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Calculation of set-up margin in frameless stereotactic radiotherapy accounting for translational and rotational patient positing error

1 Department of Medical Physics, Apollo Proton Cancer Centre, Chennai; Department of Medical Physics, Bharathiar University, Coimbatore, Tamil Nadu, India
2 Department of Radiation Oncology, Apollo Multispeciality Hospital, Kolkata, West Bengal; Department of Physics, GLA University, Mathura, UP, India
3 Department of Radiation Oncology, Manipal Hospital, Delhi, India
4 Department of Radiation Oncology, Fortis Memorial Research Institute, Delhi, India
5 Department of Radiotherapy, Government General Hospital, Guntur, Andhra Pradesh, India
6 Director, Centre of Cosmology, Astrophysics and Space Science (CCASS), GLA University, Mathura, UP, India
7 Department of Medical Physics, Bharathiar University, Coimbatore, Tamil Nadu, India

Correspondence Address:
Biplab Sarkar,
Apollo Multispeciality Hospitals, Kolkata, West Bengal
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.jcrt_359_21

Context: Rotation corrected set-up margins in stereotactic radiotherapy (SRT). Aims: This study aimed to calculate the rotational positional error corrected set-up margin in frameless SRT. Settings and Design: 6D setup errors for the steriotactic radiotherapy patients were converted to 3D translational only error mathematically. Setup margins were calculated with and without considering the rotational error and compared. Materials and Methods: A total of 79 patients of SRT each received >1 fraction (3–6 fractions) incorporated in this study. Two cone-beam computed tomography (CBCT) scans were acquired for each session of treatment, before and after the robotic couch-aided patient position correction using a CBCT. The postpositional correction set-up margin was calculated using the van Herk formula. Further, a planning target volume_R (PTV_R) (with rotational correction) and PTV_NR (without rotational correction) were calculated by applying the rotation corrected and uncorrected set-up margins on the gross tumor volumes (GTVs). Statistical Analysis Used: General. Results: A total of 380 sessions of pre- (190) and post (190) table positional correction CBCT was analyzed. Posttable position correction mean positional error for lateral, longitudinal, and vertical translational and rotational shifts was (x)-0.01 ± 0.05 cm, (y)-0.02 ± 0.05 cm, (z) 0.00 ± 0.05 cm, and (θ) 0.04° ± 0.3°, (Φ) 0.1° ± 0.4°, (Ψ) 0.0° ± 0.4°, respectively. The GTV volumes show a range of 0.13 cc–39.56 cc, with a mean volume of 6.35 ± 8.65 cc. Rotational correction incorporated postpositional correction set-up margin the in lateral (x), longitudinal (y) and vertical (z) directions were 0.05 cm, 0.12 cm, and 0.1 cm, respectively. PTV_R ranges from 0.27 cc to 44.7 cc, with a mean volume of 7.7 ± 9.8 cc. PTV_NR ranges from 0.32 cc to 46.0 cc, with a mean volume of 8.1 ± 10.1 cc. Conclusions: The postcorrection linear set-up margin matches well with the conventional set-up margin of 1 mm. Beyond a GTV radius of 2 cm, the difference between PTV_NR and PTV_R is ≤2.5%, hence not significant.

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