Utility Evaluation of Split VMAT Treatment Planning for Nasopharyngeal cancer

Purpose : IMRT using Tomotherapy during nasopharyngeal cancer radiation therapy irradiate an accurate dose to tumor tissues and is effective to reduce a dose rapidly in normal tissues. However, this has high MU and long Beam On Time. This study aims to analyze differences in tumors, normal tissues and low-dose distributions and the efficiency of Split VMAT after applying Helical IMRT (Tomotherapy), VMAT (Linac : 2Arc) and Split VMAT (Linac : 4Arc) plans. Materials and Methods : This study targeted ten nasopharyngeal cancer patients of this hospital and compared three treatment plans (Helical IMRT, VMAT, Split VMAT). For Helical IMRT planning, Precision^® (Version 1.1.1.1, Accuray, USA) was used, and for VMAT and Split VMAT planning, Pinnacle^3® (Version 9.10, Philips, USA) was used. The total dose applied was 38.4 Gy / 32 Gy (Daily Dose 2.4 Gy (GTV + 0.3 cm) / 2 Gy (CTV + 0.3 cm) 16Fx), and for GTV + 0.3 cm (P_GTV), 95% of V_38.4Gy was prescribed. VMAT with an angle of 360° 2Arc was applied, and for Split VMAT, the field was divided into the right, the left, the top and the bottom and an angle of 360° 4Arc, 6MV was set. For evaluating the quality of the treatment plans, differences in tumors, normal tissues and low-dose area were compared, and Beam On Time was measured to analyze the efficiency. Results : When calculating the mean values of evaluation items of the three treatment plans (Helical IMRT, VMAT, Split VMAT) for the patients, the H.I (Homogeneity Index) of P_GTV was 1.04, 1.11 and 1.1 respectively, and the C.I (Confomity Index) of P_CTV was 1.03, 0.99 and 1.00 respectively. The mean dose of RT Parotid Gland (Gy) was 14.54, 17.06 and 14.76 respectively, the mean dose of LT Parotid Gland (Gy) was 14.32, 17.32 and 15.09 respectively, the maximum dose of P_Cord (Spinal Cord + 0.3 cm) (Gy) was 20.57, 22.59 and 21.06 respectively, and the maximum dose of Brain Stem (Gy) was 22.35, 23.99 and 21.68 respectively. The 50% isodose curve (cc) was 1332, 1132.5 and 1065.2 respectively. Beam On Time (sec) was 373.7, 130.7 and 254.4 respectively. Conclusion : Displaying a similar treatment plan quality to Helical IMRT, which is used a lot for head and neck treatment, Split VMAT reduced the low-dose area and Beam On Time and produced a better result than VMAT. Therefore, it is considered that Split VMAT is effective not only for nasopharyngeal cancer but also for other head and neck cancers.

비인두암 Split VMAT 치료계획 유용성 평가

목 적: 비인두암 (Nasopharyngeal cancer) 방사선 치료 시 Tomotherapy을 이용한 IMRT는 종양 조직에 정확한 선량을 전달하며 정상조직에는 급격한 선량 감소의 효과가 있지만 MU가 높고 Beam On Time이 긴 단점이 있다. 본 연구는 Helical IMRT (Tomotherapy)와 VMAT (Linac : 2Arc), Split VMAT (Linac : 4Arc)의 치료계획을 시행하여 종양 및 정상 조직, 저 선량 분포의 차이를 분석하고 Split VMAT의 효율성을 평가해 보고자 한다. 대상 및 방법: 본원의 비인두암 환자 10명을 대상으로 하였으며 3개 치료계획 (Helical IMRT, VMAT, Split VMAT)을 비교하였다. Helical IMRT 치료 계획은 Precision^® (Version 1.1.1.1, Accuray, USA)을 이용하였고 VMAT, Split VMAT 치료계획은 Pinnacle3^® (Version 9.10, Philips, USA)을 이용하였다. 선량은 Daily Dose 2.4 Gy (GTV + 0.3 cm) / 2 Gy (CTV + 0.3 cm) 16 Fx으로 Total Dose 38.4 Gy / 32 Gy를 적용하였고 GTV + 0.3 cm (P_GTV)가 V_38.4Gy이 95%가 되도록 처방하였다. VMAT은 360° 2Arc, Split VMAT은 좌, 우, 상, 하로 Field를 나누고 360° 4Arc, 6MV를 사용하였다. 평가는 종양 및 정상조직, 저 선량 영역의 차이를 비교하여 치료 계획의 Quality를 평가하였고 Beam On Time을 비교하여 효율성을 평가하였다. 결 과: 대상 환자들의 3개 치료계획 (Helical IMRT, VMAT, Split VMAT)의 평가 항목 평균 값은 P_GTV의 H.I (Homogeneity Index)는 (1.04, 1.11, 1.1), P_CTV의 C.I (Conformity Index)는 (1.03, 0.99, 1.00)이었다. RT Parotid Gland의 평균 선량 (Gy)은 (14.54, 17.06, 14.76), LT Parotid Gland의 평균 선량 (Gy)은 (14.32, 17.32, 15.09), P_Cord (Spinal Cord + 0.3 cm)의 최대 선량 (Gy)은 (20.57, 22.59, 21.06), Brain Stem의 최대 선량 (Gy)은 (22.35, 23.99, 21.68)이었다. 50% isodose curve (cc)는 (1332, 1132.5, 1065.2)이었다. Beam On Time (sec)은 (373.7, 130.7, 254.4)이었다. 결 론: Split VMAT은 두경부치료에 많이 사용하는 Helical IMRT와 비교했을 때 유사한 치료계획 Quality를 보이면서도 저 선량 영역을 줄여주며 Beam On Time이 줄어든 결과를 보여주었고 VMAT보다 좋은 결과를 보였다. Split VMAT은 비인두암 뿐만 아니라 다른 두경부암에서도 효과적이라 사료된다.