The Non-Gaussian Nature of Prostate Motion Based on Real-Time Intrafraction Tracking

Abstract

Purpose-The objective of this work is to test the validity of the Gaussian approximation for prostate motion through characterization of its spatial distribution.Methods and Materials-Real-time intrafraction prostate motion was observed using Calypso 4-dimensional (4D) nonradioactive electromagnetic tracking system. We report the results from a total of 1024 fractions from 31 prostate cancer patients. First, the correlation of prostate motion in right/left (RL), anteroposterior (AP), and superoinferior (SI) direction were determined using Pearson's correlation of coefficient. Then the spatial distribution of prostate motion was analyzed for individual fraction, individual patient including all fractions, and all patients including all fractions. The displacement in RL, AP, SI, oblique, or total direction is fitted into a Gaussian distribution, and a Lilliefors test was used to evaluate the validity of the hypothesis that the displacement is normally distributed.Results-There is high correlation in AP/SI direction (61% of fractions with medium or strong correlation). This is consistent with the longitudinal oblique motion of the prostate, and likely the effect from respiration on an organ confined within the genitourinary diaphragm with the rectum sitting posteriorly and bladder sitting superiorly. In all directions, the non-Gaussian distribution is more common for individual fraction, individual patient including all fractions, and all patients including all fractions. The spatial distribution of prostate motion shows an elongated shape in oblique direction, indicating a higher range of motion in the AP and SI directions.Conclusions-Our results showed that the prostate motion is highly correlated in AP and SI direction, indicating an oblique motion preference. In addition, the spatial distribution of prostate motion is elongated in an oblique direction, indicating that the organ motion dosimetric modeling

Respiratory-Induced Prostate Motion Using Wavelet Decomposition of the Real-Time Electromagnetic Tracking Signal

“…For that purpose, wavelet decomposition is used in our study because it is ideal for situations where a repetitive pattern caused by respiration exists amidst background noise caused by prostate drifting and bladder or rectal filling. We have shown in our previous publication (12) that the prostate motion in the AP/SI direction is highly correlated, which is consistent with the longitudinal oblique motion of the prostate, and likely due to the effect of respiration on an organ confined between the bladder and rectum. For this reason, the magnitude and peak frequency of the respiratory motion are extracted from the displacement in the oblique direction ( $\sqrt{y^2+z^2}$) (y-axis for AP and z-axis for SI).…”

An Accurate Method to Quantify Breathing-induced Prostate Motion for Patients Implanted with Electromagnetic Transponders

“…The first contributing factor has been widely investigated in the literature and the results are generally in agreement, pointing out a global motion towards the posterior and caudal directions and a much less notable lateral component of motion. These findings are consistent with the theory that the morphologic modifications of bladder and rectum affect prostate position, thus determining the main direction of the observed motion (15-16-17).…”

“…The global motion of the prostate during RT treatments has been extensively investigated with different techniques by several groups (13,(15)(16)(17)(25)(26)(27). However, few studies focused on the analysis of the intrafraction BIPM (18)(19)(20)(21)23).…”

The effect of prostate motion during hypofractionated radiotherapy can be reduced by using flattening filter free beams

“…Several independent studies, using different methods, have found similar prostate motion trajectories. There are six different prostate motion patterns described [9][10][11][12][13]. The pattern of an individual patient is unpredictable and any of the six motion patterns can occur during each fraction.…”

“…The pattern of an individual patient is unpredictable and any of the six motion patterns can occur during each fraction. In any case, the positional uncertainty increases with time [12][13][14][15][16]. The increased dose per fraction and thereby increased treatment time for hypofractionation could lead to larger positional uncertainty due to the prostate motion.…”