Effect of body mass index on intrafraction prostate displacement monitored by real-time electromagnetic tracking

Monitoring Intrafraction Motion of the Prostate During Radiation Therapy: Suggested Practice Points From a Focused Review

PURPOSE

External beam radiation therapy to the prostate is typically delivered after verification of prostatic position with image guidance. Prostate motion can occur during the delivery of each radiation treatment between the time of localization imaging and completion of treatment. The objective of this work is to review the literature on intrafraction motion (IFM) of the prostate during radiation therapy and offer clinical recommendations on management.

METHODS AND MATERIALS

A comprehensive literature review was conducted on prostate motion during prostate cancer radiation therapy. Information was organized around 3 key clinical questions, followed by an evidence-based recommendation.

RESULTS

IFM of the prostate during radiation therapy is typically ≤3 mm and is unlikely to compromise prostate dosimetry to a clinically meaningful degree for men treated in a relatively short treatment duration with planning target volume (PTV) margins of ≥3 to 5 mm. IFM of 5 mm or more has been observed in up to ∼10% of treatment fractions, with limited dosimetric effect related to the infrequency of occurrence and longer fractionation of therapy. IFM can be monitored in continuous or discontinuous fashion with a variety of imaging platforms. Correction of IFM may have the greatest value when tighter PTV margins are desired (such as with stereotactic body radiation therapy or intraprostatic nodule boosting), ultrahypofractionated courses, or when treatment time exceeds several minutes.

CONCLUSIONS

This focused review summarizes literature and provides practical recommendations regarding IFM in the treatment of prostate cancer with external beam radiation therapy.

Obesity does not influence prostate intrafractional motion

Introduction

Motion of the prostate is problematic in the accurate delivery of external beam radiation therapy (EBRT) for prostate cancer. This study investigated the relationship between body mass index (BMI), an easily measured indicator of obesity, and prostate motion.

Methods

Prostate motion during EBRT was assessed by measuring the displacement of fiducial markers implanted within the prostate in 130 prostate cancer patients. Interfractional motion was corrected on daily imaging through pre‐treatment cone‐beam‐computed tomography (CBCT) and intrafractional motion measured using movie sequences captured using an electronic portal imaging device (EPID) during treatment delivery.

Results

There was no statistically significant relationship between the mean intrafractional motion and BMI, except in the left‐right (LR) translation (P = 0.049) over the study population. For each BMI category, there was no statistical significance (P > 0.05) between any of the translations/rotations except LR (P = 0.003).

Conclusion

While intrafractional motion is an important consideration, prostate motion cannot be reliably predicted through measurement of patient's BMI.

Does body mass index or subcutaneous adipose tissue thickness affect interfraction prostate motion in patients receiving radical prostate radiotherapy?

Aim

It is unclear whether body mass index (BMI) is a useful measurement for examining prostate motion. Patient’s subcutaneous adipose tissue thickness (SAT) and weight has been shown to correlate with prostate shifts in the left/right direction. We sought to analyse the relationship between BMI and interfraction prostate movement in order to determine planning target volume (PTV) margins based on patient BMI.

Materials and methods

In all, 38 prostate cancer patients with three implanted gold fiducial markers in their prostate were recruited. Height, mass and SAT were measured, and the extent of interfraction prostate movement in the left/right, superior/inferior and anterior/posterior directions was recorded during each daily fiducial marker-based image-guided radiotherapy treatment. Mean corrective shift in each direction for each patient, along with BMI values, were calculated.

Results

The median BMI value was 28·4 kg/m2 (range 21·4–44·7). Pearson’s product-moment correlation analysis showed no significant relationship between BMI, mass or SAT and the extent of prostate movement in any direction. Linear regression analysis also showed no relationship between any of the patient variables and the extent of prostate movement in any direction (BMI: R2=0·006 (ρ=0·65), 0·002 (ρ=0·80) and 0·001 (ρ=0·86); mass: R2=0·001 (ρ=0·87), 0·010 (ρ=0·54) and 0·000 (ρ=0·99); SAT: R2=0·012 (ρ=0·51), 0·013 (ρ=0·50) and 0·047 (ρ=0·19) for shifts in the X, Y and Z axis, respectively). Patients were grouped according to BMI, as BMI<30 (n=25, 65·8%) and BMI≥30 (n=13, 34·2%). A two-tailed t-test showed no significant difference between the mean prostate shifts for the two groups in any direction (ρ=0·320, 0·839 and 0·325 for shifts in the X, Y and Z axis, respectively).

Findings

BMI is not a useful parameter for determining individualised PTV margins. Gold fiducial marker insertion should be used as standard to improve treatment accuracy.

Real-time in vivo rectal wall dosimetry using plastic scintillation detectors for patients with prostate cancer

We designed and constructed an in vivo dosimetry system using plastic scintillation detectors (PSDs) to monitor dose to the rectal wall in patients undergoing intensity-modulated radiation therapy for prostate cancer. Five patients were enrolled in an Institutional Review Board-approved protocol for twice weekly in vivo dose monitoring with our system, resulting in a total of 142 in vivo dose measurements. PSDs were attached to the surface of endorectal balloons used for prostate immobilization to place the PSDs in contact with the rectal wall. Absorbed dose was measured in real time and the total measured dose was compared with the dose calculated by the treatment planning system on the daily computed tomographic image dataset. The mean difference between measured and calculated doses for the entire patient population was -0.4% (standard deviation 2.8%). The mean difference between daily measured and calculated doses for each patient ranged from -3.3% to 3.3% (standard deviation ranged from 5.6% to 7.1% for four patients and was 14.0% for the last, for whom optimal positioning of the detector was difficult owing to the patient's large size). Patients tolerated the detectors well and the treatment workflow was not compromised. Overall, PSDs performed well as in vivo dosimeters, providing excellent accuracy, real-time measurement and reusability.

[Obesity and radiation: technical difficulties, toxicity and efficacy]

The number of obese patients has increased in France over the last two decades, which has had an impact on the incidence of numerous types of cancer. The treatment of cancer by radiotherapy is impacted by obesity as a result of the physical, technical and dosimetric constraints, the acute and late toxicity, local control and the survival of patients.

Intrafraction displacement of prone versus supine prostate positioning monitored by real-time electromagnetic tracking

Implanted radiofrequency transponders were used for real-time monitoring of the intrafraction prostate displacement between patients in the prone position and the same patients in the supine position. Thirteen patients had three transponders implanted transperineally and were treated prone with a custom-fitted thermoplastic immobilization device. After collecting data from the last fraction, patients were realigned in the supine position and the displacements of the transponders were monitored for 5-7 minutes. Fourier transforms were applied to the data from each patient to determine periodicity and its amplitude. To remove auto correlation from the stream of displacement data, the distribution of short-term and long-term velocity components were calculated from Poincaré plots of paired sequential vector displacements. The mean absolute displacement was significantly greater prone than supine in the superior-inferior (SI) plane (1.2 ± 0.6 mm vs. 0.6 ± 0.4 mm, p= 0.015), but not for the lateral or anterior-posterior (AP) planes. Displacements were least in the lateral direction. Fourier analyses showed the amplitude of respiratory oscillations was much greater for the SI and AP planes in the prone versus the supine position. Analysis of Poincaré plots confirmed greater short-term variance in the prone position, but no difference in the long-term variance. The centroid of the implanted transponders was offset from the treatment isocenter by &gt; 5 mm for 1.9% of the time versus 0.8% of the time for supine. These results confirmed significantly greater net intrafraction prostate displacement of patients in the prone position than in the supine position, but most of the difference was due to respiration-induced motion that was most pronounced in the SI and AP directions. Because the respiratory motion remained within the action threshold and also within our 5 mm treatment planning margins, there is no compelling reason to choose one treatment position over the other.