P30. Experience feedback of a radiation protection incident in radiotherapy: Irradiation of a pregnant patient

A. Batalla et al. / Physica Medica 32 (2016) 367–383

already show the software’s capabilities to detect the variations as well as its limitations. Future work remains to improve precision with smaller step shifts, rotation and reduction of bolus measurements. http://dx.doi.org/10.1016/j.ejmp.2016.11.039

P28. A set of HU for ‘‘Exact IGRT” couch in Eclipse TPS F. Mazoyer, M. Gonod, A. Petitfils, F. Bidault, S. Naudy Medical Physics Department, G.F. Leclerc Center, Dijon, France Introduction. The planning system Eclipse contains a couch model library. The objective of this study is to determine the HU parameters to apply for the Exact IGRT couch. The validity of these parameters was also examined with a comparison of the impact of the couch on the calculated and measured dose. Materials and methods. Couch modeling of the ‘‘Exact IGRT” is customizable with two adjustable parameters by user: the Hounsfield units of the internal structure and those of external structure. A CT scan image was acquired and HU of these structures was measured. The obtained set of HU was used for dose calculation with couch modeling. Impact of the presence of the dose of 6 MV and 18 MV direct beams was evaluated with a calculation performed with Acuros XB V11 (Varian) with and without the couch. Results are compared to the measure. The thin and thick models were considered. Results. Hounsfield units of the internal structure were measured at 945 ± 5. Those of the external structure were measured at 300 ± 20. Differences between the calculated dose with and without thin couch modeling were respectively 1.8% and 1.0% for the 6 MV and 18 MV beams. For the thick model, differences were 2.4% and 1.4%. During measurements, the difference was established at 1.8% and 1.1% for the thin part of couch (landmark H4) and 2.1% and 1.0% or the thick part of couch (landmark H0). Conclusions. The results show a good correlation between calculated and measured dose difference. Geometrical model build-in the planning system Eclipse associated with the determined HU enable a correct modeling of treatment couch. http://dx.doi.org/10.1016/j.ejmp.2016.11.040

P29. Proposal of technique for whole-brain radiotherapy with hippocampal sparing F. Mazoyer a, G. Truc b, F. Bidault a, M. Gonod a, S. Naudy a a b

Medical Physics Department, G.F. Leclerc Center, Dijon, France Radiotherapy Department, G.F. Leclerc Center, Dijon, France

Introduction. Irradiation of the hippocampi is responsible for neurocognitive deficits, whereas metastasis incidence in this part of brain is low [1]. In this study, the objective is to develop a treatment technique with acceptable dosimetric coverage of the brain while minimizing the dose to hippocampi. Materials and methods. Treatment planning of 10 patients was performed. Volumes were delineated according to the RTOG 0933 protocol. The prescribed dose was 30 Gy in 10 fractions and plans were normalized to the average of the PTV, minus the hippocampi’s PRV. For each dosimetry, goal was minimize the value of D40% and the maximum dose at hippocampi without unacceptable degradation of PTV’s coverage. VMAT technique is proposed with 5 non coplanar arcs. Isocenter is positioned at the gravity center of the two hippocampi. The first pair of arcs consist of two whole arcs with 10° couch rotation and 30° collimator rotation. The second pair of arc

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is identical but with an opposite rotation of table and collimator. Each arc of one pair has an asymmetric collimation so that one of the jaws, having a direction of movement identical to MLC leaves, is closed about 1 cm from the axis of the beam. The sum of two arcs of one pair covers the PTV. The fifth arc with table rotation and collimator rotation at 90° is an half-arc. It is adjusted so that the edges of jaws with MLC intersect the hippocampal volumes. For each patient, this technique was compared with an usually VMAT technique, based on 2 coplanar wholes arcs in clockwise and counterclockwise directions and collimator rotation at 30° and 330° respectively [2]. For each optimization, we used a dummy structure, between hippocampi, to control the dose. Results. The average gain on the D40% of the hippocampus is 1.5 Gy and 3.2 Gy on the maximum dose. There is also an increase on the D98% of the PTV of 0.7 Gy and a diminution of 0.35 Gy to the D2%. The V95% is improved by 2%. Conclusions. This technique with 5 arcs, although time-consuming during the treatment, seems to show a superiority in the hippocampal avoidance, but also in the covering of whole brain.

References 1. Truc & al., Quelle place pour l’irradiation panencéphalique avec épargne des hippocampes ?, Cancer Radiothérapie, n° 117, pp. 419–423, 2013. 2. Prokic & al., Whole brain irradiation with hippocampal sparing and dose escalation on multiple brain metastases: a planning study on treatment concepts, Int. J. Radiat. Oncol. Biol. Phys., 85, n° 11, pp. 264–270, 2013. http://dx.doi.org/10.1016/j.ejmp.2016.11.041

P30. Experience feedback of a radiation protection incident in radiotherapy: Irradiation of a pregnant patient A. Batalla a, B. Menard a, B. Gery b a b

Centre Francois BACLESSE – Service de Radiophysique, Caen, France Centre Francois BACLESSE – Service de Radiotherapie, Caen, France

Introduction. A woman, treated by radiotherapy in the Centre Francois Baclesse in Caen for a head and neck cancer was proving pregnant (3rd month of pregnancy during the treatment). The pregnancy was revealed after the completion of the treatment (30 fractions – 2 Gy / fraction). In order to know the dose received by the fetus, measurements have been achieved with identical conditions as for the treatment. Materials and methods. Irradiation technique: VMAT on a Clinac iX (Varian) linac. 2 arcs of 307 UM and 278 UM with 6 MV photons. Phantom: Compact polystyrene to simulate the body and head and neck cylindrical phantom to simulate the irradiated volume. The ionization chamber was positioned at 45 cm of the field edge. This distance was measured by the medical doctor on the patient. The measurements were achieved with the same parameters recorded on the ”record and verify” software (Mosaiq – Elekta). The measurement cumulated 5 repetitions to improve the accuracy. Results. For 1 fraction: 0.83 mGy For the whole treatment (30 fractions): 24.9 mGy Discussion. 1/The dose far from the irradiation field has 3 components [1] -Photons scattered by the irradiated volume of the patient (internal) -Photons scattered by the irradiated materials (couch) and the walls of the treatment vault -Leakage radiation from the linac

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2/The 1st component of the dose (scattered photons inside the patient) is very low because of the large distance between the field edge and the measuring point. In the ICRP report #84 (Pregnancy and medical radiation) [2] off axis values of dose are given for cobalt 60:approximately 0.25% at 45 cm but for linacs this off axis dose is lower 2–5 times (depending on beam energy). Our measurements (0.04%) are so, in good agreement with the ICRP #84 data. In the literature, several authors reported equivalent fetal doses for head and neck irradiations [3][4][7]. For irradiated volumes closer to the uterus (Hodgkin desease), the doses are obviously much greater [5][6]. Conclusion. The value of the effective dose measured ( 25 mSv) is superior to the upper legal limit for the fetus (1 mSv). In consequence, this case was declared as an incident to the French Nuclear Safety Authority (ASN) but it is assumed not to be critical for the children: – Malformations have a threshold of 100–200 mGy or higher and are typically associated with central nervous system problems [1] – A fetal dose of 100 mGy has a small individual risk of radiationinduced cancer. There is over a 99% chance that the exposed fetus will NOT develop childhood cancer or leukaemia [2]

References 1. Fetal dose from radiotherapy with photons beams: Report AAPM TG n°36 MP vol. 22–1 jan1995. 2. Pregnancy and medical radiation. ICRP 84. 3. Fetal radiation monitoring and dose minimization during intensity modulated radiation therapy for glioblastoma in pregnancy. Horowitz DP J. Neurooncol. 2014. 4. A study of the shielding used to reduce leakage and scattered radiation to the fetus in a pregnant patient treated with a 6-MV external X-ray beam - Han, Health Phys. 2009. 5. In utero exposure to therapeutic radiation for Hodgkin lymphoma. Klieger-Grossmann - Can Fam Physician. 2009. 6. High fetal irradiation: about one pregnant woman receiving infradiaphragmatic radiotherapy for Hodgkin lymphoma - Moreau - Cancer Radiother. 2007. 7. Determination of possible doses to the gonads or fetus in pregnant patients during radiation therapy, Lin FJ, 1989.

(to export beacons info from CT scan, import etc). Commissioning QA involved checking: the system calibration and accuracy (isocenter Calypso R versus linac), the optical system monitoring the electromagnetic array, the localization and tracking functionality, the dynamic edge gating, the correct creation of the reports and beam on radiation detection. Daily QA is performed with the QA fixture phantom and system calibration uses the Isocenter calibration fixture, both delivered with the system. Five patients with different targets are actually under treatment with volumetric modulated arc therapy plans (2 pts for whole prostate gland, 2 pts for pelvic lymph nodes + prostate and 1 pt for prostatic bed). Beacons are implanted with a trans-perineal procedure (not trans-rectal, as suggested by vendor). All patients are positioned supine with a feet immobilization and a knee cousin element on the dedicated Kevlar table top. Calypso R is first used to position the patient and to monitor CBCT image acquisition, for patient overall positioning, and after for patient tracking. Results. Commissioning was made in half a day, with values within vendor’s specifications (Calypso R-Linac isocenter< 0.1 cm). Daily QA shows a stability of the system (over more than 50 days, Calypso R-Linac isocenter < 0.13 cm). Concerning patients, no beacons implant complications was registered, nor transponder migration seen on CBCT images or detected by Calypso R, even after several weeks of treatment. All patients could be treated using ”absolute” isocenter localization and tracking. Care was needed to position the array to avoid possible knee collisions if readjusting the patient or moving the table. When patient’s bladder and rectal filling reproduce simulation conditions, only 5 min extra are necessary to the session for array positioning, tracking and imaging. Clearly, one patient is reproducible on a daily basis (within 0.3 cm) and one patients shows a continuous drift of the tracking curve. Other patients do not present a distinct tracking curve behavior. Conclusions. Overall Calypso R usage is easy and fast. A lower knee cousin will be used in the future to increase clearance between patient and array. Calypso R, detecting intra-fraction prostate movements, is introducing in our department a new way to perceive organ motion. http://dx.doi.org/10.1016/j.ejmp.2016.11.043

http://dx.doi.org/10.1016/j.ejmp.2016.11.042

P32. Impact of CT acquisition parameters on the electronic density curve M. Gonod, F. Mazoyer, I. Bessière, S. Naudy

P31. Commissioning, quality assurance and initial experience of a Calypso R system: Geneva experience G. Dipasquale

Service Radiophysique – CGF Leclerc, Dijon, France

Hopitaux Universitaires de Geneve, Geneva, Switzerland Introduction. The Calypso R system (Varian medical systems), that uses radiofrequency signals for wireless tumor localization (absolute or relative to a zero set on the day) and tracking during radiotherapy, was installed in Geneva in 2015. Since January 2016 it is used to treat prostate patients (pts) with dynamic edge gating (allows to set motion thresholds which disable radiation delivery if the beacons, representing the target, move outside these thresholds). Here we present our early experience with its use with prostate implanted beacons as well as commissioning results and daily quality assurance (QA). Materials/methods. For commissioning, specific tools were made available by the vendor. Furthermore our Rando R phantom was implanted with prostatic transponders to verify Calypso R versus cone beam computed tomography (CBCT) beacon’s position and isocenter correspondence as well as to realize an end-to end QA test

Introduction. The electronic density curve is the basis of dosimetric calculation. This affects the dose calculated in planning. We purpose to study the variations of the CT Number generated by different acquisition parameters. Materials and Methods. We used Lightspeed RT16 (GE) at 120 kV. The first point was to check the performance of the scanner through the water phantom (diameter 32 cm) provided by the manufacturer. Repeatability, reproductibility, uniformity, noise and water CT number were explored. In a second step we evaluated the CT number variations due to different acquisition parameters using phantom CIRS 062 M with various electron densities. The 2 geometry (16 cm and 32 cm diameter) were tested. Following parameters were studied for FOV (Field of View) Large and Small: - Differences between two identical density insert; - Insert position; - Acquisition type: axial or helix; - Tube rotation time: 0.8–4 s;