Interfractional Geometric Variations and Dosimetric Benefits of Stereotactic MRI Guided Online Adaptive Radiotherapy (SMART) of Prostate Bed after Radical Prostatectomy: Post-Hoc Analysis of a Phase II Trial

Minsong Cao; Yu Gao; Stephanie M. Yoon; Yingli Yang; Ke Sheng; Leslie K. Ballas; Vincent Basehart; Ankush Sachdeva; Carol Felix; Daniel A. Low; Michael L. Steinberg; Amar U. Kishan

doi:10.3390/cancers13112802

Cancers (Jun 2021)

Interfractional Geometric Variations and Dosimetric Benefits of Stereotactic MRI Guided Online Adaptive Radiotherapy (SMART) of Prostate Bed after Radical Prostatectomy: Post-Hoc Analysis of a Phase II Trial

Minsong Cao,
Yu Gao,
Stephanie M. Yoon,
Yingli Yang,
Ke Sheng,
Leslie K. Ballas,
Vincent Basehart,
Ankush Sachdeva,
Carol Felix,
Daniel A. Low,
Michael L. Steinberg,
Amar U. Kishan

Affiliations

Minsong Cao: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Yu Gao: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Stephanie M. Yoon: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Yingli Yang: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Ke Sheng: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Leslie K. Ballas: Department of Radiation Oncology, University of Southern California, Los Angeles, CA 90089, USA
Vincent Basehart: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Ankush Sachdeva: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Carol Felix: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Daniel A. Low: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Michael L. Steinberg: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA
Amar U. Kishan: Department of Radiation Oncology, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA

DOI: https://doi.org/10.3390/cancers13112802
Journal volume & issue: Vol. 13, no. 11
p. 2802

Abstract

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Purpose: To evaluate geometric variations of patients receiving stereotactic body radiotherapy (SBRT) after radical prostatectomy and the dosimetric benefits of stereotactic MRI guided adaptive radiotherapy (SMART) to compensate for these variations. Materials/Methods: The CTV and OAR were contoured on 55 MRI setup scans of 11 patients treated with an MR-LINAC and enrolled in a phase II trial of post-prostatectomy SBRT. All patients followed institutional bladder and rectum preparation protocols and received five fractions of 6−6.8 Gy to the prostate bed. Interfractional changes in volume were calculated and shape deformation was quantified by the Dice similar coefficient (DSC). Changes in CTV-V95%, bladder and rectum maximum dose, V32.5Gy and V27.5Gy were predicted by recalculating the initial plan on daily MRI. SMART was retrospectively simulated if the predicted dose exceeded pre-set criteria. Results: The CTV volume and shape remained stable with a median volumetric change of 3.0% (IQR −3.0% to 11.5%) and DSC of 0.83 (IQR 0.79 to 0.88). Relatively large volumetric changes in bladder (median −24.5%, IQR −34.6% to 14.5%) and rectum (median 5.4%, IQR − 9.7% to 20.7%) were observed while shape changes were moderate (median DSC of 0.79 and 0.73, respectively). The median CTV-V95% was 98.4% (IQR 94.9% to 99.6%) for the predicted doses. However, SMART would have been deemed beneficial for 78.2% of the 55 fractions based on target undercoverage (16.4%), exceeding OAR constraints (50.9%), or both (10.9%). Simulated SMART improved the dosimetry and met dosimetric criteria in all fractions. Moderate correlations were observed between the CTV-V95% and target DSC (R2 = 0.73) and bladder mean dose versus volumetric changes (R2 = 0.61). Conclusions: Interfractional dosimetric variations resulting from anatomic deformation are commonly encountered with post-prostatectomy RT and can be mitigated with SMART.

Published in Cancers

ISSN: 2072-6694 (Online)
Publisher: MDPI AG
Country of publisher: Switzerland
LCC subjects: Medicine: Internal medicine: Neoplasms. Tumors. Oncology. Including cancer and carcinogens
Website: https://www.mdpi.com/journal/cancers/

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