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Contouring & planning variability in stereotactic radiosurgery: How to assess and address the weakest link in stereotactic radiosurgery?
Stockholm University, Faculty of Science, Department of Physics.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of stereotactic radiosurgery (SRS) employing one or a few fractions of high doses of radiation has continuously increased due to the technical development in dose delivery and morphological and functional imaging. As the target volume in SRS is usually defined without margins, the treatment success critically depends on accurate definition and contouring of the target volume and organs at risk (OARs) which are commonly situated in the proximity of the target making their precise delineation particularly important in order to limit possible normal tissue complications. Subsequent treatment planning is reliant on these volumes, which makes the accurate contouring a requisite to high quality treatments. 

The purpose of this work was to evaluate the current degree of variability for target and OAR contouring and to establish methods for analysing multi-observer data regarding structure delineation variability. Furthermore, this was set in a broader picture including the importance of contouring studies, the clinical implications of contouring errors and the possible mitigation of the variability in contouring by robust treatment planning.

A multi-centre target and OAR contouring study was initiated. Four complex and six common cases to be treated with SRS were selected and subsequently distributed to centres around the world performing Gamma Knife® radiosurgery for delineation and treatment planning. The resulting treatment plans and the corresponding delineated structures were collected and analysed.

Results showed a very high variability in contouring for the four complex radiosurgery targets. Similar results indicating high variability in delineating the common targets and OARs were also reported. This emphasised the need of continuous work towards consistent and standardized SRS treatments. Consequently, the results of the OAR analysis were incorporated in an effort to standardize stereotactic radiosurgery (SRS). Variations in treatment planning were as well analysed for several of the indications included in the initial study on contour delineation and the results showed a high variability in planned doses including several plans presenting large volumes of the brain receiving a higher dose than 12 Gy, indicating an elevated risk of normal tissue complications.

The results of the contouring work were, as a last step of this thesis, used as input for a robust treatment planning approach considering the variability in target delineation. The very preliminary results indicate the feasibility of the probabilistic approach and the potential of robust treatment planning to account for uncertainties in target extent and location.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2019. , p. 86
Keywords [en]
stereotactic radiosurgery, contouring variability, robust treatment planning
National Category
Other Physics Topics
Research subject
Medical Radiation Physics
Identifiers
URN: urn:nbn:se:su:diva-173275ISBN: 978-91-7797-785-8 (print)ISBN: 978-91-7797-786-5 (electronic)OAI: oai:DiVA.org:su-173275DiVA, id: diva2:1352278
Public defence
2019-11-01, CCK lecture hall, building R8, Karolinska University Hospital Solna, Solna, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Submitted. Paper 5: Manuscript.

Available from: 2019-10-09 Created: 2019-09-18 Last updated: 2019-10-09Bibliographically approved
List of papers
1. Variability in target delineation for cavernous sinus meningioma and anaplastic astrocytoma in stereotactic radiosurgery with Leksell Gamma Knife Perfexion
Open this publication in new window or tab >>Variability in target delineation for cavernous sinus meningioma and anaplastic astrocytoma in stereotactic radiosurgery with Leksell Gamma Knife Perfexion
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2014 (English)In: Acta Neurochirurgica, ISSN 0001-6268, E-ISSN 0942-0940, Vol. 156, no 12, p. 2303-2313Article in journal (Refereed) Published
Abstract [en]

Background: Radiosurgery clinical practice relays on empirical observations and the experience of the practitioners involved in determining and delineating the target and therefore variability in target delineation might be expected for all the radiosurgery approaches, independent of the technique and the equipment used for delivering the treatment. The main aim of this study was to quantify the variability of target delineation for two radiosurgery targets expected to be difficult to delineate. The secondary aim was to investigate the dosimetric implications with respect to the plan conformity. The primary aim of the study has therefore a very general character, not being bound to one specific radiosurgery technique.

Materials and methods: Twenty radiosurgery centers were asked to delineate one cavernous sinus meningioma and one astrocytoma and to plan the treatments for Leksell Gamma Knife Perfexion. The analysis of the delineated targets was based on the calculated 50% agreement volume, AV50. The AV50 was compared to each delineated target by the concordance index and discordance index. The differences in location, size, and shape of the delineated targets were also analyzed using the encompassing volume compared to the common volume, i.e., the AV100, of all delineated structures.

Results: Target delineation led to major differences between the participating centers and therefore the AV50 was small in comparison to each delineated target volume. For meningioma, the AV50 was 5.90 cm3, the AV100 was 2.60 cm3, and the encompassing volume was 13.14 cm3. For astrocytoma, the AV50 was 2.06 cm3 while the AV100 was extremely small, only 0.05 cm3, and the encompassing volume was 43.27 cm3. These variations translate into corresponding discrepancies in plan conformity.

Conclusions: Significant differences in shape, size, and location between the targets included in this study were identified and therefore the clinical implications of these differences should be further investigated.

Keywords
Leksell Gamma Knife Perfexion; Gamma Knife radiosurgery; Radiosurgery; Target delineation; Astrocytoma; Meningioma; Inter-observer variability
National Category
Cancer and Oncology
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-107221 (URN)10.1007/s00701-014-2235-1 (DOI)000345097700012 ()
Available from: 2014-09-06 Created: 2014-09-06 Last updated: 2019-09-18Bibliographically approved
2. Multi-institutional study of the variability in target delineation for six targets commonly treated with radiosurgery
Open this publication in new window or tab >>Multi-institutional study of the variability in target delineation for six targets commonly treated with radiosurgery
2018 (English)In: Acta Oncologica, ISSN 0284-186X, E-ISSN 1651-226X, Vol. 57, no 11, p. 1515-1520Article in journal (Refereed) Published
Abstract [en]

Background: Although accurate delineation of the target is a key factor of success in radiosurgery there are no consensus guidelines for target contouring.

Aim: The aim of the present study was therefore to quantify the variability in target delineation and discuss the potential clinical implications, for six targets regarded as common in stereotactic radiosurgery.

Material and methods: Twelve Gamma Knife centers participated in the study by contouring the targets and organs at risks and performing the treatment plans. Analysis of target delineation variability was based on metrics defined based on agreement volumes derived from overlapping structures following a previously developed method. The 50% agreement volume (AV50), the common and the encompassing volumes as well as the Agreement Volume Index (AVI) were determined.

Results: Results showed that the lowest AVI (0.16) was found for one of the analyzed metastases (range of delineated volumes 1.27–3.33 cm3). AVI for the other two metastases was 0.62 and 0.37, respectively. Corresponding AVIs for the cavernous sinus meningioma, pituitary adenoma and vestibular schwannoma were 0.22, 0.37 and 0.50.

Conclusions: This study showed that the variability in the contouring was much higher than expected and therefore further work in standardizing the contouring practice in radiosurgery is warranted.

National Category
Cancer and Oncology
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-158198 (URN)10.1080/0284186X.2018.1473636 (DOI)000451615600013 ()2-s2.0-85047265665 (Scopus ID)
Available from: 2018-07-23 Created: 2018-07-23 Last updated: 2019-09-18Bibliographically approved
3. Assessment of organs-at-risk contouring practices in radiosurgery institutions around the world - The first initiative of the OAR Standardization Working Group
Open this publication in new window or tab >>Assessment of organs-at-risk contouring practices in radiosurgery institutions around the world - The first initiative of the OAR Standardization Working Group
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2016 (English)In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 121, no 2, p. 180-186Article in journal (Refereed) Published
Abstract [en]

Materials and methods

Imaging datasets for typical lesions (cavernous sinus meningioma, vestibular schwannoma, pituitary adenoma) treated with Leksell Gamma Knife Perfexion were circulated to 12 centers. Observers were asked to contour the target and OARs as per their standard clinical practice. The analyzed parameters were the intersection (AV100), union volumes (AV100/N) and the 50% agreement volume (AV50). The ratio of AV100 and AV100/N (the Agreement Volume Index, AVI) was used as a measure of agreement level together with a generalized conformity index (CIgen) and a pairwise averaged conformity index (CIpairs). The maximum doses were also determined.

Results

Results showed a wide variability in terminology, choice of structures contoured and in the size and shape of the contoured structures. The highest variability was observed for the left and right optic tract for cavernous sinus meningioma where the AV100 was zero. The highest consistency was observed for the right optic nerve in the cavernous sinus case followed by the cochlea for the vestibular schwannoma case for which the AVI was still only 0.13 and 0.054, respectively. Corresponding results for the CIgen and CIpairs also showed the highest variability for the right optic tract and the highest consistency in contours for the right optic nerve, both in the cavernous sinus meningioma case.

Conclusion

The results quantify the large variability in OAR contouring in clinical practice across Gamma Knife radiosurgery centers with respect to the choice of OARs to be contoured, nomenclature and size and shape of OARs. This motivates future effort to standardize practices to enable more effective collaboration.

Keywords
Optic nerve, Chiasm, Cochlea, Stereotactic radiosurgery, Inter-observer variability
National Category
Cancer and Oncology
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-135331 (URN)10.1016/j.radonc.2016.10.014 (DOI)000390517600002 ()27816408 (PubMedID)
Available from: 2016-11-03 Created: 2016-11-03 Last updated: 2019-09-18Bibliographically approved
4. Simultaneous truth and performance level estimation method for evaluation of target contouring in radiosurgery – feasibility test and robustness analysis
Open this publication in new window or tab >>Simultaneous truth and performance level estimation method for evaluation of target contouring in radiosurgery – feasibility test and robustness analysis
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2019 (English)In: Physica medica (Testo stampato), ISSN 1120-1797, E-ISSN 1724-191XArticle in journal (Refereed) Submitted
National Category
Other Physics Topics
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-174734 (URN)
Available from: 2019-10-09 Created: 2019-10-09 Last updated: 2019-10-09Bibliographically approved
5. Treatment planning for Gamma Knife radiosurgery – assessment of variability and mitigation through probabilistic robust planning
Open this publication in new window or tab >>Treatment planning for Gamma Knife radiosurgery – assessment of variability and mitigation through probabilistic robust planning
(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-174735 (URN)
Available from: 2019-10-09 Created: 2019-10-09 Last updated: 2019-10-09Bibliographically approved

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