A Planning Benchmark Study for SBRT: Helical Radiation Therapy Versus Intensity Modulated Arc Therapy

Abstract

This study compares and analyzes fractionated stereotactic body radiotherapy (SBRT) using helical radiation therapy system (HT) and linac-based intensity modulated arc therapy (IMAT) so our aim is to develop an efficient and effective planning technique. Twenty patients with lung and liver cancers were previously treated in our clinic with helical tomotherapy, were randomly selected to this study. A robust IMAT planning technique was developed with identical anatomic contouring and prescription. IMAT plans were generated using 10 MV flattening filter-free beams. HT treatment plans and IMAT were optimized using commercial planning systems Tomoplan and Eclipse (AAA algorithm), respectively. Plans were calculated with prescription dose of 35 Gy in 5 fractions at the 65% isodose line with Dmax 100%. The plans were compared using coverage Paddick index (CI), homogeneity index (HI) and the external index (EI); indicating the exposure of healthy tissue at reference dose, also the volume of ITV receiving V90%(cm3), and planning tumor volume (PTV) receiving D2%, D98%, D50%. Nearly all tomotherapy plans delivered almost in mean 2.27 Gy (range 0.38-4.36) more doses to PTV_Dmean compared to IMAT plans. Median PTV_Dmean was 43.51 Gy in IMAT and 45.78 Gy in HT. Fraction of tissue surrounded the tumor volume (dose gradient between plans) received less dose in IMAT regards to statistical evaluation of the results from EI: for HT and IMAT were, 0.55, 0.34 respectively. It happens because HT delivers more doses in superior-inferior direction. HT plans resulted in better dose conformity with lower CI scores compared to IMAT. Median CI was 0.77 and 0.69. HI, ITV_Dmax, D2%, D98%, D50% were recorded for all plans and were in mean for IMAT, 0.35, 53.48, 51.62, 36.24, 43.55 and in HT 0.33, 53.46, 52.14, 37.25, 45.46, respectively. Statistical analysis showed that plans were done in HT took the longest treatment time (P<0.001) and the number of monitor units were increased. All treatment plans met the prescription requirements for the OARs limits according to the RTOG and German DEGRO guidelines, there were no significant difference. (P<0.05). This study demonstrates that IMAT is dosimetrically advantageous in treating fractionated SBRT compared to HT, while providing significantly shorter treatment times. Despite helical radiation therapy treatment planning system showed significantly better results in dose conformity. IMAT plans resulted in better steeper dose drop-off outside the targets.