Dosimetric Impact of Interfractional Variations in Prostate Cancer Radiotherapy-Implications for Imaging Frequency and Treatment Adaptation

Tilman Bostel; Ilias Sachpazidis; Mona Splinter; Nina Bougatf; Tobias Fechter; Constantinos Zamboglou; Oliver Jäkel; Peter E Huber; Dimos Baltas; Jürgen Debus; Nils H Nicolay

doi:10.3389/fonc.2019.00940

Dosimetric Impact of Interfractional Variations in Prostate Cancer Radiotherapy-Implications for Imaging Frequency and Treatment Adaptation

Front Oncol. 2019 Sep 27:9:940. doi: 10.3389/fonc.2019.00940. eCollection 2019.

Authors

Tilman Bostel^{1

2

3}, Ilias Sachpazidis^{4

5}, Mona Splinter^{6

7}, Nina Bougatf^{1

2

6}, Tobias Fechter^{4

5}, Constantinos Zamboglou^{4

5}, Oliver Jäkel^{6

7}, Peter E Huber^{1

2

6}, Dimos Baltas^{4

5}, Jürgen Debus^{1

2

6}, Nils H Nicolay^{1

4

5}

Affiliations

¹ Clinical Cooperation Unit "Radiation Oncology", German Cancer Research Center, Heidelberg, Germany.
² Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany.
³ Department of Radiation Oncology, University Medical Center Mainz, Mainz, Germany.
⁴ Department of Radiation Oncology, University of Freiburg Medical Center, Freiburg, Germany.
⁵ German Cancer Consortium (DKTK), Partner Site Freiburg, German Cancer Research Center, Heidelberg, Germany.
⁶ Heidelberg Institute of Radiation Oncology, National Center for Radiation Research in Oncology, Heidelberg, Germany.
⁷ Medical Physics in Radiation Oncology, German Cancer Research Center, Heidelberg, Germany.

Abstract

Background and purpose: To analyze deviations of the applied from the planned doses on a voxel-by-voxel basis for definitive prostate cancer radiotherapy depending on anatomic variations and imaging frequency. Materials and methods: Daily in-room CT imaging was performed in treatment position for 10 patients with prostate cancer undergoing intensity-modulated radiotherapy (340 fraction CTs). Applied fraction doses were recalculated on daily images, and voxel-wise dose accumulation was performed using a deformable registration algorithm. For weekly imaging, weekly position correction vectors were derived and used to rigidly register daily scans of that week to the planning CT scan prior to dose accumulation. Applied and prescribed doses were compared in dependence of the imaging frequency, and derived TCP and NTCP values were calculated. Results: Daily CT-based repositioning resulted in non-significant deviations of all analyzed dose-volume, conformity and uniformity parameters to the CTV, bladder and rectum irrespective of anatomic changes. Derived average TCP values were comparable, and NTCP values for the applied doses to the bladder and rectum did not significantly deviate from the planned values. For weekly imaging, the applied D₂ to the CTV, rectum and bladder significantly varied from the planned doses, and the CTV conformity index and D₉₈ decreased. While TCP values were comparable, the NTCP for the bladder erroneously appeared reduced for weekly repositioning. Conclusions: Based on daily diagnostic quality CT imaging and voxel-wise dose accumulation, we demonstrated for the first time that daily, but not weekly imaging resulted in only negligible deviations of the applied from the planned doses for prostate intensity-modulated radiotherapy. Therefore, weekly imaging may not be adequately reliable for adaptive treatment delivery techniques for prostate. This work will contribute to devising adaptive re-planning strategies for prostate radiotherapy.

Keywords: dosimetry; image-guided radiotherapy; organs-at-risk; prostate cancer; tumor control probability.