Chongqing University Cancer Hospital Chongqing, N/A
H. Luo, F. Jin, Y. Wang, M. Q. Yang, T. Qiu, W. Hu, T. Wang, S. Li, and X. Tan; Radiation Physics Center, Chongqing University Cancer Hospital, Chongqing, China
Purpose/Objective(s): In spatially fractionated radiation therapy (SFRT), there is an interaction between irradiated cells, and high-dose irradiated cells might inhibit the growth of low-dose irradiated cells, defined as cohort effect. Given the inherent dose ripple in Helical Tomotherapy (HT), the cohort effect might be triggered by thread effect of HT. Therefore, in this study, a new HT SFRT scheme was proposed and tested by plan simulation, cell experiment and clinical cases. Materials/
Methods: This study adopted Hela cells, and enrolled fourteen patients diagnosed as carcinoma of uterine cervix with stage IA to IIIC. CT images were scanned and both planning target volume (PTV) and organs at risk (OARs) were delineated by the same experienced radiation oncologist. TomoHD system with a 6-MV photon beam simulated HT SFRT plans and also simulated both conventional HT (CHT) and SBRT plans for comparison. The dose threshold triggering the cohort effect was explored via proliferative capacity changes of cells and then regarded as the daily dose of HT SFRT and SBRT plans. The total dose was clinically considered. This study analyzed and compared the physical and biological doses of PTV and OARs, treatment efficiency, TCP and NTCP. The universal survival curve (USC) model was used. Meanwhile, the Poisson and Lyman-Kutcher-Burman (LKB) model calculated TCP and NTCP, respectively. Results: The HT SFRT plan chosen the combination of FW=5.084 cm, Pitch=0.5 and MF=2.0. A significant decrease in cell viability was observed in HT SFRT with an 8-Gy dose, compared to the CHT. In delivery time, the HT SFRT outperformed SBRT (15.10 minutes vs. 26.43 minutes, p<0.05). In total time, both HT SFRT and SBRT have the significant advantages. For Dmax, Dmean, Dmedian, and D2% of PTV, HT SFRT in 3 fractions and SBRT was lower than CHT, but HT SFRT in 4 fractions has higher metrics. For bladder and bladder wall, the D15cc, Dmean and Dmax of HT SFRT and SBRT were less than CHT. There were similar values for colon, small intestine and spinal cord between HT SFRT in 4 fractions and CHT. The dose metrics of rectum and femoral head from HT SFRT in 4 fractions were higher than CHT, but this phenomenon can be changed by tightening dose constraints. TCP reached 99.91%-100.00% in all schemes. NTCP of bladder and bladder wall from HT SFRT in 4 fractions was better than CHT, and the two schemes were similar for NTCP of the rectum, colon, and small bowel. But, NTCP of femoral head was greater in HT SFRT with 4 fractions and the phenomenon was also flipped by tightening dose constraints. Conclusion: The thread effect of HT would trigger the cohort effect. The scheme of HT SFRT in 4 fractions is an excellent option for prospective clinical trial design in cases where dose constraints of normal tissue become more stringent.
