ESTRO 2020 Abstract book

S846 ESTRO 2020

depth of PTV, in addition to the 90% isodose line from PLAN. The bolus was created as a contour with corresponding density and re-imported into Raystation RTP (Figure 1) to create a new treatment plan (MEB). While keeping the coverage of PTV for both plans (CONV & MEB) the same, namely 95% of planning target volume receiving 14.4Gy in 8 fractions, the doses to heart, left lung, left ribs, monitor unit and conformity index were compared (Figure 2).

The plans of the different VILMA configurations with collapsed cone algorithm are represented graphically to know which are the radii between which the solution of the density override is better. Results When plotting the dose difference (Fig.2), under modulated fluence, between the displaced and non- displaced tumor, and the dose difference in the same positions but under non-modulated fluence curve, we found these two curves intersect in two PTV diameters, approximately 2.5cm and 10cm.

Conclusion From the results between 2.5cm to 10cm PTV diameter we can plan without modulating fluence due to the density inhomogeneity of the PTV margin. Thus, we avoid overdosing and ensure tumor coverage. The modulation is low so Interplay Effects should not appear. For larger diameters, where the margin of PTV is small compared to the tumor (the larger the tumors tend to move less), scattered radiation from the tumor provides sufficient doses to the areas of the PTV with lung density as to be, without override density, we do not need a strong modulation of fluence at the edges. For diameters smaller than 2.5cm when the tumor radii is far from containing the electronic equilibrium, the override density must follow other strategies. PO-1482 Feasibility of 3D-printed electron-modulated bolus for post lumpectomy left breast cancer patients W.Y.V. Lee 1 , K.W.K. Cheng 1 , C.H.A. Liu 1 , T.C. Lam 2 , H.F.V. Lee 2 1 Tuen Mun Hospital, Department of Clinical Oncology, New Territories, Hong Kong SAR China ; 2 The University of Hong Kong, Clinical Oncology, Hong Kong, Hong Kong SAR China Purpose or Objective Electron radiotherapy is usually used for boosting the tumor bed after tangential photon therapy as an adjuvant treatment in post lumpectomy breast cancer patients. Due to the availability of Monte Carlo dose calculation in treatment planning system (RTP), electron dose can be reviewed in CT images prior to treatment. However, the electron dose cannot conform to the tumor bed as no optimization is available in the RTP. 3D-printed electron- modulated bolus allows optimization of electron beams so as to minimize the dose to the organs-at-risk whilst keeping the same coverage on tumor bed when comparing with conventional electron therapy. Material and Methods Thirteen patients with left breast cancers who underwent lumpectomy were recruited in the study. For each patient, a tangential photon plan in Raystation RTP (v8.0b, Sweden) with prescription of 2.17Gy x 21 fractions was created as the base plan. Different electron plans were created with summation of the photon plan for comparison. Firstly, a conventional plan (CONV) without bolus using electron energy based on the depth of planning target volume (PTV) was constructed as reference. Secondly, another plan (PLAN) was created using 1cm thick uniform bolus, which was contoured retrospectively with reference to the size and location of PTV in CT. An electron-modulated bolus was then generated using Adaptiiv 3D bolus software (v2.0), which used shape and

Figure 1 Regarding MEB plan (upper) vs CONV plan (lower), arrows represents dose difference and purple contour represents electron-modulated bolus

Figure 2 The dose volume histogram of organs-at-risk and PTV for CONV and MEB plan for the electron beam. Results For the left lung, the average percentage difference of percentage volume receiving equal to or greater than 5Gy (V 5Gy ), 10Gy (V 10Gy ) and 30Gy (V 30Gy ) for MEB are 15.6%, 14.6% and 4.3% respectively lower than CONV. While for the heart, the average percentage difference of MEB for V 5Gy and V 10Gy are 15.7% and 20.2% respectively less than CONV. Moreover, the average maximum dose of MEB for left ribs is 2.7% lower CONV and the conformity index improved 17.9% in average for MEB. However, the monitor units were averagely increased by 2.5% for MEB plans. All results are statistical significant (p<0. 05) except monitor units. Conclusion