ESTRO 2024 - Abstract Book

S4124

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

modulated arc therapy (VMAT) plan (7 patients). Radiation treatment plans created on the diagnostic CT scan were then transferred on patient’s planning CT scan for dosimetric comparison, and the dose in the PTV, CTV, GTV and OAR on the planning CT scan was derived. The clinical goals included PTV: V95%≥99 % and V107%<1 % CTV: V98% ≥ 95 %, bladder V80%<40% and derived bowelbag structure V80%<100 cc. To assess the significance of differences in dosimetric parameters on the diagnostic CT scan and the planning CT scan a Wilcoxon signed-rank test was performed.

Results:

Mean GTV volume did not significantly differ between delineations on diagnostic CT scans and planning CT scans, yielding 62.79 cc (SD 24.03) and 58.68 cc (SD 21.48), respectively (p-value = 0.319). Mean CTV and PTV volumes were significantly larger on the diagnostic CT scans (p-values = ≤ 0.001), while for the organs at risk no significant differences were observed. On the diagnostic CT scans, all treatment plans had adequate PTV and CTV coverages and acceptable doses to OARs (Table 1). After recalculating these treatment plans on the planning CT scans, median PTV and CTV coverages were significantly lower (p-values < 0.003), with a median PTV V95% of 95.87% (interquartile range 5.43%) and a median CTV V98% of 78.6% (interquartile range 26.4%). None of the plans calculated on the planning CT scan scans met the prescribed planning constraints. For all except one the PTV coverage was too low (i.e. V95%<99 %); the other one had unacceptable hotspots (i.e., V107%>1 %). In addition, for the bladder and derived bowel bag the dosimetric criteria were significantly worse for the treatment plans recalculated on planning CT scans (Table 1).

Table 1. Dosimetric characteristics of treatment plans on the diagnostic CT scan and ones recalculated on the planning CT. Data is represented as median (interquartile range). P-values are reported for the Wilcoxon signed-rank test, where a star sign (*) points at significant differences between the dosimetric parameters.

Treatment

plan

on

Treatment plan on planning CT scan scan

p-value

diagnostic CT scan scan

PTV V 95% in %

99.98 (0.21)

95.87 (5.43)

< 0.001*

PTV V 107% in %

0.00 (0.01)

1.80 (6.01)

0.003

CTV V 98% in %

98.3 (1.00)

78.6 (26.40)

0.002*

Bladder V 80% in %

8.60 (7.62)

19.90 (10.05)

0.001*

Bowelbag V 80% in cc

33.04 (24.42)

106.34 (79.05)

< 0.001*

Conclusion:

Diagnostic CT scan based treatment plans showed inadequate target coverage and organs at risk sparing when recalculated on the planning CT scan in patients with rectal cancer. To overcome this, an online adaptive workflow to daily adapt the treatment plan on the diagnostic CT scan to the actual patient anatomy at the treatment machine is advocated. This online adaptive worfklow is currently being investigated in a prospective clinical trial in patients with rectal cancer receiving preoperative (chemo)radiotherapy (MEC 2023-0445).