ESTRO 38 Abstract book

S1005 ESTRO 38

Pinnacle3 TPS) and a dual arc VMAT of 240°± 20° starting from gantry at 180° (6MV - Elekta Monaco TPS). Both plans were optimised in order to obtain the best compromise between target dose distribution and OAR sparing. For each planning CT were registered: target coverage of the two PTVs in terms of V 95 , V 98 , mean dose, Conformity Index (CI) and Homogeneity Index (HI); V 20 , V 5 and mean dose for ipsilateral, controlateral and total lung; V 30 , V 5 and mean dose for heart; right breast V 5 and mean dose; spine maximal dose; total MU. In order to encompass the inter- subjects variability, for each planning CT the differences of each parameter in the two competitive plans were evaluated by means of two tailed paired t-test. Results Mean ± SD of each parameter in each group of 26 plans (3D and VMAT) are reported in Table, together with the statistical analysis. VMAT plans show highly significant better coverage both for breast or chest wall and lymphnodes and result in highly significant better CI and HI. Moreover, in VMAT, due to the better conformity of the technique, heart V 30 is significantly lower than in 3D (p=0.002) and V 20 of ipsilateral and total lung are lower, although not significant, than in 3D. OAR low doses are higher for VMAT: V 5 for ispilateral, controlateral and total lung are significantly higher (p<0.001), similarly heart V 5 and mean dose (p<0.001) and also right breast V 5 and mean dose (p<0.001). Finally, total MU are significantly higher in VMAT (p=0.01). Conclusion Our results show that VMAT technique for the irradiation of left breast or chest wall plus supra\infraclavicular nodes offers better target coverage and superior CI and HI than 3D-CRT. VMAT shows better V 30 for heart and V 20 for lung and higher low dose parameters for all the observed OAR. In our department VMAT is becoming the favourite technique because of its better target conformity and coverage and considering that, as reported in recent literature, nor low dose bath, nor MU increase, with consequent out-of-field dose, observed in VMAT, seems to determine higher detriment to healthy tissues, which is instead frequently reported in the medium-high dose region usually larger in the 3D technique.

EP-1852 Dosimetric comparison of techniques for left- sided breast and regional lymph node radiotherapy T. Joslin-Tan 1 , R. Maggs 1 , C. Pembroke 2 , J. Lambert 3 , K. O'Neill 3 , J. Staffurth 2 1 Velindre Cancer Centre, Medical Physics, Cardiff, United Kingdom ; 2 Velindre Cancer Centre, Clinical Oncology, Cardiff, United Kingdom ; 3 Rutherford Cancer Centre South Wales, Physics, Newport, United Kingdom Purpose or Objective Inclusion of supraclavicular fossa (SCF), axillary, and internal mammary nodes (IMN) as target volumes in breast cancer radiotherapy (RT) has been shown to give clinical benefits for certain patient cohorts. However, heart and ipsilateral lung doses have been shown to increase when treating the nodes with 3D conformal radiotherapy (3D- CRT) using a wide tangent and matched field approach. Concerns also exist over planning complexity with 3D-CRT. Deep inspiration breath hold (DIBH) has been shown to reduce heart and lung doses while maintaining target coverage. Intensity modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), and spot scanning proton beam therapy (PBT) have been shown to improve plan quality over 3D-CRT with free-breathing patients. A dosimetric study was performed to compare these three techniques in DIBH. Material and Methods CT datasets of 10 patients previously treated with RT in DIBH for left-sided breast cancer were used in the study. IMRT (four-field), VMAT (two 200° arcs), and PBT (anterior and 'en-face' beams) plans were created in Nucletron Oncentra Masterplan v4.3 with a view to creating class solutions to treat the breast (40Gy [100%] in 15 fractions) and regional nodes (36Gy [90%] in 15 fractions). Target coverage, organ at risk (heart, lungs, contralateral breast), and other normal structure (including the left anterior descending coronary artery and brachial plexus) doses were reviewed and statistical significance was evaluated using paired two-tailed t-tests. Results Plans were created for 9/10 patients. VMAT and PBT plans met all mandatory planning objectives compared with 7/9 IMRT plans. PBT plans had lower mean heart dose, left lung V17Gy, and mean contralateral breast dose than IMRT and VMAT while achieving better coverage. VMAT had superior target coverage, conformity, and breast PTV uniformity over IMRT with similar heart and left lung doses, but with increased contralateral breast mean dose and V5Gy. Dosimetric values are summarised in Table 1.

Table 1 – Summary of dosimetric parameters for OARs considered during plan optimisation and PTVs. Conclusion PBT gave better overall plan quality than both photon modalities but is not currently available to all regional nodal RT patients in the UK. The dosimetric benefits in target coverage coupled with a more robust class solution made VMAT the preferred choice over IMRT. Where VMAT is not possible and lower nodal target coverage is acceptable IMRT may be suitable for most patients. Lower contralateral breast doses may make IMRT advantageous when secondary cancers are of concern. EP-1853 Dosimetric comparison of helical tomotherapy and intensity-modulated radiotherapy in cervical cancer