Dosimetric analysis of cervical cancer stage IIB patients treated with volumetric modulated arc therapy using plan uncertainty parameters module of Varian Eclipse treatment planning system

Introduction. The present study aims to investigate the dosimetric and radiobiological impact of patient setup errors (PSE) on the target and organs at risk (OAR) of the cervix carcinoma stage IIB patients treated with volumetric-modulated arc therapy (VMAT) delivery technique using plan uncertainty parameters module of Varian Eclipse treatment planning system and in-house developed DVH Analyzer program. Materials and Methods. A total of 976 VMAT plans were generated to simulate the PSE in the base plan that varies from -10 mm to 10 mm in a step size of 1 mm in x- (lateral), y- (craniocaudal), and z- (anteroposterior) directions. The different OAR and tumor (PTV) volumes were delineated in each case. Various plan quality metrics, such as conformity index (CI) and homogeneity index (HI), as well as radiobiological quantities, such as tumor control probability (TCP) and normal tissue control probability (NTCP), were calculated from the DVH bands generated from the cohort of treatment plans associated with each patient case, using an in-house developed 'DVH Analyzer' program. The extracted parameters were statistically analyzed and compared with the base plan's dosimetric parameters having no PSE. Results. The maximum variation of (i) 2.4%, 21.5%, 0.8%, 2.5% in D-2cc of bladder, rectum, small bowel and sigmoid colon respectively; (ii) 19.3% and 18.9% in D-max of the left and right femoral heads (iii) 16.9% in D-95% of PTV (iv) 12.1% in NTCP of sigmoid colon were observed with change of PSE in all directions. TCP was found to be considerably affected for PSEs larger than 4 mm in x(+), y(+), z(+) directions and 7 mm in x(-), y(-) and z(-) directions, respectively. Conclusion. This study presents the effect of PSE on TCP and NTCP for the cervix carcinoma cases treated with VMAT technique and also recommends daily image guidance to mitigate the effects of PSE.