Planning Margins Determined by Inter Fractional Errors for Patients with Hepatocarcinoma—A Real-World Practice

Abstract

<h3>Purpose/Objective(s)</h3> With the progress of radiation techniques, radiotherapy is increasingly widely used in the treatment of hepatocarcinoma. Previous studies showed high radio-sensitivity and frequently motion of hepatobiliary zone. Although the application of magnetic resonance simulation (MR) and four-dimensional computed tomography (4D-CT) help us to clearly the gross tumor volume (GTV), accurate calculation of set-up margin is required to define the clinical to planning target volume margin. This study aimed at calculate the suitable margin between clinical target volume (CTV) and planning target volume (PTV) after the evaluation of the set-up errors using cone beam computed tomography (CBCT). <h3>Materials/Methods</h3> Patients aged 18-85 years old with pathologically diagnosed as primary hepatocarcinoma, Eastern Cooperative Oncology Group performance score ≤ 3 points, accepted the radiation recommendation by multi-disciplinary team, with only one tumor located in liver will be enrolled. Both 4D-CT and MR simulations were delivered to determine GTV. CTV was GTV plus a 5 mm margin in liver and GTV plus a 10mm margin along vessels if tumor embolus were included. PTV was CTV plus a 5mm margin. All the patients received image guided intensity modulated radiotherapy with conventional fractionation. CBCT was prescribed daily at the first week and then 2-3 times at every following week. Patients whose tumor could not be clearly classified in CBCT will be excluded. CBCT The setup errors in translational directions were recorded based on the image registration between the CBCT and the free breath CT images at simulation as reference images. Mean value and standard deviations (∑) of each patient were obtained. After that, we calculated the margin between CTV and PTV by a formula MPTV=2.5∑+0.7σ (σ means rootmeansquare of ∑). <h3>Results</h3> From November 2020 to April 2021, 35 patients fulfilled the criteria. The median age of these patients were 65 years old. The median value of GTV and PTV were 512.91 ml (45.09-1987.56 ml) and 1074.77ml (221.09-3028.40 ml), respectively. The median body mass index (BMI)was 23.98. The median CBCT times was 8 (5-12). After alignments by one senior radiation oncologist and one senior radiation therapist, we analyzed 284 sets of CBCT images. M±SD shifts in translational were 0.04±0.25 mm (LR), 0.05±0.51 mm (AP), and 0.06±0.26 mm (SI), respectively. Accordingly, the margins in LR, SI and AP directions were 5 mm, 6 mm and 10 mm, respectively. For patients whose BMI <24, the margins in LR, SI and AP directions were 4 mm, 5 mm and 12 mm. For patients whose BMI ≥24, the corresponding margins should be 6 mm,7 mm and 10 mm, respectively. <h3>Conclusion</h3> For patients with hepatocarcinoma, the setup errors were different in LR, SI and AP directions. We recommended that the margins from CTV to PTV should be 5mm, 6mm and 10 mm in LR, SI and AP directions. For patients with BMI≥24, larger margins in LR and SI directions are needed.