ESTRO 36 Abstract Book

S886 ESTRO 36 2017 _______________________________________________________________________________________________

EP-1654 Clinical set up and first results of EPID in vivo dosimetry in an overload Chinese Radiotherapy J. Li 1 , A. Piermattei 2 , P. WANG 1 , S. Kang 1 , M. Xiao 1 , B. Tang 1 , X. Liao 1 , X. Xin 1 , L.C. Orlandini 1 1 Sichuan Cancer Hospital, Radiation Oncology, Chengdu, China 2 Fondazione Policlinico Universitario Agostino Gemelli, UOC Fisica Sanitaria, Rome, Italy Purpose or Objective In vivo dosimetry (IVD) is an important tool able to verify the accuracy of the treatment delivered. In an environment where several linacs of different types support daily heavy treatment workload over different shifts of therapists, physicists and Radiation oncologists, IVD checks can be strongly recommended to avoid important dosimetric discrepancies. The work describes the setup of IVD procedure with electronic portal imaging devices (EPID) in an overload radiotherapy clinical workflow, and the preliminary results obtained. Material and Methods 64 patients that underwent a VMAT or IMRT treatments for head and neck, brain, breast, lung, thorax, abdomen and pelvis where scheduled for in vivo dosimetry procedure with EPID. A commercial software (SOFTDISO, Best Medical, Italy) was used at this purpose. Two indexes were analysed: the ratio R between the reconstructed (Diso) and planned (Dtps) isocenter dose (R=Diso/Dtps) and Pγ% obtained performing a gamma analysis between the first EPID image and the next ones acquired. The acceptance criteria adopted for the ratio R was ±5%, while for the 2D γ-analysis in term of Pγ index, we adopted Pγ > 90% with a passing criteria of 3% global difference and 3mm distance to agreement for head and neck treatment and 5%, 5mm for the others districts. The percentage of patients P% with Rmean and Pgmean in the tolerance level P%(Rmean) P%(Pγmean)respectively, and the percentage of IVD test T% with R and Pγ in the tolerance level T%(R) and T%(Pγ), were evaluated. For each district P% take into account the patients with the mean values of the indexes within the tolerance levels, while the T% is referred to the number of tests. If one of the indexes resulted out of tolerance, corrective actions were performed and the test repeated at the next fraction. Results The results of 1211 IVD tests over 64 patients, were reported in Table 1. All the patients analysed shown both indexes (Rmean and Pγmean) in tolerance with the exception of breast and thorax treatments. For VMAT and IMRT thorax treatments P%(Pγ) decreased to 67%. The thorax patients were revised considering the high gradient regions of the isocenter and the positioning set up was optimized. For IMRT breast treatment, P%(Pγ) decreased to 50%: two (over four) IMRT breast patients were revised adjusting the bolus positioning over the mask in order to realign the reproducibility of the treatment (Pγ index) in the tolerance level. Adopting the appropriate corrections, the successive IVD tests guaranteed at the end of the treatment P% values within the tolerance levels. For thorax and breast treatments, due to the limitation of IVD tests acquired, the mean P%(Py) index values after the correction, were again out of tolerance but the effect of

the

correction

was

always

efficient.

Conclusion IVD with EPID, is a powerful tool that can be inserted in an overload radiotherapy department. It can be helpful daily to monitor the accuracy of the treatment and enable a quickly correction of misalignment or discrepancies occurred during the treatment course. EP-1655 Improved patient setup for breast cancer patients using the predicted (absolute) couch position. M. Essers 1 , S. Hol 2 , I. Maurits 2 , W. Kruijf 1 1 Dr. Bernard Verbeeten Instituut, Department of Medical Physics, Tilburg, The Netherlands 2 Dr. Bernard Verbeeten Instituut, Radiotherapy, Tilburg, The Netherlands Purpose or Objective Usually, patient setup is performed by obtaining a reference position at the first treatment fraction (“relative couch position”) and then applying on-line or off-line setup protocols . In our institute, a method is used in which the couch position is predicted before the treatment (“absolute couch position”) 1 . The purpose of this work was to investigate whether the patient setup for breast cancer patient is improved using the ‘absolute At the time of this study, accurate patient setup was ensured by applying an on-line setup protocol using the patient anatomy (mainly vertebrae, lung tops and sternum) visible on orthogonal (AP and lateral) MV images, and checking the residual deviation of the lung wall on an MV image in the direction of the mediolateral tangential field (ML image). 1. For 83 patients positioned using relative couch positioning as well as 83 patients positioned using absolute couch positioning, the difference in image registration (MV images compared to DRRs) using sternum only or vertebrae only was determined. 2. For the same patients, the residual deviation of the lung wall on the ML image was determined. Results 1. Using relative couch positioning, the difference between sternum and vertebrae match was smaller than 2 mm for 80% and larger than 5 mm for 12% of the fractions, and for absolute couch positioning, 90% of the fractions showed a match difference smaller than 2 mm and 7% larger than 5 mm. These figures indicate that the patient couch position”method. Material and Methods