Precision of 2 Low-dose Abdomen/Pelvis Cone Beam Computed Tomography Protocols for Alignment to Bone and Soft Tissue in Pediatric Patients Receiving Image Guided Radiation Therapy.

Abstract

To evaluate the precision of 2 low-dose cone (LD) beam computed tomography (CBCT) protocols to align to bone and soft tissue for pediatric patients receiving image guided radiation therapy (IGRT) to the abdomen and pelvis. Image-quality evaluation was done for 858 CBCT scans from 46 pediatric patients treated with IGRT from January 2015 to December 2017. The evaluations guided the development of 2 significantly dose-reduced protocols, LD-CBCT1 and a further dose-reduced LD-CBCT2. Representative scans from LD-CBCT1 and LD-CBCT2 from 8 patients with at least 1 CBCT scan from both protocols were registered separately to a bone and soft-tissue landmark on the simulation computed tomography scan. Eighteen identical blinded random offsets were applied to each patient's LD-CBCT1 and LD-CBCT2 from a starting registration that was then realigned using rigid registration. The residual offset between the baseline registration and the final registration attempt was calculated and analyzed using a 1-sided, 1 sample t test to evaluate whether LD-CBCT1, delivering a higher dose, was superior to the lower-dose LD-CBCT2 for bone and soft-tissue alignment. In comparing 288 registrations with a bone landmark across 8 patients, no difference was found in the vector magnitude offsets using LD-CBCT 1 (mean [x¯], 0.73 mm; standard deviation [σ], 0.39 mm) and LD-CBCT2 (x¯, 0.74 mm; σ, 0.40 mm; P = .425). Comparing 216 registrations with a soft-tissue landmark across 6 patients, alignment using LD-CBCT2 (x¯, 1.55 mm; σ, 1.08 mm) resulted in larger differences in the vector magnitude of the offsets compared with LD-CBCT1 (x¯, 1.37 mm; σ, 0.74 mm; P = .049). Clinics treating pediatric patients should consider implementing a protocol mirroring LD-CBCT2 for abdomen and pelvis IGRT bone alignment. Further evaluation of the precision of LD-CBCTs for soft-tissue alignment is necessary.