粒子植入术后肿瘤缩小时不同布源方式对剂量的影响

目的 假定¹²⁵I粒子植入术后肿瘤每月以20%速度退缩时，研究等间距与周边密集、中间稀疏2种布源方式对剂量的影响。方法 利用计算机三维治疗计划系统（TPS）勾画4 cm×5 cm的圆柱形肿瘤，根据布源方式不同分为等间距组和周边密集、中间稀疏组，以处方剂量125 Gy载入放射性活度1.85×10⁷Bq ¹²⁵I粒子行术前计划。假设¹²⁵I粒子植入肿瘤后肿瘤高不变，直径每月以20%的速度缩小，粒子随肿瘤均匀向心性集中，分别计算¹²⁵I粒子植入术后0、1、2、3个月时90%靶体积吸收剂量（D₉₀）、90%处方剂量覆盖体积占靶体积的百分比（V₉₀）、150%处方剂量覆盖体积占靶体积的百分比（V₁₅₀）。结果 等间距组载入粒子85颗，植入0、1、2、3个月后的D₉₀分别为126.20、130.41、133.82和139.48 Gy，V₉₀分别为97.0%、98.1%、99.3%和100%，V₁₅₀分别为70.2%、69.9%、71.1%和71.5%；周边密集、中间稀疏组植入粒子75颗，植入0、1、2、3个月后的D₉₀分别为126.46、125.41、123.50和128.83 Gy，V₉₀分别为95.2%、95.7%、94.9%和97.6%，V₁₅₀分别为52.8%、60.4%、62.7%和59.3%。结论 当肿瘤直径每个月以20%速度退缩时，粒子等间距分布时肿瘤预期吸收剂量逐渐增加；周边密集、中间稀疏的布源方式肿瘤预期吸收剂量不变，且高剂量区范围明显小于等间距布源。

The dosimetric effect of different source patterns in case of tumor shrinkage after 125I seed implantation

Objective To study the dosimetric effect of two source patterns, including equal spacing and peripheral dense intermediate sparse by assuming a tumor shrinking speed of 20% per month after ¹²⁵I seed implantation.Methods A virtual cylindrical tumor with 4 cm in height and 5 cm in diameter was contoured on a three-dimensional treatment planning system (TPS). Two groups of preoperative plans were made with 1.85×10⁷ Bq ¹²⁵I seeds using two source patterns respectively. It was assumed that the tumor height was unchanged, while the diameter of tumor would decrease at a speed of 20% per month, and the locations of seeds would concentrate towards the tumor core. The 90% target volume dose (D₉₀), the ratio of 90% isodose volume over the target volume (V₉₀), and the ratio of 150% isodose volume over the target volume (V₁₅₀) were calculated at 0, 1, 2, 3 months after ¹²⁵I implantation respectively.Results In equal spacing group, 85 seeds were implanted. The values of D₉₀ were 126.20, 130.41, 133.82 and 139.48 Gy after 0, 1, 2 and 3 months respectively. The values of V₉₀ were 97.0%, 98.1%, 99.3% and 100%, while those of V₁₅₀ were 70.2%, 69.9%, 71.1% and 71.5%. The dense in-periphery and sparse-in the middle group was loaded with 75 seeds. The D₉₀ values were 126.46, 125.41, 123.50 and 128.83 Gy, the V₉₀ 95.2%, 95.7%, 94.9% and 97.6%, and the V₁₅₀ 52.8%, 60.4%, 62.7% and 59.3% after 0, 1, 2 and 3 months, respectiviely.Conclusions When the tumor diameter reduces at a rate of 20% per month after ¹²⁵I seed implantation, the expected tumor dose absorption will gradually increase using the equal spacing sources pattern. However, the expected dose does not vary with source distribution of dense-in the-surrounding and sparse-in-middle, which also reduces high dose volume more than the equal spacing pattern.