ESTRO 37 Abstract book

S960

ESTRO 37

EP-1788 A complete solution for HDR brachytherapy measurements G.P. Fonseca 1 , M. Podesta 1 , R. Voncken 1 , M.R. Van den Bosch 1 , B.G.L. Vanneste 1 , L. Lutgens 1 , F. Verhaegen 1 1 Department of Radiation Oncology MAASTRO- GROW- School for Oncology and Developmental Biology- Maastricht University Medical Center, Physics, Maastricht, The Netherlands Purpose or Objective Dose measurements in brachytherapy require high positioning accuracy due to the very steep dose gradients. Daily QA and applicator commissioning are a heavy burden in the clinic and an accurate in vivo dose verification method is not available. This work proposes the use of an Imaging Panel (IP) as a complete solution for brachytherapy measurements ranging from daily QA, applicator commissioning up to in vivo dosimetry. Material and Methods Two different IPs (Varex and Perkin Elmer) were calibrated using a HDR 192 Ir source with a robotic arm for accurate positioning. The IPs were used to acquire 192 Ir gamma-ray images of ring applicators, register the source movement inside the applicator and to fully verify a treatment plan with multiple catheters (3D Cartesians coordinates, dwell time and air kerma strength). Additionally, a Monte Carlo (MC) model capable of predicting IP responses during a brachytherapy treatment (Figure 1a) was created and verified using heterogeneous phantoms (Figure 1b).

plan from EPID images acquired during treatment. The measured dose (Diso) is compared with the calculated dose (DTPS) by R parameter defined as Diso/DTPS. A suitable phantom was made to simulate the female anatomy by modifying the ALDERSON RANDO with silicon breast prosthesis (figure 1a). 3DCRT plan of left breast was created with Pinnacle3 Professional TPS (Philips Medical Systems, Madison, WI) (figure 1b) and modified to simulate common errors: MU were perturbed to mimic an output error (adding 2-3-5-10MU); jaws positions were perturbed mimicking a calibration jaw bank error (opening and closing one jaw of 2-3-5-7mm). Both devices were employed during the irradiation of all plans. The deviations of IQM signal and R parameter from the original values were evaluated.

Results The short term reproducibility of R parameter of SoftDiso system was checked by repeating twenty times the original plan with the phantom located on the couch during irradiation. The reproducibility on terms of standard deviation/R mean value was resulted 0.6%. Same tests were performed using IQM [R&O, Volume 119,S215-S216]. Figure 2 shows the deviations of IQM signal and R value from the original for all delivery errors induced. The IQM signal and the R values linearly increase with the increment of MU (R 2 =0.99 for IQM and R 2 = 0.97 for SoftDiso) (Fig2a). The IQM signal is linearly correlated with the position of jaws too (R 2 =0.97 for jaws open and R 2 = 0.93 for jaws close), while R parameter is less sensitive to jaws positioning variations (Fig2b-2c).

Results The time-resolved applicator commissioning method has 0.2 mm accuracy (2D coordinates). Dwell positons can be visualized on top of a projection of the applicator and compared against the treatment planning system. Measurements with the applicator within a water phantom (Figure 1c) can detect positioning errors larger than 1 mm (3D coordinates), swapped catheters and dwell time errors (≥0.1 s). Measurements with heterogeneous phantom showed a difference ranging from 1.6% between adipose tissue and muscle up to 11.2% between bone and muscle. Figure 1d shows a predicted IP image (MC) for a head and neck patient (base of tongue tumor). The highlighted region (rectangle) shows a high intensity region due to the dwell position and anatomical information mostly related to the spine and air cavities. Conclusion The proposed system can be used for commissioning applicators (imaging and dosimetry) with an accurate, efficient and time-resolved method. Moreover, the same system can be used for in vivo dosimetry. Experimental results shows that the IP can detect differences in tissue composition even for soft tissues such as muscle and adipose tissue. MC predicted images show that

Conclusion Towards safer radiotherapy, centres should have protocols in place for in vivo dosimetry. The two systems exhibit good performances for in vivo monitoring of the external beams breast irradiation. The IQM device is able to detect small delivery errors in terms of output and field size of beams. The SoftDiso software shows a good capacity to detect small output errors while is less sensitive to jaws positioning variations. On the other hand SoftDiso has the potential of detecting set-up errors, which cannot be detected with IQM. The concurrent use of the two tested systems allow for a check of the correct functioning of all components in the radiotherapy chain, including the treatment planning, the delivery system and the patient positioning and thus play an important role in meeting the needs of modern and upcoming radiotherapy QA.