Comparison of setup errors between two immobilization techniques for image-guided pelvic radiotherapy: Can an ankle fixator be used as an alternative to a thermoplastic mask for immobilization without compromising treatment?

Abstract

Objectives Pelvic malignancies have reported large setup variations, requiring good treatment positioning and immobilization. Various immobilization techniques have been reported in literature for pelvic malignancies. In this study, two different immobilization techniques have been compared for translational setup errors during radiation delivery. Material and Methods Kilovoltage cone-beam computed tomography (kV CBCT) scans were taken and 3D-3D matching was done during the first three days of radiation treatment; thereafter once a week, and as per requirement of the particular treatment. Data on translational shifts was tabulated from treatment record. The mean and standard deviation of all individual shifts were calculated using descriptive statistics. The means of the translational shifts of the two arms were compared using Mann–Whitney U test. The population’s systematic and random errors were calculated. Clinical target volume to planning target volume (CTV-PTV) margins were calculated using Van Herk’s method. Results A total of 855 cone-beam computed tomography (CBCT) scans were analyzed. The mean setup errors were not significantly different between two arms in all three translational axes. The CTV-PTV margins in the anteroposterior, superoinferior, and lateral directions are 0.59, 0.94, and 0.78 cm for the TPI arm and 0.84, 0.92, and 0.87 cm for the AFI arm. To have a uniform CTV-PTV margin in all directions, the minimum expansion required is 0.94 cm and 0.92 cm in the TPI and AFI arm, respectively. Conclusion An ankle fixator or similar lower leg immobilization technique can be used as an alternative to thermoplastic mask immobilization without compromising setup accuracy. This has the potential to reduce treatment cost per patient, by excluding the cost of thermoplastic mask.