volume encompassed by the 95 % isodose surface. An homogeneity index (HI) for the PTV was calculated using the following formula: HI = (D2% - D98%)/D50 ...
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volume encompassed by the 95 % isodose surface. An homogeneity index (HI) for the PTV was calculated using the following formula: HI = (D2% - D98%)/D50% Additionally, values for Dmean, D0.1% and D2% are reported for selected organs-at-risk (OARs) according to ICRU 83 recommendations. Results: An excellent correlation between the median and mean doses to the PTV (0.2%) is noted. ICRU 83 recommends prescribing to the median dose as it should correlate closely with the ICRU reference point dose. The median dose to the PTV varies from the dose to the ICRU reference point by less than 2%. In the majority of plans, the median dose to the PTV exceeds the dose at the ICRU reference point resulting in a reduction in the values of D2% and D98% when prescribing to the median dose rather than to the ICRU reference point. Furthermore, a slight reduction in the coverage of the PTV (approximately 1%), as assessed by calculating the V95%, is noted when prescribing to the median dose to the PTV. The homogeneity index does not change irrespective of the method of prescription. A good correlation between the maximum dose (Dmax) and D0.1% to OARs is shown. It is noted, however, that the value for D2% can differ significantly from the maximum dose (Dmax) currently reported. Conclusions: A prescription model incorporating dose-volume specification would likely lead to greater consistency in the reporting of IMRT treatment plans. An audit of 176 delivered IMRT treatment plans at St. Luke’s Hospital reveals that ICRU 83 recommendations could be implemented for dose prescription and reporting with a minimal amount of change from the current practice. As tumour dose outcomes using the ICRU 83 recommendation of prescribing to median dose are similar to that obtained using prescription to the ICRU reference point, adaptation to the ICRU 83 dose prescription recommendations should be achieved relatively easily. Greater care is needed for ICRU 83 reporting of dose to OARs. EP-1555 TREATMENT PLANNING OF LUNG TUMORS USING TWO DESIGNS OF PLANNING TARGET VOLUME A. Cámara Turbí1, M. Melchor1, F. Candela1, D. Martínez1, M. Asensio1, J.L. Monroy2 1 Hospital Universitario de la Ribera, Radiation Physics, Alzira, Spain 2 Hospital Universitario de la Ribera, Radiation Oncology, Alzira, Spain Purpose/Objective: To evaluate two radiotherapy lung treatments comparing 'old' dose distribution when only one CT study was used for delineating CTV with the actual technique where three CT scans (physiological breathing, inspiration enforced and expiration enforced) are used to delineate the ITV. Materials and Methods: A group of 20 patients, treated from January to September 2011 were prospectively analyzed. In all cases ITV was generated combining the CTVs of the three CT studies (CTVn, CTVe, CTVi). The PTV was defined adding 0.7cm in all directions to this ITV. 3D treatment planning was realized for this PTV. We defined later, the 'standard' PTV (PTVn) generated by adding 1 cm margin to the CTVn delineated in the physiological breathing study, as it used to be done when only one CT scan was made. A plan was performed for this PTVn, and the results on the PTV and the all the CTVs were analyzed. Of these 20 patients, 9 had a single course (cases 1-9) treatment and the other 11 (cases 10-20), two courses, with the first one including tumor and selected nodes. Results: The resultant coverages of the 'standard' plan are exposed in the next table. These data correspond to 95% of the prescribed dose to the PTVn.
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Case 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
PTV (%) 99.73 100 70.6 65.2 99.96 100 90.15 100 99.99 99.9 100 98.89 99.92 100 100 100 100 99.42 90.8 99.87
CTVn (%) 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
CTVe (%) 100 100 100 62.78 100 100 93.1 100 100 100 100 100 100 100 100 100 100 100 100 100
CTVi (%) 100 100 73.73 71.54 100 100 100 100 100 100 100 100 100 100 100 100 100 100 90.38 100
Conclusions: In this study, we can conclude that using only one CT study, there are cases where part of the CTV can be missed and that’s why we decided to perform three CT scans for all lung tumors treatments. EP-1556 EXAMINATION OF VARIOUS TREATMENT TECHNIQUES FOR CHEST WALL AND LYMPHATIC IRRADIATION M. Gultekin1, M. Karabuga2, F. Yildiz1, M. Gurkaynak1 1 Hacettepe University Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey 2 Hacettepe University Faculty of Medicine, Nuclear Medicine, Ankara, Turkey Purpose/Objective: The aim of this trial is to find the most suitable technique for patients treated with chest wall and lymphatic irradiation after mastectomy among various techniques that differ in target volume and critical organ dosage dispersal to be used in the clinic. Materials and Methods: Three-D (3D) planning has been made on cross sections from computerized tomography (CT) scans of 10 patients diagnosed with left breast cancer using 4 different techniques (partial wide tangent, 30/70 photon electron, 20/80 photon electron, chest wall- internal mammary only electron) and dosimetric measurements have been made on Alderson Rando phantom using each technique. On dosage volume histograms (DVH) obtained from 3D planning, minimal, maximal and average dosages received by the chest wall, supraclavicular, axilla and internal mammary lymphatics have been measured for each patient, V20 for the lungs determined as the critical organ, average right and left lung dosages, for the heart the average dosage it receives and V10, V30 values have been examined. On Alderson Rando phantom each technique has been separately simulated, 3D planning has been made and the accuracy of the planning has been tested by placing TLD dosimeters on spots for each volume. For estimation of dosages on surface, gafchromic film dosimeters have been used. Results: The most homogeneous dosage dispersion on the chest wall has been found in partial wide tangent technique. Similarly, lung and heart dosages are significantly less in partial wide technique than other techniques. However, right breast dosages have been found significantly higher in this technique. Approximately 0.6-7.9% difference has been found between dosimetric evaluation and TDL estimations. In the measurement of surface dosages via gafchromic film dosimeters, tangential photon areas have been radiated using naked, 0.5cm and 1cm tissue equivalent bolus, dosages have been measured as 161.8± 2.7cGy, 241.0 ± 1.5cGy and 255.3 ± 2.7cGy respectively. Conclusions: As result of this study, partial wide tangent technique has been found to be the most convenient technique in terms of dosage dispersion, setup and planning. The main disadvantage of this technique is that the dosage on the right breast is higher than the other techniques.
