Session Item

Reduction of respiratory motion is critical in liver stereotactic body radiotherapy (SBRT)  to reduce organ at risk dose, facilitating increased tumour dose and resultant local control. In our institution we assess all liver SBRT patients prior to simulation to determine appropriate motion management (MM). This retrospective review aims to evaluate the patterns of use of MM techniques and their impact on treatment time.

For each patient we assess suitability, in order of priority, of voluntary exhale breath hold (EBH), free breathing gating, abdominal compression (AC), or free breathing (FB), according to the flowchart in Figure 1. We use an external surrogate to assess breath hold capability, followed by anterior-posterior fluoroscopy to assess liver dome position consistency. From MM assessment session records, we extracted liver dome position consistency in repeat EBHs, liver dome motion change with AC and decision of MM used for simulation and treatment. All patients were treated on a TrueBeam linear accelerator with flattening-filter free photon beams using arc (VMAT) or static gantry (3DCRT, IMRT) techniques, with between 6 and 20 Gy per fraction. CBCT image guidance was used in all cases, with acquisition matched to the MM technique (free breathing, gated or breath hold CBCT). From the record and verify system we extracted treatment time, defined as time from first setup image to last treatment beam off. Treatment time was assessed as a function of MM, technique and number of CBCTs.

Figure 1: (a) Workflow for assessment of appropriate MM and (b) example ant-post fluoroscopy liver dome position

From September 2017 to September 2021, 145 patients underwent MM assessment. Of these, 67.6% were suitable for EBH, 20.0% for AC, 6.9% for gating and 5.5% FB. 88/100 patients assessed with fluoroscopy for liver dome consistency in EBH could achieve repeat EBH with <4mm variation. Reduction in liver motion with AC compared with FB was highly variable (mean 3.5 mm, range -0.3-16 mm). Treatment time was available for 613 fractions (Figure 2) and was largest for EBH (40 ± 14 min) and gated treatments (46 ± 19 min), compared with AC (32 ± 11 min, p<0.001) and FB (27 ± 12 min, p<0.001). Treatment time was independent of fraction size. Four patients originally assessed as suitable for EBH could not achieve reproducible EBH at treatment and were treated in FB (3) or with AC (1). Arc treatments were quicker than static gantry treatments (35 ± 14 min vs 43 ± 15 min, p = <0.001). Average treatment time was increased by 7 mins with each CBCT acquired.

Figure 2: Treatment time per fraction as a function of motion management approach

MM in liver SBRT must be assessed per-patient to determine the most appropriate technique for that patient. Extended treatment times must be taken into account when deciding on optimal MM strategy. Methods to reduce intra-fraction volumetric imaging, such as real-time imaging or tracking, may be beneficial to reduce treatment time.