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Year : 2014  |  Volume : 10  |  Issue : 1  |  Page : 29-37

Critical neurological structure sparing radiosurgery of vestibular schwannoma: Dosimetric comparison of different techniques and dose prescription methods

1 Department of Radiation Oncology, Kokilaben Dhirubhai Ambani Hospital and Medical Research Institute, Andheri, Mumbai, India
2 Department of Neurosurgery, Kokilaben Dhirubhai Ambani Hospital and Medical Research Institute, Andheri, Mumbai, India

Correspondence Address:
Shamurailatpam Dayananda Sharma
Department of Radiation Oncology, Room No AS/55024, Kokilaben Dhirubhai Ambani Hospital and Medical Research Institute, Four Bungalows, Andheri (W), Mumbai - 400 053
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-1482.131353

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Aim: To investigate potential sparing of critical neurological structures (CNSs) during radiosurgery of vestibular schwannoma (VS) employing different techniques and dose prescription methods. Materials and Methods: Fused CT and MRI datasets of eight patients with unilateral VS representing a wide range of target volume (0.48 to 12.08 cc; mean = 3.56 cc), shape and proximity to CNSs such as cochlea, trigeminal nerve and brainstem were re-planned employing static conformal field (SCF), dynamic conformal arc (DCA) and intensity modulated radiosurgery (IMRS) techniques. For every patient, five plans were created for a fixed margin dose of 12 Gy prescribed at 80% in three plans (SCF_80%, DCA_80%, and IMRS_80%) and 50% in another two plans (SCF_50% and DCA_50%). All plans were compared using standard dosimetric indices. Results: Primary goal of every plan to cover ≥99% of target volume with 12 Gy was fulfilled for all patients with minimum significant dose to target (D 99 ) ≥11.99 Gy. Best conformity index (CI Paddick = 0.62 ± 0.12) was observed in SCF_80% and DCA_80% plans whereas; sharpest dose gradient index of 3.40 ± 0.40 was resulted from DCA_50%. All five plans resulted similar maximum dose to brainstem (11.04 ± 2.23 to 11.53 ± 1.10 Gy), cochlea (9.02 ± 1.79 to 10.15 ± 1.26 Gy) and trigeminal nerve (11.55 ± 1.38 to 12.19 ± 2.12 Gy). Among 80% prescription plans, IMRS_80% reduces mean and D 5 (P < 0.05) to all CNSs. Prescription of dose at 50% isodose sharpened the dose gradient and significantly (P < 0.05) reduced mean dose and D 5 to all CNSs at the cost of target conformity (P = 0.01). Mean dose to cochlea and trigeminal nerve were least at 4.53 ± 0.86 and 6.95 ± 2.02 Gy from SCF_50% and highest at 6.65 ± 0.70 and 8.40 ± 2.11 Gy from DCA_80% plans respectively. Conclusion: This dosimetric data provides a guideline for choosing optimum treatment option and scope of inter institutional dosimetric comparison for further improvement in radiosurgery of Vestibular Schwannoma (VS).

Abstract in Chinese

前庭神经鞘瘤放射治疗中关键神经结构的保护:不同技术和处方剂量的剂量测定对照 目的:通过不同技术和处方剂量的方法评估前庭神经鞘瘤(VS)放疗过程中关健神经结构(CNSs)的潜在保护。 材料和方法:将8例单侧前庭神经鞘瘤病人CT和MRI融合资料,发现他们靶区容量差别较大(0.48cc到12.08cc;平均3.56cc),靠近关键神经结构如耳蜗、三叉神经和脑干处的靶区予重新计划,分别通过普通放疗(SCF),适形放疗(DCA),以及调强放疗(IMRS)技术。对每例病人,5次计划分别给予12Gy的固定剂量范围,三次给予80%等剂量线(SCF_80%, DCA_80%, IMRS_80%),另两次予以50%等剂量线(SCF_50%,DCA_50%)。所有计划用标准剂量指数来比较。 结果:每次计划的的主要目标,以12Gy剂量覆盖≥99%的靶区容量(D99),使所有病人病灶的最小有效剂量≥11.99Gy。最佳符合指数在SCF_80%和DCA_80%计划中可以得到(CIPaddick= 0.62 ± 0.12),而在DCA_50%计划中得出最锐利的剂量梯度曲线:3.40± 0.40。5次计划得出相似的最大剂量:脑干(11.04± 2.23 到11.53± 1.10Gy),耳蜗(9.02± 1.79到10.15 ± 1.26 Gy),三叉神经(11.55± 1.38 到12.19± 2.12Gy)。80%等剂量线处方计划中,调强80%减少了关键神经结构的平均剂量和D5(P<0.05)。50%处方等剂量线明显改变剂量梯度(P<0.05),减少关键神经结构的平均剂量和D5,但是改变了靶区一致性(P=0.01)。耳蜗和三叉神经的平均剂量至少为SCF_50%的4.53± 0.86和 6.95± 2.02Gy,最高达DCA_80%的6.65± 0.70和 8.40± 2.11Gy 。 结论:本次剂量测定的数据提供了最佳治疗选择方案的指导方针,以及机构间进一步改进治疗前庭神经鞘瘤剂量的范围。 关键词:耳蜗,放射量测定,调强放疗,放疗外科学,三叉神经,前庭神经鞘瘤

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