P Mobit

What do Dosimetric Errors Encountered in Prostate Implant Brachytherapy tell us about α/β?

Objective: To determine the α/β ratio of prostate cancer using the linear quadratic model taking into account different radiation prescription doses and RBE for low energy gamma rays. Methods and material: The linear quadratic model was used to evaluate the α/β ratio for prostate cancer taking into account the dosimetric errors resulting from seed displacements in prostate permanent implant brachytherapy with 125I and 103Pd. The study assessed the variability of the α/β ratio with different prescribed external beam radiation therapy doses and published values of the relative biological effectiveness (RBE) for both 125I and 103Pd. The biological effective dose (BED) for prostate implant brachytherapy was equated to the external beam radiation therapy dose to derive an equation for α/β ratio. Results: The results showed that the α/β ratio for prostate cancer varied between 1 and 4.5 for an RBE of 1.0 when an external beam dose of 78.0 Gy was prescribed. When published values of RBEs were incorporated into the analysis, the α/β ratio varied between 0.37 and 4.4. The α/β ratio changed by 30% when the external beam radiation dose was increased from 72 Gy to 80 Gy. Conclusions: Assuming an average reduction in implanted seeds brachytherapy dose between 10–20% using 125I or 103Pd, the realistic value of the α/β ratio for prostate tumors likely lies between 0.7 and 2.0.

3D Image based Customized versus Standard Treatment Planning for Cervical Cancer High Dose Rate Brachytherapy with Tandem and Ovoids

Purpose and Objective(s): To investigate the advantages of volumetric treatment planning in HDR brachytherapy for cervical carcinoma compared to standardized loading based on 2-D planning techniques. Materials and Methods: Our institution uses volume-based 3-D planning for each tandem and ovoid (T&O) insertion for HDR brachytherapy in the treatment of advanced cervical carcinoma. Here, we attempt to define the benefits of this approach. We re-planned 48 CT-based treatment plans on 12 patients (treated in our facility between February, 2009 and February, 2010) using a commonly used 2-D standard HDR loading of the T&O. All patients had received 4 fractions of 6.5 Gy or 5 fractions of 5.5 Gy to point H or A. The following organs at risk (OARs) were contoured: rectum, bladder, sigmoid, and small bowel. Our customized planning approach required the adjustment of source dwell times and positions to keep doses to the OARs below 80% of the prescription dose. The standardized HDR planning, however, bases the loading time on the length of the tandem. The dwell time for each tandem source position is the same. The dwell time multipliers for the ovoids were 0.33, 0.665 and 1.0, proportionate to the 2 cm, 4 cm, and 6 cm tandem length, respectively. The dose to the highest 2 cc (D2cc) of the OARs were also determined and analyzed. Results: There was a marked change in the value and location of the D2cc for all OARs from one HDR session to the next in both the standard and customized plans. When the data for the 48 plans were analyzed together, there were no significant differences between the customized plans and the standardized plans. However, when data for the individual plans were analyzed, 35% of the 2-D based plans did not meet our treatment planning objectives. Conclusion: Using customized plans for HDR T&O brachytherapy did not always reduce the doses to the rectum, bladder, sigmoid, and small bowels compared to the standardized plans. The dose to the small bowel could be up to 15% higher than the dose to point H or A in the standard plans indicating that customized plans may be superior to the standardized ones for the treatment of patients where this dose is critical.

Comparison of Axxent-Xoft, 192Ir and 60Co high-dose-rate brachytherapy sources for image-guided brachytherapy treatment planning for cervical cancer

OBJECTIVE To evaluate the dosimetric differences and similarities between treatment plans generated with Axxent-Xoft electronic brachytherapy source (Xoft-EBS), (192)Ir and (60)Co for tandem and ovoids (T&O) applicators. METHODS In this retrospective study, we replanned 10 patients previously treated with (192)Ir high-dose-rate brachytherapy. Prescription was 7 Gy × 4 fractions to Point A. For each original plan, we created two additional plans with Xoft-EBS and (60)Co. The dose to each organ at risk (OAR) was evaluated in terms of V(35%) and V(50%), the percentage volume receiving 35% and 50% of the prescription dose, respectively, and D(2cc), highest dose to a 2 cm(3) volume of an OAR. RESULTS There was no difference between plans generated by (192)Ir and (60)Co, but the plans generated using Xoft-EBS showed a reduction of up to 50% in V(35%), V(50%) and D(2cc). The volumes of the 200% and 150% isodose lines, however, were 74% and 34% greater than the comparable volumes generated with the (192)Ir source. Point B dose was on average only 16% of the Point A dose for plans generated with Xoft-EBS compared with 30% for plans generated with (192)Ir or (60)Co. CONCLUSION The Xoft-EBS can potentially replace either (192)Ir or (60)Co in T&O treatments. Xoft-EBS offers either better sparing of the OARs compared with (192)Ir or (60)Co or at least similar sparing. Xoft-EBS-generated plans had higher doses within the target volume than (192)Ir- or (60)Co-generated ones. ADVANCES IN KNOWLEDGE This work presents newer knowledge in dosimetric comparison between Xoft-EBS, (192)Ir or (60)Co sources for T&O implants.

SU‐GG‐T‐44: Evaluation of Xoft Electronic Brachytherapy System for Intraoperative Treatments

Purpose: The main objective was to evaluate and commission the Xoft Electronic Brachytherapy System for intraoperative treatments.Method and Materials: Using the manufacturer supplied phantom, we evaluated and commissioned the Xoft Electronic Brachytherapy system. We tested well‐chamber constancy and intercomparison, beam output stability with time, start/end effects, and performed radiation surveys. Other checks recommended by the AAPM TG152 were evaluated. Results The Ir‐192 calibrated well chamber is 3.7 times more sensitive when irradiated with the Xoft source than the Xoft calibrated well chamber. Expectation was that both chambers would give approximately the same reading because the Xoft integrated well chamber is cross calibrated in I‐125 source. It takes about 28s for the doserate from the Xoft Unit to ramp up to the treatmentdoserate. The unit delivers 9s worth of treatment during the ramp up phase. For treatment times less than 100s, this would introduce a dosimetry error of about 10% which will be repeated if there is a treatment interruption. Xoft Unit output may vary by up to 5% between 0.25 and 30 minutes. This variation is source dependent. So the minimum time used to collect charges for the AAPM‐TG61 calibration should be 1 minute, not 0.25 minute. Radiation surveys during treatment indicate that surface and intraoperative treatments give rise to exposure rates of 200mR/hr, 30cm from treatment area. Conclusions: Well chamber calibration inconsistency for Xoft Unit needs further investigation. Ramp up time accounts for 9s equivalent treatment time. Correction needed for treatment time less than 150 seconds. Charges for TG61 calibration should be collected for at least 1 minute, not 0.25 minute. Intraoperative and surface treatment produces exposure rate of 200mR/hr at distances of 30 cm. Exercise extreme caution by standing behind lead shield or wearing a lead apron.

SU‐E‐T‐414: TG‐129 Implementation On BrachyvisionTM

PURPOSE To outline the steps taken to clinically implement TG-129 on Brachyvision™ Treatment Planning System and to show the dosimetric differences that can be expected from the original COMS Eye Plaque model. METHODS The original COMS-Eye Plaque protocol was based on the following simplification: point source model in infinite water medium, 1D geometric function, 1D radial dose function and no anisotropy. Recently, AAPM Task Group 129 had made two specific recommendations: 1> Upgrade to TG-43U1 line source model; and 2> Report the heterogeneity corrected dose. Upgrading to line TG-43U1 line source model was done by a creating a seed collection in Brachyvision (version 11) for each plaque size. For each seed in the collection, both end coordinates had to be entered into the software. Full line source model was followed to compute the homogeneous dose at the prescription point. This homogeneous dose was converted to heterogeneous dose using a conversion table. Dose to prescription points for five different plaque sizes, including one customized notched plaque, was calculated using the implemented TG-129 model and compared against actual COMS derived doses from previously treated patient plans. Conversion to heterogeneous doses was done based on table 2 of TG-129 report. RESULTS As expected, dose difference between COMS and TG-129 was relatively minor, ranging from -0.15% to 1.91% at the prescription point. On the other hand, heterogeneous dose, which should be used for reporting purpose only, can be up to ∼17% lower than the corresponding homogeneous dose. CONCLUSION Conversion to TG-129 can be rapidly accomplished in 1-2 weeks. The initial time investment can be quickly recouped since the TPS plans created can be modified for different patient cases. Dosimetric difference between TG-129 and the original COMS model is small, generally less than 2%.

SU‐E‐T‐44: Phantom 3D Dose Calculation and Anatomy Based DVH Evaluation On VMAT Patient QA Using the Newest Version of Delta4 Dosimetry System

PURPOSE To validate the two new patient QA methods on ScandiDos Delta4 system: (1) Phantom 3D dose calculation (2) Anatomy DVH evaluation, compared with conventional gamma analysis criteria. METHODS Phantom 3D dose calculation and anatomy DVHs evaluation have the capability to perform gamma analysis on critical organ contours and calculate DVHs from Delta4 measurement and calculation on either phantom or patient anatomy. The calculation speed, performance time and algorithm accuracy were investigated on these two novel methods. Elekta Synergy beam PDDs and output factors were characterized by importing plans from Pinnacle (v9.2). VMAT H&N QA plans were performed. The patient CT images and contours were transferred from Pinnacle to Delta4. Three critical structures (PTV, Brainstem and Cord) were analyzed using gamma analysis, phantom 3D dose calculation and anatomy DVH evaluation. RESULTS Conventional gamma pass rate is analyzed by combining the dose difference and distance to agreement on each detector. 3D dose calculation is implemented on the homogeneous phantom using ray-tracing algorithm. Patient anatomy dose is calculated using the fluence map and kernel-based pencil beam algorithm. The average calculation time is 1.1, 5.3 and 25.5 minutes for each method. The performance time is 22, 30, and 55 minutes for each patient QA, since anatomy evaluation needs CT images and anatomy structures. Regarding the algorithm accuracy, conventional gamma analysis and phantom based PTV 3D-dose are correlated. The passing rate difference are insignificant (P>0.2, t test). There are discrepancies when the phantom 3D-Dose gamma and DVH are compared with anatomy based evaluation. The passing rate difference are significant (P<0.005, t test). CONCLUSION Conventional gamma analysis has weak correlation to critical DVH errors. Phantom 3D dose and anatomy DVH evaluation on Delta4 are sensitive and specific for VMAT patient QA. Further investigation on accuracy and potential errors of anatomy DVH will be performed.

SU-C-500-02: Comparison of a Commercial Electronic Brachytherapy Source and Ir-192/Co-60 HDR Source for Endometrial and Cervical Cancers

PURPOSE To evaluate the dosimetric differences between Iridium-192, Co-60 based HDR plans and plans generated by electronic brachytherapy source (EBS) for cervical and endometrial cancers. MATERIAL/METHODS Our institution uses volume based planning for each tandem and ovoid (T&O) or vaginal cylinder insertion and treatment. We attempt to characterize the differences between the three sources (Ir-192, Co-60 and EBS). For vaginal cylinder cases, patients receive 6 Gy x3 prescribed to a depth 5 mm. For T&O cases, the prescription is 8Gy x3 to point A. Organs at risk (OARs) contoured were rectum, bladder, sigmoid & small bowel. Our planning protocol requires adjustment of source dwell times and positions to keep doses to the OARs below 70% of the prescription dose. For vaginal cylinder, dose coverage is optimized to a line drawn 5mm from the vaginal cylinder surface. This is a retrospective study of previously planned Ir-192 treated patients. RESULTS There is no difference between the Co-60 and Ir-192 plans but plans generated using the EBS offer better sparing of OARs as evaluated using the dose to highest 2cc of the organ at risk but the differences are not statistically significant. However, there are significant differences in the DVH between EBS and the other two sources. There was a reduction of up to 50% between EBS and other two sources using v15%, v25%, v50% v75% (% of volume getting 15, 50, 75 percent of the prescription dose). CONCLUSION All three sources are suitable for HDR Brachytherapy but the use of EBS can reduce the organ at risk dose while achieving similar dose coverage. Even though D2cc is the recommended dose volume point for HDR plans evaluation, V15%, V25%, V50% and V75% are also useful indicator of the plan quality as they characterize the lower dose region of the DVH curves more adequately.

The influence of pregnancy on heterotopic ossification post-displaced acetabular fractures surgical repair

Pregnancy is associated with maternal bone mineral density loss and modulation of calcium metabolism. We hypothesized that pregnancy may decrease the risk of heterotopic ossification (HO) after trauma. This is a single‐institution, University of Mississippi Medical Center, retrospective study investigating the effect of pregnancy on the incidence HO after surgical repair (SR) of displaced acetabular fractures. Between January 1998 and 2010, 257 non‐pregnant women (Group A) and 16 pregnant women (Group B) were identified. All the non‐pregnant women received radiation therapy (RT) ± indomethacin. None of the pregnant women in group B received any prophylaxis. After a median follow‐up of 6.6 years the incidence of HO in all patients was 27% (75/273). In Group A, non‐pregnant, women who received RT ± indomethacin, 29% developed HO; HO risk was 0.4. In Group B, 16 pregnant patients, only one developed HO (6%); HO risk was 0.06. Thus, the risk of HO appears to be nearly six‐fold higher in non‐pregnant women despite prophylactic RT ± indomethacin. Our data suggest that pregnancy may be associated with a reduced risk of HO after SR of displaced acetabular fractures. Further analysis with a larger pregnant patient sample is necessary to confirm this finding.

Computerized tomography-based radiotherapy improves heterotopic ossification outcomes☆

PURPOSE To report the impact of computerized tomography (CT) based radiotherapy (RT) on heterotopic ossification (HO) outcomes. METHODS This is a single institution, retrospective study of 532 patients who were treated for traumatic acetabular fractures (TAF). All patients underwent open-reduction internal-fixation (ORIF) of the TAF followed by RT for HO prophylaxis. Postoperative RT was delivered within 72h, in a single fraction of 7Gy. The patients were divided into 2 groups based on RT planning: CT (A) vs. clinical setup (B). RESULTS At a median follow up of 8years the incidence of HO was 21.6%. Multivariate regression analysis revealed that group (A) vs. (B) had HO incidence of 6.6% vs. 24.6% (p<0.001), respectively. Furthermore, HO Brooker grade ≥3 was observed in 2.2% vs. 10.8% (p=0.007) in group (A) vs. (B), respectively. Thus, the odds of developing HO and Brooker grades ≥3 were 4.7 and 4.5 times higher, respectively, in patients who underwent clinical setup. CONCLUSION Our data suggest that using CT based RT allowed more accurate delineation of the tissues and better clinical outcomes. Although CT-based RT is associated with additional cost the efficacy of CT-based RT reduces the risk of HO, thereby decreasing the need for additional surgical interventions.

SU‐E‐T‐299: Verification of High Density Concrete Transmission Characteristics under User Beams

Purpose: To verify transmission factors of the high density concrete (HDC) used in modification of an existing radiation shielding for a new linear accelerator and to measure transmission factors of other types of HDC under user beams before shielding construction started. Method and Materials:Six HDC 300 PCF (Pounds per Cubic Feet) blocks in size of 6″×6″×12″, randomly picked up from a shipment for radiation shielding at the user site, in addition to one sample of each 288 PCF, 250 PCF and 240 PCF were measured for their transmission factors using a 0.6 cc Farmer chamber and the Dose 1 electrometer. User beams used for measurement includes 6MV, 10 MV and 18 MV photon beams from an Elekta Synergy machine. Transmission factors are measured under both narrow (3×3 cm2)(NBG) and broad beam geometry (40×40 cm2)(BBG) with a close condition to a real shielding geometry. Results: Weight measurement indicates that the actual PCF is higher than stated: 313.3±4.5 (one SD) in the range from 306 to 318 PCF for 300 PCF and 291 PCF for 288 PCF, 251 PCF for 250 PCF, 247 PCF for 240 PCF. Transmission measured at seven spots under NBG (18 MV) for one 300 PCF sample is 0.087±0.002 (Min: 0.084 and Max: 0.089). Transmission measured under NBG (18 MV) for six 300 PCF sample blocks is 0.089±0.004 (Min: 0.086 and Max: 0.096). The first tenth value layer under BBG (18 MV) is determined as 7.4″, which is smaller than 8.5″ used in the shielding design calculation. Conclusions: We have confirmed that the HDC blocks used in constructing the radiation shielding meet and actually succeeded the transmission requirement in the shielding calculation. We provided the commercial company with updated measurement of its shielding materials under our user beams.