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Penile bulb sparing in prostate cancer radiotherapy

Dose analysis of an in-house MRI system to improve contouring

Schonen der Peniswurzel während der Strahlentherapie bei Prostatakrebs

Dosisanalyse eines im Hause befindlichen MRT-Systems zur Verbesserung der Konturierung

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Abstract

Objective

This study aimed to assess the reduction in dose to the penile bulb (PB) achieved by MRI-based contouring following drinking and endorectal balloon (ERB) instructions.

Patients and methods

A total of 17 prostate cancer patients were treated with intensity-modulated radiation therapy (IMRT) and interstitial brachytherapy (IBT). CT and MRI datasets were acquired back-to-back based on a 65 cm3 air-filled ERB and drinking instructions. After rigid co-registration of the imaging data, the CT-based planning target volume (PTV) used for treatment planning was retrospectively compared to an MRI-based adaptive PTV and the dose to the PB was determined in each case. The adapted PTV encompassed a caudally cropped CT-based PTV which was defined on the basis of the MRI-based prostate contour plus an additional 5 mm safety margin.

Results

In the seven-field IMRT treatment plans, the MRI-based adapted PTV achieved mean (Dmean) and maximum (Dmax) doses to the PB which were significantly lower (by 7.6 Gy and 10.9 Gy, respectively; p <0.05) than those of the CT-contoured PTV. For 6 patients, the estimated PB Dmax (seven-field IMRT and IBT) for the adapted PTV was <70 Gy, whereas only 1 patient fulfilled this criterium with the CT-based PTV.

Conclusion

MRI-based contouring and seven-field IMRT-based treatment planning achieved dose sparing to the PB. Whereas the comparison of MRI and CT contouring only relates to external beam radiotherapy (EBRT) sparing, considering EBRT and IBT shows the improvement in PB sparing for the total treatment.

Zusammenfassung

Ziel

Ziel war es, eine Dosisreduktion an der Peniswurzel auf Grundlage einer Magnetresonanztomographie-(MRT-)unterstützten Konturierung unter Einhaltung eines Trink- und Rektalballonprotokolls (ERB) zu erreichen.

Patienten und Methoden

Insgesamt wurden 17 Patienten mit Prostatakrebs intensitätsmoduliert bestrahlt (IMRT) und erhielten anschließend eine interstitielle Brachytherapie (IBT). Computertomographie- (CT) und MRT-Datensätze wurden kurz nacheinander auf Basis eines 65-cm3-ERB- und Trinkprotokolls akquiriert und danach rigide registriert. In beiden Datensätzen wurde jeweils die Peniswurzeldosis bestimmt. Ein CT-basiertes Planungszielvolumen (PTV) wurde für die Bestrahlungsplanung verwendet und retrospektiv mit einem MRT-basierten adaptiven PTV verglichen. Das adaptive PTV umfasst ein kaudal gekürztes CT-basiertes PTV, welches auf Grundlage einer MRT-basierten Prostatakontur und einem zusätzlichen 5‑mm-Sicherheitsraum definiert wurde.

Ergebnisse

Das MRT-basierte adaptive PTV erzielte bei der 7‑Felder-IMRT-Planung 7,6 Gy (Dmean) und 10,9 Gy (Dmax) weniger Dosis an der Peniswurzel als das CT-konturierte PTV. Für 6 Patienten, geplant mit dem adaptiven PTV, war die abgeschätzte Maximaldosis Dmax (7-Felder-IMRT + IBT) an der Peniswurzel <70 Gy, wohingegen beim CT-basierten PTV nur 1 Patient das Kriterium erfüllte.

Schlussfolgerung

Die MRT-basierte Konturierung und die 7‑Felder-IMRT-Planung reduzierte die Peniswurzeldosis. Die Gesamtdosis der Peniswurzel bestehend aus der 7‑Felder-IMRT- und IBT-Planung zeigte sich reduziert, wobei die MRT- und CT- Konturierungsanalyse nur auf die externe 7‑Felder-Bestrahlungsplanung basiert.

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Acknowledgements

The presented work was performed in partial fulfillment of the requirements for obtaining the degree Dr. rer. biol. hum. at the Friedrich-Alexander-Universität (FAU). The authors would like to thank Asa Ammarin and Stefan Vasiliniuc for language editing and improving the quality of the manuscript.

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Authors and Affiliations

  1. Department of Radiation Oncology, Universitätsklinikum Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg, Universitätsstraße 27, 91054, Erlangen, Germany

    F. Böckelmann, S. Lettmaier, R. Fietkau,  C. Bert & F. Putz

  2. Department of Radiology, Universitätsklinikum Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg, Maximiliansplatz 1, 91054, Erlangen, Germany

    M. Hammon

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  1. F. Böckelmann
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Correspondence to C. Bert.

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F. Böckelmann, M. Hammon, S. Lettmaier, R. Fietkau, C. Bert, and F. Putz declare that they have no competing interests.

Caption Electronic Supplementary Material

66_2018_1377_MOESM1_ESM.docx

Supplementary I: Commissioning of an in-house 1.5 T MRI hyperthermia hybrid system; Table 1: MRI sequence parameters for ACR phantom protocol performed with CP Head Array coil; Table 2: Results of MRI Quality Assurance.

Supplementary II: Table 1: Sequence parameters for MRI prostate protocol performed with CP Body Array Flex coil at 1.5 Tesla; Figure 1: Example of a standard temporary IBT boost irradiation with needles in situ.

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Böckelmann, F., Hammon, M., Lettmaier, S. et al. Penile bulb sparing in prostate cancer radiotherapy. Strahlenther Onkol 195, 153–163 (2019). https://doi.org/10.1007/s00066-018-1377-0

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