Evaluation of plan robustness on the dosimetry of volumetric arc radiotherapy (VMAT) with set-up uncertainty in Nasopharyngeal carcinoma (NPC) radiotherapy

Abstract

Purpose To evaluate the sensitivity to set up the uncertainty of VMAT plans in Nasopharyngeal carcinoma (NPC) treatment by proposing a plan robustness evaluation method. Methods 10 patients were selected for this study. A 2-arc volumetric-modulated arc therapy (VMAT) plan was generated for each patient using Varian Eclipse (13.6 Version) treatment planning system (TPS). 5 uncertainty plans (U-plans) were recalculated based on the first 5 times set-up errors acquired from cone-beam computer tomography (CBCT). The dose differences of the original plan and perturbed plan corresponded to the plan robustness for the structure. Tumor control probability (TCP) and normal tissues complication probability (NTCP) were calculated for biological evaluation. Results The mean dose differences of D98% and D95% (ΔD98% and ΔD95%) of PTVp were respectively 3.30 Gy and 2.02 Gy. The ΔD98% and ΔD95% of CTVp were 1.12 Gy and 0.58 Gy. The ΔD98% and ΔD95% of CTVn were 1.39 Gy and 1.03 Gy, distinctively lower than those in PTVn (2.8 Gy and 2.0 Gy). The CTV-to-PTV margin increased the robustness of CTVs. The ΔD98% and ΔD95% of GTVp were 0.56 Gy and 0.33 Gy. GTVn exhibited strong robustness with little variation of D98% (0.64 Gy) and D95% (0.39 Gy). No marked mean dose variations of Dmean were seen. The mean reduction of TCP (ΔTCP) in GTVp and CTVp were respectively 0.4% and 0.3%. The mean ΔTCPs of GTVn and CTVn were 0.92% and 1.3% respectively. The CTV exhibited the largest ΔTCP (2.2%). In OARs, the brain stem exhibited weak robustness due to their locations in the vicinity of PTV. Bilateral parotid glands were sensitive to set-up uncertainty with a mean reduction of NTCP (ΔNTCP) of 6.17% (left) and 7.70% (right). The Dmax of optical nerves and lens varied slightly. Conclusion VMAT plans had a strong sensitivity to set-up uncertainty in NPC radiotherapy, with increasing risk of underdose of tumor and overdose of vicinal OARs. We proposed an effective method to evaluate the plan robustness of VMAT plans. Plan robustness and complexity should be taken into account in photon radiotherapy.

Physical and Radiobiological Evaluation of Patient Setup Errors in the Radiotherapy Procedure for Prostate Cancer

“…The average EUD values for femoral head-R in the VMAT plans without error 10.29 Gy were higher than those in the VMAT plans with error 10.61 Gy the average NTCP values of femoral head-R without error in the VMAT plans, 0.0% and with error in the VMAT plans 0.0% are shown in Table (17). For dosimetric response, The average volume in VMAT plans without errors is 171.2 cc, the average minimum dose is 2.20 Gy, the average maximum dose is 35.01 Gy, the average mean dose is 16.65 Gy, the average modal dose is 13.07 GY, the average of median dose 17.59 Gy, the average of STD 7.04 Gy, and the average of equivalent sphere diameter 6.9cm are shown in Table (18).…”

“…In literature, some works were focused on the dosimetric or radiobiological assessment of systematic isocenter shift errors for different types of cancers. [17,18,24]. This work aimed to evaluate both the dosimetric and radiobiological effects of the random setup error that usually occurs during the position of prostate patients.…”

“…On the other hand, treatment plan robustness is the degree of resilience of the required dose distribution to these uncertainties and varies with the treatment site, technique, and method. TCP and NTCP are useful parameters in the evaluation of plan robustness [17]. Therefore in our study, both parameters will be used to evaluate the difference between the corrected and uncorrected patient setup errors in patient positioning during the radiotherapy treatments.…”