Preliminary Clinical Evaluation of Intrafraction Prostate Displacements for Two Immobilization Systems.

Farshad Mostafaei,Russell J Hamilton,Shona T Dougherty

doi:10.7759/cureus.10206

Abstract

Immobilization systems and their corresponding set-up errors influence the clinical target volume to the planning target volume (CTV-PTV) margins, which is critical for hypofractionated prostate stereotactic body radiotherapy (SBRT). This preliminary study evaluates intrafraction prostate displacement for two immobilization systems (A and B). Six consecutive patients having localized prostate cancer and implanted prostate marker seeds were studied. Planar X-ray images were acquired pre- and post-treatment to find the intrafraction prostate displacement. The average absolute displacements (lateral, longitudinal, vertical) were 0.9 ± 0.4 mm, 1.7 ± 0.1 mm, 1.3 ± 0.3 mm (system A), and 0.5 ± 0.2 mm, 0.6 ± 0.1 mm, 0.8 ± 0.3 mm (system B), with average three-dimensional displacements of 2.6 ± 0.2 mm (system A) and 1.3 ± 0.2 mm (system B). The computed CTV-PTV margins (lateral, longitudinal, vertical) were 2.5 mm, 2.5 mm, 3.6 mm and 1.4 mm, 1.6 mm, 2.4 mm for systems A and B, respectively. This suggests that the immobilization system influences intrafraction prostate displacement and, therefore, the margins applied. However, the margins found for both systems are comparable to the margins used for hypofractionated prostate SBRT.

Highlights

Prostate carcinoma is a prevalent malignant disease in men [1]
The average absolute displacements for patients who were immobilized with system A were 0.9 ± 0.2 mm, 1.7 ± 0.4 mm, and 1.3 ± 0.3 mm, and for patients who were immobilized with system B, they were 0.5 ± 0.2 mm, 0.6 ± 0.2 mm, and 0.8 ± 0.2 mm in the lateral, longitudinal, and vertical directions, respectively
In order to obtain statistically significant conclusions, further investigation with more patients is necessary. Despite this limitation of having few patients, it is encouraging that the intrafraction displacements and calculated margins found here are similar, and the standard deviations of 0.3-0.5 mm for prostate displacements are smaller than those found in a large study of stereotactic body radiotherapy (SBRT) patients [10] using the same van Herk formulation [11]

Summary

Introduction

Prostate carcinoma is a prevalent malignant disease in men [1]. In general, prostate cancer can be treated by external beam radiotherapy while sparing organs-at-risk (OARs), such as the rectum, small bowel, bladder, bilateral femoral heads, and penile bulb, from high-dose radiation [2,3].Received 07/07/2020 Review began 08/04/2020 Review ended 09/02/2020 Published 09/02/2020Studies have emphasized the importance of immobilization to reduce set-up uncertainties [4]. For prostate cancer treatment, improved set-up uncertainties will result in smaller field margins, higher local control rates, and reduce the occurrence of normal tissue toxicities such as diarrhea and cystitis [2,6]. Patient positioning and immobilization would place a patient in the same position during treatment as during simulation when the data used for treatment planning was collected This requires that a patient’s position would be reproducible for every treatment fraction (no interfraction variation) and that, once positioned, the patient’s position would not change during treatment (no intrafraction variation). Without careful training in the fabrication of custom treatment devices, significant geometric errors in radiotherapy treatment can occur Immobilization systems and their corresponding set-up errors influence the CTV to PTV margins and, may result in undesirable treatment outcomes [7,8,9]

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