Intercranial stereotactic RT
A comprehensive evaluation of treatment accuracy, including end-to-end tests and clinical data, applied to intracranial stereotactic radiotherapy

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Abstract

Background and purpose

A methodology is presented to quantify the uncertainty associated with linear accelerator-based frameless intracranial stereotactic radiotherapy (SRT) combining end-to-end phantom tests and clinical data.

Methods and materials

The following steps of the SRT chain were analysed: planning computed tomography (CT) and magnetic resonance (MR) scans registration, target volume delineation, CT and cone beam CT (CBCT) registration and intrafraction-patient displacement. The overall accuracy was established with an end-to-end test. The measured uncertainties were combined, deriving the total systematic (ΣT) and random (σT) error components, to estimate the GTV-PTV margin.

Results

The uncertainty in the MR-CT registration was on average 0.40 mm (averaged over AP, CC and LR directions). Rotational variations were smaller than 0.5° in all directions.

Interobser variation in GTV delineation was on average 0.29 mm.

The uncertainty in the CBCT-CT registration was on average 0.15 mm. Again, rotational variations were smaller than 0.5° in all directions.

The systematic and random intrafraction displacement errors were on average 0.55 mm and 0.45 mm, respectively.

The systematic and random positional errors from the end-to-end test were on average 0.49 mm and 0.53 mm, respectively.

Combining these uncertainties resulted in an average ΣT = 0.9 mm and σT = 0.7 mm and an average GTV-PTV margin of 2.8 mm.

Conclusion

This comprehensive methodology including end-to-end tests enabled a GTV-PTV margin calculation considering all sources of uncertainties. This generic method can also be used for other treatment sites.

Section snippets

Methods and materials

In Fig. 1 the stereotactic radiotherapy treatment chain at Catharina Hospital, Eindhoven, the Netherlands, is depicted.

For each step included in the uncertainty analysis, the method used to quantify the uncertainty is explained in the following sections. Subsequently, the end-to-end test is described. The last section describes how the data were combined to derive the GTV-PTV margin. All analyses have been performed and reported for each direction separately (AP: anterior-posterior; CC:

Registration MR-CT

The uncertainty in the MR-CT registration was found to be 0.32 mm in AP-direction, 0.57 mm in CC-direction and 0.33 mm in LR-direction (see Table 1). Rotational variations were smaller than 0.5° in all three directions (data not shown).

Delineation study

In Fig. 2 the GTV volumes delineated by the three observers are depicted. Observer 1 delineated systematically the largest volumes while observer 3 delineated mostly the smallest volume. The average CI was 0.76, range: 0.59–0.87. The CI among observers becomes

Discussion

In this work we presented a method to evaluate the overall uncertainty of a linear accelerator based frame-less stereotactic irradiation technique for brain metastasis using clinical data and performing end-to-end tests. The obtained information was used to assess the adequateness of the employed GTV-PTV margin. No GTV-CTV margin (GTV = CTV) is used because no microscopic expansions of brain metastasis are to be expected.

The analysis of MR-CT and CBCT-CT registration accuracy was in agreement

Conclusion

The comprehensive methodology presented here has enabled us to calculate the GTV-PTV margin we need to accommodate all uncertainties in the treatment chain. It also enables the detection of the most important sources of uncertainty. A 2.4–3.1 mm GTV-PTV margin is necessary to accommodate all uncertainties in our intracranial SRT treatment chain. We believe the presented approach can also be used to evaluate the accuracy of the treatment chain and the adequateness of the employed margins for

Conflicts of interest notification

No actual or potential conflicts of interest exist.

Acknowledgments

The authors like to thank the medical physicist engineers Dennis Hellegers, Jan Weterings and Sander van het Schip of the Catharina Hospital, Eindhoven for their support during the measurements. The authors are also grateful to the three radiation technologists Marjon Reinders, Fanny van Aarle and Maarten van Lieshout for the MR-CT registration study and the radiation oncologists Tom Budiharto en Marijel Monasch who took part in the delineation study.

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    Now at Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.

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