Nuclear Medicine & Radiation Therapy

Variations of the Tumor Position in Frameless Lung Sbrt: Assessment of Predictive Factors Including Tumor Volume Changes

Abstract

Jessica Schuster, Michael Myers, Mihaela Rosu, Nitai Mukhopadhyay and Elisabeth Weiss

Purpose: To quantify inter- and intrafractional variations of tumor position and analyze the relationship between these changes and respiratory motion amplitude, volume changes and tumor location, in frameless stereotactic body radiotherapy (SBRT) of lung tumors.
Materials and methods: Tumor volumes and bony landmarks were contoured manually by a single physician on 174 pre- and under treatment cone-beam computed tomographies (CBCTs) of 17 patients. The interfraction variation of the tumor position was measured by comparing the centroid position of the tumor relative to bony anatomy of each fraction to the pretreatment CBCT scans. The intrafraction variation was measured by comparing the pretreatment tumor location to under treatment CBCTs for every fraction. Respiratory motion was analyzed on planning 4D fan beam CTs for all patients. The change in tumor volume was determined by comparing the contoured tumor volumes on sequential pretreatment CBCTs.
Results: The average interfraction/intrafraction tumor displacementrelative to bony landmark in mm was 0.6 (SD 2.3) /-0.3 (SD 0.7) in mediolateral, -0.7 (SD 3.8) /0.0 (SD 2.1) in anteroposterior, and -0.6 (SD 5.9) /-0.2 (SD 2.3) in craniocaudal direction. Inter-/intrafraction tumor-to-bone variations >3 mm were observed in 60%/14% of scans respectively. On the initial CBCT, the average tumor volume was 9 cm3 (range 1-37 cm3) with a mean volume reduction over the treatment course of 12% (range, +14% to -54%). Patients with a pretreatment motion amplitude > 9 mm (p=0.002), peripheral tumor location (p=0.04), and volume change >12% (p=0.009) had larger interfraction displacement in lateral direction.
Conclusions: Frameless set up is comparable to patient positioning with more elaborate fixation devices. Tumor position variations relative to bony anatomy are an important source of geometric uncertainty providing a rationale for repeated soft tissue-based image guidance, particularly in patients identified in this study to be at higher risk for variations.