Clinical evaluation of an endorectal immobilization system for use in prostate hypofractionated Stereotactic Ablative Body Radiotherapy (SABR)

Abstract

Background: The objective of this study was to evaluate a novel prostate endorectal immobilization system (EIS) for improving the delivery of hypofractionated Stereotactic Ablative Body Radiotherapy (SABR) for prostate cancer.

Methods: Twenty patients (n = 20) with low- or intermediate-risk prostate cancer (T1-T2b, Gleason Score < 7, PSA ≤ 20 ng/mL), were treated with an EIS in place using Volumetric Modulated Arc Therapy (VMAT), to a prescription dose of 26 Gy delivered in 2 fractions once per week; the intent of the institutional clinical trial was an attempt to replicate brachytherapy-like dosimetry using SABR. EBT3 radiochromic film embedded within the EIS was used as a quality assurance measure of the delivered dose; additionally, prostate intrafraction motion captured using pre- and post-treatment conebeam computed tomography (CBCT) scans was evaluated. Treatment plans were generated for patients with- and without the EIS to evaluate its effects on target and rectal dosimetry.

Results: None of the observed 3-dimensional prostate displacements were ≥ 3 mm over the elapsed treatment time. A Gamma passing rate of 95.64 ± 4.28 % was observed between planned and delivered dose profiles on EBT3 film analysis in the low-dose region. No statistically significant differences between treatment plans with- and without-EIS were observed for rectal, bladder, clinical target volume (CTV), and PTV contours (p = 0.477, 0.484, 0.487, and 0.487, respectively). A mean rectal V80% of 1.07 cc was achieved for plans using the EIS.

Conclusions: The EIS enables the safe delivery of brachytherapy-like SABR plans to the prostate while having minimal impact on treatment planning and rectal dosimetry. Consistent and reproducible immobilization of the prostate is possible throughout the duration of these treatments using such a device.

Fig. 1

Endorectal Immobilization System. 3D Mechanical drawing of the EIS system highlighting the three main components of the system: The carbon-fibre base (CB), the indexed mechanical system (IS), and the probe apparatus (PR)

Fig. 2

Intrafraction prostate displacement with EIS. Inverse Cumulative histogram of the 3D displacement, and left-right (L/R), superior-inferior (S/I), and anterior-posterior (A/P) axis displacements. Less than 1 % of 3D displacements were greater than 2.9 mm, and no displacements were ≥ 3.0 mm. Similarly, no translational displacements were ≥ 2.0 mm in the L/R and A/P axes, and none were ≥ 2.3 mm in the S/I axis

Fig. 3

Target and OAR doses with- and without-EIS. a & b Comparison of average Dose-Volume Histogram (DVH) results for 10 patients planned with- and without EIS (dashed lines). Prescription is 26 Gy ≥ 99 % to the clinical target volume (CTV) and 22 Gy ≥ 99 % to the planning target volume (PTV), respectively. Average DVH results for the Rectal (c), and Bladder (d), contour with- and without EIS are also presented. Standard deviation bands (1SD) are displayed as grey areas (without EIS), and hatched areas (with EIS). Overlapping standard deviation bands are displayed as grey

Fig. 4

Sample treatment plans of patients treated with- and without-EIS. a Treatment plan showing isodose distribution for patient treated with the EIS (purple). Displayed are the prostate CTV (orange), and PTV (blue), as well as the overall rectal contour. Percent isodose lines displayed are 105 % (maroon), 100 % (red), 95 %(dark blue), 85 % (green), and 50 % (light blue). b Isodose distribution for a patient treated without the EIS. CTV prescription dose is 2600 cGy

Fig. 5

Longitudinal dose profile for EBT3 film sample. Absolute dose for a single patient as a function of the longitudinal film length plotted against the computed dose model. Good agreement was seen between EBT3 and the treatment planning system (TPS) model for the first and second fractions (Fr). Slight differences in curves were likely due to minute changes in EIS positioning from that at planning