Factors Affecting Prostate Displacement During Volumetric Modulated Arc Therapy in Prone Position After High-Dose-Rate Brachytherapy for Prostate Cancer

Purpose

In irradiating the prostate and pelvic lymph node regions, registration based on bony structures matches the pelvic lymph node regions but not necessarily the prostate position, and it is important to identify factors that influence prostate displacement. Therefore, we investigated factors influencing prostate displacement during volumetric modulated arc therapy after single-fraction high-dose-rate brachytherapy (HDR-BT) for prostate cancer and the trends in displacement for each fraction.

Methods and Materials

Seventy patients who underwent pelvic volumetric modulated arc therapy of 46 Gy in the prone position 15 days after 13 Gy HDR-BT were included. Prostate displacement relative to bony structures was calculated using cone beam computed tomography. Systematic error (SE) and random error (RE) were evaluated in the right-left (RL), craniocaudal (CC), and anteroposterior (AP) directions. The association with clinical and anatomic factors on the planning computed tomography or magnetic resonance imaging was analyzed. Prostate volume change (PVC) was defined as the volume change at 2 days after HDR-BT. Displacement trends were individually examined from the first to 23rd fractions.

Results

The mean SE in the RL, CC, and AP directions was -0.01 mm, -2.34 mm, and -0.47 mm, respectively. The root mean square of the RE in the RL, CC, and AP directions was 0.44 mm, 1.14 mm, and 1.10 mm, respectively. SE in the CC direction was independently associated with bladder volume (P = .021, t statistic = 2.352) and PVC (P < .001, t statistic = -8.526). SE in the AP direction was independently associated with bladder volume (P = .013, t statistic = -2.553), PVC (P < .001, t statistic = 5.477), and rectal mean area (P = .008, t statistic = 2.743). RE in the CC direction was independently associated with smoking (P = .035). RE in the AP direction was associated with PVC (P = .043). Gradual displacement caudally and posteriorly occurred during the irradiation period.

Conclusions

Anatomic characteristics of the bladder, rectum, and prostate predict SE. Smoking and PVC predict RE. In particular, whether PVC is ≥140% affects setting internal margins.

Planning study: prone versus supine position for stereotactic body radiotherapy in prostate by CyberKnife

This study aimed to clarify the differences in radiotherapy dose characteristics and delivery efficiency between the supine and prone positions in patients with prostate cancer using the CyberKnife. The planning computed tomography (CT) and delineations of the prone position were obtained by rotating the supine CT images with delineations of 180° using image processing software. The optimization parameters for planning target volume (PTV) and organs at risk (OARs) were based on the prone position. The optimization parameters determined for the prone position were applied to the supine position for optimization and dose calculation. The dosimetric characteristics of the PTV and OARs, and delivery efficiency were compared between the two different patient positions. The plans in the prone position resulted in better PTV conformity index (nCI), rectum V90%, V80%, V75%, V50% and bladder V50%. A significant difference was observed in treatment time and depth along the central axis (dCAX) between the two plans. The mean treatment time per fraction and dCAX for the supine and prone positions were 20.9 ± 1.7 min versus 19.8 ± 1.3 min (P = 0.019) and 151.1 ± 33.6 mm versus 233.2 ± 8.8 mm (P < 0.001), respectively. In this study the prone position was found to improve dosimetric characteristics and delivery efficiency compared with the supine position during prostate cancer treatment with the CyberKnife.

Soft-tissue prostate intrafraction motion tracking in 3D cine-MR for MR-guided radiotherapy

To develop a method to automatically determine intrafraction motion of the prostate based on soft tissue contrast on 3D cine-magnetic resonance (MR) images with high spatial and temporal resolution. Twenty-nine patients who underwent prostate stereotactic body radiotherapy (SBRT), with four implanted cylindrical gold fiducial markers (FMs), had cine-MR imaging sessions after each of five weekly fractions. Each cine-MR session consisted of 55 sequentially obtained 3D data sets ('dynamics') and was acquired over an 11 s period, covering a total of 10 min. The prostate was delineated on the first dynamic of every dataset and this delineation was used as the starting position for the soft tissue tracking (SST). Each subsequent dynamic was rigidly aligned to the first dynamic, based on the contrast of the prostate. The obtained translation and rotation describes the intrafraction motion of the prostate. The algorithm was applied to 6270 dynamics over 114 scans of 29 patients and the results were validated by comparing to previously obtained fiducial marker tracking data of the same dataset. Our proposed tracking method was also retro-perspectively applied to cine-MR images acquired during MR-guided radiotherapy of our first prostate patient treated on the MR-Linac. The difference in the 3D translation results between the soft tissue and marker tracking was below 1 mm for 98.2% of the time. The mean translation at 10 min were X: 0.0 [Formula: see text] 0.8 mm, Y: 1.0 [Formula: see text] 1.8 mm and Z: [Formula: see text] mm. The mean rotation results at 10 min were X: [Formula: see text], Y: 0.1 [Formula: see text] 0.6° and Z: 0.0 [Formula: see text] 0.7°. A fast, robust and accurate SST algorithm was developed which obviates the need for FMs during MR-guided prostate radiotherapy. To our knowledge, this is the first data using full 3D cine-MR images for real-time soft tissue prostate tracking, which is validated against previously obtained marker tracking data.

Fiducial marker based intra-fraction motion assessment on cine-MR for MR-linac treatment of prostate cancer

We have developed a method to determine intrafraction motion of the prostate through automatic fiducial marker (FM) tracking on 3D cine-magnetic resonance (MR) images with high spatial and temporal resolution. Twenty-nine patients undergoing prostate stereotactic body radiotherapy (SBRT), with four implanted cylindrical gold FMs, had cine-MR imaging sessions after each of five weekly fractions. Each cine-MR examination consisted of 55 sequentially obtained 3D datasets ('dynamics'), acquired over a 11 s period, covering a total of 10 min. FM locations in the first dynamic were manually identified by a clinician, FM centers in subsequent dynamics were automatically determined. Center of mass (COM) translations and rotations were determined by calculating the rigid transformations between the FM template of the first and subsequent dynamics. The algorithm was applied to 7315 dynamics over 133 scans of 29 patients and the obtained results were validated by comparing the COM locations recorded by the clinician at the halfway-dynamic (after 5 min) and end dynamic (after 10 min). The mean COM translations at 10 min were X: 0.0 [Formula: see text] 0.8 mm, Y: 1.0 [Formula: see text] 1.9 mm and Z: 0.9 [Formula: see text] 2.0 mm. The mean rotation results at 10 min were X: 0.1 [Formula: see text] 3.9°, Y: 0.0 [Formula: see text] 1.3° and Z: 0.1 [Formula: see text] 1.2°. The tracking success rate was 97.7% with a mean 3D COM error of 1.1 mm. We have developed a robust, fast and accurate FM tracking algorithm for cine-MR data, which allows for continuous monitoring of prostate motion during MR-guided radiotherapy (MRgRT). These results will be used to validate automatic prostate tracking based on soft-tissue contrast.

Monitoring of mechanical errors and their dosimetric impact throughout the course of non-coplanar continuous volumetric-modulated arc therapy

BACKGROUND

Volumetric-modulated Dynamic WaveArc therapy (VMDWAT) is a non-coplanar continuous volumetric modulated radiation therapy (VMAT) delivery technique. Here, we monitored mechanical errors and their impact on dose distributions in VMDWAT using logfiles throughout the course of treatment.

METHODS

Fifteen patients were enrolled (2 skull base tumor patients and 13 prostate cancer patients). VMDWAT plans were created for the enrolled patients. The prescribed dose for the skull base tumor was set as 54 Gy at 1.8 Gy per fraction, and that for the prostate cancer was set as 72 to 78 Gy at 2 Gy per fraction. We acquired logfiles to monitor mechanical errors and their impact on dose distribution in each fraction. The root mean square error (RMSE) in the multi-leaf collimator (MLC), gantry angle, O-ring angle and monitor unit (MU) were calculated using logfiles throughout the course of VMDWAT for each patient. The dosimetric impact of mechanical errors throughout the course of VMDWAT was verified using a logfile-based dose reconstruction method. Dosimetric errors between the reconstructed plans and the original plans were assessed.

RESULTS

A total of 517 datasets, including 55 datasets for the 2 skull base tumor patients and 462 datasets for the 13 prostate cancer patients, were acquired. The RMSE values were less than 0.1 mm, 0.2°, 0.1°, and 0.4 MU for MLC position, gantry angle, O-ring angle, and MU, respectively. For the skull base tumors, the absolute mean dosimetric errors and two standard deviations throughout the course of treatment were less than 1.4% and 1.1%, respectively. For prostate cancer, these absolute values were less than 0.3% and 0.5%, respectively. The largest dosimetric error of 2.5% was observed in a skull base tumor patient. The resultant dosimetric error in the accumulated daily delivered dose distribution, in the patient with the largest error, was up to 1.6% for all dose-volumetric parameters relative to the planned dose distribution.

CONCLUSIONS

MLC position, gantry rotation, O-ring rotation and MU were highly accurate and stable throughout the course of treatment. The daily dosimetric errors due to mechanical errors were small. VMDWAT provided high delivery accuracy and stability throughout the course of treatment.

TRIAL REGISTRATION

UMIN000023870 . Registered: 1 October 2016.

A comparison of clinical outcomes between three-dimensional conformal radiotherapy and intensity-modulated radiotherapy for prostate cancer

BACKGROUND

Intensity-modulated radiation therapy (IMRT) reduces the dose delivered to organs at risk. However, there have been few direct comparisons of IMRT with conventional three-dimensional conformal radiotherapy (3DCRT). The aim of this study was to evaluate the clinical benefit of IMRT in terms of toxicity and biochemical control.

METHODS

The medical records of 203 consecutive patients with localized to non-metastatic (stage T1a-T3bN0M0) prostate cancer between 2007 and 2011 were retrospectively reviewed. The prescribed dose was 76 Gy delivered in 38 fractions in both the 3DCRT and IMRT treatment groups. The frequency of grade 2 or greater late gastrointestinal (GI) and genitourinary toxicity and biochemical control were estimated by the log-rank test and Cox proportional hazards model with and without adjustment by the propensity score for treatment choice.

RESULTS

A total of 159 patients were included in the study (3DCRT: 70 patients, IMRT: 89 patients). The median follow-up period was 4.7 years. The estimated 5-year cumulative risk of late GI toxicity was significantly lower in the IMRT group than in the 3DCRT group (3.6 vs 13.2%, respectively, p = 0.022). After adjustment by propensity score, IMRT remained associated with a lower risk of late GI toxicity (hazard ratio 0.22; 95% confidence interval 0.058-0.85; p = 0.028). The 5-year biochemical failure-free rate was 93.2% in the 3DCRT group and 95.4% in the IMRT group (non-significant difference).

CONCLUSIONS

The incidence of late GI toxicity was significantly lower in the IMRT group than in the 3DCRT group, while the biochemical control rates were no different between the two groups. These clinical data suggest the benefit of IMRT in the reduction of late GI toxicity.

MRI-guided prostate adaptive radiotherapy - A systematic review

Dose escalated radiotherapy improves outcomes for men with prostate cancer. A plateau for benefit from dose escalation using EBRT may not have been reached for some patients with higher risk disease. The use of increasingly conformal techniques, such as step and shoot IMRT or more recently VMAT, has allowed treatment intensification to be achieved whilst minimising associated increases in toxicity to surrounding normal structures. To support further safe dose escalation, the uncertainties in the treatment target position will need be minimised using optimal planning and image-guided radiotherapy (IGRT). In particular the increasing usage of profoundly hypo-fractionated stereotactic therapy is predicated on the ability to confidently direct treatment precisely to the intended target for the duration of each treatment. This article reviews published studies on the influences of varies types of motion on daily prostate position and how these may be mitigated to improve IGRT in future. In particular the role that MRI has played in the generation of data is discussed and the potential role of the MR-Linac in next-generation IGRT is discussed.

Pilot study on interfractional and intrafractional movements using surface infrared markers and EPID for patients with rectal cancer treated in the prone position

OBJECTIVE

To evaluate interfractional and intrafractional movement of patients with rectal cancer during radiotherapy with electronic portal imaging device (EPID) and surface infrared (IR) markers.

METHODS

20 patients undergoing radiotherapy for rectal cancer with body mass index ranging from 18.5 to 30 were enrolled. Patients were placed in the prone position on a couch with a leg pillow. Three IR markers were put on the surface of each patient and traced by two stereo cameras during radiotherapy on a twice-weekly basis. Interfractional isocentre movement was obtained with EPID images on a weekly basis. Movement of the IR markers was analysed in correlation with the isocentre movement obtained from the EPID images.

RESULTS

The maximum right-to-left (R-L) movement of the laterally located markers in the horizontal isocentre plane was correlated with isocentre translocation with statistical significance (p = 0.018 and 0.015, respectively). Movement of the surface markers was cyclical. For centrally located markers, the 95% confidence intervals for the average amplitude in the R-L, cranial-to-caudal (C-C) and anterior-to-posterior (A-P) directions were 0.86, 2.25 and 3.48 mm, respectively. In 10 patients, intrafractional movement exceeding 5 mm in at least one direction was observed. Time-dependent systematic movement of surface markers during treatment, which consisted of continuous movement towards the cranial direction and a sail back motion in the A-P direction, was also observed.

CONCLUSION

Intrafractional movement of surface markers has both cyclic components and time-dependent systematic components. Marker deviations exceeding 5 mm were mainly seen in the A-P direction. Pre- or post-treatment EPID images may not provide adequate information regarding intrafractional movement because of systematic movement in the A-P direction during radiotherapy.

ADVANCES IN KNOWLEDGE

This work uncovered a sail back motion of patients in the A-P direction during radiotherapy. Pre- or post-treatment EPID images may not provide accurate positioning of patients in the A-P direction because of this time-dependent intrafractional motion.