B. Mason

Simple Carotid-Sparing Intensity-Modulated Radiotherapy Technique and Preliminary Experience for T1-2 Glottic Cancer

PURPOSE To investigate the dosimetry and feasibility of carotid-sparing intensity-modulated radiotherapy (IMRT) for early glottic cancer and to report preliminary clinical experience. METHODS AND MATERIALS Digital Imaging and Communications in Medicine radiotherapy (DICOM-RT) datasets from 6 T1-2 conventionally treated glottic cancer patients were used to create both conventional IMRT plans. We developed a simplified IMRT planning algorithm with three fields and limited segments. Conventional and IMRT plans were compared using generalized equivalent uniform dose and dose-volume parameters for in-field carotid arteries, target volumes, and organs at risk. We have treated 11 patients with this simplified IMRT technique. RESULTS Intensity-modulated radiotherapy consistently reduced radiation dose to the carotid arteries (p < 0.05) while maintaining the clinical target volume coverage. With conventional planning, median carotid V35, V50, and V63 were 100%, 100%, and 69.0%, respectively. With IMRT planning these decreased to 2%, 0%, and 0%, respectively (p < 0.01). Radiation planning and treatment times were similar for conventional radiotherapy and IMRT. Treatment results have been excellent thus far. CONCLUSIONS Intensity-modulated radiotherapy significantly reduced unnecessary radiation dose to the carotid arteries compared with conventional lateral fields while maintaining clinical target volume coverage. Further experience and longer follow-up will be required to demonstrate outcomes for cancer control and carotid artery effects.

Accelerated partial breast irradiation using the strut-adjusted volume implant single-entry hybrid catheter in brachytherapy for breast cancer in the setting of breast augmentation

PURPOSE Accelerated partial breast irradiation (APBI) has gained popularity as an alternative to adjuvant whole breast irradiation; however, owing to limitations of delivery devices for brachytherapy, APBI has not been a suitable option for all the patients. This report evaluates APBI using the strut-adjusted volume implant (SAVI) single-entry catheter to deliver brachytherapy for breast cancer in the setting of an augmented breast. METHODS AND MATERIALS The patient previously had placed bilateral subpectoral saline implants; stereotactic core biopsy revealed estrogen receptor- and progesterone receptor-positive ductal carcinoma in situ of intermediate nuclear grade. The patient underwent needle-localized segmental mastectomy of her left breast; pathologic specimen revealed no residual malignancy. An SAVI 8-1 device was placed within the segmental resection cavity. Treatment consisted of 3.4 Gy delivered twice a day for 5 days for a total dose of 34 Gy. Treatments were delivered with a high-dose-rate (192)Ir remote afterloader. RESULTS Conformance of the device to the lumpectomy cavity was excellent at 99.2%. Dosimetric values of percentage of the planning target volume for evaluation receiving 90% of the prescribed dose, percentage of the planning target volume for evaluation receiving 95% of the prescribed dose, volume receiving 150% of the prescribed dose, and volume receiving 200% of the prescribed dose were 97.1%, 94.6%, 22.7 cc, and 11.6 cc, respectively. Maximum skin dose was 115% of the prescribed dose. The patient tolerated treatment well with excellent cosmetic results, and limited acute and late toxicity at 8 weeks and 6 months, respectively. CONCLUSIONS Breast augmentation should not be an exclusion criterion for the option of APBI. The SAVI single-entry catheter is another option to successfully complete APBI using brachytherapy for breast cancer in the setting of an augmented breast.

Commissioning results of an automated treatment planning verification system

A dose calculation verification system (VS) was acquired and commissioned as a second check on the treatment planning system (TPS). This system reads DICOM CT datasets, RT plans, RT structures, and RT dose from the TPS and automatically, using its own collapsed cone superposition/convolution algorithm, computes dose on the same CT dataset. The system was commissioned by extracting basic beam parameters for simple field geometries and dose verification for complex treatments. Percent depth doses (PDD) and profiles were extracted for field sizes using jaw settings 3 × 3 cm2 ‐ 40 × 40 cm2 and compared to measured data, as well as our TPS model. Smaller fields of 1 × 1 cm2 and 2 × 2 cm2 generated using the multileaf collimator (MLC) were analyzed in the same fashion as the open fields. In addition, 40 patient plans consisting of both IMRT and VMAT were computed and the following comparisons were made: 1) TPS to the VS, 2) VS to measured data, and 3) TPS to measured data where measured data is both ion chamber (IC) and film measurements. Our results indicated for all field sizes using jaw settings PDD errors for the VS on average were less than 0.87%, 1.38%, and 1.07% for 6x, 15x, and 18x, respectively, relative to measured data. PDD errors for MLC field sizes were less than 2.28%, 1.02%, and 2.23% for 6x, 15x, and 18x, respectively. The infield profile analysis yielded results less than 0.58% for 6x, 0.61% for 15x, and 0.77% for 18x for the VS relative to measured data. Analysis of the penumbra region yields results ranging from 66.5% points, meeting the DTA criteria to 100% of the points for smaller field sizes for all energies. Analysis of profile data for field sizes generated using the MLC saw agreement with infield DTA analysis ranging from 68.8%–100% points passing the 1.5%/1.5 mm criteria. Results from the dose verification for IMRT and VMAT beams indicated that, on average, the ratio of TPS to IC and VS to IC measurements was 100.5 ± 1.9% and 100.4 ± 1.3%, respectively, while our TPS to VS was 100.1 ± 1.0%. When comparing the TPS and VS to film measurements, the average percentage pixels passing a 3%/3 mm criteria based gamma analysis were 96.6 ± 4.2% and 97 ± 5.6%, respectively. When the VS was compared to the TPS, on average 98.1 ± 5.3% of pixels passed the gamma analysis. Based upon these preliminary results, the VS system should be able to calculate dose adequately as a verification tool of our TPS. PACS number: 87.55.km

Lower mean heart dose with deep inspiration breath hold-whole breast irradiation compared with brachytherapy-based accelerated partial breast irradiation for women with left-sided tumors

PURPOSE For left-sided breast cancer, radiation to the heart is a concern. We present a comparison of mean heart and coronary artery biologically effective dose (BED) between accelerated partial breast irradiation (APBI) and whole breast irradiation with deep inspiration breath-hold technique (DIBH-WBI). METHODS AND MATERIALS A total of 100 patients with left-sided, early-stage breast cancer were identified. Fifty underwent single-entry catheter-based APBI and 50 underwent DIBH-WBI. The heart, left anterior descending/interventricular branch, left main, and right coronary artery were delineated. BEDs were calculated from APBI treatment plans (34 Gy in 3.4 Gy twice daily fractions) and for 4 separate plans generated for each DIBH-WBI patient: 50 Gy in 25 fractions (50/25), 50/25 + 10/5 boost, 40/15, and 40/15 + 10/5 boost. RESULTS BED to the heart and coronary vessels were statistically significantly higher with APBI than with any of the DIBH-WBI dose/fractionation schedules. CONCLUSIONS For women with left-sided early-stage breast cancer, DIBH-WBI resulted in statistically significantly lower mean BED to the heart and coronary vessels compared with APBI. This is likely due to increased physical separation between the heart and tumor bed afforded by the DIBH-WBI technique. Long-term assessment of late effects in these tissues will be required to determine whether these differences are clinically significant.

Contralateral breast dose from partial breast brachytherapy.

The purpose of this study was to determine the dose to the contralateral breast during accelerated partial breast irradiation (APBI) and to compare it to external beam‐published values. Thermoluminescent dosimeter (TLD) packets were used to measure the dose to the most medial aspect of the contralateral breast during APBI simulation, daily quality assurance (QA), and treatment. All patients in this study were treated with a single‐entry, multicatheter device for 10 fractions to a total dose of 34 Gy. A mark was placed on the patients skin on the medial aspect of the opposite breast. Three TLD packets were taped to this mark during the pretreatment simulation. Simulations consisted of an AP and Lateral scout and a limited axial scan encompassing the lumpectomy cavity (miniscan), if rotation was a concern. After the simulation the TLD packets were removed and the patients were moved to the high‐dose‐rate (HDR) vault where three new TLD packets were taped onto the patients at the skin mark. Treatment was administered with a Nucletron HDR afterloader using Iridium‐192 as the treatment source. Post‐treatment, TLDs were read (along with the simulation and QA TLD and a set of standards exposed to a known dose of 6 MV photons). Measurements indicate an average total dose to the contralateral breast of 70 cGy for outer quadrant implants and 181 cGy for inner quadrant implants. Compared to external beam breast tangents, these results point to less dose being delivered to the contralateral breast when using APBI. PACS number: 87.55.D‐The purpose of this study was to determine the dose to the contralateral breast during accelerated partial breast irradiation (APBI) and to compare it to external beam-published values. Thermoluminescent dosimeter (TLD) packets were used to measure the dose to the most medial aspect of the contralateral breast during APBI simulation, daily quality assurance (QA), and treatment. All patients in this study were treated with a single-entry, multicatheter device for 10 fractions to a total dose of 34 Gy. A mark was placed on the patients skin on the medial aspect of the opposite breast. Three TLD packets were taped to this mark during the pretreatment simulation. Simulations consisted of an AP and Lateral scout and a limited axial scan encompassing the lumpectomy cavity (miniscan), if rotation was a concern. After the simulation the TLD packets were removed and the patients were moved to the high-dose-rate (HDR) vault where three new TLD packets were taped onto the patients at the skin mark. Treatment was administered with a Nucletron HDR afterloader using Iridium-192 as the treatment source. Post-treatment, TLDs were read (along with the simulation and QA TLD and a set of standards exposed to a known dose of 6 MV photons). Measurements indicate an average total dose to the contralateral breast of 70 cGy for outer quadrant implants and 181 cGy for inner quadrant implants. Compared to external beam breast tangents, these results point to less dose being delivered to the contralateral breast when using APBI. PACS number: 87.55.D.

SU-F-T-487: On-Site Beam Matching of An Elekta Infinity with Agility MLC with An Elekta Versa HD

PURPOSE Historically, beam matching of similar Linear Accelerators has been accomplished by sending beam data to the manufacturer to match at their factory. The purpose of this work is to demonstrate that fine beam matching can be carried out on-site as part of the acceptance test, with similar or better results. METHODS Initial scans of a 10 × 10 Percent depth dose (PDD) and a 40 × 40 beam profile at the depth of Dmax, for 6MV and 10 MV were taken to compare with the standard beam data from the Versa. The energy was then adjusted and the beam steered to achieve agreement between the depth dose and the horns of the beam profile. This process was repeated until the best agreement between PDD and profiles was achieved. Upon completion, all other clinical data were measured to verify match. This included PDD, beam profiles, output factors and Wedge factors. For electron beams PDDs were matched and the beam profiles verified for the final beam energy. Confirmatory PDD and beam profiles for clinical field sizes, as well as Output Factors were measured. RESULTS The average difference in PDDs for 6MV and 10MV were within 0.4% for both wedged and open fields. Beam profile comparisons over the central 80% of the field, at multiple depths, show agreement of 0.8% or less for both wedged and open fields. Average output factor agreement over all field sizes was 0.4% for 6MV and 0.2 % for 10MV. Wedge factors agreement was less than 0.6% for both photon energies over all field sizes. Electron PDD agreed to 0.5mm. Cone ratios agreed to 1% or less. CONCLUSION This work indicates that beam matching can be carried out on-site simply and quickly. The results of this beam matching can achieve similar or better results than factory matching.

Dosimetric impact of setup accuracy for an electron breast boost technique

PURPOSE To determine the setup error on an electron breast boost technique using daily cone beam computed tomography (CBCT). Patient and setup attributes were studied as contributing factors to the accuracy. METHODS AND MATERIALS Reproducibility of a modified lateral decubitus position breast boost setup was verified for 33 patients using CBCT. Three-dimensional matching was performed between the CBCT and the initial planning CT for each boost fraction by matching the tumor bed and/or surgical clips. The dosimetric impact of the daily positioning error was achieved by rerunning the initial treatment plans incorporating the recorded shifts to study the dose differences. Breast compression, decubitus angle, tumor bed location and volume, and cup size were studied for their contribution to setup error. RESULTS The range of setup errors was: 1.5 cm anterior to 9 mm posterior, 1.3 cm superior to 2.3 cm inferior, and 3.2 cm medial to 2.4 cm lateral. Seven patients had setup errors that were ≥2-cm margin placed on the tumor bed and scar. Four of those 7 patients had unacceptable coverage as defined by the volume of the tumor bed plus scar that is covered by the 90% isodose line (V90) compared with the original plan. All other patients had no discernible difference in the coverage (V90). The use of compression, tumor bed location, or volumes >20 mL showed no effect on coverage. CONCLUSIONS In general, this study supported that a 2-cm margin was adequate (29 of 33 patients) when patients are treated under typical conditions. Care should be taken when high electron energies are selected because the coverage at depth is more difficult to maintain in the clinical environment.

SU-E-T-392: Evaluation of Ion Chamber/film and Log File Based QA to Detect Delivery Errors

Purpose: Ion chamber and film (ICAF) is a method used to verify patient dose prior to treatment. More recently, log file based QA has been shown as an alternative for measurement based QA. In this study, we delivered VMAT plans with and without errors to determine if ICAF and/or log file based QA was able to detect the errors. Methods: Using two VMAT patients, the original treatment plan plus 7 additional plans with delivery errors introduced were generated and delivered. The erroneous plans had gantry, collimator, MLC, gantry and collimator, collimator and MLC, MLC and gantry, and gantry, collimator, and MLC errors. The gantry and collimator errors were off by 4⁰ for one of the two arcs. The MLC error introduced was one in which the opening aperture didn’t move throughout the delivery of the field. For each delivery, an ICAF measurement was made as well as a dose comparison based upon log files. Passing criteria to evaluate the plans were ion chamber less and 5% and film 90% of pixels pass the 3mm/3% gamma analysis(GA). For log file analysis 90% of voxels pass the 3mm/3% 3D GA and beam parameters match what was in the plan. Results: Two original plans were delivered and passed both ICAF and log file base QA. Both ICAF and log file QA met the dosimetry criteria on 4 of the 12 erroneous cases analyzed (2 cases were not analyzed). For the log file analysis, all 12 erroneous plans alerted a mismatch in delivery versus what was planned. The 8 plans that didn’t meet criteria all had MLC errors. Conclusion: Our study demonstrates that log file based pre-treatment QA was able to detect small errors that may not be detected using an ICAF and both methods of were able to detect larger delivery errors.

SU‐E‐T‐258: Commissioning of a Commercial Treatment Planning System Verification Software Package

Purpose: A commercial software package (Mobius3d, Mobius Medical Systems) was acquired for use as an additional QA tool of our current treatment planning system (TPS). The system reads DICOM RT files generated from the TPS and performs an independent dose calculation for comparison to the TPS. This work summarizes our methods for commissioning a system such as this. Methods: Preliminary comparisons of our TPS and measured data to the QA softwares beam model were done by comparing PDDs and profiles with field sizes ranging from 4&×;4 to 40×40. Static fields and step and shoot IMRT plans were generated on a homogeneous medium for comparisons of 6x photons. For the homogeneous IMRT cases, ion chamber measurements were compared to the dose calculated in the TPS and in the verification system. Results: Analysis of PDD data showed that the average PDD error of the verification model (relative to the TPS) was 0.5% compared to our measured data which was 0.1%. The average infield profile error over all field sizes and depths of the TPS to our reference measured data set was 0.3% whereas the verification system relative to the TPS was 1.7%. Simple beam geometries between the TPS and the verification system were 0.8% difference on average. Ion chamber measurements on a homogeneous phantom differed from the TPS on average by 0.5% whereas the verification system was 2%. The results indicate an adjustment in the beam model of the verification system is needed to better match our current beam model/measured data. Conclusion: Preliminary commissioning results of a commercial QA dose algorithm were completed. Adjustment of the verification softwares beam model is necessary before further testing is done. Further analysis is needed to fully investigate this system to be used clinically as a QA tool of our current TPS. “Evaluation equipment provided by Mobius Medical Systems, LP”

SU‐E‐T‐315: Preliminary Results of the Dosimetric Impact of Set‐Up Accuracy for An Electron Breast Boost Technique

PURPOSE To analyze the dosimetric impact of set-up accuracy of an electron breast boost technique using a lateral decubitus position with and without a breast compression device. METHODS Reproducibility of the breast boost set-up was verified for 19 patients and 99 fractions using Cone Beam CT (CBCT). 3D-3D matching was performed between the CBCT and the initial planning CT for each boost fraction by matching the tumor bed and clips. Shifts in all three dimensions were recorded for each fraction. The dosimetric impact of the daily positioning error was achieved by rerunning the initial treatment plans incorporating the shifts recorded for each fraction. Comparison of the tumor bed and scar coverage was analyzed for both plans. RESULTS The range of set-up errors based on CBCT was: 1.5 cm anterior to 8 mm posterior, 1.3 cm superior to 2.3 cm inferior, and -2.4 cm to 2.1 cm laterally. Three patients had set-up errors that were greater than or equal to 2 cm which is the normal margin placed on the tumor bed and scar. Two of these three patients had unacceptable coverage as defined by the V90 when compared to the original plan. The remaining 17 patients had no discernible difference in coverage (V90). Whether patients had breast compression seemed to have little impact on reproducibility of the set-up. CONCLUSION 14 patients with a breast compression device and 5 patients without a breast compression device were studied using CBCT to quantify set-up error from 99 fractions. A 2 cm margin around the tumor bed plus scar volume seems to be adequate to account for set-up error. No appreciable difference was seen in set-up error between patients with breast compression or without. The two patients with unacceptable coverage were large breasted with deep seated tumors.