Eval Volume Research Articles

Low Toxicity and Excellent Cosmesis with Highly Conformal Accelerated Partial Breast Irradiation Delivered Either Twice Daily or Every Other Day

<h3>Purpose/Objective(s)</h3> Two landmark randomized controlled trials (RCT) of external beam accelerated partial breast irradiation (EB-APBI) using 38.5 Gy/10 fractions (F) BID demonstrated excellent cancer control outcomes in appropriately selected patients but conflicting results regarding cosmesis. A recent RCT reported high rates of acceptable cosmesis using 30 Gy/5F QOD, making the most appropriate schedule for EB-APBI unclear. We utilized the BID regimen largely until the COVID-19 pandemic with strict contouring, dosimetric, and planning guidelines for gross/clinical/planning target volumes (GTV/CTV/PTV) and organs-at-risk (OAR). We report our experience with BID and QOD EB-APBI with a hypothesis that our treatment planning approach would result in acceptable acute toxicity and cosmesis. <h3>Materials/Methods</h3> We identified patients that received EB-APBI from 4/2017 through 12/2021. Clinical, pathologic, acute toxicity, cosmesis and dosimetric data for the lumpectomy (Lump) GTV/CTV/PTV and key OARs (ipsilateral breast [IB V50%, V80% and V100% of the Rx dose] heart (mean dose, V5% Rx dose; V3 Gy], ipsilateral lung [IL V30% Rx dose] and mean total lung dose [TLD]) were collected. Cosmesis was physician-reported using the 4-point NRG Oncology/RTOG Global Cosmetic Score (GCS): Excellent (E)/Good (G)/Fair (F)/Poor (P). We report descriptive statistics to summarize our results. <h3>Results</h3> 245 patients were included with median follow-up 19 months (IQR, 9-30 months): median age, 66 y (IQR, 59-71 y); 51% left-sided; 82% invasive; 100% invasive tumors HR+/HER2-; 95% of DCIS HR+; median invasive tumor size 9.5 mm (IQR, 6-13 mm) and DCIS size 8mm (IQR, 4-12 mm); 96% nodal surgery in invasive disease. Fractionation was BID in 55%, QOD in 45%. 3DCRT was used in 88% with median 6 fields (IQR, 5-7) and 96% were treated prone. Median Lump PTV eval volume was 176 mL and median breast volume 1335 mL resulting in median breast V50%Rx Dose=40.3% (IQR, 34.5-45.8%) and median breast V100=15.6% (IQR, 10.2-18.6%). Lump PTV coverage was high (median V95=100%). The mean heart dose was 35 cGy (IQR, 15-59 cGy), heart V5%=1.1% (IQR, 0-8.1%), and median heart V3Gy=0% (0-0.5%). The IL V30% (median 0%, IQR 0-0.4%) and TLD (median 49 cGy, IQR 26-93.1 cGy) were also low. The majority of patient had no acute toxicity (55% grade 0 dermatitis; 57% grade 0 fatigue; 97% grade 0 pruritis). The rate of E/G cosmesis was 97.1% (N=238) and F/P 2.9% (N=7). In patients with at least 2 years follow-up, rates were 96% E/G (N=95) and 4% F/P (N=4). The IB V100 was marginally associated with increased odds of F/P cosmesis (OR=1.18, 95% CI 0.99-1.42, p=0.07). <h3>Conclusion</h3> With multiple-field 3DCRT in the prone position, EB-APBI can be delivered with low toxicity and great cosmetic results with BID or QOD treatment. Given the low rate of F/P cosmesis, longer follow-up is needed to confirm stability of these results and to help identify optimal dose-volume parameters to minimize the rate of F/P cosmesis.

Per-fraction positional and dosimetric performance of prone breast tangential radiotherapy on Halcyon\u2122 linear accelerator assessed with daily rapid kilo-voltage cone beam computed tomography: a single-institution pilot study

BackgroundThis study investigates daily breast geometry and delivered dose to prone-positioned patients undergoing tangential whole breast radiation therapy (WBRT) on an O-ring linear accelerator with 6X flattening filter free mode (6X-FFF), planned with electronic compensation (ECOMP) method. Most practices rely on skin marks or daily planar image matching for prone breast WBRT. This system provides low dose daily CBCT, which was used to study daily robustness of delivered dose parameters for prone-positioned WBRT.MethodsEight patients treated with 16-fraction prone-breast WBRT were retrospectively studied. Planning CTs were deformed to daily CBCT to generate daily synthetic CTs, on which delivered dose distributions were calculated. A total of 8 × 16 = 128 synthetic CTs were generated. Consensus ASTRO definition was used to contour Breast PTV Eval for each daily deformed CT. Breast PTV Eval coverage (V90%) and hotspot (V105% and Dmax) were monitored daily to compare prescription dose with daily delivered dose. Various predictors including patient weight, breast width diameter (BWD), and Dice similarity coefficient (DSC) were fit into an analysis of covariance model predicting V90% and V105% deviation from prescribed (ΔV90%, ΔV105%). Statistical significance is indicated with asterisks (* for p < 0.05; ** for p < 0.001).ResultsDaily delivered Breast PTV Eval V90% was moderately smaller than prescribed (median ΔV90% = − 0.1%*), while V105% was much larger (median ΔV105% = + 10.1%** or + 92.4 cc**). Patient’s weight loss correlated with significantly increased ΔV105% (+ 4.6%/ − 1% weight, R2 = 0.4**) and moderately decreased ΔV90% (− 0.071%/ − 1% wt., R2 = 0.2**). Comprehensive ANCOVA models indicated three factors affect ΔV90% and ΔV105% the most: (1) BWD decrease (− 0.09%* and + 10%**/ − 1 cm respectively), (2) PTV Eval volume decrease (− 0.4%** and + 9%**/ − 100 cc), and for ΔV105% only, (3) the extent of breast deformation (+ 10%**/ − 0.01 DSC). Breast PTV Eval volume also decreased with time (− 2.21*cc/fx), possibly indicating seroma resolution and increase in V105% over time.ConclusionsDaily CBCT revealed key delivered dose parameters vary significantly for patients undergoing tangential prone breast WBRT planned with ECOMP using 6X-FFF. Patient weight, BWD, and breast shape deformation could be used to predict dosimetric variations from prescribed. Preliminary findings suggest an adaptive plan based on daily CBCT could reduce excessive dose to the breast.

Daily Dosimetric Robustness of Prone-Position Whole Breast Radiotherapy (WBRT) Planned With Electronic Tissue Compensation (ECOMP) and 6MV Flattening Filter Free (6X-FFF) Beam Energy: Dose Variation and Predictive Factors

This study investigates daily dose to prone-positioned patients undergoing tangential WBRT on an O-ring LINAC with 6X-FFF energy. For prone breast patients, most practices rely on skin marks or daily planar image matching. This system provides low dose (<1mGy CTDIvol) daily CBCT, which was used to verify daily robustness of target coverage and hotspot dose metrics for prone-position WBRT. Eight patients treated with 16-fraction prone breast WBRT planned with ECOMP were retroactively studied. During treatment, patients were positioned under CBCT guidance by matching to the chest wall. Planning CTs were deformed to daily CBCT to generate daily synthetic CTs, on which daily dose distributions were calculated. A total of 8 patients x 16 fractions = 128 synthetic CTs were generated. Consensus ASTRO definition was used to contour Breast PTV Eval for each daily deformed CT. Target coverage (V90%) and hotspot (V105%) to Breast PTV Eval contour was monitored each day. Various predictors (patient weight, breast separation, breast deformation / dice similarity coefficient DSC, etc.) were fit into a mixed effect linear regression model predicting V90% and V105% deviation from planned values (ΔV90%, ΔV105%). Statistical significance is indicated with asterisks (* for p<0.05; ** for p<0.001). For all synthetic CTs studied, V90% (as % of Breast PTV Eval volume) was moderately smaller than planned (median ΔV90% = -0.1%*), while V105% was much larger (median ΔV105% = +10.1%** or +92.4cc**). Among all fractions studied, only 40% met PTV Breast Eval V105%<15% criteria and 59% met V90%>95%. V90% and V105% did not correlate with time. Breast PTV Eval volume decreased with time (-2.21cc per fx*), possibly indicating general weight loss or seroma resolution. Patient’s weight loss correlated with significantly increased V105% (+4.6% / -1% weight., R = 0.4**) and moderately decreased V90% (-0.071% / -1% wt., R = 0.2**). However, comprehensive models indicate three factors influence ΔV90% and ΔV105% the most (with R = 0.86**, 0.95** respectively): (1) Breast separation decrease (-0.09%* and +10%** per -1cm), (2) PTV Eval volume decrease (-0.4%** and +9%** per -100cc), and for ΔV105% only, (3) more breast deformation (+10%** per -1% ΔDSC). For patients undergoing tangential prone breast WBRT with ECOMP-planned parallel opposed beams, daily CBCT revealed key dosimetric parameters vary significantly, especially for V105% associated with negative cosmesis. Patient weight loss substantially increased hotspots and slightly decreased coverage over treatment course. Breast separation and shape changes could be used to predict dosimetric variations. Preliminary findings suggest some patients could benefit from an adaptive plan based on daily CBCT.

Patterns of Failure Observed in the 2-Step Institution Credentialing Process for NRG Oncology/Radiation Therapy Oncology Group 1005 (NCT01349322) and Lessons Learned

PurposeTo investigate patterns of failure in institutional credentialing submissions to NRG/RTOG 1005 with the aim of improving the quality and consistency for future breast cancer protocols. Methods and MaterialsNRG/RTOG 1005 allowed the submission of 3-dimensional conformal radiation therapy (3DCRT), intensity-modulated radiation therapy (IMRT), and simultaneous integrated boost (SIB) breast plans. Credentialing required institutions to pass a 2-step quality assurance (QA) process: (1) benchmark, requiring institutions to create a plan with no unacceptable deviations and ≤1 acceptable variation among the dose volume (DV) criteria, and (2) rapid review, requiring each institution’s first protocol submission to have no unacceptable deviations among the DV criteria or contours. Overall rates, number of resubmissions, and reasons for resubmission were analyzed for each QA step. ResultsIn total, 352 institutions participated in benchmark QA and 280 patients enrolled had rapid review QA. Benchmark initial failure rates were similar for 3DCRT (18%), IMRT (17%), and SIB (18%) plans. For 3DCRT and IMRT benchmark plans, ipsilateral lung most frequently failed the DV criteria, and SIB DV failures were seen most frequently for the heart. Rapid review contour initial failures (35%) were due to target rather than organs at risk. For 29% of the rapid review initial failures, the planning target volume boost eval volume was deemed an unacceptable deviation. ConclusionsThe review of the benchmark and rapid review QA submissions indicates that acceptable variations or unacceptable deviations for the ipsilateral lung and heart dose constraints were the most commonly observed cause of benchmark QA failure, and unacceptable deviations in target contouring, rather than normal structure contouring, were the most common cause of rapid review QA failure. These findings suggest that a rigorous QA process is necessary for high quality and homogeneity in radiation therapy in multi-institutional trials of breast cancer to ensure that the benefits of radiation therapy far outweigh the risks.

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.

SU‐F‐T‐22: Clinical Implications When Using TG‐186 (ACE) Heterogeneity Software

Purpose:The purpose of this study is to compare dosimetric calculations using traditional TG‐43 formalism and Oncentra Brachy Advanced Collapsed cone Engine (ACE) TG‐186 calculation algorithm in clinical setting.Methods:We analyzed dosimetry of four patients treated with accelerated partial breast irradiation using a multi‐channel intracavitary device (SAVI). All patients were treated to 34 Gy in 10 fractions using a high‐dose‐rate (192) Ir source. The plans were designed and treated using the TG‐43 model. ACE was used to assess the effect heterogeneity correction on various dosimetric parameters. Mass density was estimated using Hounsfield units.Results:Compared to TG‐43 formalism, ACE estimated lower doses to targets and organs at risk. The mean difference was 19.8% (range 15.3–24.1%) for PTV_eval V200, 12.0% (range 9.7–17.7%) for PTV_eval V150, 4.3% (range 3.3–6.5%) for PTV_eval D95, 3.3% (range 1.4–5.4%) for PTV_eval D90, 5.4% (range 2.9–9.9%) for maximum rib dose, and 5.7% (2.4–7.4%) for maximum skin dose. There was no correlation between the magnitude of the difference and the PTV_eval volume, air volume, or tissue‐applicator conformance.Conclusion:Based on our preliminary study, the TG‐43 algorithm appears to overestimate the dose to targets and organs at risk when compared to the ACE TG‐186 software. We hypothesize that air adjacent to the SAVI struts contributes to lack of scatter thereby contributing a significant difference in dose calculation when using ACE. We believe that ACE calculation provides a more realistic isodose distribution than TG‐43. We plan to further investigate the impact of heterogeneity correction on brachytherapy planning for a wide variety of clinical scenarios, include skin, cervix/uterus, prostate, and lung

Long-Term Outcome of Accelerated Partial Breast Irradiation Using Multi-Lumen Applicators in the Setting of Breast Augmentation

Long-Term Outcome of Accelerated Partial Breast Irradiation Using Multi-Lumen Applicators in the Setting of Breast Augmentation

Prescription dose evaluation for APBI with noninvasive image-guided breast brachytherapy using equivalent uniform dose.

Noninvasive image-guided breast brachytherapy (NIBB) is an attractive novel approach to deliver accelerated partial breast irradiation (APBI). Calculations of equivalent uniform dose (EUD) were performed to identify the appropriate APBI dose for this technique. APBI plans were developed for 15 patients: five with three-dimensional conformal APBI (3D-CRT), five with multi-lumen intracavitary balloons (m-IBB), and five simulating NIBB treatment. Prescription doses of 34.0 and 38.5 Gy were delivered in 10 fractions for m-IBB and 3D-CRT, respectively. Prescription doses ranging from 34.0 to 38.5 Gy were considered for NIBB. Dose-volume histogram data from all 3D-CRT, m-IBB, and NIBB plans were used to calculate the biologically effective EUD and corresponding EUD to the PTV_eval using the following equation: EUD = EUBED/(n [1 + EUD/α/β]). An α/β value of 4.6 Gy was assumed for breast tumor. EUD for varying NIBB prescription doses were compared with EUD values for the other APBI techniques. Mean PTV_eval volume was largest for 3D-CRT (372.9 cm(3)) and was similar for NIBB and m-IBB (88.7 and 87.2 cm(3), respectively). The EUD value obtained by prescribing 38.5 Gy with 3D-CRT APBI was 38.6 Gy. The EUD value of 34.0 Gy prescribed with m-IBB was 34.4 Gy. EUD values for NIBB ranged from 33.9 to 38.2 Gy for prescription doses ranging from 34.0 to 38.5 Gy. Using EUD calculations to compare APBI techniques and treatment doses, a prescription dose of 36.0 Gy in 10 fractions using NIBB has a comparable biologic equivalent dose to other established brachytherapy techniques.

Retrospective review of Contura HDR breast cases to improve our standardized procedure

To retrospectively review our first 20 Contura high dose rate breast cases to improve and refine our standardized procedure and checklists. We prepared in advance checklists for all steps, developed an in-house Excel spreadsheet for second checking the plan, and generated a procedure for efficient contouring and a set of optimization constraints to meet the dose volume histogram criteria. Templates were created in our treatment planning system for structures, isodose levels, optimization constraints, and plan report. This study reviews our first 20 high dose rate Contura breast treatment plans. We followed our standardized procedure for contouring, planning, and second checking. The established dose volume histogram criteria were successfully met for all plans. For the cases studied here, the balloon-skin and balloon-ribs distances ranged between 5 and 43mm and 1 and 33mm, respectively; air_seroma volume/PTV_Eval volume≤5.5% (allowed≤10%); asymmetry<1.2mm (goal≤2mm); PTV_Eval V90%≥97.6%; PTV_Eval V95%≥94.9%; skin max dose≤98%Rx; ribs max dose≤137%Rx; V150%≤29.8cc; V200%≤7.8cc; the total dwell time range was 225.4 to 401.9 seconds; and the second check agreement was within 3%. Based on this analysis, more appropriate ranges for the total dwell time and balloon diameter tolerance were found. Three major problems were encountered: balloon migration toward the skin for small balloon-to-skin distances, lumen obstruction, and length change for the flexible balloon. Solutions were found for these issues and our standardized procedure and checklists were updated accordingly. Based on our review of these cases, the use of checklists resulted in consistent results, indicating good coverage for the target without sacrificing the critical structures. This review helped us to refine our standardized procedure and update our checklists.

Investigation of Interfraction Variance of a SAVI6-1 Mini APBI Applicator in Patients with Reduced Chest Wall and Skin Sparing

 Routine pre-treatment CT scan checks are unlikely to point out hard-to-quantify local anatomy inter-fraction variances, which are identified as the main impact factor for APBI treatments in patients with reduced chest wall and skin sparing that leads to considerable PTV-to-PTV_EVAL volume reduction (if VR = 1 – [PTV_EVALVolume/PTVVolume] proposed action value for VR = 0.30) Meticulous pre-treatment QA procedures are mandatory for the correct delivery of a SAVI6-1 mini APBI treatment and for the achievement of a high degree of conformance between the Initial reference TPS plan and the clinically delivered plan. Inter-fractional TPS plan re-evaluations should become a routine when dealing with difficult cases where the applicator is proximal to organs of risk and where highly asymmetrical PTV_EVAL volumes are employed, in order to avoid overlooking prohibitive sensitive structure doses and unacceptable PTV_EVAL coverage cutbacks. To evaluate the effect on target coverage and organ of risk sparing caused by inter-fraction variance of a SAVI6-1 mini APBI applicator in patients with small breast tissue volumes and limited skin/chest wall spacing. PURPOSE

628 poster SHORT-COURSE ACCELERATED PARTIAL BREAST IRRADIATION (OVER 2 DAYS) IS FEASIBLE WITH THE CONTURA(r) MULTI-LUMEN BRACHYTHERAPY DEVICE

PTV_EVAL volume did not correlate with aspirated seroma volume in our study. Conclusions: In our study, seroma accumulation seemed to have a variable pattern of development with no discernible predictors of occurrence. During a course of fractionated partial breast brachytherapy, serial aspirations via vacuum port, when available, should be considered for all patients to potentially improve dosimetry and reproducibility of PTV_EVAL. Further study is needed to establish factors predictive for seroma accumulation and resultant clinical significance.

SU‐FF‐T‐468: Comparison of Target Dose Inhomogeneity for Four Different Techniques of Breast Cancer Treatment

Purpose: To evaluate target dose inhomogeneity for four different breast cancer treatment techniques. Method and Materials: External beam radiotherapy (EBRT) was performed for 12 patients each using either whole or 3D conformal partial breast irradiation (WBI or PBI), 19 patients with high dose rate (HDR) MammoSite® brachytherapy and 13 patients with Contura® multi‐lumen applicator. The PTV_EVAL volume was defined by following NSABP B‐39/RTOG 0413 for PBI, MammoSite® and Contura® while retrospectively delineated for WBI. The differential dose volume histogram was evaluated using mean dose (Dm) and most probable dose (Dmp) relative to prescribed dose (Dpr) and root mean square deviation (RMSD). The biologically effective dose (BED) accounting for non‐uniform dose distribution was computed. Generalized equivalent uniform dose (gEUD) was calculated together with dose homogeneity index (DHI) and conformal index (COIN). Results: Compared to EBRT, the target received higher dose with HDR (Dm: 143% vs. 101%, Dmp: 111% vs. 101%) which increased BED values (77 Gy vs. 73 Gy for fibrosis, 56 Gy vs.47 Gy for erythema, and 63 Gy vs. 57 Gy for breast carcinoma, respectively) and dose inhomogeneity (RMSD: 12.1 vs. 1.8). Due to steeper dose fall‐off, HDR plans were more conformal than EBRT (COIN: 0.84 vs. 0.45). Compared to MammoSite®, Contura® yielded slightly higher dose (BED difference &lt; 0.5 Gy and 0.3 Gy higher in gEUD) which increased DHI by 5% and COIN by 1%. The PBI had large inter‐patient variability of BED values (2–8 times &gt; others) and the smallest BED and gEUD values. The WBI had large inter‐patient variability of gEUD values (14 times &gt; others) due to cold spots. Conclusion: Despite greater inhomogeneity, HDR plans have higher target dose conformality and are less patient‐specific than EBRT plans. However, based on the biological and dosimetric data, the four different techniques are clinically comparable.

A spreadsheet to determine the volume ratio for target and breast in partial breast irradiation*

The technical feasibility of Partial Breast Irradiation (PBI) using external beam radiotherapy depends on the ratio between the evaluation planning target volume (PTV_eval) and the whole breast volume ("PBI volume ratio" = PVR). We aimed to develop a simple method to determine PVR using measurements performed at the time of the planning CT scan. A PVR calculation tool was developed using a Microsoft Excel spreadsheet to determine the PTV from three orthogonal dimensions of the seroma cavity and a given margin on the CT scans. The breast volume is estimated from the separation and breast height in five equally spaced CT slices. The PTV_eval and whole breast volume were determined for 29 patients from two centres using the spreadsheet calculation tool and compared to volumes delineated on computerised treatment planning systems. Both the PTV_eval and whole breast volumes were underestimated by approximately 25% using the spreadsheet. The resulting PVRs were 1.05 +/- 0.35 (mean +/- 1SD) times larger than the ones determined from planning. Estimations of the PVR using the calculation tool were achievable in around 5 minutes at the time of CT scanning and allow a prompt decision on the suitability of the patients for PBI.

Investigation of Interfraction Variations of MammoSite Balloon Applicator in High-Dose-Rate Brachytherapy of Partial Breast Irradiation

To measure the interfraction changes of the MammoSite applicator and evaluate their dosimetric effect on target coverage and sparing of organs at risk. A retrospective evaluation of the data from 19 patients who received 10 fractions (34 Gy) of high-dose-rate partial breast irradiation was performed. A computed tomography-based treatment plan was generated for Fraction 1, and a computed tomography scan was acquired just before the delivery of each fraction to ensure a consistent shape of the balloon. The eccentricity, asymmetry, and planning target volume (PTV) for plan evaluation purposes (PTV_EVAL), as well as trapped air gaps, were measured for all patients. Furthermore, 169 computed tomography-based treatment plans were retrospectively generated for Fractions 2-10. Interfraction dosimetric variations were evaluated using the %PTV_EVAL coverage, target dose homogeneity index, target dose conformal index, and maximum doses to the organs at risks. The average variation of eccentricity and asymmetry from Fraction 1 values of 3.5% and 1.1 mm was -0.4% +/- 1.6% and -0.1 +/- 0.6 mm. The average trapped air gap volume was dramatically reduced from before treatment (3.7 cm(3)) to Fraction 1 (0.8 cm(3)). The PTV_EVAL volume change was insignificant. The average variation for the %PTV_EVAL, target dose homogeneity, and target dose conformal index from Fraction 1 values of 94.7%, 0.64, and 0.85 was 0.15% +/- 2.4%, -0.35 +/- 2.4%, and -0.34 +/- 4.9%, respectively. The average Fraction 1 maximum skin and ipsilateral lung dose of 3.2 Gy and 2.0 Gy varied by 0.08 +/- 0.47 and -0.16 +/- 0.29 Gy, respectively. The interfraction variations were patient specific and fraction dependent. Although the average interfraction dose variations for the target and organs at risk were not clinically significant, the maximum variations could be clinically significant.

2047: Partial Breast Irradiation With Helical Tomotherapy Using RTOG 0413 Planning Constraints

To investigate the ability to meet RTOG protocol 0413 dose volume constrains for partial breast treatments using helical tomotherapy. CT studies from 10 breast cancer patients (5 right, 5 left sided) in the supine position were selected. The tumor bed was contoured and RTOG Protocol 0413 guidelines were used to expand the gross target volume, and generate clinical target, planning target, and PTV_EVAL volumes. PTV_EVAL is defined as the planning target volume excluding the part outside the ipsilateral breast, the first 5 mm of tissue under the skin, and any expansion beyond the posterior extent of the breast tissue (chest wall, pectoralis and lung). A dose of 38.5 Gy to the PTV_EVAL was planned in 10 fractions. A 5 cm jaw length and a pitch of 0.215 were used for planning. The doses to the ipsilateral breast, contralateral breast, heart, and both lungs were restricted during plan optimization to meet protocol guidelines. The treatment time and PTV_EVAL doses are reported. Table 1 lists the average (range) of volumes that received the allowable dose according to protocol. For various structures, the RTOG volume limits are based on the percentage of the prescription dose (PD) that is allowed to a percentage volume. The second row lists the allowable volume limit. The third row lists the average and range of volumes that received the dose limit. In all cases, the maximum dose to the contralateral lung was <3% of the prescription dose. The average (range) treatment time was 10 (7–14) minutes. In 9 of the 10 cases, all PTV_EVAL and sensitive structure volume limits were acceptable (i.e. within 5% of specified value). In one right-sided case, the contralateral lung volume to 5% of the PD was 40% and the respective heart volume was 60%. This PTV_EVAL was located medially and dose limits to the contra lateral breast likely compromised the delivery. A solution with helical tomotherapy could be theoretically found with this particular situation, however, treatment times exceeded practical limits.Table 1Contralat. Lung, 5% of PDIpsilat. Lung, 30% of PDHeart, 5% of PDNormal Breast, 50% PDPTV_Eval, Max. DosePTV_Eval, 95% of PDRTOG vol. limit15%15%R: 5%, L: 40%60%120% of PD—Median13%10%R: 5%, L: 37%40%108%99%Average14%10%R: 15%, L: 33%41%108%99%Range0-40%8-17%R:0-60%, L:17-38%28-65%103-120%95-100% Open table in a new tab In the large majority of patients, partial breast treatment helical tomotherapy plans were acceptable by RTOG Protocol 0413 guidelines. Although not the focus of the present study, megavoltage CT scanning prior to each fraction would also add the benefit of daily visualization and localization of the seroma, i.e. the tumor bed (Langen et al, IJROBP, 63S, p. S179, 2005), thus potentially allowing a decrease in treatment margins. Partial breast accelerated irradiation is feasible with helical tomotherapy delivery.