Abstract
PurposeTo assess the effects of daily adaptive MR-guided replanning in stereotactic body radiation therapy (SBRT) of liver metastases based on a patient individual longitudinal dosimetric analysis.MethodsFifteen patients assigned to SBRT for oligometastatic liver metastases underwent daily MR-guided target localization and on-table treatment plan re-optimization. Gross tumor volume (GTV) and organs at risk (OARs) were adapted to the anatomy-of-the-day. A reoptimized plan (RP) and a rigidly shifted baseline plan (sBP) without re-optimization were generated for each fraction. After extraction of DVH parameters for GTV, planning target volume (PTV), and OARs (stomach, duodenum, bowel, liver, heart) plans were compared on a per-patient basis.ResultsMedian pre-treatment GTV and PTV were 14.9 cc (interquartile range (IQR): 7.7–32.9) and 62.7 cc (IQR: 42.4–105.5) respectively. SBRT with RP improved PTV coverage (V100%) for 47/75 of the fractions and reduced doses to the most proximal OARs (D1cc, Dmean) in 33/75 fractions compared to sBP. RP significantly improved PTV coverage (V100%) for metastases within close proximity to an OAR by 4.0% (≤ 0.2 cm distance from the edge of the PTV to the edge of the OAR; n = 7; p = 0.01), but only by 0.2% for metastases farther away from OAR (> 2 cm distance; n = 7; p = 0.37). No acute grade 3 treatment-related toxicities were observed.ConclusionsMR-guided online replanning SBRT improved target coverage and OAR sparing for liver metastases with a distance from the edge of the PTV to the nearest luminal OAR < 2 cm. Only marginal improvements in target coverage were observed for target distant to critical OARs, indicating that these patients do not benefit from daily adaptive replanning.
Highlights
The implementation of stereotactic body radiation therapy (SBRT) was an important milestone in local treatment for oligometastatic and medically inoperable cancers [1, 2]
Median Gross tumor volume (GTV) and planning target volume (PTV) changes compared to baseline were 0 cc (IQR: − 0.6 to 0) and 0.4 cc (IQR: 0–2.5) respectively
The volume of the GTV was not adjusted from the baseline plan in 34% of all fractions
Summary
The implementation of stereotactic body radiation therapy (SBRT) was an important milestone in local treatment for oligometastatic and medically inoperable cancers [1, 2]. When treating abdominal malignancies, such as liver metastases, the dose of SBRT is often limited by the proximity of gastrointestinal organs [6, 7] and PTV compromises are necessary to minimize the risk of radiationinduced gastrointestinal toxicity. This may translate in Mayinger et al Radiat Oncol (2021) 16:84 reduced local control if a minimum BED of 100 Gy cannot be achieved [8,9,10]. Stereotactic MR-guided online adaptive radiation therapy (SMART) has been suggested to overcome the limitations of low soft-tissue contrast IGRT by combining daily MR based treatment adaptation and replanning with MR based target localization and continuous real-time tracking of the moving target
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