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PO-0833: A method of identifying planning system accuracy limitations in step and shoot IMRT
ESTRO 33, 2014 dedicated to SBRT and one of the first group project was to compare and evaluate SBRT plans for prostate cancer among different Italian centers. Materials and Methods: Seventeen centres of the SBRT Italian working group shared the contours of five patients with prostate cancer, candidated to SBRT. Each center, with own personal treatment planning systems, was asked to plan them according to a reference paper for dose prescription (35 Gy in 5 fractions) and dose constraints to organs at risk (OAR) [1]. Different techniques were used (fixed-Field IMRT, VMAT by Monaco, RapidArc, and CyberKnife). Plans were compared in terms of DVH. Data are presented as averages over the five patients. Results: Eighty-five plans from 17 italian hospital were analyzed. Maximum dose to the body was 107.9±4.5% (range: 101.5-116.3%). Dose at 98% (D98%) and mean dose to the CTV were: 101.2±3.2% (range: 96.1106.5%) and 103.9±3.4% (range: 99.1-110.5%). Similar trends were found for D95% and mean dose to PTV, D0.1% to rectum and D0.1% to bladder. Significant differences were found for mean doses to rectum and bladder: 43.0±12.5% (range: 34.2-53.0%) and 33.7±16.7% (range: 26.344.2%). The subgroups with the same TPS showed analogous differences.
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Results: Results of the comparisons between CK and VMAT plans are summarized below. If the normalized quantity is greater than unity then the LB absolute value is larger and vice versa. Although cord DIs as well as cauda max doses in the CK plans resulted in lower doses by up to 33%, the actual limit for a particular DI was not reached for any patient. Therefore, despite the large percent differences, both techniques should result in comparable toxicities. Of note, linac targets (first row) are, on average, 34% larger than CK targets due to the 0.1 cm expansion, and still for maximum cauda and 200 cc kidney doses, LB plans are dosimetrically advantageous. The average linac gradient index is 3.7 while for CK it is 4.2, showing larger dose gradients and lower peripheral doses with LB plans. Similarly, the average linac conformity index was 0.82 compared to 0.79 for the CK plans, showing 'tighter' dose distributions with the LB plans.
Conclusions: Despite larger target volumes of almost 35%, LB SBRT for spinal lesions with a single modulated arc is as dosimetrically effective as with CK. Addition of more arcs will increase the treatment time somewhat, but will certainly decrease the differences between the two techniques. Nonetheless, LB plans would be delivered 2-3 times faster than CK plans with much fewer MUs. Further investigation is underway.
Conclusions: This project showed differences for all the parameters considered with possible clinical implication, in particular related to mean doses to OAR. The SBRT working group will develop protocols to help the homogenization of the SBRT planning process. Reference: [1] R. King et al. - Int J. Radiation Oncology Biol. Phys., Vol 82, No. 2, pp. 877-882, 2012 PO-0832 Dosimetric comparison between robotic and linac-based stereotactic body radiation therapy for spinal tumors M.I. Monterroso1, E. Bossart1, B. Ly1, I. Mihaylov1 1 University of Miami, Radiation Oncology, Miami Florida, USA Purpose/Objective: Metastatic bone disease is a significant source of morbidity for cancer patients. Stereotactic Body Radiotherapy (SBRT) is ideal for the treatment of spinal tumors because of the close proximity and high radiosensitivity of many organs at risk (OARs) around the spinal column. The purpose of this work is to compare the efficacy of robotic and linac-based (LB) SBRT for spinal tumors. Materials and Methods: Ten patients with spinal lesions were retrospectively studied. For each patient a Cyberknife (CK) and a Volumetric Arc Therapy (VMAT) treatment plan was generated, such that 27 Gy in 3 fractions covered 90% of the planning target volume (PTV). The CK PTV was uniformly expanded by 0.1 cm to form a PTV for the LB SBRT plans. Each plan aimed to keep volumetric and maximum doses to spinal cord/cauda equina, and kidneys within published limits. CK plans were generated in the Multiplan (Accuray) treatment planning system (TPS) and used 160 to 250 beams with the variable aperture Iris collimator, while VMAT plans were generated in the Eclipse (Varian) TPS and used a single modulated arc for the 120-leaf Millennium multi-leaf collimator. PTV coverage and dose indices (DIs) used as optimization goals were tallied to estimate the efficacy of each technique. DIs included: Maximum cord/cauda doses, as well as doses to 0.25 cc and 1.2 cc of cord, to 5cc of cauda, and to 200 cc of kidneys. DIs were normalized with respect to CK doses since different patients have different absolute DIs. Dose gradient and dose conformality were also evaluated.
PO-0833 A method of identifying planning system accuracy limitations in step and shoot IMRT L.S.H. Bendall1, M. Trainer1, C.J. Boylan1, M.J. Hardy1 1 The Christie NHS Foundation Trust, Radiotherapy CMPE, Manchester, United Kingdom Purpose/Objective: Due to suspected limitations of the TPS to model complex step-and-shoot IMRT fields, clinical practice has previously involved manually inspecting and adjusting segment shapes after optimisation. In this study we aimed to determine the limitations of the TPS model with a view to avoiding unnecessary manual adjustments, reducing inter-planner variation and improving planning efficiency. Materials and Methods: 10 clinical IMRT treatment plans for both H&N and Prostate were re-optimised within the TPS (Pinnacle v9) using the final planning objectives but with no limitation on segment modulation. These covered the wide range of linacs in current clinical use, including Elekta and Varian models with MLC leaf widths ranging from 0.25-1cm at isocentre. Film measurements of each field were compared to TPS fluences using gamma analysis (4%/4mm & 3%/3mm of maximum dose in field, within 20% isodose). A modulation complexity score (MCS) was calculated for each plan, with and without manual adjustments. To test beam model accuracy a novel bar test of varying widths and separations, down to the smallest single leaf and gap, was also delivered on each linac. Finally, the plan rejection rate for the four months before and after the introduction of a new IMRT segment advice document was investigated, using rejection codes input by the plan checker. Results: For both sites, the mean MCS was larger in the clinical plans, indicating reduced plan complexity. However, all of the re-optimised plans delivered on Elekta linacs passed the gamma index at the appropriate clinical tolerance. For Varian linacs, the bar test data showed excellent beam model accuracy, with all features >0.5cm modelled to within 2%. However some of the revised TPS optimised fields failed to pass the clinical tolerance. The mean MCS for those beams which passed was 0.481 compared to 0.442 for those which failed. Analysing these fields showed the failures to be readily predictable, using segment weighting information and bar test data. Changes to clinical practice were made based on this work and an audit of prostate IMRT plans rejections showed the rejection rate of plans for segment issues dropped from 6.0% to 1.8% of plans produced.
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Min Mean (range) Plan # failed gamma Max Total leaf MCS analysis ? Site # width bar beams Clinical Unconstrained Clinical Unconstrained (cm) test 0.460 0.437 0.25 25 (0.4150 7 2% (0.427-0.523) Prostate 0.542) (8 & 0.492 10MV) 0.475 1.0 25 (0.3660 0 6% (0.364-0.583) 0.616) Head & 0.282 0.244 Neck 1.0 28 (0.2160 0 6% (0.226-0.257) (6MV) 0.289) Table 1. Results obtained during the study for both Prostate and Head and Neck IMRT plans. Conclusions: We have demonstrated an effective method of establishing the level of accuracy within the 2D dose distribution of step and shoot IMRT plans without the need for pre-treatment verification. Guidance on limitations of modulation and bar test data have allowed planners and checkers to make decisions about model accuracy for individual plans. New guidance has resulted in increased confidence in the accuracy of IMRT step and shoot treatments, a reduction in planning time and improved consistency between planners. PO-0834 Investigation of an inverse planning system for MERT as a function of beam setup and dose constraints A. Joosten1, D. Henzen1, W. Volken1, D. Frei1, K. Lössl1, P. Manser1, M.K. Fix1 1 Inselspital, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland Purpose/Objective: To investigate the possibilities of a direct aperture optimization (DAO) algorithm for modulated electron radiation therapy (MERT) as a function of selected beam ports, beam energies and dose constraints and to compare the results with those from the original photon plans. Materials and Methods: A chest wall and a larynx case were investigated for MERT using the homemade DAO algorithm. Up to 4 and 7 different beam ports were defined for the chest wall and larynx cases, respectively. For each beam port, beamlets were generated for one to three different electron beam energies. For a given beam port and energy, apertures can be defined and optimized using the DAO by changing the shape and weight of the apertures. Different combinations of apertures were investigated with different dose constraints on the PTV and organs at risk. Dosimetric parameters such as the homogeneity index (HI=V95%-V107%) for the PTV as well as mean and max doses to organs at risk were derived and compared to the values of the original photon plans. Results: 133 and 73 different DAO optimizations were run for the chest wall and larynx cases, respectively. The HI ranged from 64% to 93% for the chest wall case and from 52% to 97% for the larynx case. The mean doses to neighboring organs at risk tend to increase with improving dose homogeneity to the PTV. For the same number of total apertures, a better dose homogeneity is achieved using multiple beam ports and different energies rather than using multiple apertures with same energy delivered from a single beam port. Our results show that it is possible to achieve a similar homogeneity to the PTV with MERT as with photons, although at the cost of increased dose to organs at risk. While the mean and max doses to organs at risk were usually higher than those obtained with the photon plans, they were still clinically acceptable given the usual dose tolerance values used. Conclusions: In contrast with previously published reports of MERT, it is possible to achieve a high HI for the PTV with an inverse planning system similar to those obtained with photon fields. The high HI achieved with MERT comes at the cost of larger doses to organs at risk. As the dosimetric results depend strongly on beam port and energy selection, further research is required to select optimal beam ports and electron beam energies to reduce doses to organs at risk while maintaining similar dose homogeneity to the PTV.
ESTRO 33, 2014 PO-0835 Dosimetric comparison between flattened and unflattened beams for lung SBRT G. Nicolini1, A. Fogliata1, A. Clivio1, E. Vanetti1, M.F. Belosi1, F. Martucci2, A. Richetti2, M.C. Valli2, L. Cozzi1 1 Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona, Switzerland 2 Oncology Institute of Southern Switzerland, Radiation Oncology Department, Bellinzona, Switzerland Purpose/Objective: To compare dosimetric differences in the treatment of lung cancer in stereotactic regimen using flattened or unflattened (Flattening Filter Free, FFF) photon beams of 6 and 10 MV. Materials and Methods: Eleven patients with early stage NSCLC were selected for stereotactical treatment, to be planned for 3 sessions of 18 Gy each at the mean dose to PTV. Target was delineated as ITV as envelope of target delineation on 10 respiratory phases from the 4DCT acquisitions. PTVs had an average diameter of 3.1±0.4 cm (range: 2.2, 3.4 cm) and a mean volume of 15.9±6.3 cm3 (range: 5.3, 20.3 cm3). All cases were planned with RapidArc technology, with two partial arcs entering from the disease side to maximally spare the contralateral lung. The same plan geometry was applied, using the same constraints, to optimize with the PRO3 algorithm four different plans: with flattened 6 and 10 MV (6 and 10X), and with unflattened 6 and 10 MV beams (6, 10 FFF) from a Varian TrueBeam unit. During the optimization process, intermediate dose calculation and re-optimization were applied to all plans. Dose distributions were calculated with Acuros XB, version 11, using a grid size of 1.25 mm. The analysis was conducted on the dose volume histograms of PTV for target coverage and dose homogeneity, and on the ipsilateral lung deducted the target volume, as well as on structures drawn as shells around the target: the mean dose in the various shells would give appraisal of the dose fall-off in the surrounding tissue for the different energies. The dose to surrounding shells was also evaluated in relation to the specific structure density. Results: Analysis of PTV data showed that plans with 6FFF beams resulted in a better target coverage and dose homogeneity, while the opposite was for 10X plans: D98%=52.7±0.2 and 51.6±0.8 Gy for 6FFF and 10X respectively. Mean lung doses were 3.5±0.7, 3.6±0.8, 3.8±0.8, 4.0±0.8 Gy for 6FFF, 6X, 10FFF, 10X respectively. Similar trend of increasing dose relative to energy was shown in the surrounding shells from 1 to 5 cm distance from target surface. Conclusions: Stereotactic treatments for small lung cancer can dosimetrically benefit from the usage of FFF modes relative to the flattened beam. The 6FFF beams showed to deliver the lowest peripheral dose among the studied plans. PO-0836 Coronary arteries preservation through VMAT use in adjuvant left breast cancer irradiation C. Bourgier1, C. Lemanski1, L. Bedos1, O. Riou1, J. Molinier1, C. LlacerMoscardo1, N. Aillères1, D. Azria1, P. Fenoglietto1 1 Institut du Cancer de Montpellier, Radiation Oncology, Montpellier, France Purpose/Objective: To lessen anterior interventricular artery (IVA) and heart exposure through the use of VMAT technique (RA) in left breast cancer (BC) treated by breast conserving surgery. Materials and Methods: From 12/2012 to 03/2013, five patients with unfavorable cardiac anatomy were treated with RA technique. Both lungs, ipsilateral (CTV1) and contralateral breasts, lumpectomy cavity (CTV2), heart and IVA were delineated. A margin of 7mm was added to CTV1 and 2 to define PTV1 and 2. Total dose delivered to PTV1was 52.2 Gy and to PTV2 63.2 Gy (concomitant boost) in 29 fractions. In all treatment planning, 95% of the dose prescribed to PTV1 encompassed 99% of CTV1 volume. We present here results of dosimetric RA compared to 3D-conformal (3D) treatment planning in BC (i.e, tangential fields and a boost by additional electron/photon irradiation). Results: The average heart length include in 3D beam was 2.6 cm [1.43.4] . Heart V35 Gy were 11% and 0.1% for 3D and RA respectively; and mean heart doses were 8.8 Gy and 6 Gy for3D and RA, respectively. For the IVA D1% was 59.34 Gy and 29.2 Gy in 3D and RA when respective Dmean reached 40.5 Gy and 14.1 Gy Dmean , V5 and V20 Gy for the ipsilateral lung, were 14.2 Gy,38.5% and 23.9% with 3D and 6.5 Gy, 34.3% and 7.46% with RA. Doses to contralateral organs were increased by RA treatment in the lung with a Dmean of 4.6 Gy in RA compare to 0.3 Gy in 3D. Similar results for the breast with3.5 Gy in RA against 1.1 Gy in 3D.
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Results: Results of the comparisons between CK and VMAT plans are summarized below. If the normalized quantity is greater than unity then the LB absolute value is larger and vice versa. Although cord DIs as well as cauda max doses in the CK plans resulted in lower doses by up to 33%, the actual limit for a particular DI was not reached for any patient. Therefore, despite the large percent differences, both techniques should result in comparable toxicities. Of note, linac targets (first row) are, on average, 34% larger than CK targets due to the 0.1 cm expansion, and still for maximum cauda and 200 cc kidney doses, LB plans are dosimetrically advantageous. The average linac gradient index is 3.7 while for CK it is 4.2, showing larger dose gradients and lower peripheral doses with LB plans. Similarly, the average linac conformity index was 0.82 compared to 0.79 for the CK plans, showing 'tighter' dose distributions with the LB plans.
Conclusions: Despite larger target volumes of almost 35%, LB SBRT for spinal lesions with a single modulated arc is as dosimetrically effective as with CK. Addition of more arcs will increase the treatment time somewhat, but will certainly decrease the differences between the two techniques. Nonetheless, LB plans would be delivered 2-3 times faster than CK plans with much fewer MUs. Further investigation is underway.
Conclusions: This project showed differences for all the parameters considered with possible clinical implication, in particular related to mean doses to OAR. The SBRT working group will develop protocols to help the homogenization of the SBRT planning process. Reference: [1] R. King et al. - Int J. Radiation Oncology Biol. Phys., Vol 82, No. 2, pp. 877-882, 2012 PO-0832 Dosimetric comparison between robotic and linac-based stereotactic body radiation therapy for spinal tumors M.I. Monterroso1, E. Bossart1, B. Ly1, I. Mihaylov1 1 University of Miami, Radiation Oncology, Miami Florida, USA Purpose/Objective: Metastatic bone disease is a significant source of morbidity for cancer patients. Stereotactic Body Radiotherapy (SBRT) is ideal for the treatment of spinal tumors because of the close proximity and high radiosensitivity of many organs at risk (OARs) around the spinal column. The purpose of this work is to compare the efficacy of robotic and linac-based (LB) SBRT for spinal tumors. Materials and Methods: Ten patients with spinal lesions were retrospectively studied. For each patient a Cyberknife (CK) and a Volumetric Arc Therapy (VMAT) treatment plan was generated, such that 27 Gy in 3 fractions covered 90% of the planning target volume (PTV). The CK PTV was uniformly expanded by 0.1 cm to form a PTV for the LB SBRT plans. Each plan aimed to keep volumetric and maximum doses to spinal cord/cauda equina, and kidneys within published limits. CK plans were generated in the Multiplan (Accuray) treatment planning system (TPS) and used 160 to 250 beams with the variable aperture Iris collimator, while VMAT plans were generated in the Eclipse (Varian) TPS and used a single modulated arc for the 120-leaf Millennium multi-leaf collimator. PTV coverage and dose indices (DIs) used as optimization goals were tallied to estimate the efficacy of each technique. DIs included: Maximum cord/cauda doses, as well as doses to 0.25 cc and 1.2 cc of cord, to 5cc of cauda, and to 200 cc of kidneys. DIs were normalized with respect to CK doses since different patients have different absolute DIs. Dose gradient and dose conformality were also evaluated.
PO-0833 A method of identifying planning system accuracy limitations in step and shoot IMRT L.S.H. Bendall1, M. Trainer1, C.J. Boylan1, M.J. Hardy1 1 The Christie NHS Foundation Trust, Radiotherapy CMPE, Manchester, United Kingdom Purpose/Objective: Due to suspected limitations of the TPS to model complex step-and-shoot IMRT fields, clinical practice has previously involved manually inspecting and adjusting segment shapes after optimisation. In this study we aimed to determine the limitations of the TPS model with a view to avoiding unnecessary manual adjustments, reducing inter-planner variation and improving planning efficiency. Materials and Methods: 10 clinical IMRT treatment plans for both H&N and Prostate were re-optimised within the TPS (Pinnacle v9) using the final planning objectives but with no limitation on segment modulation. These covered the wide range of linacs in current clinical use, including Elekta and Varian models with MLC leaf widths ranging from 0.25-1cm at isocentre. Film measurements of each field were compared to TPS fluences using gamma analysis (4%/4mm & 3%/3mm of maximum dose in field, within 20% isodose). A modulation complexity score (MCS) was calculated for each plan, with and without manual adjustments. To test beam model accuracy a novel bar test of varying widths and separations, down to the smallest single leaf and gap, was also delivered on each linac. Finally, the plan rejection rate for the four months before and after the introduction of a new IMRT segment advice document was investigated, using rejection codes input by the plan checker. Results: For both sites, the mean MCS was larger in the clinical plans, indicating reduced plan complexity. However, all of the re-optimised plans delivered on Elekta linacs passed the gamma index at the appropriate clinical tolerance. For Varian linacs, the bar test data showed excellent beam model accuracy, with all features >0.5cm modelled to within 2%. However some of the revised TPS optimised fields failed to pass the clinical tolerance. The mean MCS for those beams which passed was 0.481 compared to 0.442 for those which failed. Analysing these fields showed the failures to be readily predictable, using segment weighting information and bar test data. Changes to clinical practice were made based on this work and an audit of prostate IMRT plans rejections showed the rejection rate of plans for segment issues dropped from 6.0% to 1.8% of plans produced.
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Min Mean (range) Plan # failed gamma Max Total leaf MCS analysis ? Site # width bar beams Clinical Unconstrained Clinical Unconstrained (cm) test 0.460 0.437 0.25 25 (0.4150 7 2% (0.427-0.523) Prostate 0.542) (8 & 0.492 10MV) 0.475 1.0 25 (0.3660 0 6% (0.364-0.583) 0.616) Head & 0.282 0.244 Neck 1.0 28 (0.2160 0 6% (0.226-0.257) (6MV) 0.289) Table 1. Results obtained during the study for both Prostate and Head and Neck IMRT plans. Conclusions: We have demonstrated an effective method of establishing the level of accuracy within the 2D dose distribution of step and shoot IMRT plans without the need for pre-treatment verification. Guidance on limitations of modulation and bar test data have allowed planners and checkers to make decisions about model accuracy for individual plans. New guidance has resulted in increased confidence in the accuracy of IMRT step and shoot treatments, a reduction in planning time and improved consistency between planners. PO-0834 Investigation of an inverse planning system for MERT as a function of beam setup and dose constraints A. Joosten1, D. Henzen1, W. Volken1, D. Frei1, K. Lössl1, P. Manser1, M.K. Fix1 1 Inselspital, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland Purpose/Objective: To investigate the possibilities of a direct aperture optimization (DAO) algorithm for modulated electron radiation therapy (MERT) as a function of selected beam ports, beam energies and dose constraints and to compare the results with those from the original photon plans. Materials and Methods: A chest wall and a larynx case were investigated for MERT using the homemade DAO algorithm. Up to 4 and 7 different beam ports were defined for the chest wall and larynx cases, respectively. For each beam port, beamlets were generated for one to three different electron beam energies. For a given beam port and energy, apertures can be defined and optimized using the DAO by changing the shape and weight of the apertures. Different combinations of apertures were investigated with different dose constraints on the PTV and organs at risk. Dosimetric parameters such as the homogeneity index (HI=V95%-V107%) for the PTV as well as mean and max doses to organs at risk were derived and compared to the values of the original photon plans. Results: 133 and 73 different DAO optimizations were run for the chest wall and larynx cases, respectively. The HI ranged from 64% to 93% for the chest wall case and from 52% to 97% for the larynx case. The mean doses to neighboring organs at risk tend to increase with improving dose homogeneity to the PTV. For the same number of total apertures, a better dose homogeneity is achieved using multiple beam ports and different energies rather than using multiple apertures with same energy delivered from a single beam port. Our results show that it is possible to achieve a similar homogeneity to the PTV with MERT as with photons, although at the cost of increased dose to organs at risk. While the mean and max doses to organs at risk were usually higher than those obtained with the photon plans, they were still clinically acceptable given the usual dose tolerance values used. Conclusions: In contrast with previously published reports of MERT, it is possible to achieve a high HI for the PTV with an inverse planning system similar to those obtained with photon fields. The high HI achieved with MERT comes at the cost of larger doses to organs at risk. As the dosimetric results depend strongly on beam port and energy selection, further research is required to select optimal beam ports and electron beam energies to reduce doses to organs at risk while maintaining similar dose homogeneity to the PTV.
ESTRO 33, 2014 PO-0835 Dosimetric comparison between flattened and unflattened beams for lung SBRT G. Nicolini1, A. Fogliata1, A. Clivio1, E. Vanetti1, M.F. Belosi1, F. Martucci2, A. Richetti2, M.C. Valli2, L. Cozzi1 1 Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona, Switzerland 2 Oncology Institute of Southern Switzerland, Radiation Oncology Department, Bellinzona, Switzerland Purpose/Objective: To compare dosimetric differences in the treatment of lung cancer in stereotactic regimen using flattened or unflattened (Flattening Filter Free, FFF) photon beams of 6 and 10 MV. Materials and Methods: Eleven patients with early stage NSCLC were selected for stereotactical treatment, to be planned for 3 sessions of 18 Gy each at the mean dose to PTV. Target was delineated as ITV as envelope of target delineation on 10 respiratory phases from the 4DCT acquisitions. PTVs had an average diameter of 3.1±0.4 cm (range: 2.2, 3.4 cm) and a mean volume of 15.9±6.3 cm3 (range: 5.3, 20.3 cm3). All cases were planned with RapidArc technology, with two partial arcs entering from the disease side to maximally spare the contralateral lung. The same plan geometry was applied, using the same constraints, to optimize with the PRO3 algorithm four different plans: with flattened 6 and 10 MV (6 and 10X), and with unflattened 6 and 10 MV beams (6, 10 FFF) from a Varian TrueBeam unit. During the optimization process, intermediate dose calculation and re-optimization were applied to all plans. Dose distributions were calculated with Acuros XB, version 11, using a grid size of 1.25 mm. The analysis was conducted on the dose volume histograms of PTV for target coverage and dose homogeneity, and on the ipsilateral lung deducted the target volume, as well as on structures drawn as shells around the target: the mean dose in the various shells would give appraisal of the dose fall-off in the surrounding tissue for the different energies. The dose to surrounding shells was also evaluated in relation to the specific structure density. Results: Analysis of PTV data showed that plans with 6FFF beams resulted in a better target coverage and dose homogeneity, while the opposite was for 10X plans: D98%=52.7±0.2 and 51.6±0.8 Gy for 6FFF and 10X respectively. Mean lung doses were 3.5±0.7, 3.6±0.8, 3.8±0.8, 4.0±0.8 Gy for 6FFF, 6X, 10FFF, 10X respectively. Similar trend of increasing dose relative to energy was shown in the surrounding shells from 1 to 5 cm distance from target surface. Conclusions: Stereotactic treatments for small lung cancer can dosimetrically benefit from the usage of FFF modes relative to the flattened beam. The 6FFF beams showed to deliver the lowest peripheral dose among the studied plans. PO-0836 Coronary arteries preservation through VMAT use in adjuvant left breast cancer irradiation C. Bourgier1, C. Lemanski1, L. Bedos1, O. Riou1, J. Molinier1, C. LlacerMoscardo1, N. Aillères1, D. Azria1, P. Fenoglietto1 1 Institut du Cancer de Montpellier, Radiation Oncology, Montpellier, France Purpose/Objective: To lessen anterior interventricular artery (IVA) and heart exposure through the use of VMAT technique (RA) in left breast cancer (BC) treated by breast conserving surgery. Materials and Methods: From 12/2012 to 03/2013, five patients with unfavorable cardiac anatomy were treated with RA technique. Both lungs, ipsilateral (CTV1) and contralateral breasts, lumpectomy cavity (CTV2), heart and IVA were delineated. A margin of 7mm was added to CTV1 and 2 to define PTV1 and 2. Total dose delivered to PTV1was 52.2 Gy and to PTV2 63.2 Gy (concomitant boost) in 29 fractions. In all treatment planning, 95% of the dose prescribed to PTV1 encompassed 99% of CTV1 volume. We present here results of dosimetric RA compared to 3D-conformal (3D) treatment planning in BC (i.e, tangential fields and a boost by additional electron/photon irradiation). Results: The average heart length include in 3D beam was 2.6 cm [1.43.4] . Heart V35 Gy were 11% and 0.1% for 3D and RA respectively; and mean heart doses were 8.8 Gy and 6 Gy for3D and RA, respectively. For the IVA D1% was 59.34 Gy and 29.2 Gy in 3D and RA when respective Dmean reached 40.5 Gy and 14.1 Gy Dmean , V5 and V20 Gy for the ipsilateral lung, were 14.2 Gy,38.5% and 23.9% with 3D and 6.5 Gy, 34.3% and 7.46% with RA. Doses to contralateral organs were increased by RA treatment in the lung with a Dmean of 4.6 Gy in RA compare to 0.3 Gy in 3D. Similar results for the breast with3.5 Gy in RA against 1.1 Gy in 3D.