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Metastasis-directed radiotherapy (MDRT) using SBRT is highly recommended for treatment of lymph node (oligo)metastases of prostate cancer. SBRT is applied using a high fraction dose, steep dose gradients and tight margins. However, treatment errors are easily induced by patient or target motion. Patient intra-fraction motion has been reported on previously, however, intra-fraction motion of lymph node oligometastasis on CBCT has not yet been described, and current margins are based on individual centre experience. The aim of this study was to analyse pelvic lymph node motion on CBCT and to derive margin estimations for SBRT.

Motion analysis included 18 pelvic lymph node metastases in 13 patients treated with IGRT in 5 fractions of 7 Gy (every other day) to the PTV. CBCT linac with 3D couch was used, performing CBCT before and after each fraction. The entire targeted lymph node (GTV) was delineated by one observer on planning CT and 179 CBCTs. CBCTs were matched with the planning CT using a rigid match with verification mask containing adjacent bony anatomy for translations only. GTV centre of mass displacement was calculated to identify lesion inter- and intra-fraction translational motions in the left-right (LR), anterior-posterior (AP) and superior-inferior (SI) patient direction. The systematic and random population errors were derived, and margins were calculated (van Herk prescription). Patient bony anatomy intra-fraction motion was independently included in our calculation. Additionally, we described target coverage of a 3 mm margin added to the planning GTV using the inclusiveness index (GTV volume covered / total GTV volume). This inclusiveness index was derived for all pre- and post-fraction GTV positions.

The maximum observed lesion intra-fraction translations LR, AP and SI were 3.3, 4.5 and 3.6 mm, respectively. (Fig 1A) The mean population systematic errors were 0.54 (LR), 0.78 (AP), and 0.47 (SI), and random errors were 0.68, 0.79, and 0.98. These intra-fraction lesion motion errors translated in margins of 1.8, 2.5 and 1.9 mm, respectively. Including intra-fraction patient motion increased the estimated margin to 2.8, 3.3 and 2.3 mm. Lesion inter-fraction translations were maximum 2.8, 6.1 and 5.3 mm, and including inter-fraction translations in margin calculation (simulating bony match only) resulted in a margin of 4.2 (LR), 5.4 (AP) and 4.0 mm (SI). (Table 1)
GTV volume on planning CT ranged from 0.17 cm3 to 3.22 cm3 (median 0.54 cm3). Lesion volume showed a significant decrease during radiotherapy. (Fig 1B) The expanded GTV (margin 3 mm) had a median volume of 2.40 cm3, and the mean inclusiveness index was 98.4%. An inclusiveness index of at least 95% was achieved in 95.9% of all target positions.

Intra-fraction motion presents an important target variation influencing treatment results in SBRT. Margin calculation for intra-fraction motion of pelvic lymph node metastasis resulted in an estimated margin of 2.8, 3.3, and 2.3 mm.