Intrafraction Prostate Motion Research Articles

A new method to reconstruct intra-fractional prostate motion in volumetric modulated arc therapy

Intra-fractional motion is a concern during prostate radiation therapy, as it may cause deviations between planned and delivered radiation doses. Because accurate motion information during treatment delivery is critical to address dose deviation, we developed the projection marker matching method (PM3), a novel method for prostate motion reconstruction in volumetric modulated arc therapy. The purpose of this method is to reconstruct in-treatment prostate motion trajectory using projected positions of implanted fiducial markers measured in kV x-ray projection images acquired during treatment delivery. We formulated this task as a quadratic optimization problem. The objective function penalized the distance from the reconstructed 3D position of each fiducial marker to the corresponding straight line, defined by the x-ray projection of the marker. Rigid translational motion of the prostate and motion smoothness along the temporal dimension were assumed and incorporated into the optimization model. We tested the motion reconstruction method in both simulation and phantom experimental studies. We quantified the accuracy using 3D normalized root-mean-square (RMS) error defined as the norm of a vector containing ratios between the absolute RMS errors and corresponding motion ranges in three dimensions. In the simulation study with realistic prostate motion trajectories, the 3D normalized RMS error was on average (range from to ). In an experimental study, a prostate phantom was driven to move along a realistic prostate motion trajectory. The 3D normalized RMS error was . We also examined the impact of the model parameters on reconstruction accuracy, and found that a single set of parameters can be used for all the tested cases to accurately reconstruct the motion trajectories. The motion trajectory derived by PM3 may be incorporated into novel strategies, including 4D dose reconstruction and adaptive treatment replanning to address motion-induced dose deviation.

Validation of dynamic treatment-couch tracking for prostate SBRT

In stereotactic body radiation therapy (SBRT) of prostatic cancer, a high dose per fraction is applied to the target with steep dose gradients. Intrafractional prostate motion can occur unpredictably during the treatment and lead to target miss. This work investigated the dosimetric benefit of motion compensation with dynamic treatment-couch tracking for prostate SBRT treatments in the presence of prostatic motion. Ten SBRT treatment plans for prostate cancer patients with integrated boosts to their index lesion were prepared. The treatment plans were applied with a TrueBeam linear accelerator to a phantom in (a) static reference position, (b) moved with five prostate motion trajectories without any motion compensation, and (c) with real-time compensation using transponder-guided couch tracking. The geometrical position of the electromagnetic transponder was evaluated in the tracked and untracked situation. The dosimetric performance of couch tracking was evaluated, using Gamma agreement indices (GAI) and other dose parameters. These were evaluated within the phantoms biplanar diode array, as well as target- and organ-specific. The root-mean-square error of the motion traces (range: 0.8-4.4 mm) was drastically reduced with couch tracking (0.2-0.4 mm). Residual motion was mainly observed at abrupt direction changes with steep motion gradients. The phantom measurements showed significantly better GAI1%/1mm with tracked (range: 83.4%-100.0%) than with untracked motion (28.9%-99.7%). Also GAI2%/2mm was significantly superior for the tracked (98.4%-100.0%) than the untracked motion (52.3%-100.0%). The organ-specific evaluation showed significantly better target coverage with tracking. The dose to the rectum and bladder showed a dependency on the anterior-posterior motion direction. Couch tracking clearly improved the dosimetric accuracy of prostate SBRT treatments. The treatment couch was able to compensate the prostatic motion with only some minor residual motion. Therefore, couch tracking combined with electromagnetic position monitoring for prostate SBRT is feasible and improves the accuracy in treatment delivery when prostate motion is present.

The first clinical implementation of a real-time six degree of freedom target tracking system during radiation therapy based on Kilovoltage Intrafraction Monitoring (KIM)

PurposeWe present the first clinical implementation of a real-time six-degree of freedom (6DoF) Kilovoltage Intrafraction Monitoring (KIM) system which tracks the cancer target translational and rotational motions during treatment. The method was applied to measure and correct for target motion during stereotactic body radiotherapy (SBRT) for prostate cancer. MethodsPatient: A patient with prostate adenocarcinoma undergoing SBRT with 36.25Gy, delivered in 5 fractions was enrolled in the study. 6DoF KIM technology: 2D positions of three implanted gold markers in each of the kV images (125kV, 10mA at 11Hz) were acquired continuously during treatment. The 2D→3D target position estimation was based on a probability distribution function. The 3D→6DoF target rotation was calculated using an iterative closest point algorithm. The accuracy and precision of the KIM method was measured by comparing the real-time results with kV-MV triangulation. ResultsOf the five treatment fractions, KIM was utilised successfully in four fractions. The intrafraction prostate motion resulted in three couch shifts in two fractions when the prostate motion exceeded the pre-set action threshold of 2mm for more than 5s. KIM translational accuracy and precision were 0.3±0.6mm, −0.2±0.3mm and 0.2±0.7mm in the Left-Right (LR), Superior-Inferior (SI) and Anterior-Posterior (AP) directions, respectively. The KIM rotational accuracy and precision were 0.8°±2.0°, −0.5°±3.3° and 0.3°±1.6° in the roll, pitch and yaw directions, respectively. ConclusionThis treatment represents, to the best of our knowledge, the first time a cancer patient’s tumour position and rotation have been monitored in real-time during treatment. The 6 DoF KIM system has sub-millimetre accuracy and precision in all three translational axes, and less than 1° accuracy and 4° precision in all three rotational axes.

Quantification of intrafraction prostate motion and its dosimetric effect on VMAT.

Intrafraction prostate motion degrades the accuracy of radiation therapy (RT) delivery. Whilst a number of metrics in the literature have been used to quantify intrafraction prostate motion, it has not been established whether these metrics reflect the effect of motion on the RT dose delivered to the patients. In this study, prostate motion during volumetric modulated arc therapy (VMAT) treatment of 18 patients and a total of 294 fractions was quantified through novel metrics as well as those available in the literature. The impact of the motion on VMAT dosimetry was evaluated using these metrics and dose reconstructions based on a previously validated and published method. The dosimetric impact of the motion on planning target volume (PTV) and clinical target volume (CTV) coverage and organs at risk (OARs) was correlated with the motion metrics, using the coefficient of determination (R 2 ), to evaluate their utility. Action level threshold for the prostate motion metric that best described the dosimetric impact on the PTV D95% was investigated through iterative regression analysis. The average (range) of the mean motion for the patient cohort was 1.5mm (0.3-9.9mm). A number of motion metrics were found to be strongly correlated with PTV D95%, the range of R 2 was 0.43-0.81. For all the motion measures, correlations with CTV D99% (range of R 2 was 0.12-0.62), rectum V65% (range of R 2 was 0.33-0.58) and bladder V65% (range of R 2 was 0.51-0.69) were not as strong as for PTV D95%. The mean of the highest 50% of motion metric was one of the best indicator of dosimetric impact on PTV D95%. Action level threshold value for this metric was found to be 3.0mm. For an individual fraction, when the metric value was greater than 3.0mm then the PTV D95% was reduced on average by 6.2%. This study demonstrated that several motion metrics are well correlated with the dosimetric impact (PTV D95%) of individual fraction prostate motion on VMAT delivery and could be used for treatment course adaptation.

Analysis of electromagnetic transponders tracking data to quantify intrafraction prostate motion during radiotherapy treatments

The Calypso tracking system (Varian, Palo Alto, CA, USA) is used to track the prostate isocenter on patients undergoing prostate radiotherapy after implantation of electromagnetic transponders. Aim of this study was to assign 226 recorded prostate tracks to different patterns of prostate intrafraction motion (i.e. stable target (ST), continuous target drift (CTD) and irregular wave motion (IWM)) and excursion (i.e. transient excursion (TE), persistent excursion (PE) and high-frequency excursion (HFE)). Relative frequencies of STs, CTDs and IWMs were 51.8%, 44.6% and 3.6%, respectively. TEs, PEs and HFEs were revealed in 9.4%, 5.4% and 14.3% cases, respectively, with maximum values of 8.0 mm, 8.7 mm and 15.5 mm, respectively. The equation D(t) = 8.0*10-3 mm/s * t + 0.93 mm was established to calculate the average prostate drift D with time t. Intrafraction prostate motion and excursions can be significant and should be in particular taken into account with treatment deliveries that require a prolonged treatment time, as for instance stereotactic body radiotherapy (SBRT) or hadrontherapy.

INTRAFRACTIONAL PROSTATE SHIFTS DURING STEREOTACTIC RADIATION THERAPY

In this study we analyzed prostate intrafraction motion during the course of stereotactic radiotherapy (STRT). STRT (5 fraction of 7.25Gy) was performed in 31 primary patients with prostate cancer. At least 3 gold fiducial markers were implanted in prostate of every patient 3-5 days before simulation. Prostate position on treatment table was verified with cone beam CT just before and immediately after the end of each radiotherapy session. This data help to determine prostate displacement in cranio-caudal, anterior-posterior and lateral axes. Average values of intra-fractional prostate displacement were as follows: 0,8+/-1,2mm - in cranio-caudal, 0.9+/-1,0mm - in lateral and 1.3+/-1,3mm - in anterior-posterior directions. In 5%-8,3% cases prostate displacement in cranio-caudal and lateral directions exceeded 2mm with maximal value of 5mm. In accordance with obtained average shifts we recommend following PTV margins: 1mm - in anterior-posterior and 3mm - in all other directions. In this case average intrafractional prostate shifts would not compromise dose delivery to prostate (V100 - 98%, D90 - 101%) and in comparison with standard (3mm, 5mm, 5mm, 5mm) margins would permit 16% reduction of rectum volume incorporated in 80% isodose. We propose that small average intrafractional displacement of prostate permits the use of narrower PTV margins without compromising coverage of the target and significant reduction of rectum volume covered by 80% isodose.

An analysis of interfractional and intrafractional prostate motion in hypofractionated precise radiotherapy for prostate cancer

Objective To investigate the interfractional and intrafractional prostate motion during hypofractionated precise radiotherapy for prostate cancer. Methods From 2013 to 2016, twenty-eight patients who received 5 Gy radiotherapy in 9 fractions for prostate cancer were enrolled as subjects. Every patient had three gold fiducials implanted into the prostate by transrectal ultrasound guidance two weeks before computed tomography (CT) simulation. All patients underwent CT scans in the supine position with full bladders and rectal balloons filled with 60 ml air. The Pinnacle planning system was used to design the treatment plans. Twenty-three patients were treated with a Synergy accelerator. Those patients underwent cone-beam CT (CBCT) scans prior to treatment. The set-up error was recorded by bone alignment between CBCT images and planning CT images. The prostate displacement was then recorded by gold fiducial alignment. The interfraction prostate displacement was defined by the difference between the above two parameters. The other 5 patients were treated with a Novalis accelerator. Based on gold fiducial alignment, the real-time tracking of gold fiducials was carried out by the ExacTrac system to evaluate the intrafractional prostate displacement. Results A total of 207 measurements of interfractional prostate displacement were made in the 23 patients before treatment. The mean interfractional prostate displacements in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were (0.05±0.10), (0.20±0.22), and (0.19±0.18) cm, respectively. The displacements larger than 0.3 cm in the above three directions were observed in 1, 52, and 49 measurements, respectively, while the displacements larger than 0.5 cm in the three directions were observed in 1, 29, and 16 measurements, respectively. A total of 225 measurements of gold fiducial displacement were made in the 5 patients during treatment. The mean intrafractional prostate displacements in LR, SI, and AP directions were (0.61±0.50), (0.68±0.69), and (0.70±0.67) mm, respectively. The displacements larger than 3 mm in the three directions were observed in 0, 1, and 1 measurement, respectively. Conclusions In hypofractionated precise radiotherapy for prostate cancer, the interfractional prostate displacement is significantly larger than the intrafractional prostate displacement. The interfractional prostate displacement must be corrected before radiotherapy. In order to avoid off-target irradiation due to postural changes in patients, the tracking of the intrafractional prostate displacement is still necessary although the displacement is relatively small. The rectal balloon can help immobilize the prostate. Key words: Prostate motion; Interfraction; Intrafraction; Prostate neoplasms/hypofraction-ated radiotherapy

Real-time Image-guided Adaptive-predictive Prostate Radiotherapy using Rectal Diameter as a Predictor of Motion

AimsTo investigate a relationship between maximum rectal diameter (MRD) on pre-treatment cone beam computed tomography (CBCT) and intra-fraction prostate motion, in the context of an adaptive image-guided radiotherapy (IGRT) method. Materials and methodsThe MRD was measured on 2125 CBCTs from 55 retrospective patient datasets and related to prostate displacement from intra-fraction imaging. A linear regression model was developed to determine a threshold MRD associated with a high probability of small prostate displacement. Standard and reduced adaptive margin plans were created to compare rectum and bladder normal tissue complication probability (NTCP) with each method. ResultsA per-protocol analysis carried out on 1910 fractions from 51 patients showed with 90% confidence that for a MRD≤3 cm, prostate displacement will be ≤5 mm and that for a MRD≤3.5 cm, prostate displacement will be ≤5.5 mm. In the first scenario, if adaptive therapy was used instead of standard therapy, median reductions in NTCP for rectum and bladder were 0.5% (from 9.5% to 9%) and 1.3% (from 6.6% to 5.3%), respectively. In the second scenario, the NTCP for rectum and bladder would have median reductions of 1.1% and 2.6%, respectively. ConclusionsWe have identified a potential method for adaptive prostate IGRT based upon predicting small prostate intra-fraction motion by measuring MRD on pre-treatment CBCT.

Development and validation of a software to evaluate breathing-induced prostate motion tracked with implanted electromagnetic transponders

Introduction A system (Calypso, Varian Medical Systems) based on the localization of electromagnetic transponders (ELM-TRNs) permanently implanted in the prostate allows real-time tracking during radiotherapy treatments. Purpose Development and validation of a dedicated software for ELM-TRNs signal analysis to quantify the amplitude of the intra-fraction prostate motion induced by patient’s breathing, both in supine and prone position. Materials and methods For each treatment session, the software automatically identifies the breathing frequency on the power density spectrum of the recorded signal and creates a bandpass filter around this frequency. The obtained filter is then applied to selectively compute the breathing-induced harmonic excursion of the prostate around its nominal position. The software was validated with a moving phantom (QUASAR, Modus Medical Devices), provided with home-made inserts containing three ELM-TRNs. Harmonic motions along the three main directions were tracked at several known frequencies and amplitudes. The calculated frequencies and amplitudes were compared to the expected ones. The software was then applied to signals of patients who underwent radiotherapy treatments in supine or prone position. Results The software automatically computed the correct frequencies and amplitudes within a 0.6% and a 110 μm uncertainty, respectively, without any significant difference among the three main directions. The software was demonstrated to properly work on signals acquired both with supine and prone patients. Conclusion A software to quantify prostate motion due to patient’s breathing was successfully developed, in-phantom validated and applied to supine and prone patients, highlighting significant motion differences between the two setups. Disclosure None. Work partially funded by AIRC-(IG-14300).

PO-0878: The effect of rectal retractor on intra-fraction motion of prostate

PO-0878: The effect of rectal retractor on intra-fraction motion of prostate

EP-1750: Monitoring of intra-fraction prostate motion with a new 4D ultrasound device

EP-1750: Monitoring of intra-fraction prostate motion with a new 4D ultrasound device

EP-1747: Assessment of PTV margins accounting for prostate intrafraction motion in SBRT with online IGRT

EP-1747: Assessment of PTV margins accounting for prostate intrafraction motion in SBRT with online IGRT

Effect of ProSpare (PS), a rectal obturator, on inter- and intrafraction prostate motion and anorectal doses in prostate radiotherapy (RT).

e633 Background: PS is a single use, self-insertable endorectal device designed as a daily image-guidance tool for prostate cancer RT. Rectal filling is a significant factor in prostate motion. We investigated the effect of PS on inter- and intrafraction prostate motion and its effect on anorectal dosimetry. Methods: 19 patients with localized prostate cancer were randomized to receive prostate RT with PS either in the 1st or 2nd half of treatment. For each patient two planning CT scans were acquired, without and with PS. Contours were delineated by one physician and planned with a forward multi-segment technique to a dose of 74Gy/37F. Dose surface histograms were created (VODCA). In all patients, 3 electromagnetic transponders (EM) were implanted into the prostate and daily online image-guided RT was performed using either cone-beam CT (Elekta) or Calypso–based localization with tracking. Interfraction systematic (Σ) and random (σ) errors were estimated for bone matching without and with PS using EM position as reference. CTV-PTV margins were generated using the Van Herk formula. In-house code was used to analyse intrafraction motion recorded by Calypso of the EM centroid in the RL, SI and AP planes for individual fractions. A 2-way ANOVA test (SPSS v22) was used to determine if PS had an effect on maximal intrafraction prostate displacement. Results: See Table. Conclusions: PS affects the dosimetric pattern to the anal canal and rectum and this may impact GI toxicity. Also, PS stabilizes the prostate, which could enable reduced planning margins. PS will now be assessed in post-prostatectomy RT within a randomized controlled trial, POPS. Clinical trial information: 11814. [Table: see text] [Table: see text] [Table: see text]

Nature of intrafractional and interfractional prostate motion during stereotactic radiation.

152 Background: To characterize the intra-fraction and inter-fraction prostate motion as tracked by the X-ray images of the implanted gold fiducials during stereotactic radiotherapy with CyberKnife. The published data have analysed the linear and angular prostate motion intrafraction and interfraction prostate motion among patients. We sought to quantify the same within each patient. Methods: Twenty Five patients with localized prostate cancer treated with CyberKnife radiosurgery between January 2013 and August 2015 were studied retrospectively. A data set constitutes the deviations derived from X-ray images obtained between two consecutive couch motions. Results: Included in the analysis were 3926 data sets. A total of 210 non-coplanar fields were used per fraction. The mean total treatment time for all fields per fraction was 36.13 minutes. The detected and corrected movements over all were in a range of ± 10.1 mm in linear direction (Right: mean 1.1±0.4 mm; Left: mean 1.0±0.6 mm; Superior: mean 0.7±0.3 mm; Inferior: mean 1.6±0.6 mm; Anterior: mean 1.6±0.7 mm; Posterior: mean 0.5±0.3 mm with maximum (max) movement range of Right max 9.9±6.4 mm, Left max 7.1±3.4 mm, Superior max 8.6±5.4 mm, Inferior max 10.1±8.5 mm, Anterior max 9.2±6.5 mm, Posterior max 8.4±2.9 mm) and angular movements were in a range of ± 6.7 deg in all directions (Right Angle: mean 0.6±0.3 deg; Left Angle: mean 0.6±0.3 deg; Head Up(H-U): mean 1.3±0.6 deg; Head Down(H-D): mean 1.4±0.6 deg; Counter-Clockwise movement (CCW): mean 0.7±0.3 deg; Clockwise movement (CW): mean 0.5±0.3 deg with max rotation range of Right angle max 2.4±2 deg, Left angle max 2.7±2 deg, H-U max 10.2±3.5 deg, H-D max 6.7±4.8 deg, CCW 4±2.9 deg, CW max 2.8±2.4 deg). There was an unpredictable change in prostate motion inter-fraction in each patient. But, a unique observation is that a predictable pattern exists for prostate motion intra-fraction within a patient. Change in the linear or angular prostate motion intra-fraction in any direction is not erratic. Conclusions: The linear and rotational prostate motion intra-fraction in any direction has a predictable pattern and any change is gradual and not erratic. The motion shows secular trend during the course of treatment.

SU‐E‐J‐181: Effect of Prostate Motion On Combined Brachytherapy and External Beam Dose Based On Daily Motion of the Prostate

Purpose:In this study, the adequacy of target expansions on the combined external beam and implant dose was examined based on the measured daily motion of the prostate.Methods:Thirty patients received an I–125 prostate implant prescribed to dose of 90Gy. This was followed by external beam to deliver a dose of 90Gyeq (external beam equivalent) to the prostate over 25 to 30 fractions. An ideal IMRT plan was developed by optimizing the external beam dose based on the delivered implant dose. The implant dose was converted to an equivalent external beam dose using the linear quadratic model. Patients were set up on the treatment table by daily orthogonal imaging and aligning the marker seeds in the prostate. Orthogonal images were obtained at the end of treatment to assess prostate intrafraction motion. Based on the observed motion of the markers between the initial and final images, 5 individual plans showing the actual dose delivered to the patient were calculated. A final true dose distribution was established based on summing the implant dose and the 5 external beam plans. Dose to the prostate, seminal vesicles, lymphnodes and normal tissues, rectal wall, urethra and lower sphincter were calculated and compared to ideal. On 18 patients who were sexually active, dose to the corpus cavernosum and internal pudendal artery was also calculated.Results:The average prostate motion in 3 orthogonal directions was less than 1 mm with a standard deviation of less than +2 mm. Dose and volume parameters showed that there was no decrease in dose to the targets and a marginal decrease in dose to in normal tissues.Conclusion:Dose delivered by seed implant moves with the prostate, decreasing the impact of intrafractions dose movement on actual dose delivered. Combined brachytherapy and external beam dose delivered to the prostate was not sensitive to prostate motion.

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

BackgroundThe 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.MethodsTwenty 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.ResultsNone 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.ConclusionsThe 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.

Intensity modulated proton therapy versus uniform scanning proton therapy: Treatment planning study of the prostate cancer in patients with a unilateral metallic hip prosthesis

The purpose of this study is to compare the dosimetric results between the uniform scanning proton therapy (USPT) and intensity modulated proton therapy (IMPT) plans for the prostate cancer in patients with a unilateral metallic hip prosthesis. Five prostate cancer cases with left (n = 3) and right (n = 2) metallic hip prostheses were included in this retrospective study. For each case, the USPT and IMPT plans were generated using two anterior-oblique beams and one lateral beam for a total dose of 79.2 Gy(RBE) to be delivered in 44 fractions. For a given case, the beam parameters, dose prescription, and delivery schema in the IMPT plan were kept identical to the ones in the USPT plan. The IMPT and USPT plans were compared for various dosimetric parameters. The mean dose to the target volume was comparable. Both the IMPT and USPT techniques achieved the target coverage goals. Dose homogeneity was found to be similar in the IMPT and USPT plans. For both the rectum and bladder, the IMPT plans produced favorable dosimetric results in the low-, medium-, and high-dose regions when compared to the USPT plans. For the high dose regions, the rectal V 70 was lower in the IMPT plans by about 3.89 cc when compared to the one in the USPT plans. The rectal V 80 in the IMPT plans (1.10 cc) was almost half than the one in the USPT plans (2.39 cc). In comparison to the USPT plans, the mean dose to the rectum, bladder, and femoral head were lower in the IMPT plans by about 8.91%, 4.15%, and 41.09%, respectively. Based on the preliminary results of five cases presented in this study, the IMPT plans provided slightly better dosimetric results compared to the USPT plans, especially in sparing the rectum and bladder in the low-, medium-, and high-dose regions, for the treatment of the prostate cancer in patients with a unilateral metallic hip prosthesis. Future studies need to address the impact of the setup uncertainties and intra-fraction prostate motion in the IMPT planning of the prostate cancer patients with prosthetic hip replacements.

Long-term results of a study using individualized planning target volumes for hypofractionated intensity-modulated radiotherapy boost for prostate cancer.

BackgroundThis is the final report of a prospective phase I study which evaluated the feasibility, toxicities, and biochemical control in prostate cancer patients treated with a hypofractionated boost utilizing a fiducial marker-based daily image guidance strategy and small patient-specific PTV margins.MethodsLow- and intermediate-risk prostate cancer patients underwent transperineal ultrasound-guided implantation of three gold fiducial markers and were treated with three-dimensional conformal radiotherapy to 42 Gy (2 Gy/day). During the first nine fractions of treatment, pre- and post-treatment electronic portal imaging was performed to calculate intrafraction prostate motion. Patient-specific PTV margins were derived and a 30 Gy (3 Gy/day) intensity modulated radiotherapy boost was delivered (Total dose = 72 Gy in 31 fractions; EQD2 = 81 Gy, α/β = 1.4).ResultsThirty-three patients completed treatment and were followed for a median of 7.2 years (range, 1.2 – 9.5). Seven patients (21%) developed Radiation Therapy Oncology Group (RTOG) late grade 2 GI toxicity and 1 patient (3%) developed late grade 2 GU toxicity. No patients developed late grade 3 GI or GU toxicity. To date, nine patients developed PSA relapse according to the Phoenix criteria. The actuarial five, seven and nine year biochemical control (BC) rates were 87% (95% confidence interval: 69–95), 77% (95% confidence interval: 56–89) and 66% (95% confidence interval: 42–82).ConclusionsOur study demonstrates that the use of prostate fiducial markers in combination with a daily online image guidance protocol permits reduced, patient-specific PTV margins in a hypofractionated treatment scheme. This treatment planning and delivery strategy was well tolerated in the intermediate time frame. The use of very small PTV margins did not result in excessive failures when compared to other radiation regimens of similar radiobiological intensity.

PO-0927: Minimizing intra-fractional prostate motion during hypofractionated radiotherapy using beacon transponders

PO-0927: Minimizing intra-fractional prostate motion during hypofractionated radiotherapy using beacon transponders

Intrafractional prostate motion during external beam radiotherapy monitored by a real-time target localization system.

This paper investigates the clinical significance of real‐time monitoring of intrafractional prostate motion during external beam radiotherapy using a commercial 4D localization system. Intrafractional prostate motion was tracked during 8,660 treatment fractions for 236 patients. The following statistics were analyzed: 1) the percentage of fractions in which the prostate shifted 2−7 mm for a certain duration; 2) the proportion of the entire tracking time during which the prostate shifted 2−7 mm; and 3) the proportion of each minute in which the shift exceeded 2−7 mm. The ten patients exhibiting maximum intrafractional‐motion patterns were analyzed separately. Our results showed that the percentage of fractions in which the prostate shifted by >2,3,5,and 7 mm off the baseline in any direction for >30 s was 56.8%, 27.2%, 4.6%, and 0.7% for intact prostate and 68.7%, 35.6%, 10.1%, and 1.8% for postprostatectomy patients, respectively. For the ten patients, these percentages were 91.3%, 72.4%, 36.3%, and 6%, respectively. The percentage of tracking time during which the prostate shifted >2,3,5,and 7 mm was 27.8%, 10.7%, 1.6%, and 0.3%, respectively, and it was 56.2%, 33.7%, 11.2%, and 2.1%, respectively, for the ten patients. The percentage of tracking time for a >3 mm posterior motion was four to five times higher than that in other directions. For treatments completed in 5 min (VMAT) and 10 min (IMRT), the proportion for the prostate to shift by >3 mm was 4% and 12%, respectively. Although intrafractional prostate motion was generally small, caution should be taken for patients who exhibit frequent large intrafractional motion. For those patients, adjustment of patient positioning may be necessary or a larger treatment margin may be used. After the initial alignment, the likelihood of prostate motion increases with time. Therefore, it is favorable to use advanced techniques (e.g., VMAT) that require less delivery time in order to reduce the treatment uncertainty resulting from intrafractional prostate motion.PACS number: 87.50.S‐, 87.53.Kn, 87.55.N‐, 87.55.ne