目的:通过基于多场MLC的方法治疗大且大体积(≥8cm)不可切除的肿瘤,评估由于患者内部运动引起的剂量对空间分割放射治疗(SFRT)计划质量的影响。
方法:对于大肿瘤,采用锥形束CT引导的基于MLC的三维适形SFRT方法,处方15Gy.MLCGTV拟合算法提供Icm直径的孔,在等中心处具有2cm的中心到中心距离。这在一小时内产生了高度异质的筛状剂量分布,使同一天SFRT治疗。对15例先前接受过治疗的SFRT患者进行了分析(5例头颈部[H&N],5胸部和肺部,和5个腹部和骨盆肿块)。对于每个计划,通过在不同的x-,-,和z方向从1到5毫米。分析的剂量学指标为:峰谷剂量比(PVDR),GTV接收7.5Gy的百分比,GTV平均剂量,和对危险器官(OAR)的最大剂量。
结果:对于±1、±2、±3、±4和±5mm等中心偏移:PVDR下降了3.9%,3.8%,4.0%,4.1%,和平均分别为5.5%。GTV(V7.5)保持在0.2%以内,GTV平均剂量平均保持在3.3%以内,与原来的计划相比。对于H&N病例,5mm移位的平均PVDR下降为4.2%,10%用于胸部和肺部,腹部和骨盆病例为2.2%。OAR剂量也增加了。在H&N计划中,脊髓的最大剂量增加了17cGy,平均肺剂量(MLD)变化小的胸部/肺,但腹部和骨盆病例的肠道剂量变化高达100cGy。
结论:由于肿瘤大小,location,以及基于MLC的SFRT的特点,在不同方向上最大±5mm的等中心偏移对H&N和骨盆肿瘤的PVDR有中等影响,对胸部肿瘤的影响更大。对OAR剂量的剂量学影响取决于治疗部位。特定地点的患者口罩,Vac-Lok袋,应使用类似于SBRT/SRT设置的适当固定装置,以最大程度地减少这些影响。
OBJECTIVE: To evaluate the dosimetric impact on spatially fractionated radiation therapy (SFRT) plan quality due to intrafraction patient motion via multi-field MLC-based method for treating large and bulky (≥8 cm) unresectable tumors.
METHODS: For large tumors, a cone beam CT-guided 3D conformal MLC-based SFRT method was utilized with 15 Gy prescription. An MLC GTV-fitting algorithm provided 1 cm diameter apertures with a 2 cm center-to-center distance at the isocenter. This generated a highly heterogeneous sieve-like dose distribution within an hour, enabling same-day SFRT treatment. Fifteen previously treated SFRT patients were analyzed (5 head & neck [H&N], 5 chest and lungs, and 5 abdominal and pelvis masses). For each plan, intrafraction motion errors were simulated by incrementally shifting original isocenters of each field in different x-, y-, and z-directions from 1 to 5 mm. The dosimetric metrics analyzed were: peak-to-valley-dose-ratio (PVDR), percentage of GTV receiving 7.5 Gy, GTV mean dose, and maximum dose to organs-at-risk (OARs).
RESULTS: For ±1, ±2, ±3, ±4, and ±5 mm isocenter shifts: PVDR dropped by 3.9%, 3.8%, 4.0%, 4.1%, and 5.5% on average respectively. The GTV(V7.5) remained within 0.2%, and the GTV mean dose remained within 3.3% on average, compared to the original plans. The average PVDR drop for 5 mm shifts was 4.2% for H&N cases, 10% for chest and lung, and 2.2% for abdominal and pelvis cases. OAR doses also increased. The maximum dose to the spinal cord increased by up to 17 cGy in H&N plans, mean lung dose (MLD) changed was small for chest/lung, but the bowel dose varied up to 100 cGy for abdominal and pelvis cases.
CONCLUSIONS: Due to tumor size, location, and characteristics of MLC-based SFRT, isocenter shifts of up to ±5 mm in different directions had moderate effects on PVDR for H&N and pelvic tumors and a larger effect on chest tumors. The dosimetric impact on OAR doses depended on the treatment site. Site-specific patient masks, Vac-Lok bags, and proper immobilization devices similar to SBRT/SRT setups should be used to minimize these effects.