Session Item

Stereotactic MR guided online adaptive radiotherapy (SMART) offers significant dosimetric advantages in the treatment of abdominal and pelvic malignancies due to complex surrounding anatomy and mobile organs at risk^1,2 Here, we report patient tolerability, acute toxicity and dosimetric outcomes of SMART in a cohort of patients with abdominal and pelvic lymph node metastases. 

Patients who underwent SMART between 01.01.2020 and 31.09.2021 were retrospectively included. Acute toxicity was prospectively assessed using Common Terminology Criteria for Adverse Events (CTCAE) v4.0.

Dose-volume histograms (DVHs) for target volumes and OARs were obtained for every treatment course at the following time points:

1. Baseline: simulation plan applied to anatomy at simulation

2. Pre-adaptation: simulation plan applied to the anatomy of the day prior to each treatment fraction

3. Post-adaptation: re-optimized plan applied to the anatomy of the day prior to each treatment fraction

Target volume coverage and organ-at-risk violation metrics for initial non-adaptive and adaptive plans were compared using non-parametric Dunn method with Bonferroni correction for multiple comparisons.

N=15 patients (median age 58, range 39-76, 7/15 females (47%)) with oligometastatic lymph nodes (coeliac, pre-sacral, aortocaval, porta hepatis, mesenteric, common iliac and peri-pancreatic) were included (Table 1).

No CTCAE v4 G3 or higher toxicities were recorded on-treat or at two-week post treatment review.

Median GTV and PTV volumes on reference plan were 13.51cc (IQR 5.73-61.44) and 50.45 (IQR 16.97-108.64) respectively. Median dose prescription was 40Gy (range 30-50Gy) in 3-5 fractions. All initial plans met hard OAR constraints.

In total, 62 fractions were delivered, using daily adaptive plans with data available for 55 fractions. Target re-contouring resulted in median 0.24% (IQR -2.68 – 3.1%) volume change in GTV. There were no OAR violations on the base plans. On pre-adaptation plans, OAR violation occurred in 37/55 (67.27%) of fractions versus 5/55 on post-adaptation plans (9.09%; p<.0001**; Figure 1) Re-optimisation achieved a mean dose reduction to duodenum and stomach within 3cm of PTV of 27.46% (95%CI 33.34-21.57) and 28.03% (95%CI 36.24-19.82) in D(0.01cc) and D(0.5cc), respectively, whilst significantly improving PTV100, PTV95 and GTV100 coverage as well as GTVmean dose (Table 1, Figure 1).

Our data confirms findings from similar series³ that SMART is well tolerated, safe and offers a favourable dosimetric profile and an improved therapeutic index in abdominal and pelvic sites