L Potter

3D Analysis of Intensity-Modulated Radiation Therapy Quality Assurance Measurement using a 2D Diode Array

Intensity-modulated radiation therapy (IMRT) quality assurance (QA) is often performed using a 2D device and compares measured and computed fluence maps to determine if a field passes or fails certain dose and position criteria. The effects of a measured deviation to the 3D patient spatial dosimetry and dose volume histogram (DVH) are largely unknown because they cannot be analyzed using commercial 2D array IMRT QA systems. We report an in-house treatment planning system (TPS) PLanUNC based 3D IMRT QA analysis approach that has been used in our institution for the past ten years when 2D fluence map IMRT QA failed. In this approach the measured 2D fluence maps are imported back to PLanUNC and used to re-compute 3D patient dosimetry including DVHs. The 2D fluence map IMRT QA criteria is that the measured dose for 95% of the detectors is within 5% of the planned dose, and that the distance-to-agreement be within 4mm (5%/4mm). 22 IMRT plans that had at least one field fail initial QA using MapCHECK 2 are examined using our 3D QA approach. The DVH analysis shows that 19/22 plans that failed initial QA were within 2% of the planned target and critical structure DVHs. 3/22 IMRT plans were found to have DVH difference greater than 2%. The 3D analysis of 2D IMRT QA result shows that when a fluence map QA fails for a single field, often it is clinically insignificant in terms of patient 3D dosimetry

SU‐GG‐T‐88: A Multi‐Institutional Retrospective Study On Clinical IMRT Treatment Delivery Efficiency

Purpose: To better understand IMRT delivery efficiency in daily clinical operation we conducted a five‐institution retrospective study on clinical IMRTtreatment delivery time and IMRT MUs as functions of a number of variables — the number of fields per IMRTtreatment,treatment site, total MUs, and total number of segment fields for MLC‐IMRT treatment. The goal is to identify major contributing factor(s) for IMRT delivery time for each IMRT approach for improvement. Method and Materials: Data from more than 350 patients were extracted from IMPAC/LANTIS R&V system. A 5‐day average value was taken for any time data. The IMRTtreatment delivery time represents the portion of total patient delivery time that is specific to IMRT delivery technique and is defined as the time elapsed between the beam‐ON of the first field/segment and the beam‐OFF of the last field/segment in an IMRTtreatment. The accelerators studied are Siemens, Varian, and Elekta. Treatment planning systems used are PLanUNC, ADAC Pinnacle, and CMS XiO. The four IMRT delivery approaches studied are: segmental MLC‐IMRT on Siemens accelerator, segmental MLC‐IMRT on Elekta accelerator, dynamic MLC‐IMRT on Varian accelerator, and compensator‐IMRT on Siemens accelerators. Results: Our initial results show that for MLC‐IMRT treatments the IMRTtreatment delivery time is closely correlated with the number of segment fields and less correlated with the total number of MUs and treatment site. There is a large variation in IMRTtreatment delivery time for IMRTtreatment of a given number of fields, depending on the accelerator and IMRT approach used. Conclusion: Our five‐institution retrospective study on clinical treatment delivery data shows that manual compensator‐IMRT treatment is among the fastest of the five IMRT delivery approaches studied. In average all IMRT delivery approaches spent only approximately 20% of the IMRT delivery time (as defined) on actual radiation delivery.

SU-FF-T-119: Comparison of Compensator-IMRT and Segmental MLC-IMRT Techniques: A Retrospective Study On Treatment Time, Monitor Units, and Dosimetry in Clinical Application

Purpose: There are more than 150 radiotherapy centers in the US using compensators to deliver IMRT treatments. However, compensator‐IMRT is still not a well‐understood technique to many; better knowledge of the technology can encourage acceptance and proper use of this valuable IMRT delivery technology. We will present our compensator‐IMRT experience in clinical application using retrospective patient treatment data from eleven years (1100 patients) of compensator‐IMRT experience and five years (500 patients) of segmental MLC‐IMRT in parallel. Method and Materials:IMRT plans are designed by the in‐house IMRT TPS PLanUNC. The resulting continuous intensity maps are used for compensator design. The intensity maps are converted to discrete maps for MLC segment generation if segmental MLC‐IMRT technique is used. Compensators are fabricated using a Par Scientific milling machine and granular compensator material. This study uses data from the treatment RV 2) both IMRT delivery techniques use similar monitor units; and 3) the high spatial resolution compensator‐IMRT generally has a similar or better dosimetric quality compared to the segmental MLC‐IMRT technique. Conclusion: We evaluated both the compensator‐IMRT and segmental MLC‐IMRT delivery techniques in terms of treatment delivery time, treatment monitor units, and dosimetric quality (DVH and EUD). Our experience demonstrated that the compensator‐IMRT technique delivered high quality IMRTdosimetry, fast IMRT treatments, and similar monitor units compared to the segmental MLC‐IMRT technique.

Prostate deformation from inflatable rectal probe cover and dosimetric effects in prostate seed implant brachytherapy.

PURPOSE Prostate brachytherapy is an important treatment technique for patients with localized prostate cancer. An inflatable rectal ultrasound probe cover is frequently utilized during the procedure to adjust for unfavorable prostate position relative to the implant grid. However, the inflated cover causes prostate deformation, which is not accounted for during dosimetric planning. Most of the therapeutic dose is delivered after the procedure when the prostate and surrounding organs-at-risk are less deformed. The aim of this study is to quantify the potential dosimetry changes between the initial plan (prostate deformed) and the more realistic dosimetry when the prostate is less deformed without the cover. METHODS The authors prospectively collected the ultrasound images of the prostate immediately preceding and just after inflation of the rectal probe cover from thirty-four consecutive patients undergoing real-time planning of I-125 permanent seed implant. Manual segmentations of the deformed and undeformed images from each case were used as the input for model training to generate the initial transformation of a testing patient. During registration, the pixel-to-pixel transformation was further optimized to maximize the mutual information between the transferred deformed image and the undeformed images. The accuracy of image registration was evaluated by comparing the displacement of the urethra and calcification landmarks and by determining the Dice index between the registered and manual prostate contours. After registration, using the optimized transformation, the implanted seeds were mapped from the deformed prostate onto the undeformed prostate. The dose distribution of the undeformed anatomy, calculated using the VariSeed treatment planning system, was then analyzed and compared with that of the deformed prostate. RESULTS The accuracy of image registration was 1.5 ± 1.0 mm when evaluated by the displacement of calcification landmarks, 1.9 ± 1.1 mm when characterized by the displacement of the centroid of the urethra, and 0.86 ± 0.05 from the determination of the Dice index of prostate contours. The magnitude of dosimetric changes was associated with the degree of prostate deformation. The prostate coverage V100% dropped from 96.6 ± 1.7% on prostate-deformed plans to 92.6 ± 3.8% (p < 0.01) on undeformed plans, and the rectum V100% decreased from 0.48 ± 0.39 to 0.06 ± 0.14 cm3 (p < 0.01). The dose to the urethra increased, with the V150% increasing from 0.02 ± 0.06 to 0.11 ± 0.10 cm3 (p < 0.01) and D1% changing from 203.5 ± 22.7 to 239.5 ± 25.6 Gy (p < 0.01). CONCLUSIONS Prostate deformation from the inflation of an ultrasound rectal probe cover can significantly alter brachytherapy dosimetry. The authors have developed a deformable image registration method that allows for the characterization of dose with the undeformed anatomy. This may be used to more accurately reflect the dosimetry when the prostate is not deformed by the probe cover.

SU-E-J-236: Feasibility of Using Infrared Imaging to Verify the Accuracy of the Radiotherapy Delivery

PURPOSE To assess whether infrared imaging can be used to verify the accuracy of the radiation treatment delivery. METHODS Radiation treatment induced skin reactions include acute changes (erythema and pigmentation) and chronic changes (telangiectasia and ulceration). Thermal imaging can detect these reactions before they become visible. A thermal camera with infrared spectral band 7.5 μm to 14 μm, and temperature measurement range from -20 °C to +150 °C and accuracy of ±2 °C was used. Beams-eye-view images were taken by the end of each treatment for a head and neck patient. The temperature changes of three points were monitored throughout the treatment course, including two points in the field (canthus and vessel) and one point outside of the field. The thermal images were warped and registered to the corresponding planning CT skin rendering. Dose map was exported from the treatment planning system and warped and registered to the planning CT skin rendering as well. The correlation between these two warped and registered images was calculated. The time spent to take the images was recorded. RESULTS The temperatures of the two points inside the field were always higher than the temperature of the point outside of the field. The temperature inside the field tended to increase as more fractions delivered to the patient. A correlation of 0.90 was found between the registered thermal image and the 3D dose map. On average, half minute was needed to take the images after each treatment. CONCLUSION Thermal imaging can monitor the temperature variations of the skin, and the temperatures were proportional to the dose map. It can therefore potentially be an inexpensive and non-ionization tool to verify the accuracy of the radiation treatment delivery.

SU‐E‐T‐771: Isodose Line Driven Semi‐Inverse Planning of High Dose Rate Brachytherapy for Cervical Cancer

Purpose: Delineation of tumor is indispensible for adequate tumor coverage in inverse planning of cervical cancerbrachytherapy. However, target definition is challenging in CT/CBCT planning images. In this project, we developed a tool to convert the isodose lines from traditional source loading, which produces dose distribution with good tumor coverage, to a surrogate treatment volume needed in the optimization. Through this, we integrate the clinical knowledge of conventional loading and advantage of inverse planning to spare organs at risks. Method and Materials: Five cervical cancer patients treated with tandem and ovoids HDR brachytherapy are studied. The clinical plans are point‐based (600cGy to point A) with Fletcher‐type loading pattern. Retrospectively, an inverse plan was made for comparisons. A software tool was developed to convert the isodose curves of the conventional plan to closed anatomic structures. In order to limit the dose of bladder and rectum less than 70% of the prescription dose, their contours were subtracted from the 70% isodose line converted‐volume and this new structure (ISD70‐bladder‐rectum ) was placed as the target in Oncentra optimization software (Nucletron). Three dosimetric endpoints, volume coverage of ISD70‐bladder‐rectum and 2cc maximal dose of the bladder and rectum, are used in compassions. Results: The isodose line converted‐ structures are compared with the original dose curves and they are accurate. The inverse planning lowers the dose on the bladder with similar coverage of ISD70‐bladder‐rectum as the conventional plan. The bladder 2cc dose of the inverse plan is 510.4± 92.7 cGy, which is significantly lower than that of conventional plan, 560.3± 93.8 cGy (p=0.05). Conclusion: Isodose surfaces from traditional source loading are good surrogate for 3D treatment volume in HDR inverse planning for cervical cancer. This approach combines the prior clinical experience and strength of inverse planning for better critical structure sparing with the similar tumor coverage.

SU‐GG‐T‐209: A Novel Method for Further Analysis of IMRT QA

Purpose:IMRT QA is often performed with a commercial device that compares 2D intensity maps to determine if a field passes certain dose and position criteria. The effect of an intensity map error on 3D dosimetry and DVH is clinically relevant but often not considered in IMRT QA. We herein exam the 3D dosimetric impact of IMRT failures measured with a 2D IMRT QA tool. Methods and Materials: 22 IMRT plans had at least one field fail QA using MapCHECK 2, and these were further analyzed using in‐house software to perform a DVH analysis. The MapCHECK 2 passing criteria was 95% of measured points had to be within 5% of the planned dose and have a distance‐to‐agreement of 4mm. Software was installed in our treatment planning system, PlanUNC, that reads intensity maps measured with the MapCHECK 2 and creates DVHs for 3D dose analysis. The software allows for field segment adjustment of MLC positions to address field edge failures and modification of prescribed MUs to address detector points that failed QA. Results: Of the 22 plans that had a field fail initial QA with MapCHECK 2, it was determined that 16/22 and 3/22 plans were within 1% and 2% of the planned CTV and critical structure DVHs. 3/22 plans were found to have DVH differences greater than 2%, and MU values were therefore altered in the treatment plan. Conclusions: Further analysis following an initial IMRT QA with the MapCHECK 2 is often needed to address the 3D dosimetric effects of the measured plan. Software has been installed into PLUNC that allows for 3D analysis for IMRT QA. Our analysis shows that using the MapCHECK 2 passing criteria of dose within 5% and 4mm DTA is sufficient, since 19/22 plans failing this criteria are within 2% of the planned DVH.

MO‐FF‐A1‐06: The Impact of Source Dwell Position Uncertainty on CT‐Based Brachytherapy Plan for Cervical Cancer

Purpose: In CT based high dose rate (HDR) brachytherapy, the longitudinal catheter tip position can be easily mis‐localized because of several reasons. These include the coarse image scanning thickness, artifact of applicators, use of ovoids shield and digitizing uncertainty during planning. With the use of anatomic structures of planning CT, this project investigates the 3D dosimetric impact when source dwell positions deviate from the true positions. Method and Materials: Five cervical cancer patients treated with intracavity HDR brachytherapy using tandem and ovoids applicators were included in this study. For each patient, we acquired CT and made clinic plan based on ICRU 38 protocol. Retrospectively, we contoured the bladder and rectum wall and replaned patients considering the scenarios of source positions shifted 5mm. For each patient we compared five plans: original clinical plan, seed positions of all three applicators shifted forward or backward and seed positions of two ovoids shifted forward or backward. Seven dosimetric endpoints are used in comparisons and they are total treatment time, ICRU bladder/rectum point dose, and 1cc/5cc bladder/rectum maximal dose.Results: The DVHs of two forward plans, all shifted or only ovoids shifted, show similar trends. Similar result was observed from two backward shifted plans. Forward shifted and backward shifted plans deviate from the original plan, but in opposite way, and the direction of deviation is structure dependent. For 1cc bladder wall maximum dose, the mean of forward plans is −3.9±3.0% and backward plans is 5.7±4.6% different from the original plan. For 1cc rectum wall maximum dose, the mean of forward plans is 14.6±6.3% and backward plans is −6.3±3.2% different from the original plan. Conclusion: Source position accuracy is a critical factor for the high quality of brachytherapytreatment planning. A 5mm discrepancy could have great impact on the dosimetric endpoints of normal structures.

SU‐FF‐T‐216: Comparison of a 2D and 3D Array of Diodes for IMRT QA

Purpose: To compare the IMRT QA pass/fail rates of a 2D diode array system MapCHECK™ and a cylindrical 3D diode array system Delta4™, and to investigate the benefit of DVH‐based IMRT QA. Methods and Materials: Eight treatment plans totaling 62 IMRT fields were measured using both MapCHECK and Delta4. The data were compared to the treatment planning data using Gamma analysis. Passing criteria was defined as 95% of measured points had to have a gamma value ⩽1.0 using a distance to agreement of 4mm, a 5% dose window, and a 10% dose threshold. Structures, including GTV and organs at risk (OAR), and dose volume histograms (DVHs) were exported from the treatment planning system to Delta4 for comparison to measured DVHs. Results: QA with Delta4 used an average of 466 detector points per field. Using gamma analysis, 60/62 (96.8%) IMRT fields passed with an average of 98.9% of detector points within a gamma value ⩽1.0 when measured on the Delta4. QA with MapCHECK used an average of 88 detector points per field. Application of the same gamma analysis resulted in 14/62 (22.6%) IMRT fields passing with an average of 91.3% of detector points with a gamma value ⩽1.0 as measured by MapCHECK. Further analysis of IMRT fields that failed using MapCHECK QA, indicated that the measured data was within 1–2% of the treatment plan. Planned GTV DVHs corresponded with the Delta4 measured GTV DVHs, however measured OAR DVHs differed from their planned DVHs. Conclusions: This study suggests that QA results acquired with Delta4 correspond more accurately to the actual treatment plan as compared to MapCHECK. Incorporating Delta4 into routine QA will decrease the overall QA analysis time. The increased pass rate with Delta4 may result from the increased amount of detectors per treatment field.

SU‐GG‐T‐24: Dose Difference of Nucletron HDR Planning with Three Source Localization Methods

Purpose: High dose rate brachytherapy (HDR) is a very effective cancer treatment method. Localization of source positions is essential for dosimetric accuracy. Nucletrons PLATO TPS provides multiple means to reconstruct and localize source positions. Three commonly used methods are: (1) catheter describing using film, (2) catheter tracking using film, and (3) seed identifying on CT slices. Although all of these methods have been used in the clinic, the dose variance among them is unknown. The purpose of this project is to study quantitively the dose discrepancy of these different source reconstruction methods. Method and Materials: A brachy phantom was constructed with CT compatible dummy sources and a tandem. Fiducials of 5mm in diameter were attached to the phantom surface. The phantom was CT scanned with 1mm slice thickness, and then was moved to a treatment couch under the reconstruction bridge. Orthogonal films were taken, and three HDR plans were created. Plan 1 and 2 used film images. Plan 1 reconstructed the source with the catheter describing method while Plan 2 employed the tracking method. Plan 3 was based on the CTimages.Doses at the four fiducial points were calculated and compared. In order to minimize the uncertainty caused by human operators, each plan was repeated by three experienced physicists; one physicist repeated planning three times on every plan. Results:Dose points from CT based plan was used as a reference. When comparing plans by the source reconstruction method, we found significant dose difference on certain dose point between film based planning and CT based planning. The biggest dose difference was 10.3% on fiducial A. Conclusion: Through a series of planning on a phantom, we found the doses can differ significantly between the film and CT based‐ method. At certain locations, the film based‐ plan may underestimate the dose.