ESTRO 2024 - Abstract Book

S4719

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

a prescription dose (D pres ) of 60 Gy in 30 fractions. The plan created for each robustness level was robustly re evaluated on all reCTs (with a setup uncertainty of 2 mm independent of setup uncertainty in the planning stage, and a range uncertainty of 2.6%) to assess whether the clinical constraints (e.g., target coverage - D 98% ≥ 0.95 x D pres in the worst-case, and OAR dose – lungs-GTV and heart D mean < 20 Gy in the nominal case, spinal cord D 2% < 40 Gy in the worst-case) were met or if adaptation was required. OAR planned dose metrics were used alongside clinical factors to calculate the NTCP for esophagitis, pneumonitis grade 2 or higher, and mortality 2 years post-treatment.

Results:

For the 3, 6 and 9 mm robustness levels, adaptation rates of 88%, 70% and 58%, respectively, were observed. This was as expected, as an increase in robustness will decrease the susceptibility to anatomical changes. Out of the 40 patients, only 4 (10%) passed our clinical thresholds for all robustness levels and all reCTs, while 5 patients (13%) only passed for 9 mm plans, and 8 patients (20%) passed only for 9 and 6 mm plans. 22 patients (55%) did not pass for any robustness level, and one (2.5%) only passed in the 3 mm plan. The two main factors leading to need of adaptation were target underdosage and spinal cord overdosage. Average gains in OAR dose sparing between the plans can be seen in Table 1. A mean absolute NTCP gain of 1.4 percentage points (pp) was calculated for pneumonitis grade ≥2 when transitioning from 9 mm plans to 6 mm plans, and of 1.5 pp when transitioning from 6 mm plans to 3 mm plans. A mean NTCP gain of 3.5 pp was calculated for esophagitis grade ≥2 when transitioning from 9 mm to 6 mm plans, and 4.1 pp when transitioning from 6 mm to 3 mm plans. For mortality risk 2 years post-treatment, a mean absolute NTCP gain of 1.2 pp was calculated when transitioning from 9 mm to 6 mm plans, and 1.3 pp when transitioning from 6 mm to 3 mm plans. On an individual patient basis, maximum NTCP gains of 10.2 pp for pneumonitis, 19.6 pp for esophagitis and 4.0 pp for mortality risk were recorded when transitioning from 9 mm to 3 mm plans. Based on the difference in NTCP between 9 mm and 3 mm cases, an NTCP gain per mm of the setup uncertainty of 0.48% for pneumonitis grade ≥2, 1.26% for esophagitis grade ≥2 and 0.43% for mortality 2 tears post-treatment was determined.

OAR

Δ6-3 mm (Gy)

Δ9-6 mm (Gy)

Lungs-GTV D mean

1.1

1.0

Heart D mean

0.8

0.8

Spinal cord D 2%

4.2

4.5

Tab. 1 - OAR sparing between robustness levels.