Lower Isodose Line Research Articles

What is the optimal isodose line for stereotactic radiotherapy for single brain metastases using HyperArc?

The study aimed to investigate the optimal isodose line (IDL) in linear accelerator-based stereotactic radiotherapy for single brain metastasis, using HyperArc. We compared the dosimetric parameters for target and normal brain tissue among six plans with different IDLs. This study included 30 patients with single brain metastasis. We retrospectively generated six plans for each tumor with different IDLs (80%, 70%, 60%, 50%, 40%, and33%) using HyperArc. All treatment plans were normalized to the prescription dose of 35Gy in five fractions which was covered by 95% of the planning target volume (PTV), defined by adding a 1.0mm margin to the gross tumor volume (GTV). The dosimetric parameters were compared among the six plans. For GTV>0.1 cm3, the ratio of brain-GTV volumes receiving 25Gy to PTV (V25Gy/PTV) was significantly lower at IDL 40%-70% than at IDL 80% and 33% (p<0.01, retrospectively). For GTV<0.1 cm3, V25Gy/PTV decreased continuously as IDL decreased. The values of D99% and D80% for GTV increased with decreasing IDL. An IDL of 50% or less was required to achieve D99% of greater than 43Gy and D80% of greater than 50Gy. The mean values of D99% and D80% for IDL 50% were 44.3 and 51.9Gy. The optimal IDL is 40%-50% for GTV>0.1 cm3. These lower IDLs could increase D99% and D80% of GTV while lowering V25Gy of normal brain tissue, which may help reduce the risk of radiation necrosis and improve local control.

Treatment of Mantle and Inverted Y Fields in Modern Era: A VMAT Approach.

Hodgkin's lymphoma has been treated with combined modality therapy (chemotherapy and radiation) with a very high degree of success. Total Nodal Irradiation (TNI) performed with large AP/PA mantle fields for treatment of axillary, cervical, and mediastinal lymphatics, provide adequate coverage to the mediastinum and bilateral axillae and hila, while blocking lungs. The para-aortic and pelvic lymph nodes are treated with the so called inverted-Y AP/PA fields, which often includes the spleen in cases of TNI. Multileaf Collimators (MLC) have been tried, but due to the irregular shape of the fields and necessity of island blocking in 3D treatment, they have not been successful in full elimination of Cerrobend blocks. We hypothesize that using two or three matched Volumetric Modulated Arc Therapy (VMAT) fields will not only eliminate a need for Cerrobend blocks or island blocks, but will also provide better target coverage and better organs at risk (OAR) sparing. Under IRB study, 10 patients were retrospectively planned using two or three matched VMAT technique for mantle and inverted-Y treatments of TNI that had been previously treated using MLC and Cerrobend block combination. Pinnacle treatment planning system version 16.2.1 was used to generate plans using mantle/inverted-Y technique and corresponding VMAT plans using 2-3 arcs per isocenter (2 isocenters per plan). Optimization was performed to cover targets with the prescribed dose of 1500 cGy in 10 fractions per institutional protocol. The VMAT plans were compared with traditional 3D plans. VMAT consistently provided better or similar results to traditional field arrangements. Target coverage: V15Gy - 95.45% vs 77.99% (p = 0.00017), OAR coverage: total lung V5Gy 63.7% vs 68.8% (p = 0.016), bone marrow mean dose 539.1 cGy vs 727.8 cGy (p = 0.00047), Integral Dose 464.1 mJ vs 573.9 mJ (p = 0.0026). Low isodose lines- mean volume of 5 Gy isodose line was not significantly different - 24036 cc vs 25091 cc (p = 0.271). Cord maximum dose was 40% lower for VMAT plans (p = 0.00006). Mean bladder dose was similar in VMAT plans compared to 3D plan - 821.7 cGy vs 804.9 cGy (p = 0.454). One counter-intuitive result is that the mean integral dose for 10 patients was 24% lower for VMAT plans. VMAT based mantle fields for TNI eliminates Cerrobend blocks and improve dosimetry significantly for target volumes and all OARs; including bone marrow, which plays important role in patient's recovery after chemotherapy, radiation and often stem cell transplantation in recurrent disease. Lower integral dose for VMAT plans is explained by the large irradiated in-fields and small out-of-field volumes. The VMAT process requires minimal effort for optimization and is economical compared to the traditional planning, while improving the target coverage and decreasing dose to OARs.

Evaluating the impact of prescription isodose line on plan quality using Gamma Knife inverse planning.

The impact of selection of prescription isodose line (IDL) on plan quality has not been well evaluated during inverse planning (IP). In this study, a total of 180 IP plans at five levels of IDL were generated for 30 brain metastases (BMs). For each BM, one round of IP was performed with typical IP settings, followed by a quick fine‐tuning to ensure the same target coverage and comparable conformality index. The impact of the IDL on the quality metrics (selectivity, gradient index [GI], and treatment time) was evaluated. The decrease of selectivity and increase of GI meant inferior target dose conformality and more dose spillage. Additionally, a metric directly correlated to the treatment time was proposed. For all cases, the mean GI decreased monotonically as IDL decreased from 70% to 30%, and the decreasing rate was significantly different based on tumor size. The mean selectivity and number of shots decreased monotonically as IDL decreased for all the tumors. From 70% to 30% IDL, the decreasing rate of the mean selectivity was 2.8% (p = 0.020), 7.7% (p = 0.005), and 15.4% (p = 0.020) and that of the number of shots was 75.4% (p = 0.001), 73.2% (p = 0.001), and 50.7% (p = 0.009), for the large, medium, and small tumors, respectively. For the medium and small tumor groups, the mean treatment time increased monotonically when IDLs decreased (increasing rate was 80.0% [p = 0.002] for medium tumors [p = 0.001] and 130.8% [p = 0.001] for small tumors from 70% to 30%). For the large tumors, the mean treatment time was the shortest at 50% IDL (59.0 min) and higher at 70% (65.9 min) and 30% (71.9 min). Overall, the GammaPlan chose smaller sectors for plans with lower IDLs except for the large size group.

Dosimetric and radiobiological comparison of treatment plan between CyberKnife and EDGE in stereotactic body radiotherapy for pancreatic cancer

To perform a comparison of the different stereotactic body radiotherapy (SBRT) plans between the Varian EDGE and CyberKnife (CK) systems for locally advanced unresectable pancreatic cancer. Fifteen patients with pancreatic cancer were selected in this study. The median planning target volume (PTV) was 28.688 cm3 (5.736–49.246 cm3). The SBRT plans for the EDGE and CK were generated in the Eclipse and Multiplan systems respectively with the same contouring and dose constrains for PTV and organs at risk (OARs). Dose distributions in PTV were evaluated in terms of coverage, conformity index (CI), new conformity index (nCI), homogeneity index (HI), and gradient index (GI). OARs, including spinal cord, bowel, stomach, duodenum and kidneys were statistically evaluated by different dose-volume metrics and equivalent uniform dose (EUD). The volume covered by the different isodose lines (ISDL) ranging from 10 to 100% for normal tissue were also analyzed. All SBRT plans for EDGE and CK met the dose constraints for PTV and OARs. For the PTV, the dosimetric metrics in EDGE plans were lower than that in CK, except that D99 and GI were slightly higher. The EDGE plans with lower CI, nCI and HI were superior to generate more conformal and homogeneous dose distribution for PTV. For the normal tissue, the CK plans were better at OARs sparing. The radiobiological indices EUD of spinal cord, duodenum, stomach, and kidneys were lower for CK plans, except that liver were higher. The volumes of normal tissue covered by medium ISDLs (with range of 20–70%) were lower for CK plans while that covered by high and low ISDLs were lower for EDGE plans. This study indicated that both EDGE and CK generated equivalent plan quality, and both systems can be considered as beneficial techniques for SBRT of pancreatic cancer. EDGE plans offered more conformal and homogeneous dose distribution for PTV, while the CK plans could minimize the exposure of OARs.

Selection of prescription isodose line for brain metastases treated with volumetric modulated arc radiotherapy.

PurposeTo exploit the optimal prescription isodose line (IDL) for brain metastases treated with volumetric modulated arc radiotherapy (VMAT) as there is no consensus on the selection of IDL with VMAT.Methods and materialsEighteen patients with 20 brain tumors, who were treated with VMAT, were enrolled in this study. For each tumor of every patient, five plans were designed with IDL ranging from 50% to 90% in 10% increments. Different IDLs were obtained through adjusting the constraint parameters during planning optimization. Prescription dose (10 × 5 Gy) were identical for all plans, and the plans were compared in terms of gradient index (GI), conformity Index (CI), V26 Gy/VPTV, and V32 Gy/VPTV in normal brain tissue, which correlate to radiation necrosis.ResultsIDL with lowest GI has a median value of 60.0% (ranging from 50% to 80%). Except for one tumor with volume larger than 10 cc, the IDL with lowest GI varies from 50% to 70%, which depends on the shape of PTV, location, and whether the target volume is adjacent to crucial OAR. Moreover, there is no significant difference for CI with varying IDL plans. The average V26 Gy/VPTV and V32 Gy/VPTV in normal brain tissue 60% IDL plans are 27.3%, 31.7% lower than 90% IDL plans separately (P < 0.05). However, by further decreasing IDL from 60% to 50%, the average V26 Gy/VPTV and V32 Gy/VPTV may increase comparing with 60% IDL plans (P > 0.05). Furthermore, a lower IDL is found to result in higher mean dose to the target volume (P < 0.05).ConclusionsPlans using VMAT with PTV smaller than 10 cc tend to be optimal with IDL around 60–70% for lower GI, smaller V26 Gy/VPTV, V32 Gy/VPTV in normal brain tissue, and higher mean dose in tumor comparing with high IDL plans which have potential benefit in reducing risk of radiation necrosis and increasing the local control. However, IDL lower than 60% is not recommended for the disadvantage of increasing V26 Gy/VPTV and V32 Gy/VPTV in normal brain tissue.

Hypopituitarism after Gamma Knife radiosurgery for pituitary adenomas: a multicenter, international study.

Recurrent or residual adenomas are frequently treated with Gamma Knife radiosurgery (GKRS). The most common complication after GKRS for pituitary adenomas is hypopituitarism. In the current study, the authors detail the timing and types of hypopituitarism in a multicenter, international cohort of pituitary adenoma patients treated with GKRS. Seventeen institutions pooled clinical data obtained from pituitary adenoma patients who were treated with GKRS from 1988 to 2016. Patients who had undergone prior radiotherapy were excluded. A total of 1023 patients met the study inclusion criteria. The treated lesions included 410 nonfunctioning pituitary adenomas (NFPAs), 262 cases of Cushing's disease (CD), and 251 cases of acromegaly. The median follow-up was 51 months (range 6-246 months). Statistical analysis was performed using a Cox proportional hazards model to evaluate factors associated with the development of new-onset hypopituitarism. At last follow-up, 248 patients had developed new pituitary hormone deficiency (86 with NFPA, 66 with CD, and 96 with acromegaly). Among these patients, 150 (60.5%) had single and 98 (39.5%) had multiple hormone deficiencies. New hormonal changes included 82 cortisol (21.6%), 135 thyrotropin (35.6%), 92 gonadotropin (24.3%), 59 growth hormone (15.6%), and 11 vasopressin (2.9%) deficiencies. The actuarial 1-year, 3-year, 5-year, 7-year, and 10-year rates of hypopituitarism were 7.8%, 16.2%, 22.4%, 27.5%, and 31.3%, respectively. The median time to hypopituitarism onset was 39 months.In univariate analyses, an increased rate of new-onset hypopituitarism was significantly associated with a lower isodose line (p = 0.006, HR = 8.695), whole sellar targeting (p = 0.033, HR = 1.452), and treatment of a functional pituitary adenoma as compared with an NFPA (p = 0.008, HR = 1.510). In multivariate analyses, only a lower isodose line was found to be an independent predictor of new-onset hypopituitarism (p = 0.001, HR = 1.38). Hypopituitarism remains the most common unintended effect of GKRS for a pituitary adenoma. Treating the target volume at an isodose line of 50% or greater and avoiding whole-sellar radiosurgery, unless necessary, will likely mitigate the risk of post-GKRS hypopituitarism. Follow-up of these patients is required to detect and treat latent endocrinopathies.

CBCT-based dosimetric verification and alternate planning techniques to reduce the normal tissue dose in SBRT of lung patients

Purpose: Confirmation of treatment delivery accuracy in stereotactic body radiotherapy (SBRT) of lung tumors suggests the possibility of treatment margin, or aperture reduction. In this investigation, the dose delivery to lung tumors using SBRT techniques was verified, and the feasibility of normal tissue sparing via aperture reduction or altered prescription isodose line was assessed. Methods: Planned and delivered doses to the gross tumor volume (GTV) and planning target volume (PTV) were compared for 10 patients using planning CT and conebeam CT image. Potential for reduction in normal tissue dose were assessed using 2 alternate treatment plans – reduced PTVs and alternate prescription techniques. Plans were assessed using conformity index, homogeneity index and the ratio of 50% / 100% isodose volumes (R 50% ). Results: The planned and delivered mean doses were consistent to within 4%. However, the mean dose delivered to the GTV exceeded the prescription dose (Rx) by 19% and is consistent with our planning technique of prescribing to the 80% isodose line. When reducing treatment margins and retaining a constant dose-volume constraint, block margins had to be increased which produced a constant effective field aperture outside of the GTV. Prescription to a lower isodose line using stereotactic-like planning techniques yielded the only method by which the volume of the prescription isodose could be affected, although this yielded increases in normal tissue dose due to the increased monitor units required. Conversely, conventional prescription techniques using wider field apertures were effective in reducing absolute values of normal tissue dose. Although dose conformity was similar across different prescription isodose lines, homogeneity index and R 50% values were significantly different in the 60%-70% prescription isodose line plans than the 80%, 90% prescription plans. Conclusion: Traditional margin reduction techniques did not affect a reduction in the volume of normal tissue irradiated to the prescribed dose. Prescribing to low isodose lines yields reduced volumes of the prescribed dose, but at the expense of normal tissue dose.

Application of the gamma evaluation method in Gamma Knife film dosimetry

Gamma Knife (GK) radiosurgery is a minimally invasive surgical technique for the treatment of intracranial lesions. To minimize neurological deficits, submillimeter accuracy is required during treatment delivery. In this paper, the delivery accuracy of GK radiosurgery was assessed with the gamma evaluation method using planning dose distribution and film measurement data. Single 4, 8, and 16 mm and composite shot plans were developed for evaluation using the GK Perfexion (PFX) treatment planning system (TPS). The planning dose distributions were exported as digital image communications in medicine - radiation therapy (DICOM RT) files using a new function of GK TPS. A maximum dose of 8 Gy was prescribed for four test plans. Irradiation was performed onto a spherical solid water phantom using Gafchromic EBT2 films in the axial and coronal planes. The exposed films were converted to absolute dose based on a 4th-order polynomial calibration curve determined using ten calibration films. The film measurement results and planning dose distributions were registered for further analysis in the same Leksell coordinate using in-house software. The gamma evaluation method was applied to two dose distributions with varying spatial tolerance (0.3-2.0 mm) and dosimetric tolerance (0.3-2.0%), to verify the accuracy of GK radiosurgery. The result of gamma evaluation was assessed using pass rate, dose gamma index histogram (DGH), and dose pass rate histogram (DPH). The 20, 50, and 80% isodose lines found in film measurements were in close agreement with the planning isodose lines, for all dose levels. The comparison of diagonal line profiles across the axial plane yielded similar results. The gamma evaluation method resulted in high pass rates of >95% within the 50% isodose line for 0.5 mm∕0.5% tolerance criteria, in both the axial and coronal planes. They satisfied 1.0 mm∕1.0% criteria within the 20% isodose line. Our DGH and DPH also showed that low isodose lines exhibited inferior gamma indexes and pass rates compared with higher isodose lines. The gamma evaluation method was applicable to GK radiosurgery. For all test plans, planning dose distribution and film measurement met the tolerance criteria of 0.5 mm∕0.5% within the 50% isodose line which are used for marginal dose prescription.

SU-E-T-638: Optimization of Prescription Isodose Selection for Stereotactic Radiation Therapy: Conventional Linac vs. Cyberknife

Purpose: To determine the optimal prescription isodose line that minimizes normal tissue dose for stereotactic irradiation of intracranial targets comparing conventional linear accelerator and robotic (Cyberknife) delivery. Material and Methods: An anthropomorphic head phantom with a centrally located target was scanned using a 16 slice large bore CT simulator. Two concentric 5mm spherical shells of normal tissue, the first from the target surface to 5 mm radically, and the other from 5 mm to 10mm, were identified/contoured around the target. For linac planning, 9 co-planar, non-opposing 3D conformal beams were designed for conformal dose delivery. By manipulating the block margin, varying prescription isodose lines were used to cover exactly 95% of tumor. After linac planning, the same CT images and contours were sent to the Cyberknife for treatment planning. Multiple plans were generated with different prescription isodose lines covering exactly 95% of the target. gEUD (generalized effective uniform dose) was used to evaluate normal tissue irradiation within the shells using different parameters for parallel and serial tissues. Results: For linac plans, the results show that the optimal prescription isodose line yielding maximum normal tissue sparing is located between 50%∼60% of the maximum dose when the organ displays parallel behavior. For serially functioning tissues, the optimal prescription isodose line increases to approximately 80%. The gEUD of normal tissue (predisposing to toxicity) increases greatly when prescription isodose lines lower than 50% or higher than 80% are used. For Cyberknife plans, the optimal prescription isodose line lies near 50% for tissue for both either parallel or serial structures. These results are consistent for different tumor sizes. Conclusion: Both conventional linac and robotic delivery provide conformal tumor coverage with excellent normal tissue sparing. To achieve optimal normal tissue sparing, a lower isodose line should be prescribed for Cyberkinfe® than for a conventional linac.

SU-E-T-631: A Systematic Study on Dosimetric Impact of Grid Size and Angular Interval in Dynamic Conformal Arc Technique for Lung SBRT

Purpose: To systematically evaluate the dosimetric impact of grid size and angular increment, in stereotactic body radiation therapy(SBRT) using dynamic conformal arc therapy (DCAT) technique. Methods: DCAT plans were obtained with varying grid size (2, 3, and 4 mm) and angular increment (5, 10, and 20°) for various planning target volumes (PTVs). As 95% of PTV volume is covered with the prescribed dose,dose variation from coarser grid size was compared to 2 mm grid size with assuming the latter could provide calculated dose accurate enough in practical aspect. The impact of angular increment variation on any organ at risk (OAR) was evaluated with placing the spinal cord in a set of 8 different locations (5 cm and 7 cm distance from the isocenter and 5°, 10°, and 20° of azimuthal angle with respect to the a location chosen as zero angle in axial plane). Results: Compared to 2 mm grid size, 3 mm grid size showed increases ranging from 2 to 3%, 2%, and 1 to 2% in monitor units, the maximum dose of the PTV, and the maximum dose of the spinal cord, respectively. For 4 mm grid size, the amount of increase was larger (4 to 7%, 4 to 6%, and 4 to 6%, respectively). When compared to 5° angular increment, dose variation with 10° increment was less than 2%, but 20° increment caused significant variations reaching up to 10% and 24% for 5 cm and 7 cm distant volume. Conclusions: Dosimetric impact of planning parameters would guide planners to use appropriate parameters with possible dose uncertainty in DCAT. With coarse grid size, planners are expected to choose lower isodose line for PTV coverage, resulting in patient overdose. Significantly inaccurate prediction of OAR dose could be made with coarse angular increment depending on location of OAR.

Energy modulated electron therapy using a few leaf electron collimator in combination with IMRT and 3D‐CRT: Monte Carlo‐based planning and dosimetric evaluation

Energy modulated electron therapy (EMET) based on Monte Carlo dose calculation is a promising technique that enhances the treatment planning and delivery of superficially located tumors. This study investigated the application of EMET using a novel few-leaf electron collimator (FLEC) in head and neck and breast sites in comparison with three-dimensional conventional radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) techniques. Treatment planning was performed for two parotid cases and one breast case. Four plans were compared for each case: 3D-CRT, IMRT, 3D-CRT in conjunction with EMET (EMET-CRT), and IMRT in conjunction with EMET (EMET-IMRT), all of which were performed and calculated with Monte Carlo techniques. For all patients, dose volume histograms (DVHs) were obtained for all organs of interest and the DVHs were used as a means of comparing the plans. Homogeneity and conformity of dose distributions were calculated, as well as a sparing index that compares the effect of the low isodose lines. In addition, the whole-body dose equivalent (WBDE) was estimated for each plan. Adding EMET delivered with the FLEC to 3D-CRT improves sparing of normal tissues. For the two head and neck cases, the mean dose to the contralateral parotid and brain stem was reduced relative to IMRT by 43% and 84%, and by 57% and 71%, respectively. Improved normal tissue sparing was quantified as an increase in sparing index of 47% and 30% for the head and neck and the breast cases, respectively. Adding EMET to either 3D-CRT or IMRT results in preservation of target conformity and dose homogeneity. When adding EMET to the treatment plan, the WBDE was reduced by between 6% and 19% for 3D-CRT and by between 21% and 33% for IMRT, while WBDE for EMET-CRT was reduced by up to 72% when compared with IMRT. FLEC offers a practical means of delivering modulated electron therapy. Although adding EMET delivered using the FLEC results in perturbation of target conformity when compared to IMRT, it significantly improves normal tissue sparing while offering enhanced target conformity to the 3D-CRT planning. The addition of EMET systematically leads to a reduction in WBDE especially when compared with IMRT.

MO-E-T-617-02: Dosimetric Evaluation of Inverse Monte Carlo-Based Modulated Electron Beam Treatment Planning and Delivery Using a Few Leaf Electron Collimator

Purpose: To investigate the potential of improving the treatment planning and delivery of treatment to superficially located tumors using energy modulated electron therapy (EMET) based on inverse techniques and Monte Carlo dose calculation algorithms. This study investigates the application of EMET using a few-leaf electron collimator (FLEC) in head & neck, sarcoma, and breast sites in comparison with three dimensional conventional radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) techniques. Method and Materials: Treatment planning was performed for a parotid case, a sarcoma case, and a breast case. Three Monte Carlo calculated plans were compared for each case: 3D-CRT, IMRT, and 3D-CRT in conjunction with EMET (EMET-CRT). For all patients, dose volume histograms (DVHs) were obtained for organs of interest. For each plan, homogeneity and conformity indices of dose distributions, sparing index (SPIN50/10) that quantifies the conformity of the low isodose lines, and the whole-body dose equivalent (WBDE) were analyzed. Results: Adding EMET delivered with the FLEC to 3D-CRT preserves target conformity and dose homogeneity and improves sparing of normal tissues. For the head & neck case the mean dose to the contralateral parotid and brain decreased relative to IMRT by 43% to 84%, and by 57% to 71%, respectively. Improved normal tissue sparing is also quantified as an increase in sparing index of 47% and 30% for the head & neck and the breast cases, respectively. The WBDE for EMET-CRT was reduced by up to 72% when compared with IMRT. Conclusion: EMET delivered with the FLEC could be a valuable addition to currently existing treatment techniques especially when applied to superficially located tumors that are inherently difficult to plan using IMRT. The addition of EMET systematically leads to a reduction in WBDE especially when compared with IMRT.

3D conformal external beam radiotherapy (3D-CRT) for accelerated partial breast irradiation (APBI): What is the correct prescription dose

APBI has traditionally been delivered with interstitial brachytherapy, with 3D-CRT emerging as a promising technique. The most widely used fractionation scheme for APBI using HDR brachytherapy consists of 34 Gy delivered in 10 fractions over five days, and 3D-CRT has been introduced utilizing 38.5 Gy delivered in 10 fractions over five days. The dose prescription used for breast brachytherapy is the current standard in the delivery of APBI. No literature currently exists to confirm the equivalence of the dose prescription for 3D-CRT. This study is a case-based evaluation of the biological equivalence of the dose prescriptions for brachytherapy and 3D-CRT APBI, based on the concept of equivalent uniform biologically effective dose (EUBED). Both 3D-CRT and multi-catheter brachytherapy plans were generated for each of five patients. A CTV and PTV were created for each patient per the current APBI protocol guidelines, with PTVs ranging from 131 to 282 cm3. Dose matrices for the brachytherapy and 3D-CRT plans were computed using the Varian Brachyvision and Pinnacle planning systems, respectively. These dose matrices were then exported and analyzed with in-house software. The EUBED is used in this study to compare a highly non-uniform dose distribution of a given total dose and fraction size (brachytherapy) with a more homogeneous one (3D-CRT). The EUBED is a combination of equivalent uniform dose (EUD) and biologically effective dose (BED), allowing a non-uniform dose distribution to be reduced to an equivalent uniform dose while also accounting for fraction size. For each patient, the biologically effective dose (BEDi) was calculated for each voxel in the brachytherapy dose matrix using the standard linear quadratic (LQ) model, with α=0.3 Gy−1 and β=10 Gy. These values were then used to generate the equivalent uniform biologically effective dose for the entire brachytherapy dose matrix (EUBEDbrachy). Since EUBED is not a value easily related to in terms of clinical experience, it can be converted into a homogeneous EUD delivered at a given dose per fraction (eud). Using n = 10 fractions, one can calculate the EUD and eud that will produce the same EUBED. The equivalent uniform dose for the 3D-CRT plans (EUD3D-CRT) was then calculated from EUBEDbrachy, with the constraint that the treatment should be delivered with the same fractionation schedule. The generalized EUD (EUDa) was calculated for breast tissue with local control as the end-point (a = −7.2) for both the HDR and 3D-CRT distributions. Despite large differences in the geometry of the implants for the five patients studied, the values of generalized EUDa for the PTV were remarkably similar (39.4 ± 0.2 Gy). For the larger volume enclosed by a lower isodose line (e.g. 80%), the calculated EUDa was closer to the prescription dose (34.3 Gy). The calculated values of EUBEDbrachy were also quite consistent (51.5 ± 0.4 Gy). Using the values of EUD3D-CRT derived from EUBEDbrachy, we obtained a mean fraction size of 3.74 ± 0.03 Gy, and an equivalent prescription dose of 37.4 Gy. The generalized EUDa computed for dose distributions which were actually delivered with 3D-CRT was slightly higher, averaging 39.4 Gy, but does not account for the effect of different fraction sizes. The published experience suggests that the presently accepted dose prescription for breast brachytherapy yields excellent local control. Utilizing established radiobiological parameters, this study suggests that the ideal fraction size needed to deliver a biologically equivalent dose using 3D-CRT is at least 3.74 Gy. Therefore, the presently employed fraction size of 3.85 Gy for 3D-CRT appears to be appropriate to achieve equivalent local control