Strut-adjusted Volume Implant Research Articles

Feasibility of accelerated partial breast irradiation with strut-adjusted volume implant brachytherapy in Japan focusing on dosimetry and acute toxicity: a Japanese multi-institutional prospective study.

A Japanese multi-institutional prospective study was initiated to investigate the effectiveness and safety of accelerated partial breast irradiation (APBI) using strut-adjusted volume implant (SAVI) brachytherapy, with subjects registered between 2016 and 2021. Herein, we report the preliminary results on the feasibility of this treatment modality in Japan, focusing on the registration process, dosimetry, and acute toxicities. Primary registration was conducted before breast-conserving surgery (BCS) and the eligibility criteria included the following: age ≥ 40years, tumor unifocal and unicentric, ≤ 3cm in diameter, cN0M0, proven ductal, mucinous, tubular, medullary, or lobular carcinoma by needle biopsy. Secondary registration was conducted after BCS had been performed leaving a cavity for device implantation and pathological evaluations, and the eligibility criteria were as follows: negative surgical margin, tumor ≤ 3cm in diameter on gross pathological examination, histologically confirmed ductal, mucinous, tubular medullary, colloid, or lobular carcinoma, pN0, L0V0, no extensive ductal component, no initiation of chemotherapy within 2weeks of the brachytherapy APBI planning with SAVI was performed for the patients successfully entered in the study by the secondary registration process, and the treatment was administered at the dose of 34Gy in 10 fractions administered twice daily. Between 2016 and 2021, 64 women were enrolled in the study through primary registration, of which 19 were excluded from the secondary registration process, and in one, it was deemed impossible to comply with the dose constraints established during treatment planning. After the exclusion of these latter 20 patients, we treated the remaining 44 patients by APBI with SAVI. The dose constraints could be adhered to in all the patients, but re-planning was necessitated in 3 patients because of applicator movement during the treatment period. Grade 2 acute toxicities were observed in 18% of all patients, but more severe acute toxicities than Grade 2 were not observed in any of the patients. APBI with SAVI brachytherapy is feasible in Japan from the aspects of compliance with dose constraints and frequency of acute toxicities.

Analysis of applicator displacement in accelerated partial breast irradiation using a strut-based design brachytherapy applicator

PURPOSEThis study aimed to identify factors associated with strut-adjusted volume implant (SAVI) displacement in accelerated partial breast irradiation (APBI) using a SAVI device. METHODS AND MATERIALSWe retrospectively analyzed computed tomography scans taken at the time of treatment planning and immediately before treatment in 61 patients (median age; 55 years, range; 40–85) treated with SAVI and determined the amount of SAVI displacement that occurred between the time from planning to the treatment. The displacement was calculated for the CT axis and SAVI axis, which is related to the SAVI structure. To investigate the cause of the displacement, multivariate analysis was performed on the calculated standard deviation and the insertion angle of SAVI with respect to the sternum in each cross-section, breast density, amount of air around the SAVI, and SAVI length inside the patient to obtain the β coefficient (p-value). RESULTSOn the CT coordinate system, positive correlations were observed between the SAVI insertion angle and air volume in the lateral (β coefficient:0.255–0.483) and rotational directions (β coefficient:0.341). On the SAVI coordinate system, positive correlations were observed between the SAVI insertion angle and air volume in all lateral (β coefficient:0.270–0.354) and rotational directions (β coefficient:0.294). A negative correlation was observed between the SAVI length inside the patient and the rotational direction (β coefficient: −0.262). CONCLUSIONSAVI insertion angle, the amount of the air outside SAVI and SAVI insertion length are factors which affect the displacement of the applicator. From the results, the applicator displacement and rotation must be <3 mm and 10o in order to meet all the dose criteria. Thus, we should be aware of these factors during insertion of the device to avoid the problem in treatment delivery for the APBI.

A Single-Institution Retrospective Study of Three-Fraction HDR Accelerated Partial Breast Irradiation

Accelerated partial breast irradiation (APBI) delivered with high-dose-rate brachytherapy is a standard of care treatment typically delivered over 10 fractions. The TRIUMPH-T multi-institutional study recently demonstrated promising results using a shorter three fraction regimen, however there are limited additional published series using this regimen. Here, we report our experience and outcomes for patients treated as per the TRIUMPH-T regimen. This study was a retrospective single-institution analysis of patients who underwent lumpectomy followed by APBI (22.5 Gy in 3 fractions delivered over 2-3 days) using a Strut Adjusted Volume Implant (SAVI) applicator between November 2016 and January 2021. Dose-volume metrics were obtained from the clinically treated plan. Chart review was performed to determine locoregional recurrence and toxicities according to CTCAE v5.0. Between 2016 and 2021, 31 patients were treated per the TRIUMPH-T protocol. Median followup was 31 months from completion of brachytherapy. There were no acute/late Grade 3 or higher toxicities. Cumulative late Grade 1 and 2 toxicities were seen in 58.1% and 9.7% of patients, respectively. Of note, four patients experienced locoregional recurrence: three ipsilateral breast tumor recurrences and one nodal recurrence. All three ipsilateral breast tumor recurrences occurred in patients who would be classified as "cautionary" based on ASTRO consensus guidelines due to age ≤50, lobular histology, or high grade. Three-fraction HDR brachytherapy APBI was well-tolerated with no grade 3 or higher toxicities and an acceptably small percentage of grade 2 toxicities. Given the small sample size, the number of recurrences suggests that attention to appropriate patient selection is necessary until more long-term followup data is available.

Characterization of Dosimetric Differences in Strut-Adjusted Volume Implant Treatment Plans Calculated With TG-43 Formalism and a Model-Based Dose Calculation Algorithm.

To comprehensively characterize dosimetric differences between calculations with a commercial model-based dose calculation algorithm (MBDCA) and the TG-43 formalism in application to accelerated partial breast irradiation (APBI) with the strut-adjusted volume implant (SAVI) applicator. Dose for 100 patients treated with the SAVI applicator was recalculated with an MBDCA for comparison to dose calculated via TG-43. For every pair of dose calculations, dose-volume histogram (DVH) metrics including V90%, V95%, V100%, V150%, and V200% for the PTV_EVAL were compared. Features were defined for each case including (1) applicator size, (2) ratio between PTV_EVAL contour and 1-cm rind surrounding SAVI applicator, (3) ratio between dwell time in central catheter and total dwell time, and (4) mean computed tomography (CT) number within the lumpectomy cavity. Wilcoxon rank sum tests were performed to test whether treatment plans could be stratified according to feature values into groups with statistically significant dosimetry differences between MBDCA and TG-43. For all DVH metrics, differences between TG-43 and MBDCA calculations were statistically significant (P < .05). Minimum (maximum) relative percent differences between the MBDCA and TG-43 for V90%, V95%, and V100% were -2.1% (0.1%), -3.1% (-0.1%), and -5.0% (-0.5%), respectively. The median relative percent difference in mean PTV_EVAL dose between the MBDCA and TG-43 was -3.9%, with minimum (maximum) difference of -6.5% (-1.8%). For V90%, V95%, and V100%, plan quality worsened beyond defined thresholds in 26, 23, and 31 cases with no instances of coverage improvement. Features 1, 2, and 4 were shown to be able to stratify treatment plans into groups with statistically significant differences in dosimetry metrics between MBDCA and TG-43. Investigated dose metrics for SAVI treatments were found to be systematically lower with MBDCA calculation in comparison to TG-43. Plans could be stratified according to several features by the magnitude of dosimetric differences between these calculations.

PO-0169 Effects of uncertainty with Strut Adjusted Volume Implant applicator in Japan.

PO-0169 Effects of uncertainty with Strut Adjusted Volume Implant applicator in Japan.

Outcomes of Breast Brachytherapy in Stage I Breast Cancer – Data from a Large Health System

Accelerated partial breast irradiation (APBI) can be accomplished with a variety of techniques. Randomized trial data exist to support the efficacy of APBI utilizing external beam radiotherapy and interstitial multi-catheter-based brachytherapy, techniques that encompass a large volume of breast tissue. Far less data is available for smaller volume APBI techniques such as intraoperative radiotherapy (IORT) and single-entry brachytherapy devices. NSABP B-39 was the only randomized trial to include single-entry brachytherapy within the APBI arm, but too few patients received this technique to allow for a subset analysis. TARGIT-A is the only trial to evaluate photon IORT with short follow-up. Current study leverages the large patient volume of a university health system to review the experience of smaller volume APBI techniques. We hypothesize that smaller volume APBI would show inferior outcomes in loco-regional tumor control compared to larger volume APBI or whole breast irradiation in stage I breast cancer patients. The shared electronic health record of multiple institutions of a university health system was queried to identify women with either DCIS, T1N0 or T2N0 breast cancer treated with BCS between 2011 and 2014. The data collection included demographic features, known prognostic factors, tumor characteristics, treatment type, and evidence of local or regional recurrence (LRR). For the purposes of this study, the analysis was restricted to those patients treated with IORT or single-entry brachytherapy. The search identified 1386 patients who met criteria. Of these, 112 (8%) were treated with IORT to 20 Gy in a single fraction, and 116 (8%) underwent brachytherapy with a strut-adjusted volume implant (SAVI) device to 34 Gy in 10 fractions delivered twice daily. The demographic and tumor variables are listed below. The five-year rate of LRR among women treated with IORT was 3.4%, and 1.9% for women treated with SAVI brachytherapy using Kaplan-Meier estimate. In the evaluation of treatments for which randomized data are scarce, non-randomized series are vital. Health system consolidation facilitates the compilation of larger multi-hospital experiences. We report on a series of 228 low-risk patients treated with either IORT or SAVI brachytherapy with a single hospital system. The resulting LRR rates are relatively low, reflecting the trend of most modern series. Our next initiative will be to compare these outcomes to those seen for external beam radiotherapy within the same system.Abstract 2089; TableIORT (n = 112)SAVI (n = 116)Median Follow-up (mo.)67 (2.4-102)68 (1-92)Age (median)67 (49-90)59 (43-90)Stage DCIS127 T1mic1- T1a2015 T1b3330 T1c5443 T231Tumor Markers ER+111109 H2N+33 Triple -16 LVI +31 Open table in a new tab

Dosimetric Limitations in Treating Breast Cancer with Accelerated Partial Breast Irradiation Using Strut Adjusted Volume Implant (SAVI)

We present one case of accelerated partial breast irradiation (APBI) using strut adjusted volume implant (SAVI) where there were limitations in delivering the dose as per the standard guidelines. The device was placed close to both the chest wall and the skin with little tissue surrounding the tip. Two plans were made in an attempt to achieve the standard therapeutic doses without over-treating the chest wall or the skin. Similar cases reported in the literature were reviewed. The dosimetry of the two plans was compared to the cases discussed in the literature.

Inter-fractional variations in the dosimetric parameters of accelerated partial breast irradiation using a strut-adjusted volume implant.

The aim of the study was to evaluate inter-fractional dosimetric variations for high-dose rate breast brachytherapy using a strut-adjusted volume implant (SAVI). For the nine patients included, dosimetric constraints for treatment were as follows: for the planning target volume for evaluation (PTV_Eval), the volume receiving 90, 150 and 200% of the prescribed dose (V90%,150%,200%) should be >90%, ≤50 cm3 and ≤20 cm3, respectively; the dose covering 1 cm3 (D1cc) of the organs at risk should be ≤110% of the prescribed dose; and the air volume should be ≤10% of PTV_Eval. Differences in V90%,150%,200%, D1cc and air volume (n}{}Delta V and n}{}Delta D) as inter-fractional dosimetric variations and SAVI displacements were measured with pretreatment and planning computed tomography (CT) images. Inter-fractional dosimetric variations were analyzed for correlations with the SAVI displacements. The patients were divided into two groups based on the distance of the SAVI from the surface skin to assess the relationship between the insertion position of the SAVI and dosimetric parameters. The median ΔV90%,150%,200% for the PTV_Eval in all patients was −0.3%, 0.2 cm3 and 0.2 cm3, respectively. The median (range) ΔD1cc for the chest wall and surface skin was −0.8% (−18.9 to 9.4%) and 0.3% (−7.6 to 5.3%), respectively. SAVI displacement did not correlate with inter-fractional dosimetric variations. In conclusion, the dose constraints were satisfied in most cases. However, there were inter-fractional dosimetric changes due to SAVI displacement.

Volume Consideration and Dose Distribution in Patients with Breast Prosthesis Treated with Accelerated Partial Breast Irradiation

Accelerated partial breast Irradiation (APBI) is one of the treatment modalities available for irradiating breast cancer patients. In these patients, irradiation could be delivered through HDR brachytherapy applicators/balloons such as Strut adjusted volume implant (SAVI), Contura balloons and Best Balloons. The applicator is surgically implanted in the patients after lumpactomy. Normally, in these patients the planning target volume (PTV) is defined as 1 cm expansion around the applicator. This volume excludes skin surface and ribs. Some of the patients have prosthesis in the breast and therefore the treatment volume is different than the patients without Prosthesis. Therefore, we are assessing the treatment technique with prosthesis included or excluded from the planning volume (PTV_Eval). Moreover, the impact of this selection on dose distribution of other organs such as skin and ribs have been evaluated.

Retrospective Analysis of the Dosimetric Consequences of a Displaced or Shifted Strut Adjusted Volume Implant (SAVI) Applicator for Accelerated Partial Breast Irradiation (APBI) HDR Brachytherapy

Retrospective Analysis of the Dosimetric Consequences of a Displaced or Shifted Strut Adjusted Volume Implant (SAVI) Applicator for Accelerated Partial Breast Irradiation (APBI) HDR Brachytherapy

EP-2118 Effects of interfraction uncertainty with Strut Adjusted Volume Implant applicator

EP-2118 Effects of interfraction uncertainty with Strut Adjusted Volume Implant applicator

The effect of breast phantom, and geometry on dose distribution in breast brachytherapy using the strut-adjusted volume implant and contura applicators

Context: Accelerated partial breast irradiation using brachytherapy is a postlumpectomy treatment to reduce cancer recurrence and a choice for people in the early-stage breast cancer. Although accurate dosimetry is necessary to obtain successful clinical outcomes, the usual commercial treatment planning systems use a simple water phantom to simulate the patient. Hence, the precise attenuated radiation and also the scattering effects occurred in real situations may be dosimetrically ignored. Aims: The purpose of this study is to use Monte Carlo simulation to obtain the effect of phantom material and geometry corrections on dose distribution of the strut-adjusted volume implant (SAVI) and Contura high-dose-rate brachytherapy applicators of breast cancer. Settings and Design: Contura with four lumens surrounding the central one and also SAVI with eight peripheral source channels are separately simulated into the breast phantoms. 192Ir high dose rate sources are located on dwell positions in each applicator. Subjects and Methods: The applicators were simulated inside three different phantom geometry and materials. The dose distribution and dose-volume histograms for each phantom were obtained for typical treatment. Gamma index evaluation is performed to examine the dose distribution according to the water phantom for each trial. Results: According to the results for SAVI and Contura applicators, breast material correction shows about 1% deviations from the calculations for water in most points of the breast. Maximum differences are not >3% that are found near the skin. Conclusion: Deviations from the water phantom in both SAVI and Contura treatments show good conformity especially in Contura and it seems that no serious dosimetric correction is necessary for simple water phantom. Although the results for SAVI were not observed with great deviations from water, areas with high-gradient dose need to be precisely considered.

A study on dosimetry accuracy of Strut-Adjust Volume Implant (SAVI) brachytherapy

Introduction: Accelerated Partial Breast Irradiation (APBI) is an effective treatment for breast carcinomas because of its limited number of fractions. Strut-adjusted volume implant (SAVI) is a HDR brachytherapy applicator to deliver the prescription dose with good PTV coverage and acceptable dose to skin, chest wall and organ at risks. Acceptable clinical outcomes depend on accurate dosimetry methods. Currently water is used as the reference dosimetry phantom material in treatment planning systems. The purpose of this study is to consider the corrections on phantom material and geometry in dose distribution of SAVI brachytherapy simulation using MCNP5 Monte Carlo code. Materials and Methods: SAVI with 8 peripheral source channels were considered expanded inside the lumpectomy cavity in breast. Three realistic phantoms were employed to perform the step by step corrections and dose distributions were compared with that of water. Considerations were included using the ICRU-44 defined breast material as the real breast composition and moreover the air-tissue interferences at the skin, lung and the air cavity inside the SAVI were studied. Results: According to the results, the dose differences due to the breast material corrections were less than 1% in the most points of breast and only 2-3% overestimation were observed close to the skin. Breast phantoms containing air cavity have shown up to 2% decreases in regions around it. Finally a phantom with both noted corrections and a curved surface for breast skin, corrected the calculations with about 3-5% underestimations at skin. Conclusion: According to the results of this study, dose distributions in breast phantoms containing different materials does not differ significantly with the dose distribution in water phantom. Conservative approximations can be provided with water phantom and it doesn’t seem necessary that the current treatment planning systems be replaced with the model-based patient specific dosimetry methods.

Disease Control and Survival Outcomes for Strut-Adjusted Volume Implant for Early-Stage Breast Cancer in ASTRO Consensus Guidelines Cautionary Groups for Accelerated Partial Breast Irradiation: A Single-Institution Experience

We report the outcomes for survival, local recurrence, and distant failure in patients treated with strut-adjusted volume implant (SAVI) for patients treated accelerated partial breast irradiation (APBI) treated at our institution groups by suitable and cautionary factors according to the ASTRO 2016 Consensus Guidelines for APBI. Patients treated from January 2013 to July 2015 with SAVI planned for 10 fractions at twice daily fraction regimen for a total dose of 34 Gy were included. Patients conforming to the ASTRO 2016 consensus guidelines for APBI were grouped into “Suitable” and respective “Cautionary” groups. Metrics for survival, recurrence, and progression were curated from chart review and imaging examinations. A total of 134 SAVI patients were treated within the analyzed period. Median follow up for all patients was 40.37 months. Those with adequate time to three-year follow-up totaled 44 patients in the ASTRO 2016 APBI suitability group, and 15, 14, 7, and 9 for age 40-49 years, estrogen receptor negative, invasive lobular carcinoma histology, and Grade 3 ductal carcinoma in situ cautionary groups, respectively. Results for overall survival, local control, and distant control are shown in Table 1.Abstract TU_5_3364; Table 1SuitableAge 40-49 yearsER negativeLobular HistologyGrade 3 DCIS3-year Overall Survival96%100%93%100%100%3-year Local Control96%73%86%100%100%3-year Distant FailureN/AN/A7%N/AN/A Open table in a new tab Our data suggest excellent local control and survival in ASTRO 2016 APBI cautionary groups with SAVI implants at 3 years. Certain cautionary features appear to portend worse outcomes, but further follow up and characterization will elucidate specifics in patient characteristics or treatment schema that may correlate with these outcomes. We will continue to document outcomes for patients enrolled within this time frame, as well as outcomes for patients enrolled after July 2015 to increase our sample size and identify pertinent outcomes and correlations.

Comparison of Two Commercially Available Model Based Dose Calculation Algorithms and TG-43 for APBI Brachytherapy

Brachytherapy dose calculation algorithms that account for heterogeneous patient geometries have recently become commercially available. However, it is as yet unknown how the treatment plans produced by these new algorithms will compare to the current clinical standard, TG-43 algorithm when applied to patients. Ten clinical APBI brachytherapy treatment plans were randomly chosen from our clinical database. These plans were recomputed with the Oncentra® Advanced Collapsed Cone Engine (ACE) version 4.5 and Acuros™ BV version 13 and compared to the current standard TG-43-based algorithms treated at our institution with a Strut-Adjusted Volume Implant (SAVI).

Efficiency of using the day-of-implant CT for planning of SAVI APBI

PurposeThe purpose of the study was to develop an optimized, efficient workflow for using the day-of-implant (DOI) CT for treatment planning of accelerated partial breast irradiation brachytherapy using the strut-adjusted volume implant (SAVI) device. Methods and MaterialsFor 62 consecutive SAVI patients, a DOI CT was acquired and used for treatment planning. A “verification” CT was acquired 24–72 h after implant and immediately before the first fraction, then registered to the DOI CT. If the DOI CT–based plan was no longer optimal, a replan was performed. An array of metrics describing the geometry of the device and its relative position in the patient from the DOI CTs for these patients was collected. These metrics from the DOI CT were evaluated to determine what features could predict for the need to replan before the first treatment fraction. Logistical regression analysis including χ2 tests was used to determine if different factors correlated with replanning. ResultsTwenty-two of 62 patients (35%) required replanning. Only the presence of splayed struts, where splay was toward the skin, and the use of a nine strut (“8-1”) SAVI were significantly correlated (p < 0.05) with replanning. Within these individual populations, no additional factors showed a significant statistical correlation for requiring replanning. ConclusionsFor strut-based accelerated partial breast irradiation brachytherapy, it was feasible to treat with a plan based on the DOI CT for a majority (65%) of patients. Some factors correlate to needing replanning; recognizing these could be used to optimize treatment workflow for certain patients, increasing clinical efficiency while enhancing the quality of patient care.

Effect of applicator rotation on dose distribution in accelerated partial breast irradiation

Objective. To investigate the effect of rotation of the strut-adjusted volume implant (SAVI) device on dose distribution in accelerated partial breast irradiation (APBI). Methods. The required dose distribution in high-dose rate (HDR) partial breast irradiation performed with a multi-catheter device (also referred to as an applicator) is achieved by varying the source position and time spent at each position. Rotation of the device about its axis (which coincides with the central catheter) can modify the planned dose distribution. In this study we introduce an analytical model to determine changes in dose coverage of the planning target volume (PTV). In addition, we report rotation-induced variations in dosimetric parameters determined with the help of commercially available software for brachytherapy calculations. Results. Our analysis of seven different cases of APBI indicates that rotation of the HDR applicator in the breast leads to changes in doses to the PTV, skin and lung. The effect of rotation can be particularly significant when the lumpectomy cavity is close (i.e., less then 1 cm separation) to the skin and/or lung. Conclusions. Rotation of the applicator can cause greater than 10% reduction in dose coverage of the target, increased doses to skin and lung even for small angle of rotation (e.g., 30°) in the case when the cavity-to-skin and cavity-to-lung distances are less than 1 cm.

Strut-adjusted volume implant (SAVI) brachytherapy-based accelerated partial breast irradiation (APBI) in African American women.

To examine the clinical outcomes of postmenopausal African American (AA) women treated with strut-adjusted volume implant brachytherapy-based accelerated partial breast irradiation for early-stage node-negative breast cancer. From January 2011 through April 2015, a total of 50 AA patients, meeting criteria to receive APBI as defined by the National Surgical Adjuvant Breast and Bowel Project B-39 (NASBP B-39), completed treatment with the SAVI breast brachytherapy device at Howard University Hospital. 4% ipsilateral breast tumor recurrence and 2% breast cancer-specific mortality was observed. Median follow-up has been 3.8years with a range of 0.29-4.69years. Dosimetry parameters yielded a median V90 of 96.22% (range 77.86-105.00%), a median V150 of 31.27cm3 (range 23.30-49.15mL), and a median V200 of 14.53cm3 (range 5.92-19.38mL). Cosmesis was excellent. There were no infections, persistent seromas, fat necrosis, or telangiectasias observed to date. This study is the first study to describe the use of SAVI as APBI in an exclusively AA population. This study has demonstrated excellent local control in appropriately selected patients, similar clinical outcomes to the general population, and good to excellent cosmesis in AA women to date.

Accelerated partial breast irradiation dosimetric criteria for the strut-adjusted volume implant

PurposeCurrent guidelines for high-dose-rate accelerated partial breast irradiation using single-entry implants are based on the National Surgical Adjuvant Breast and Bowel Project B-39/Radiation Therapy Oncology Group 0413 protocol, which assumed a balloon implant geometry. We have developed robust plan evaluation criteria specifically for the strut-adjusted volume implant (SAVI). Methods and MaterialsPlan evaluation criteria were established using a “training data set” of 62 SAVI treatment plans and included the percentage volume of target receiving 90%, 95%, and 100% of the prescription dose (V90, V95, and V100), the absolute volume of target receiving 150% and 200% of prescription (V150 and V200), and the maximum doses to skin (Dskin max) and ribs (Drib max). “Ideal” and “expected” (routinely achievable) thresholds were determined for each criterion and compared to B-39 guidelines. A “test data set” collected from the next 25 patients was analyzed using the developed plan evaluation criteria. ResultsIdeal (expected) dosimetric thresholds established from the training data set were V90 ≥ 98% (95%), V95 ≥ 95% (92%), V100 ≥ 91% (88%), Dskin max < 90% (100%), and Drib max < 100%. Thresholds for V150 and V200 were stratified by SAVI size: V150 ≤ 30 cc (50 cc) and V200 ≤ 15 cc (20 cc) for 6–1 and Mini; V150 ≤ 50 cc (50 cc) and V200 ≤ 20 cc (20 cc) for 8–1 and 10–1. There was no significant difference between the training and test data sets in the fractions of patients meeting the criteria. Overall, 58% and 95% of 87 plans met all ideal and all expected criteria, respectively. ConclusionsThe plan evaluation criteria developed specifically for the SAVI device are robust, providing increased target coverage and increased skin sparing compared to B-39 guidelines.

SU-G-201-14: Is Maximum Skin Dose a Reliable Metric for Accelerated Partial Breast Irradiation with Brachytherapy?

Purpose: To evaluate the reliability of the maximum point dose (Dmax) to the skin surface as a dosimetric constraint, we investigated the correlation between Dmax at the skin surface and dose metrics at various definitions of skin thickness. Methods: 42 patients treated with APBI using a Strut Adjusted Volume Implant (SAVI) applicator between 2010 and 2014 were retrospectively reviewed. Target (PTV_EVAL) and organs at risk (OARs: skin, lung, and ribs) were delineated on a CT following NSABP B-39 guidelines. Six skin structures were contoured: a rind 3cm external to the body surface and 1, 2, 3, 4, and 5mm thick rinds deep to the body surface. Inverse planning simulated annealing optimization was used to deliver 32–34Gy in 8-10 fractions to the target while minimizing OAR doses. Dmax, D0.1cc, D1.0cc, and D2.0cc to the various skin structures were calculated. Linear regressions between the metrics were evaluated using the coefficient of determination (R2). Results: The average±SD PTV_EVAL volume and cavity-to-skin distances were 71.1±28.5cc and 6.9±5.0mm. The target V90 and V95 were 97.3±2.3% and 95.1±3.2%. The Dmax to the skin structures were 78.7±10.2% (skin surface), 82.2±10.7% (skin-1mm), 89.4±12.6% (skin-2mm), 97.9±15.4% (skin-3mm), 114.1±32.5% (skin-4mm), and 157.0±85.3% (skin-5mm). Linear regression analysis showed D1.0cc and D2.0cc to the skin 1mm and Dmax to the skin-4mm and 5mm were poorly correlated with other metrics (R2=0.413±0.204). Dmax to the skin surface was well correlated (R2=0.910±0.047) and D1.0cc to the skin-3mm was strongly correlated with all subsurface skin layers (R2=0.935±0.050). Conclusion: Dmax to the skin surface is a relevant metric for breast skin dose. Contouring discontinuities in the skin with a 1mm subsurface rind and the active dwells in the skin 4 and 5mm introduced significant variations in skin DVH. D0.1cc, D1.0cc, and D2.0cc to a 3mm skin rind are more robust metrics in breast brachytherapy.