Gross Tumour Volume dynamics during MRI-guided stereotactic body radiation therapy.
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Gross Tumour Volume dynamics during MRI-guided stereotactic body radiation therapy Background One of the challenges of stereotactic body radiation therapy (SBRT) is managing tumour and organ motion, with the use of motion-encompassing planning target volumes potentially increasing the risk of toxicity. MR-guided radiotherapy (MRgRT) enables better visualisation of targets and OARs whilst allowing real-time adaptations to treatment plans based on daily anatomy, with the Unity MR-Linac allowing reference plan creation and daily plan modification via adaption to position and shape. Real-time adaptation using MRgRT has been shown to be beneficial for multiple tumour sites, including for example improved dosimetry and toxicities in malignancies of the pancreas, thorax and prostate and in the oligometastatic context. Purpose This study evaluated the degree of gross tumour volume (GTV) changes in patients who received MR-guided SBRT to abdominal and pelvic targets to assess the degree of GTV changes between simulation and fractions. Methods and Materials This analysis of a single-institution registry assessed patients treated with SBRT on the Unity MR-Linac. Between 2022-2025, 277 cases who received 25-50Gy in 2-6 fractions were enrolled. MR simulation imaging was performed on the MR-Linac and co registered with the CT planning scan for target delineation. Treatment sites and GTV changes were recorded between simulation and first fraction and all fractions. Patients who did not complete their course of treatment were excluded from the study. Results The most common treatment sites were the liver (36.8% n=102) followed by abdominal/pelvic lymph nodes (26.0%, n=72), pancreas (11.9%, n=33) and adrenals (10.4%, n=29). 85.2% of patients (n=236) received 5-6 fractions while 14.4% (n=40) received 3 fractions and 0.4 % (n=1) received 2 fractions. For the entire cohort, the median baseline GTV at simulation was 6.16 cm3 (range 0.15 to 252.72 cm3). The median GTVs for fraction 1 and final fraction were 6.50 cm3 and 7.14 cm3 respectively. The mean change in GTV volume between simulation and fraction 1 was +10.0% (0.73 cm3, range -66.2 to +277.7%). The mean change in GTV volume from simulation to final fraction was reduced at +9.9% (1.48 cm3, range -66.35 to +401.03%). 44.8% (n=124) of cases had GTVs decreasing in volume between the first and final fractions. Table 1 shows that the percentage of cases with decreasing GTV volume between first and final fractions is similar across treatment regions, with abdominal/pelvic nodes showing the most cases decreasing. Large variations >20% occur for more than one third of patients for all treatment regions except adrenals, with relatively more cases showing increased GTV volumes >20%. Table 1: Median GTV volume and the prevalence of decrease in GTV volume by treatment region between first and final fraction Treatment site Median GTV volume, cm3 (range) % of cases with decreasing GTV volume between first and final fractions % of cases with increase in GTV volume > 20% between first and final fraction % cases with decrease in GTV volume > 20% between first and final fraction All (n=277) 6.2 (0.2-225.1) 44.8% (n=124) +23.8% (n=66) -14.4% (n=40) Liver (n=102) 5.1 (0.2-252.7) 43.1% (n=44) +23.5% (n=24) -12.7% (n=13) Abdominal/ pelvic nodes (n=72) 2.7 (0.5-100.5) 48.6% (n=35) +25% (n=18) -19.4% (n=14) Pancreas (n=33) 20.6 (2.3-73.0) 45.5% (n=15) +18.2% (n=6) -18.2% (n=6) Adrenals (n=29) 10.2 (3.4-82.6) 44.8% (n=13) +20.7% (n=6) -6.9% (n=2) Turning to GTV changes between simulation to final fraction, 40.7% (n=113) of all cases had decreased volumes with 46.9% (n=130) having variations of >20%. In cases involving treatment to the liver, abdominal/pelvic nodes, pancreas and adrenals, decreasing GTVs between simulation to final fraction were observed in 40.7% (n=113), 36.3% (n=37), 45.8% (n=33), 42.4% (n=14) and 41.4% (n=12) of patients respectively. Conclusion The variations in GTV volume observed in MRgRT-guided SBRT across different treatment sites, both between the simulation and the first fraction, as well as throughout the course of treatment, emphasise the importance of adaptive replanning for enhancing dosimetry. The correlation between GTV variation and outcome data will be evaluated in future studies. References 1. van Sörnsen de Koste JR, Palacios MA, Bruynzeel AME, Slotman BJ, Senan S, Lagerwaard FJ. MR-guided Gated Stereotactic Radiation Therapy Delivery for Lung, Adrenal, and Pancreatic Tumors: A Geometric Analysis. Int J Radiat Oncol Biol Phys. 2018 Nov 15;102(4):858-866. doi: 10.1016/j.ijrobp.2018.05.048. Epub 2018 May 29. 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