Assessment of interfractional prostate motion in patients immobilized in the prone position using a thermoplastic shell

Abstract

The aim of this study was to evaluate the interfractional prostate motion of patients immobilized in the prone position using a thermoplastic shell. A total of 24 patients with prostate calcifications detectable using a kilo-voltage X-ray image-guidance system (ExacTrac X-ray system) were examined. Daily displacements of the calcification within the prostate relative to pelvic bony structures were calculated by the ExacTrac X-ray system. The average displacement and standard deviation (SD) in each of the left–right (LR), anterior–posterior (AP), and superior–inferior (SI) directions were calculated for each patient. Based on the results of interfractional prostate motion, we also calculated planning target volume (PTV) margins using the van Herk formula and examined the validity of the PTV margin of our institute (a 9-mm margin everywhere except posteriorly, where a 6-mm margin was applied). In total, 899 data measurements from 24 patients were obtained. The average prostate displacements ± SD relative to bony structures were 2.8 ± 3.3, −2.0 ± 2.0 and 0.2 ± 0.4 mm, in the SI, AP and LR directions, respectively. The required PTV margins were 9.7, 6.1 and 1.4 mm in the SI, AP and LR directions, respectively. The clinical target volumes of 21 patients (87.5%) were located within the PTV for 90% or more of all treatment sessions. Interfractional prostate motion in the prone position with a thermoplastic shell was equivalent to that reported for the supine position. The PTV margin of our institute is considered appropriate for alignment, based on bony structures.

Target margins in radiotherapy of prostate cancer

“…One study examined the use of prostatic calcifications as natural, internal fiducials for tracking with a kilovoltage X-ray IG system (ExacTrac® X-ray system; BrainLab Inc., Feldkirchen, Germany). 26 In this study, the required PTV margins were found to be 1.4, 9.7 and 6.1 mm in the LR, SI and AP directions, respectively. Although the ability to use intrinsic fiducials is attractive, not all patients have such calcifications, and the PTV margins seem larger (6-10 mm) than in other IG strategies.…”

Particle therapy of moving targets—the strategies for tumour motion monitoring and moving targets irradiation

“…33 When considering the interfractional prostate motion in patients who are immobilized, the smallest movement occurs in the LR direction and the maximal displacement of .10 mm is observed only in the superior-inferior (SI) direction. 34 The variation in pancreas position during quiet breathing is closely linearly correlated with external abdominal movement. 31 Even though intrafractional pancreatic motion, which is induced principally by respiration, has the highest amplitude (,20.6 mm) in the CC direction, the AP (,10.1 mm) and LR (,6.7 mm) movements are also important.…”

Highly hypofractionated intensity-modulated radiation therapy for nonmetastatic prostate cancer with a simultaneous integrated boost to intraprostatic lesions: a planning study

“…The concept of the BTV was first suggested in 2000 [14], but it was considered difficult to achieve a BTV boost at that time because it was difficult to reproduce the BTV position with skin markings or bone matching using on-board kV imaging. The BTV position at the time of treatment often deviates from the position in the simulation CT due to interfractional error derived from the amount of urine and rectal gas present (e.g., [24]). Recently, cone-beam computed tomography (CBCT) on a linear accelerator has become more common, and organ-matching using CBCT can be widely implemented.…”