Department of Medical Physics and Engineering, Strahlenklinik, Klinikum Offenbach GmbH, 63069 Offenbach, Germany.
Department of Medical Physics, Attikon University Hospital, 12462 Athens, Greece
Med Phys. 2010 Jun;37(6):2572-86. doi: 10.1118/1.3426027.
The present report addresses the question of what could be the appropriate dose and dose rate for 125I and 103PD permanent seed implants for breast cancer as monotherapy for early stage breast cancer. This is addressed by employing a radiobiological methodology, which is based on the linear quadratic model, to identify a biologically effective dose (BED) to the prescription point of the brachytherapy implant, which would produce equivalent cell killing (or same cell survival) when compared to a specified external radiotherapy scheme.
In the present analysis, the tumor and normal tissue BED ratios of brachytherapy and external radiotherapy are examined for different combinations of tumor proliferation constant (K), alpha/beta ratios, initial dose rate (R0), and reference external radiotherapy scheme (50 or 60 Gy in 2 Gy per fraction). The results of the radiobiological analysis are compared against other reports and clinical protocols in order to examine possible opportunities of improvement.
The analysis indicates that physical doses of approximately 100-110 Gy delivered with an initial dose rate of around 0.05 Gyh(-1) and 78-80 Gy delivered at 0.135 Gyh(-1) for 125I and 103Pd permanent implants, respectively, are equivalent to 50 Gy external beam radiotherapy (EBRT) in 2 Gy per fraction. Similarly, for physical doses of approximately 115-127 Gy delivered with an initia dose rate of around 0.059 Gyh(-1) and 92 Gy delivered at 0.157 Gyh(-1) for 125I and 103Pd, respectively, are equivalent to 60 Gy EBRT in 2 Gy per fraction. It is shown that the initial dose rate required to produce isoeffective tumor response with 50 or 60 Gy EBRT in 2 Gy per fraction increases as the repopulation factor K increases, even though repopulation is also considered in EBRT. Also, the initial dose rate increases as the value of the alpha/beta ratio decreases. The impact of the different alpha/beta ratios on the ratio of the tumor BEDs is significantly large for both the 125I and 103Pd implants with the deviation between the alpha/beta = 10.0 Gy ratios and those using the 4.0 and 3.5 Gy values ranging between 18% and 22% in most of the cases.
For the cases of 125I and 103Pd, the equivalent physical doses to 50 Gy EBRT in 2 Gy per fraction are associated with an overdosage of the involved normal tissue in the range of 4%-16% and an underdosage by 10%-15% for a BED for normal tissue, using an alpha/beta value of 3.0 Gy (BEDNT,3 Gy) of 100 Gy. These values are lower by 10%-20% than the published value of 124 Gy for 125I and by about 13% when compared to the published isoeffective dose of 90 Gy for 103Pd. Similarly, the equivalent physical doses to 60 Gy EBRT in 2 Gy per fraction are associated with an overdosage of the involved normal tissue by 10%-20% and an underdosage by 4%-10% for BEDNT,3 Gy of 110 Gy.
本报告探讨了 125I 和 103Pd 永久性种子植入物作为早期乳腺癌单一疗法的合适剂量和剂量率。这是通过采用基于线性二次模型的放射生物学方法来确定放射性治疗植入物处方点的生物有效剂量（BED）来实现的，该剂量与指定的外部放射治疗方案相比，将产生等效的细胞杀伤（或相同的细胞存活）。
在目前的分析中，对于不同的肿瘤增殖常数（K）、α/β比值、初始剂量率（R0）和参考外部放射治疗方案（50 或 60 Gy，每次 2 Gy）组合，检查了近距离放射治疗和外部放射治疗的肿瘤和正常组织 BED 比值。将放射生物学分析的结果与其他报告和临床方案进行比较，以检查可能的改进机会。
分析表明，对于 125I 和 103Pd 永久性植入物，初始剂量率约为 0.05 Gyh(-1) 时，物理剂量约为 100-110 Gy，初始剂量率约为 0.135 Gyh(-1) 时，物理剂量约为 78-80 Gy，与 50 Gy 外部束放射治疗（EBRT），每次 2 Gy 等效。同样，对于 125I 和 103Pd 分别初始剂量率约为 0.059 Gyh(-1) 时，物理剂量约为 115-127 Gy，初始剂量率约为 0.157 Gyh(-1) 时，物理剂量约为 92 Gy，与 60 Gy EBRT，每次 2 Gy 等效。结果表明，与 50 或 60 Gy EBRT，每次 2 Gy 等效的肿瘤反应所需的初始剂量率随着再增殖因子 K 的增加而增加，尽管 EBRT 中也考虑了再增殖。此外，随着α/β比值的降低，初始剂量率也会增加。不同的α/β比值对肿瘤 BED 比值的影响在 125I 和 103Pd 植入物中都非常显著，α/β=10.0 Gy 比值与使用 4.0 和 3.5 Gy 值之间的偏差在大多数情况下在 18%至 22%之间。
对于 125I 和 103Pd 的情况，与 50 Gy EBRT，每次 2 Gy 等效的物理剂量与涉及的正常组织的超剂量相关，范围在 4%-16%之间，对于 BEDNT，3 Gy 的 100 Gy 的剂量不足 10%-15%。这些值比 125I 公布的 124 Gy 值低 10%-20%，比 103Pd 公布的 90 Gy 等效应剂量低约 13%。同样，与 60 Gy EBRT，每次 2 Gy 等效的物理剂量与涉及的正常组织的超剂量相关，范围在 10%-20%之间，对于 BEDNT，3 Gy 的 110 Gy 的剂量不足 4%-10%。
