Endorectal Balloon Research Articles

Prostate Imaging: Evaluation of a Reusable Two-Channel Endorectal Receiver Coil for MR Imaging at 1.5 T

To prospectively compare image quality with use of a two-channel solid reusable phased-array endorectal receiver coil (SPAC) with that of the single-channel inflatable endorectal balloon coil currently in widespread use for 1.5-T magnetic resonance (MR) imaging of the prostate. Institutional review board approval and informed consent were obtained. Multiparametric prostate MR imaging at 1.5 T was performed in patients who were suspected of having cancer. Thirty consecutive patients were included (mean age, 66.1 years; range, 49-76 years). The first 15 patients were imaged by using a balloon coil and an eight-channel external array, and the remaining 15 were imaged with a SPAC alone. One patient was imaged with both techniques. Axial T2-weighted images acquired at both standard and high spatial resolution were used to compare image quality between coils. Qualitative assessments of image quality were made separately by three radiologists. Signal-to-noise ratio (SNR) profiles were determined on a pixel-by-pixel basis in a 1-cm central band in the prostate by using T1-weighted axial images at the apex, midgland, and base. Interrater reliability was determined by using a two-way intraclass correlation coefficient, qualitative scores were compared by using the Student t test for independent samples, and SNR profiles were plotted by using a Biot-Savart curve approximation. SNR of the SPAC was significantly better compared with that of the balloon coil at distances up to 3.0 cm at the apex and 3.5 cm at the base and midgland (P < .001). There was a 7% improvement in SNR at the mean maximal anteroposterior prostate dimension in this cohort and a 96% improvement at half this distance. At both standard and high spatial resolution, significant improvements in overall image quality (P = .015 and P < .001, respectively), visibility of the anterior gland (P = .009 and P < .001, respectively), and noise (P < .001 and P < .001, respectively) were seen when the SPAC was used. Interrater reliability was 0.536 (95% confidence interval: 0.461, 0.603). Both SNR and image quality were significantly improved with use of the SPAC at 1.5 T compared with use of the single-channel inflatable endorectal balloon coil.

Effect of endorectal balloon positioning errors on target deformation and dosimetric quality during prostate SBRT

An inflatable endorectal balloon (ERB) is often used during stereotactic body radiation therapy (SBRT) for treatment of prostate cancer in order to reduce both intrafraction motion of the target and risk of rectal toxicity. However, the ERB can exert significant force on the prostate, and this work assessed the impact of ERB position errors on deformation of the prostate and treatment dose metrics. Seventy-one cone-beam computed tomography (CBCT) image datasets of nine patients with clinical stage T1cN0M0 prostate cancer were studied. An ERB (Flexi-Cuff, EZ-EM, Westbury, NY) inflated with 60 cm3 of air was used during simulation and treatment, and daily kilovoltage (kV) CBCT imaging was performed to localize the prostate. The shape of the ERB in each CBCT was analyzed to determine errors in position, size, and shape. A deformable registration algorithm was used to track the dose received by (and deformation of) the prostate, and dosimetric values such as D95, PTV coverage, and Dice coefficient for the prostate were calculated. The average balloon position error was 0.5 cm in the inferior direction, with errors ranging from 2 cm inferiorly to 1 cm superiorly. The prostate was deformed primarily in the AP direction, and tilted primarily in the anterior–posterior/superior–inferior plane. A significant correlation was seen between errors in depth of ERB insertion (DOI) and mean voxel-wise deformation, prostate tilt, Dice coefficient, and planning-to-treatment prostate inter-surface distance (p < 0.001). Dosimetrically, DOI is negatively correlated with prostate D95 and PTV coverage (p < 0.001). For the model of ERB studied, error in ERB position can cause deformations in the prostate that negatively affect treatment, and this additional aspect of setup error should be considered when ERBs are used for prostate SBRT. Before treatment, the ERB position should be verified, and the ERB should be adjusted if the error is observed to exceed tolerable values.

Dosimetric effects of endorectal balloons on intensity-modulated radiation therapy plans for prostate cancer

We used an endorectal balloon (ERB) for prostate immobilization during intensity-modulated radiotherapy (IMRT) for prostate cancer treatment. To investigate the dosimetric effects of ERB-filling materials, we changed the ERB Hounsfield unit (HU) from 0 to 1000 HU in 200-HU intervals to simulate the various ERB fillings; 0 HU simulated a water-filled ERB, and 1000 HU simulated the densest material-filled ERB. Dosimetric data (coverage, homogeneity, conformity, maximal dose, and typical volume dose) for the tumor and the organs at risk (OARs) were evaluated in prostate IMRT treatment plans with 6-MV and 15-MV beams. The tumor coverage appeared to differ by approximately 1%, except for the clinical target volume (CTV) V100% and the planning target volume (PTV) V100%. The largest difference for the various ERB fillings was observed in the PTV V100%. In spite of increasing HU, the prostate IMRT plans at both energies had relatively low dosimetric effects on the PTV and the CTV. However, the maximal and the typical volume doses (D25%, D30%, and D50%) to the rectal wall and the bladder increased with increasing HU. For an air-filled ERB, the maximal doses to the rectal wall and the monitor units were lower than the corresponding values for the water-filled and the densest material-filled ERBs. An air-filled ERB spared the rectal wall because of its dosimetric effect. Thus, we conclude that the use of an air-filled ERB provides a dosimetric benefit to the rectal wall without a loss of target coverage and is an effective option for prostate IMRT treatment.

Image Guided Method for TLD-Based In Vivo Rectal Dose Verification With Endorectal Balloon in Proton Therapy for Prostate Cancer

Image Guided Method for TLD-Based In Vivo Rectal Dose Verification With Endorectal Balloon in Proton Therapy for Prostate Cancer

Interfraction Prostate Motion With Endorectal Balloon Placement

Inter-fraction prostate motion has been well established, requiring daily imaging guidance to localize the prostate. Limited soft tissue contrast in most available in-room-CT imagers such as kilo-voltage or mega-voltage cone beam CT may lead to localization uncertainties. Implanted radio-opaque markers are often applied to reduce uncertainties of soft tissue localization. Pre-treatment endorectal balloon (RB) placement has been used to potentially reduce intra-fractional prostate motion, particularly for stereotactic body radiation therapy (SBRT) of prostate cancer.

Endorectal balloons in the post prostatectomy setting: Do gains in stability lead to more predictable dosimetry?

PurposeTo perform a comparative study assessing potential benefits of endorectal-balloons (ERB) in post-prostatectomy patients. Method and materialsTen retrospective post-prostatectomy patients treated without ERB and ten prospective patients treated with the ERB in situ were recruited. All patients received IMRT and IGRT using kilovoltage cone-beam computed tomography (kVCBCT). kVCBCT datasets were registered to the planning dataset, recontoured and the original plan recalculated on the kVCBCTs to recreate anatomical conditions during treatment. The imaging, structure and dose data were imported into in-house software for the assessment of geometric variation and cumulative equivalent uniform dose (EUD) in the two groups. ResultsThe difference in location (ΔCOV) for the bladder between planning and each CBCT was similar for each group. The range of mean ΔCOV for the rectum was 0.15–0.58cm and 0.15–0.59cm for the non-ERB and ERB groups. For superior-CTV and inferior-CTV the difference between planned and delivered D95% (mean±SD) for the non-ERB group was 2.1±6.0Gy and −0.04±0.20Gy. While for the ERB group the difference in D95% was 8.7±12.6Gy and 0.003±0.104Gy. ConclusionsThe use of ERBs in the post-prostatectomy setting did improve geometric reproducibility of the target and surrounding normal tissues, however no improvement in dosimetric stability was observed for the margins employed.

SU‐E‐J‐146: Effectiveness of Daily Endorectal Balloon for Post‐Prostatectomy Patients Undergoing Pencil Beam Scanning Proton Therapy

Purpose: Pencil beam scanning proton therapy was found more sensitive to organ motion as compared to conventional IMRT. Significant target under dose may be caused by interfractional tumor motion. The objectives of this study are to investigate the role of endorectal balloon (ERB) in daily patient setup throughout the course of proton therapy and its dosimetric impact. Methods: Ten consecutive post‐prostatectomy patients enrolled on a prospective IRB approved institutional study were analyzed. Patients underwent CT/MRI simulation and treatment with daily ERB. Six T2‐MRI verification scans were performed during the treatment course. A pencil beam scanning proton therapy plan was generated for each patient. Organ at risks and CTVs were contoured by physician on each of the weekly MRI scans. The MRI scans were subsequently rigidly fused to the CT simulation images in order to simulate 2 possible daily kV‐kV patient setups: 1) alignment to bony anatomy and 2) alignment to pelvic bone in cranial caudal direction and ERB along the AP and lateral direction. The dosimetric impact of interfraction target motion on target coverage was compared for both scenarios. Results: We found that target interfraction motion in the AP direction (average 4.1mm) is consistently larger than those in the lateral and cranial caudal directions (2.9 mm and 3mm respectively) most likely due to variations in bladder volume. ERB can be used to further improve the patient bony alignment(∼1mm), in AP and lateral directions. Overall when both ERB pelvic bony structures are used for patient alignment, we observed a median 4.03% decrease in V98% of CTV as compared to 8.27% when only bony alignment is used. Conclusion: In post‐prostatectomy patients undergoing pencil beam scanning proton therapy, daily patient setup via bony alignment and ERB can stabilize the target and maintain the dose coverage within 5% relative to the initial treatment plans.

SU‐E‐J‐27: Residual Interfraction Errors for Prostate SBRT with 6 Dimensional Couch Shifts Using Stereoscopic X‐Ray and Cone Beam CT

Purpose: To evaluate residual interfraction alignment errors following prostate SBRT patient setup with stereoscopic kV‐X‐ray (Exactrac), kV‐CBCT, and a 6‐D robotic treatment couch (RTC). Methods: Data was reviewed from seven prostate cancer patients undergoing five‐fraction SBRT with pre‐and daily‐treatment endorectal balloon placement. Setup protocol, using stereoscopic X‐ray images, consisted of an initial RTC displacement based on bony registration (3 shifts, 3 rotations) followed by a RTC displacement based on implanted gold seeds (3 shifts only). Final image verification of the prostate position was performed with kV‐CBCT, upon which additional shifts prior to treatment delivery were permitted. The prostate (CTV), rectum wall and bladder were delineated on each daily CBCT and planning CT by the same physician. Shifts recorded at the time of treatment (X‐ray and X‐ray plus CBCT) were applied. Planning CT and CBCT contours were then superimposed and evaluated with dice similarity coefficient (DSC). Daily CTVs were also compared with the PTV (simulation CTV + 3mm). Paired student t test was used for statistical comparison. ResultsThe average of 96 stereoscopic X‐ray shifts were 0.96±4.08cm, 0.72±3.42cm and 0.58±3.64cm in the lateral, vertical and longitudinal directions, and 0.78±2.42°, 0.55±1.60° and 0.21±1.09° for the pitch, roll and yaw angles, respectively. CBCT‐guided shifts were performed in 25.7% of the fractions, with an average of 0.01±0.03cm, 0.23±0.21cm, and 0.14±0.26cm in the lateral, vertical, and longitudinal directions, respectively. With X‐ray only and X‐ray plus CBCT shifts, 95.1±3.7% and 96.2±3.2% (p=0.01) of daily CTVs were covered by PTV, respectively. The DSC were 77.1±5.8% and 79.2±4.6% for CTV (p&lt;0.01), 40.1±12.8% and 42.1±12.7% for rectal wall (p=0.03), and 65.2±17.5% and 64.5±18.0% for bladder (p=0.34), respectively. Conclusion Interfraction error following Exactrac‐guided RTC shifts was &lt;3mm, resulting in protocol‐appropriate CTV coverage at treatment based on a 3mm PTV margin. Further CBCT‐based shifts improve prostate alignment, but not significantly.

Reducing rectal injury during external beam radiotherapy for prostate cancer

Rectal bleeding and faecal incontinence are serious injuries that men with prostate cancer who receive radiotherapy can experience. Although technical advances--including the use of intensity-modulated radiotherapy coupled with image-guided radiotherapy--have enabled the delivery of dose distributions that conform to the shape of the tumour target with steep dose gradients that reduce the dose given to surrounding tissues, radiotherapy-associated toxicity can not be avoided completely. Many large-scale prospective studies have analysed the correlations of patient-related and treatment-related parameters with acute and late toxicity to optimize patient selection and treatment planning. The careful application of dose-volume constraints and the tuning of these constraints to the individual patient's characteristics are now considered the most effective ways of reducing rectal morbidity. Additionally, the use of endorectal balloons (to reduce the margins between the clinical target volume and planning target volume) and the insertion of tissue spacers into the region between the prostate and anterior rectal wall have been investigated as means to further reduce late rectal injury. Finally, some drugs and other compounds are also being considered to help protect healthy tissue. Overall, a number of approaches exist that must be fully explored in large prospective trials to address the important issue of rectal toxicity in prostate cancer radiotherapy.

Sa2037 Image Guided Radiation Therapy (IGRT) With Endorectal Balloon (ERB) for Carcinoma of the Prostate (CAP) Reduces Acute Gastrointestinal (GI) Sequelae Compared With Igrt Alone

Sa2037 Image Guided Radiation Therapy (IGRT) With Endorectal Balloon (ERB) for Carcinoma of the Prostate (CAP) Reduces Acute Gastrointestinal (GI) Sequelae Compared With Igrt Alone

Image‐guided method for TLD‐based in vivo rectal dose verification with endorectal balloon in proton therapy for prostate cancer

To present a practical image-guided method to position an endorectal balloon that improves in vivo thermoluminiscent dosimeter (TLD) measurements of rectal doses in proton therapy for prostate cancer. TLDs were combined with endorectal balloons to measure dose at the anterior rectal wall during daily proton treatment delivery. Radiopaque metallic markers were employed as surrogates for balloon position reproducibility in rotation and translation. The markers were utilized to guide the balloon orientation during daily treatment employing orthogonal x-ray image-guided patient positioning. TLDs were placed at the 12 o'clock position on the anterior balloon surface at the midprostatic plane. Markers were placed at the 3 and 9 o'clock positions on the balloon to align it with respect to the planned orientation. The balloon rotation along its stem axis, referred to as roll, causes TLD displacement along the anterior-posterior direction. The magnitude of TLD displacement is revealed by the separation distance between markers at opposite sides of the balloon on sagittal x-ray images. A total of 81 in vivo TLD measurements were performed on six patients. Eighty-three percent of all measurements (65 TLD readings) were within +5% and -10% of the planning dose with a mean of -2.1% and a standard deviation of 3.5%. Examination of marker positions with in-room x-ray images of measured doses between -10% and -20% of the planned dose revealed a strong correlation between balloon roll and TLD displacement posteriorly from the planned position. The magnitude of the roll was confirmed by separations of 10-20 mm between the markers which could be corrected by manually adjusting the balloon position and verified by a repeat x-ray image prior to proton delivery. This approach could properly correct the balloon roll, resulting in TLD positioning within 2 mm along the anterior-posterior direction. Our results show that image-guided TLD-based in vivo dosimetry for rectal dose verification can be perfomed reliably and reproducibly for proton therapy in prostate cancer.

Editorial Comment to Salvage intensity modulated radiotherapy using endorectal balloon after radical prostatectomy: Clinical outcomes

Editorial Comment to Salvage intensity modulated radiotherapy using endorectal balloon after radical prostatectomy: Clinical outcomes

Prostate and Patient Intrafraction Motion: Impact on Treatment Time-Dependent Planning Margins for Patients With Endorectal Balloon

To investigate intrafraction prostate and patient motion during different radiation therapy treatments as a function of treatment time; included were prostate patients with an endorectal balloon (ERB). Margins accounting for setup uncertainties and intrafraction motion were determined. The study included 17 patients undergoing prostate cancer radiation therapy. All patients received 3 fiducial gold markers implanted in the prostate and were then immobilized in the supine position with a knee support and treated with an ERB. Twelve patients with intermediate risk for pelvic lymph node metastases received intensity modulated radiation therapy (IMRT), and 5 patients at low risk received a 4-field box treatment. After setup based on skin marks, patients were imaged with a stereoscopic imaging system. If the marker displacement exceeded a 3-mm tolerance relative to planning computed tomography, patients were shifted and verification images were taken. All patients underwent additional imaging after treatment; IMRT patients also received additional imaging at halftime of treatment. Prostate and bone drifts were evaluated as a function of treatment time for more than 600 fractions, and margins were extracted. Patient motion evaluated by bone match was strongly patient dependent but in general was smallest in the superior-inferior (SI) direction. Prostate drifts were less patient dependent, showing an increase with treatment time in the SI and anterior-posterior (AP) directions. In the lateral (LAT) direction, the prostate stayed rather stable. Mean treatment times were 5.5 minutes for 4-field box, 10 minutes for 5-field boost IMRT, and 15 minutes or more for 9-field boost and 9-field pelvic IMRT treatments. Margins resulted in 2.2 mm, 3.9 mm, and 4.3 mm for 4-field box; 3.7 mm, 2.6 mm, and 3.6 mm for 5-field boost IMRT; 2.3 mm, 3.9 mm, and 6.2 mm for 9-field boost IMRT; and 4.2 mm, 5.1 mm, and 6.6 mm for 9-field pelvic IMRT in the LAT, SI, and AP directions, respectively. Intrafraction prostate and patient displacement increased with treatment time, showing different behaviors for the single directions of movement. Repositioning of the patients during long treatments or shorter treatment times will be necessary to further reduce the treatment margin.

Salvage intensity modulated radiotherapy using endorectal balloon after radical prostatectomy: Clinical outcomes

To evaluate biochemical non-evidence of disease and adverse events of salvage intensity-modulated radiotherapy using an endorectal balloon for prostate cancer patients after radical prostatectomy. Data of 107 patients (median age 65 years) with persistent (>0.1 ng/mL) or rising prostate-specific antigen after radical prostatectomy were retrospectively analyzed. The mean dose to clinical target volume was 70 Gy in 32 fractions (the equivalent dose in 2 Gy fraction is 73.2 Gy based on α:β = 2). Biochemical non-evidence of disease and predictive factors were assessed. Genitourinary toxicity and gastrointestinal toxicity were also evaluated using the Radiation Therapy Oncology Group toxicity criteria. The median follow up was 37 months (range 6-126 months). A total of 48 patients developed biochemical recurrence. The 3- and 5-year biochemical non-evidence of disease rates of all patients were 52.6% and 48.8%, respectively. The Gleason score (≥4 + 3, ≤3 + 4) and pre-intensity-modulated radiotherapy prostate-specific antigen level (≥0.5 ng/mL, <0.5 ng/mL) were significant predictive factors for biochemical non-evidence of disease in univariate analysis. In multivariate analysis, only the Gleason score was detected as a significant variable. The highest late genitourinary toxicities were grade 2 in 13% and grade 3 in 6% of patients. The highest late gastrointestinal toxicities were grade 2 in 6% and grade 3 in 3% of patients. Despite a relatively high radiation dose, intensity-modulated radiotherapy with an endorectal balloon can be delivered with acceptable toxicity and efficacy for patients developing biochemical recurrence after radical prostatectomy.

PO-0727: Prostate bed motion using an endorectal balloon and CBCT during postprostatectomy radiotherapy

PO-0727: Prostate bed motion using an endorectal balloon and CBCT during postprostatectomy radiotherapy

PO-0723: Stability of dose to the rectum during salvage radiotherapy with endorectal balloon after prostatectomy

PO-0723: Stability of dose to the rectum during salvage radiotherapy with endorectal balloon after prostatectomy

Rectal dose-volume differences for scanning proton beam therapy with or without an endorectal balloon for the treatment of prostate cancer.

243 Background: Endorectal balloons (ERB) are routinely used in proton radiation for prostate cancer (PCa). The dosimetric implications, however, in conjunction with scanning proton beam therapy (SPT) have not been studied. We sought to describe dose-volume values with the use of an ERB vs no ERB for SPT for PCa. Methods: We analyzed 10 patients with localized PCa. Each patient underwent two CT simulations, one with a water-filled ERB (50-60 ml) and one with no ERB. For 8 patients, the prostate and proximal 1 cm of the seminal vesicles was the clinical target volume (CTV), with the prostate only as CTV for 2 patients. We defined 3 structures: rectum (R), anterior rectal wall (ARW) as a 3 mm rim of the anterior half of the rectum, and AR-CTV as the ARW only on slices with a contoured CTV. SPT plans were created in both cases for each patient, with the creation of a scanning target volume (STV), which expanded the CTV by 6 mm in all dimensions except 4 mm posteriorly and 12 mm laterally. Two opposed lateral beams were used to plan to the STV. Total dose was 78 Co-60 Gy equivalent in 39 fractions, to cover 95% of the STV. RBE was 1.1. We compared the dose-volume values for the R, ARW, and AR-CTV between the two arms. Results: The ERB significantly increased the R volume for all cases (61 ± 11 ml vs 105 ± 10 ml (p&lt;0.01)). Rectal volumes radiated for all cases: V10, 55%; V50, 23%; and V70, 11%. The ERB significantly decreased the R volume radiated for all dose levels V5-V82 (p&lt;0.05). The absolute difference was larger at low dose levels (8% at V10) and intermediate dose levels (5% at V50) compared to high dose levels (3% at V70). For the ARW, the ERB did not significantly change the volume radiated at any dose level. For the AR-CTV, the ERB significantly decreased the volume radiated for dose levels V20-V60 (p&lt;0.05). Conclusions: The ERB provided a significant decrease in R volume radiated at all dose levels, and significantly decreased the AR-CTV volume, which represents a high risk volume, radiated at intermediate dose levels. Rectal doses with SPT, however, were low whether using an ERB or not. In the setting of the low rectal doses delivered, further study regarding the clinical benefit is warranted.

Whole pelvic intensity-modulated radiotherapy for high-risk prostate cancer: a preliminary report

PurposeTo assess the clinical efficacy and toxicity of whole pelvic intensity-modulated radiotherapy (WP-IMRT) for high-risk prostate cancer.Materials and MethodsPatients with high-risk prostate cancer treated between 2008 and 2013 were reviewed. The study included patients who had undergone WP-IMRT with image guidance using electronic portal imaging devices and/or cone-beam computed tomography. The endorectal balloon was used in 93% of patients. Patients received either 46 Gy to the whole pelvis plus a boost of up to 76 Gy to the prostate in 2 Gy daily fractions, or 44 Gy to the whole pelvis plus a boost of up to 72.6 Gy to the prostate in 2.2 Gy fractions.ResultsThe study cohort included 70 patients, of whom 55 (78%) had a Gleason score of 8 to 10 and 50 (71%) had a prostate-specific antigen level > 20 ng/mL. The androgen deprivation therapy was combined in 62 patients. The biochemical failure-free survival rate was 86.7% at 2 years. Acute any grade gastrointestinal (GI) and genitourinary (GU) toxicity rates were 47% and 73%, respectively. The actuarial rate of late grade 2 or worse toxicity at 2 years was 12.9% for GI, and 5.7% for GU with no late grade 4 toxicity.ConclusionWP-IMRT was well tolerated with no severe acute or late toxicities, resulting in at least similar biochemical control to that of the historic control group with a small field. The long-term efficacy and toxicity will be assessed in the future, and a prospective randomized trial is needed to verify these findings.

Emerging evidence for the role of an endorectal balloon in prostate radiation therapy

Purpose: To reassess and update the role of an endorectal balloon (ERB) in prostate radiotherapy (RT) based on emerging evidence by reviewing various aspects of treatment methodologies and clinical outcomes. Methods and materials: A literature review based on a PubMed/MEDLINE database search using keywords such as: ERB, prostate RT, toxicities, real-time, image-guided radiotherapy (IGRT), radiofrequency-guided radiotherapy (RGRT), and inter- and intrafraction prostate motion for articles published over the past two years. Ten articles were identified and subdivided into three categories: (I) Issue of Motion, (II) Dosimetry, (III) Clinical Outcomes. Results: With the advent of real-time prostate tracking, analysis of intrafraction motion as a function of treatment time for patients treated with a daily-ERB was performed and revealed an overall reduction in 3D prostate motion, especially in the anterior-posterior direction. Two different groups of authors found that this reduction in intrafraction prostate motion allowed for tighter internal margins. Dosimetric studies showed overall improved dose distributions which for proton therapy were maximized when using ERBguided range verification for anteriorly oriented beams. Clinical outcomes showed favorable early GI toxicities however late toxicity results are still awaited. Conclusions: Utilizing a daily-ERB shows favorable early GI toxicity as well as reduced prostate intrafraction motion based on real-time tracking data. Reduced intrafraction motion improves the feasibility to use anteriorly oriented proton beams, which may further improve dosimetry.

Assessment of Using Endorectal Balloon in Prostate Radiation Therapy on Daily Kilovoltage Cone Beam Computed Tomography Localization

To assess the effect of using endorectal balloon (ERB) in prostate radiation therapy on daily kilovoltage (kV) cone beam computed tomography (CBCT) shifts. Thirty consecutive patients treated for prostate cancer with VMAT or IMRT techniques were localized using daily kV CBCT. Half of the patients were simulated and treated using ERB with air volume of 80 to 90 cc. Selection of patients receiving ERB was at the discretion of the physician. With the exception of the ERB, all patients were simulated using the same conditions: supine, with full bladder and empty rectum, and immobilized from the knees down using a Vac-Lok cushion. All patients had three to four fiducials implanted for image guidance and localization using CBCT. Treatment isocenter was marked on the skin during simulation and the marks were used for initial setup at treatment. Daily CBCT shift records of the superior-inferior (SI), left-right (LR), anterior-posterior (AP), and overall 3-dimensional (3D) displacements were collected and analyzed for a total of 1251 treatment sessions. The statistical significance in the absolute value of the shift data of the two groups was investigated using two-tailed unequal-variance t-test. Random (σ) errors, defined as the standard deviation (SD) of the distribution of shifts of all patients within a group, and systematic errors (∑), defined as the SD of the distribution of the means of patients' shifts in a group, and PTV margins, defined as 2.5∑+0.7σ, were derived for both patient groups. The statistical results of the daily CBCT shifts are tabulated below. The mean of the 3D shifts for the patient group treated with ERB was 1.4 mm lower than that for the group treated without ERB. The t-test analysis of the data yielded statistically significant difference in shift data between the two groups in the SI (p<0.0001), AP (p = 0.02), and 3D displacement (p<0.0001). No significant difference was observed in the LR direction. Random and systematic errors and derived margins were consistently lower for patients treated with ERB than for those treated without ERB.Poster Viewing Abstract 3327; TableParameter/DirectionCBCT absolute shift (mm) with ERBCBCT absolute shift (mm) without ERBSILRAP3DSILRAP3DMinimum00000000Maximum1013121412112020Mean3.12.43.55.94.52.43.97.3Random error (σ)2.22.02.62.82.92.13.33.4Systematic error (∑)1.40.81.11.41.90.92.32.2Derived PTV margins (2.5∑+0.7σ)53557488 Open table in a new tab Using ERB in prostate radiation therapy yields a statistically significant reduction in the 3D displacement of the CBCT setup shift. The demonstrated reduction in the systematic and random errors in the patient group treated with ERB can potentially translate in a reduction of PTV margins for this group using published formulation.