ESTRO 37 Abstract book

S1076

ESTRO 37

Conclusion Coverage of the CTV by the voxel-wise minimum dose is closely related to coverage of the PTV in the nominal dose evaluation and the V95 criteria used for the PTV can be applied approximately 1:1 to the voxel-wise minimum dose. EP-1975 Comparison of Field in Field and Tangential Wedged Beams techniques in radiotherapy of breast cancer M. Yarahmadi 1 , A. Haghparast 2 , B. Faramarzi 2 , Z. Saalehi 1 1 Kurdistan University of Medical Sciences, Medical physics, Sanandaj, Iran Islamic Republic of 2 Kermanshah University of Medical Sciences, Medical physics, kermanshah, Iran Islamic Republic of Purpose or Objective In this study dose distribution of the chest wall in post- mastectomy breast cancer patients were evaluated and compared in the tangential wedged beams (TWB) and field-in-field (FIF) plans. Material and Methods 36 patients with left-sided breast cancer were enrolled in this study. The FIF and TWB plans were generated for each patient to compare dosimetric parameters of the chest wall. The maximum dose (D max ), the mean dose (D mean ), the homogeneity index (HI), the conformity index (CI) and the uniformity index (UI) were defined and used for comparison of the dosimetric parameters of the planning target volume (PTV) in both the FIF and the TWB plans. D mean and the percentage of volumes receiving at least 10, 20, 30 and 40 Gy of the left lung and 5, 10, 20, 25 and 30 Gy of the heart were used to compare the dosimetric results of the organs at risk. All statistical analysis was performed using the SPSS version 20 software. Results The FIF plan had significantly lower HI (p = 0.000) than TWB plan, which means that FIF plan was better than TWB plan in the PTV. The V 10lung (25.28±5.91 vs. 27.19±6.22), V 40lung (15.36±4.35 vs. 18.37±4.42), V 10heart (11.34±4.40 vs. 14.06±4.31) and V 30heart (8.15±3.75 vs. 10.94±3.94) were significantly lower in the FIF plan than in the TWB plan. D mean heart (5.08±1.84 vs. 6.39±1.95) and D mean left lung (10.50±2.51 vs. 11.70±2.69) with (p=0.000), were significantly lower in the FIF plan than in the TWB plan. Also the Monitor Unit (MU) was significantly lower in the FIF plan than the TWB plan (227.76 vs. 323.59). The comparison of plan Dose Volume Histograms (DVH) curves for PTV and OARs of a patient for FIF and TWB plans are shown in Figure 1. Fig1. An example of Dose Volume Histograms (DVH) curves for PTV and OARs of a patient; comparison of FIF and TWB plan.

EP-1976 An automated tool for perpendicular beam incidence in electron radiation therapy H. Afsharpour 1 , M. Desbiens 2 1 Peel regional cancer center, Radiation Oncology, Mississauga, Canada 2 CISSS Monteregie-Centre, Radio-Oncologie, Greenfield Park, Canada Purpose or Objective The purpose of this work is to introduce an automatic method for determining the required gantry and couch rotations for a perpendicular beam incidence on patient’s surface for electron radiation therapy. Introduction Electron beam radiation therapy is an efficient method for eradicating tumours near the skin. This technique is gaining popularity because of the widespread availability of linear accelerators in expense of orthovoltage photon beams. Unless otherwise stated, a basic requirement for an electron beam is to have a perpendicular incidence on the surface to reproduce, as much as possible, the commissioning data measurement conditions (water tank). But not all cases could be handled by rotating the gantry only and usually a couch rotation is also required. Thus, one of the challenges for planners is to find out the good combination of gantry and couch angle to produce a perpendicular beam incidence on patient’s surface being the skin, a bolus or any other equipment. Currently, planners can only rely on the state-of-the-art 3D visualization tools in the TPS for this endeavor which is time-consuming and not accurate. The purpose of this work is to provide the planners with an automatic tool (a script) to determine the proper gantry and couch rotation angles to achieve a perpendicular incidence on the skin, quickly and accurately. Material and Methods Two major treatment planning systems namely Pinnacle 3 (Philips) and Eclipse (Varian) were considered in this study. Each patient is scanned and the scans are sent to the TPS. A physician contours the CTV/PTV to define the extent and the depth of treatment. A planner will then place the isocenter at the center of PTV projected to the skin’s surface. Next step is to define four points at the surface vis-à-vis the four extremities of the PTV being the superior, inferior, right and left directions. The coordinates of those points are then input to the script which will immediately give instructions for gantry and couch rotations for achieving a perpendicular beam incidence. Results Two cases are reported as examples for the usefulness of this macro in our clinical routine. First case was a right breast boost with a surgical cavity placed medially (Fig. 1). The second case was a scalp skin lesion (Fig. 2) with a non-standard patient setup (prone with face turned towards left). Using the 3D visualization tool, it took about 10 minutes for a planner to finalize the beam setup. However, this tool allowed us to achieve an accurate beam setup within a minute.

Conclusion Using the FIF plan significantly reduced the dose volume of the left lung and heart in chest wall radiotherapy of post-mastectomy patients compared using the TWB plan. Therefore the FIF plan is recommended for post- mastectomy radiotherapy.

Figure 1; A breast boost plan with a superior-medial surgical bed.