ESTRO 2024 - Abstract Book

S2906

Interdiscplinary - Other

ESTRO 2024

Results:

region

# CTs

dDRL

75th perc.

med.

std.

abdomen & pelvis 645

12.0

12.1

9.7

3.8

head & neck

348

15.0

17.9

17.9

0

skull

391

55.0

58.2

56.6

2.3

thorax

453

8.0

8.4

6.8

2.6

Table 1: Results of the dosimetric analysis. The dDRLs are shown in column 3 in mGy. The 75th percentile, median value and standard deviation of the CTDI are displayed in columns 4-6, all in mGy.

Results of the dosimetric analysis are shown in table 1. The 75th percentiles of the CTDIs were in general found to be slightly higher than the respective dDRLs. For acquisitions in the head & neck region, dose modulation is disabled following recommendations of the manufacturer. Therefore, the CTDI was always 17.9 mGy, exceeding the dDRL by ca. 19%. For skull and thorax, the dDRL was exceeded by 6% and 5%, respectively. For abdomen & pelvis, the difference between the 75th percentile and the dDRL was smaller than 1%. The exceeding of the dDRLs by the 75th percentiles found in this analysis may be due to the differences between CT acquisitions in diagnostics and radiotherapy: Firstly, a fixed tube voltage must be used for planning CTs in radiotherapy, excluding the possibility to optimise CT protocols by varying the tube voltage. Furthermore, the priority for patients undergoing radiotherapy lies in acquiring a high-quality CT scan well suited for dose calculation, rather than minimising imaging dose at all costs. Following the ALARA principle, planning CT dose in radiotherapy should nevertheless be closely monitored and optimised. Custom-made DMSs like PyCT can help to achieve that goal.

Conclusion:

A custom-made DMS was successfully developed and implemented. Evaluating CTDIs in comparison to dDRLs yielded slightly higher values of the 75th percentile for all regions. These findings may be used to optimise planning CT dose in the future by monitoring it and comparing it to the local DRLs. Data comparison between different radiotherapy departments in Germany could help to further optimise planning CT doses and establish national DRLs for CT acquisitions in radiotherapy.

Keywords: DRLs, radiotherapy CT, custom programming

References:

[1] Ordinance on Protection against the Harmful Effects of Ionising Radiation (Radiation Protection Ordinance). Chapter 7 § 122. (2018). url: https://www.bfs.de/EN/bfs/laws-regulations/ordinances-radiation-protection ion/ordinances-on-radiation-protection_node.html.