ESTRO 2024 - Abstract Book

S4780

Physics - Quality assurance and auditing

ESTRO 2024

made in the CT acquisition phase. IVD is performed during each treatment fraction at 6 different anatomical locations (groin (G), oral cavity (C), neck (N), apical (AL) and basal lung (BL), hips (H)) with silicon diodes or thermoluminescent dosimeters (TLD) and used as monitoring. Also, IVD measurements could be used to adjust the MUs during the treatment course if necessary. Midline dose is estimated at these anatomical locations from entry and exit dose readouts. Midline doses and delivered deviation from estimate MUs (ΔMU) were retrieved from 103 patients treated between 2012 and 2023 analyzing a total of 537 treatment fractions. Two different myeloablative prescription schedules were considered: 12 Gy in 6 bi-daily fractions or 9.9 Gy in 3 daily fractions. Dose difference consistency has been evaluated between treatment fractions, focusing on the first and last fractions to get an idea of the dose delivery pattern during therapy. A Wilcoxon-Mann-Whitney signed-ranked test (p<0.05) is used to evaluate significant differences.

Results:

The median ΔMU and minimum and maximum range was -1.2% [-7.1% – 3.6%] and in 5% of patients the thickness of lung compensators was changed during treatment. Median deviation and minimum and maximum range between measured and prescribed doses at the end of treatment were G 0.1% [-6.7% – 8.7%], C -1.6% [-6.9% – 3.0%], N 0.8% [- 5.8% – 11.4%], AL -0.1% [-8.6% – 4.1%], BL -0.6% [-8.1% – 4.7%] and H -1.0% [-7.2% – 5.7%] ( Figure 1 ). Significant differences between first and last treatment fractions were found in patients undergoing 3 fractions in 4 out of 6 anatomical locations (G, AL, BL, H) while for patients with the 6 fractions schedule significative differences were found only in the C region. Considering midline doses, total estimates percentage dose difference resulted outside the ±10% range in 2% of treatment fractions. Cumulative percentage dose difference has been evaluated session after session showing a tendency to decrease towards the end of treatment ( Figure 2 ).