life T1/2 of the used isotope. During the irradiation tumor cells may repopulate, an effect which is well known in fractionated radiotherapy. The longer the radiation treatment lasts the more dose will be needed to kill these repopulating tumor cells. A differential equation has been constructed where cell kill is determined by an alpha-beta term and cell proliferation by a constant tumor cell doubling time Tpot. The solution of the differential equation gives the number of clonogenic tumor cells as a function of time.

RESULTS

For our model an analytic solution has been found. The number of repopulating tumor cells shows a minimum if the dose rate is sufficiently high. Otherwise cell kill by radiation is overcompensated by growing tumor cells. The minimum value Nmin of tumor cells depends on the initial cell number N0, the total dose Dtot, the half-life T1/2 of the implant, the half-life Tr for repair of the sublethal damage, the alpha and beta values of the tumor cells, and the effective clonogen doubling time Tpot of the tumor. Assuming that the tumor is cured if no clonogenic cell survives, the tumor control probability (TCP) is determined by the Poisson statistics TCP = exp(-Nmin). Isoeffective doses are doses with constant TCP. Isoeffective doses Dtot have been calculated for different Tpot as a function of the half-life T1/2 of the isotope.

CONCLUSIONS

The model allows the calculation of isoeffective doses for permanent implants. Care must be taken to use the results in clinical practice unless all of the radiobiological parameters are known for a specific tumor.

目的

确定不同半衰期的永久性植入物的等效剂量。

方法

为计算等效剂量，使用了一个模型，该模型将克隆源性肿瘤细胞数量描述为时间的函数。永久性植入物的有效照射时间取决于所用同位素的半衰期T1/2。在照射期间，肿瘤细胞可能会再增殖，这是分割放疗中众所周知的一种效应。放射治疗持续时间越长，杀死这些再增殖肿瘤细胞所需的剂量就越大。构建了一个微分方程，其中细胞杀伤由α-β项决定，细胞增殖由恒定的肿瘤细胞倍增时间Tpot决定。微分方程的解给出了克隆源性肿瘤细胞数量作为时间的函数。

结果

对于我们的模型，已找到解析解。如果剂量率足够高，再增殖肿瘤细胞的数量会显示出最小值。否则，肿瘤细胞的生长会过度补偿辐射造成的细胞杀伤。肿瘤细胞的最小值Nmin取决于初始细胞数N0、总剂量Dtot、植入物的半衰期T1/2、亚致死损伤修复的半衰期Tr、肿瘤细胞的α和β值以及肿瘤的有效克隆原倍增时间Tpot。假设如果没有克隆源性细胞存活则肿瘤被治愈，肿瘤控制概率（TCP）由泊松统计确定，TCP = exp(-Nmin)。等效剂量是具有恒定TCP的剂量。已针对不同的Tpot计算了等效剂量Dtot作为同位素半衰期T1/2的函数。