ESTRO 36 Abstract Book

S883 ESTRO 36 _______________________________________________________________________________________________

to validate that this technique is safe, robust and well tolerated by the patient. Material and Methods The standard conventional irradiation technique is adapted to become less sensitive to different intrafractional DIBH levels. A less steep penumbra at the junction is obtained by creating several subbeams with different MLC-positions. In order to reduce the number of beams, a selection of these beams are grouped to one beam using the Field in Field (FiF) forward IMRT functionality of Varian Eclipse TM TPS. This resulted in treatment plans with 6 to 9 beams. The study consists of the following steps: 1 The field match is validated using film measurements in a phantom. 2 Dose distributions and relative movements for 6 patients were assessed in vivo. For this a strip of film (with 6 mm buildup) was placed on the patient skin at the junction. In addition markers were placed on the skin above, on and below the junction, next to the film. Relative movements of the markers in each DIBH were determined using MV- images. 3 The relative movements of the marker measured in each DIBH were used to position the beams in the corresponding location in the TPS, resulting in a realistic dose distribution for each fraction and for the total treatment. 4 The in-vivo film measurements were used as a validation for the skin dose distribution calculated as explained in 3. Results The measured relative marker positions of all patients are shown in fig.1.

Results Tumor visibility on CBCT images was limited to a small range of projection angles, corresponding to about 5 seconds of each CBCT scan. Table 1 reports the tracking errors measured along the vertical image dimension, i.e. the projection of the superior-inferior tumor motion, and along the horizontal image dimension, combining antero- posterior and medio-lateral tumor motion. The absolute tracking error, averaged over all CBCT scans of all patients, measured 1.0 ± 0.9 mm and 0.8 ± 0.7 mm for the horizontal and vertical image dimensions, respectively. The comparison of real and estimated tumor trajectories along the vertical dimension is depicted in Figure 1. A significant correlation (p-value < 0.005) was found between real and estimated tumor motion, with correlation coefficients higher than 0.75 and 0.69 for the horizontal and vertical dimensions, respectively. Conclusion We developed and tested a novel approach for intra- fraction lung tumors tracking, using CBCT-based motion models driven by an external breathing surrogate obtained from non-invasive optical surface imaging. Compared to CT-based motion models, the proposed method does not need to compensate for inter-fraction motion variations that can occur between planning and treatment phases. The validation of the proposed approach on a wider patient population is currently ongoing. EP-1630 The impact of DIBH on dose to the junction in loco-regionally treated left-sided breast patients M. Van Hinsberg 1 , I. De Bree 2 , E. Osté 2 , D. De Ronde 2 1 Zuidwest Radiotherapeutisch Instituut, Klinische Fysica, Vlissingen, The Netherlands 2 Zuidwest Radiotherapeutisch Instituut, RTT, Vlissingen, The Netherlands Purpose or Objective Irradiating loco-regional left sided breast cancer patients using a field matching technique is common practice in clinical routine. Possible under- and overdosages at the junction are normally reduced by the natural breathing of the patient. Adding a (deep inspiration) breathhold strategy introduces extra risks of under- and overdosage in this region. A modified conventional treatment technique for the irradiation of loco-regional left-sided breast patients combined with a voluntary DIBH technique has been introduced in our institute. The objective of this pilot is

Fig. 1 Marker positions relative to the fraction mean. Distribution mean 0.4 mm; sd 2mm; range; -7 to 8 mm. A negative sign means the patient has moved in caudal direction. Analysing the marker positions of each patient shows no trend between the DIBH level of the first and last beam. No hyperventilating nor exhaustion occurred, indicating that DIBH with 6 to 9 beams is well tolerated. Planned dose-values and actual, total dose values for the CTV are presented in table 1: