S. Teodoli

Hybrid Tri-Co-60 MRI radiotherapy for locally advanced rectal cancer: An in silico evaluation

Highlights • Tri-Co-60 MRI radiotherapy (RT) is feasible in locally advanced rectal cancer.• Larger volumes of normal tissue receive low-moderate doses in Tri-Co-60 MRI RT.• Further studies on functional imaging applications and LinacMRI approach are needed.• Tri-Co-60 MRI might represent a safe irradiation technique for pelvic tumors.

Predicting tumour motion during the whole radiotherapy treatment: a systematic approach for thoracic and abdominal lesions based on real time MR

INTRODUCTION Aim of this study was to investigate the ability of pre-treatment four dimensional computed tomography (4DCT) to capture respiratory-motion observed in thoracic and abdominal lesions during treatment. Treatment motion was acquired using full-treatment cine-MR acquisitions. Results of this analysis were compared to the ability of 30 seconds (s) cine Magnetic Resonance (MR) to estimate the same parameters. METHODS A 4DCT and 30 s cine-MR (ViewRay, USA) were acquired on the simulation day for 7 thoracic and 13 abdominal lesions. Mean amplitude, intra- and inter-fraction amplitude variability, and baseline drift were extracted from the full treatment data acquired by 2D cine-MR, and correlated to the motion on pre-treatment 30 s cine-MR and 4DCT. Using the full treatment data, safety margins on the ITV, necessary to account for all motion variability from 4DCT observed during treatment, were calculated. Mean treatment amplitudes were 2 ± 1 mm and 5 ± 3 mm in the anteroposterior (AP) and craniocaudal (CC) direction, respectively. Differences between mean amplitude during treatment and amplitude on 4DCT or during 30 s cine-MR were not significant, but 30 s cine-MR was more accurate than 4DCT. Intra-fraction amplitude variability was positively correlated with both 30 s cine-MR and 4DCT amplitude. Inter-fraction amplitude variability was minimal. RESULTS Mean baseline drift over all fractions and patients equalled 1 ± 1 mm in both CC and AP direction, but drifts per fraction up to 16 mm (CC) and 12 mm (AP) were observed. Margins necessary on the ITV ranged from 0 to 8 mm in CC and 0 to 5 mm in AP direction. Neither amplitude on 4DCT nor during 30 s cine MR is correlated to the magnitude of drift or the necessary margins in both directions. CONCLUSION Lesions moving with small amplitude show limited amplitude variability throughout treatment, making passive motion management strategies seem adequate. However, other variations such as baseline drifts and shifts still cause significant geometrical uncertainty, favouring real-time monitoring and an active approach for all lesions influenced by respiratory motion.

Experimental evaluation of the impact of low tesla transverse magnetic field on dose distribution in presence of tissue interfaces

PURPOSE Aim of this study is to experimental evaluate the impact of a 0.35 T transverse magnetic field on dose distribution in presence of tissue-air and tissue-lung interfaces. METHODS The investigation was carried out using MRIdian (ViewRay, Cleveland, Ohio) and it consisted of comparing experimental measurements performed by Gafchromic EBT3 film dosimetry, to Montecarlo simulations, carried out in the presence and, as well as, the absence of the magnetic field. A preliminary dose calibration was planned on MRIdian, arranging 3 × 3 cm2 film pieces in a water slab phantom and exposing them at different beam-on times, in a dose range equal to 0.1-12.1 Gy. All experimental measurements were then carried out using the calibrated films and delivering one single beam orthogonally to three different phantoms: without inhomogeneity, with an air gap and with a lung inhomogeneity. The dose distributions measured by EBT3 films in presence of magnetic field were compared to those calculated in the presence and, as well as, the absence of the magnetic field, in terms of gamma analysis. A quantification of electron return effect (ERE) was also performed. RESULTS All the tested plans considering the magnetic field show a gamma-passing rate higher than 98% for 3%/3 mm gamma analysis. In presence of tissue-air interface, the electron return effect causes an over-dosage of +31.9% at the first interface and an under-dosage of -33% at the second interface. The dosimetric variations in presence of tissue-lung interface results to be smaller (+0.8% first interface, -1.3% second interface). CONCLUSION The impact of 0.35 T magnetic field is not negligible and it can be effectively modelled by the Montecarlo dose calculation platform available in the MRIdian TPS.

Use of Indirect Target Gating in Magnetic Resonance-guided Liver Stereotactic Body Radiotherapy: Case Report of an Oligometastatic Patient

The case of a 73-year-old woman affected by anal canal cancer with concomitant liver metastases is presented here. The patient was addressed to stereotactic body radiotherapy (SBRT) on two hepatic secondary lesions after the first radiochemotherapy treatment of the primary tumor. A Tri-60-Co magnetic resonance hybrid radiotherapy unit was used for SBRT treatment delivery. Both liver lesions were not clearly visible on the setup magnetic resonance imaging (MRI) due to their limited dimensions (maximum diameter 13 mm); however, the presence of two cysts adjacent to the metastases allowed the use of an indirect target gating approach. Treatment was delivered in deep inspiration breath-hold conditions using the visual feedback technique for breathing control optimization. Post radiotherapy imaging assessed the complete response.

EP-1798: Is there a true dosimetric improvement in lung SBRT using a 6-Degree of Freedom couch in IGRT era?

Material and Methods: The planning CT scans (CT0) and 7 per-treatment weekly CT scans of 20 patients having received IMRT for prostate cancer were used. The main pelvic vessels were manually delineated: common iliac (CI), external iliac (EI) and internal iliac (II) of both sides. The central lines of the vessels were first defined thanks to a 3D workstation (EndoSize®, Therenva) dedicated to the preoperative sizing before endovascular interventions. A pelvic bone registration was then performed. For a given vascular segment, the distance between its central line CL0 from CT0 and its central line CLi from the weekly CTs were calculated.The central line CL0 of each vascular segment was sampled every mm. The distance corresponded to the mean value of the distances between corresponding points of the two central lines (CL0 and CLi). The correspondance was established by considering the cross-section plane othogonal to CL0 at a given point and its intersection with CLi. For each patient, the mean and the standard deviation (SD) of the measurements of the 7 fractions were determined. The systematic error (∑) of the whole population was calculated as the SD of the mean values. The random error (σ) of the whole population was calculated as the root mean square of the standard deviation values. The margins were calculated both with M. Van Herk formula (IJROBP 2000) and by geometrically computing margins covering 99% of the vessels displacements.

PO-1008: In silico implementation of MRI-60Co RT. A dosimetrical comparison in cervical cancer (SIMBAD-02)

Material and Methods: 25 Patients were included, with regional nodes level I-IV. Twelve were left-sided with breathhold treatments. All were planned with both original 3DCRT and a new hybrid 3DCRT-IMRT techniques. Delineations were made according to ESTRO guidelines. Comparison was based on DVH parameters for OARs, namely lung, heart, oesophagus, contra-breast (eg V20, Dmean) and the PTV (V95%, D2%, D98%, conformity). Analysis was performed using SPSS. Further analysis focussed on the efficacy for breathhold treatments and efficiency in planning and delivery.

EP-1950: Monte Carlo dose calculation of Viewray hybrid MRI-Co60 radiotherapy system: a repeatability study

Conclusion: This study shows an high consistency of small field dosimetry in the involved radiotherapy departments using this new generation silicon diode; consequently, the values reported may actually be used by other centers as indicative values, especially in the case of small fields when suitable detector are not commonly available. Moreover, these results confirm that the new RAZOR silicon diode can be used to assess dosimetric accuracy in small-field delivery. In general, the adopted methodology removes much of the ambiguity in reporting and interpreting small field dosimetric quantities and facilitates a clear dosimetric comparison across a population of linacs.