ESTRO 2024 - Abstract Book

S5501

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

Material/Methods:

Seven patients with peripheral lung metastases underwent single-fraction 34 Gy SBRT. The diameter of the GTV was required to be less than 2 cm and tumor motion was limited (size of internal gross target volume, iGTV) was limited to less than 2 times the length of GTV. The treatment plan consisted of three FFF VMAT arcs of about 180°. The PTV was defined as the iGTV with a 5 mm expansion. A minimum of five CBCTs were acquired during the single fraction: initial set-up verification, initial correction verification, in-between arc 1 and 2, in-between arc 2 and 3 and post treatment. First a 4D CBCT was obtained to evaluate the tumor movement, subsequently only 3D CBCTs were acquired. An applied set-up correction was always verified by an (additional) CBCT. A dual match approach was applied. An initial bone match (translations + rotations) was succeeded by a grey value match (translations only), limited to the volume of the iGTV +5 mm. The action level for initial set-up was 0 mm vector and 3 mm vector for setup verification and in-between arcs. Treatment duration was defined as the difference between the acquisition time of the first and last CBCT. For each of the three VMAT arcs, the IA motion was assessed for both patient and tumor by subtracting the match results of the last CBCT before the arc and the first CBCT after the arc, for respectively the bone and soft tissue (tumor) match. The IF motion was measured as the difference between match results of the initial correction verification and the post treatment CBCT, increased with the sum of all setup-corrections applied between the arcs.

Results:

A total of 42 CBCTs were analysed. Couch corrections were carried out after initial set-up verification, in one patient after initial correction verification and in five patients also after in-between arcs. The average treatment duration was 25.5 ± 2.5 min. The average IF motion of the patient (bone match) amounted 1.7 ± 1.0 mm vector. The average vector length IA motion of the tumor amounted 1.7 ± 0.8 mm, 1.8 ± 1.0 mm and 1.0 ± 0.5 mm for the first, second and third arc respectively (average 1.5 mm). If inter-arc set-up corrections would not have been applied, this vector length would have been 3.2 ± 1.9 mm, more than twice the average IA motion.

Conclusion:

To assess the importance of inter-arc position verification for single-fraction 34 Gy lung SBRT, the total tumor motion that would have occurred without applied corrections should be compared to the average IA motion. Our results show that the inter-arc position verification leads to a reduction of the position error of 50%, underpinning the importance of inter-arc CBCT position verification.

Keywords: Single-fraction, position verification, SBRT lung

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