Search

Introduction Stereotactic radiotherapy is known to deliver a very high dose per fraction. The corresponding peripheral dose has to be taken into consideration as it increases late toxicities and the risk of second cancer. A recent study has been conducted for Cyberknife® (CK), evaluating peripheral doses according to collimator type and aperture [1] . The purpose of this study is to compare peripheral doses during brain stereotactic treatments with two linear accelerators: Versa HD® (Elekta) equipped with Exactrac® (Brainlab) versus Cyberknife® M6™ (Accuray). Methods The doses were measured with GR 200A (LiF:Mg, Cu, P) thermoluminescent dosimeters (TLD). Each TLD was individually calibrated in a 6 MV beam. The TLD responses were analysed with a PCL3 automatic reader (FIMEL). The doses were measured for several points representing organs at risk: thyroid, breast, umbilicus and gonads. The peripheral dose evaluation in Versa HD® was performed on an anthropomorphic phantom (CIRS ATOM 701-c) for 10 treatment plans, calculated with Pinnacle v9.10 (Philips) with 4 VMAT non-coplanar arcs delivering 30 Gy (in 3 fractions). The peripheral doses in CK® were in vivo measurements for 23 patients with a 3-fractions treatment (total dose between 24 and 30 Gy). Treatment plans were performed with Multiplan TPS v5.3 (Accuray) with 3 collimator types (fixed, Iris™ and MLC Incise2™). The peripheral doses were expressed as a proportion of dose per monitor unit (% cGy/UM). Results Table 1 compares the peripheral dose for both linear accelerators. The doses measured in the thyroid were 2 to 3 times higher in VersaHD® treatments than in CK®: 0.16% cGy/UM and 0.05% cGy/UM (mean for 2 collimators type), respectively. The breast, umbilicus and gonads dose measurements were not significantly different between both equipment. With CK® the thyroid doses were 2 to 3 times higher with MLC than with Iris™ and fixed collimators. Due to the fact that CK treatment plans need 2.7 times more MU, the thyroid doses measured for the entire treatment were not significantly different between 2 linear accelerators: 76.5 mGy with VersaHD® instead of 61.5 mGy with CK® (mean for 3 collimators types). This dose decreased with distance in VersaHD®: 6 mGy delivered to the gonads instead of 32 mGy with CK® (mean for 3 collimator types). Download : Download high-res image (269KB) Download : Download full-size image Conclusions For a same dose prescribed to the target volume, the peripheral dose delivered during treatment depends on accelerator type, collimator type and also MU number. The doses delivered to the thyroid are equivalent for both accelerator types and can reach 75 mGy. However, VersaHD® stereotactic treatments can divide by five the peripheral dose at a large distance compared to CK®. The peripheral dose should therefore be evaluated in paediatric and benign treatments and should be a criterion for the optimisation of treatment plans.

Introduction The Regional Cancer Center of Rennes (Centre Eugene Marquis, CEM) is one of 31 French institutions hosting students preparing the final diploma of medical physicist (DQPRM) for 22-month internship in the three area of medical physics: radiotherapy (RT), nuclear medicine (NM) and radiology (RX). The CEM is accredited by the National Institute for Nuclear Science and Technology (INSTN) to train two students in the first year and two students in the second year. The objective is to share the feedback from the CEM on the management of students during these 22 months of internship. Methods During the first week of the internship, a meeting of the entire physics team is organized to introduce the DQPRM students to all the members of the team, to plan the weeks dedicated to each field and to define the reference physicist for each activity to be acquired. During this meeting the general organization of the team and the specific organization for the DQPRM students are presented. The scientific work of the second year is defined among the list of projects of the team in dialogue between the student and the referent physicist of the training center. The study is managed by a senior physicist specialized in the area of the work. Some parts of the document presenting the functional schedule of the team are dedicated to the DQPRM students to reveal clearly their participation in the clinical activities. The students are invited to all meetings in which the medical physicists participate. A bi-monthly point of 30 min is organized between the students and the referent physicist of the training center. Results During the first month, the students have to define with the referent physicist for every activity the annual organization to acquire the expected skills. Regarding to functional schedule of 2017, the students of 1st year spent 49% of days in RT, 20% in NM and 10 in RX. These percentage were 51%, 16% and 11% respectively for the students of 2nd year. A delegation of some functional tasks are possible after validation of specific skills base. Depending of the subject of the scientific work performed during 2nd year of training, writing of the report is encouraged in English in order to facilitate the submission in an international journal. Conclusions Concurrently to the acquisition of basic skills of the medical physicist, the objective of the 22 months of internship is to transform the DQPRM students in a young professional in a progressive and supervised way by teaching them the teamwork, the empowerment and time management thanks to clinical activities and projects.

Introduction Recently, in-treatment 4D cone-beam CT (4D-CBCTin-treat) has been implemented in clinic allowing the acquisition of respiratory correlated CBCT projections concurrently with the beam delivery. With this imaging modality, the actual 4D target locations during treatment can be visualized at the end of each fraction. 4D-CBCTin-treat provides reliable information to evaluate actual target coverage during treatment and to verify planning target volume (PTV) adequacy for lung stereotactic body radiotherapy (SBRT). The purpose of this study was to compare in-treatment target coverage for two motion-encompassing PTV strategies: an internal target volume (ITV) based approach and the mid-ventilation (MidV) based approach. Methods Data from 27 lung cancer patients treated with a frameless SBRT technique using a volumetric modulated arc therapy (VMAT) were selected. For each patient, a 4D-CT was used for treatment planning. For the ITV-based approach, we applied the PTV setting currently used at our institute for lung SBRT: ITV was determined by contouring the target on 10 respiration phases and a generic 4 mm margin was then added to create the PTV. For the MidV-based approach, PTV was defined around the tumor closest to its time-weighted mean position (MidV CT phase) using the Van Herk margin recipe [1] . At each fraction, the target was localized using a 4D-CBCT on-line registration procedure and a 4D-CBCTin-treat was acquired during the VMAT delivery. The clinical target volume (CTV) was delineated in 10 respiration phases on all 4D-CBCTin-treat images reconstructed during the treatment course. We defined target coverage per treatment as the percentage of the CTV included within the PTV averaged over respiration phases and fractions. Results A total of 93 4D-CBCTin-treat were delineated. Mean target coverage per treatment was 98.6% (min: 93.4%) and 98.9% (min: 93.5%) for ITV and MidV approaches respectively. Mean PTV volume was 18.1 cc (min: 2.9 cc, max: 50.7 cc) and 18.6 cc (min: 5.4 cc, max: 49.9 cc) for ITV and MidV approaches respectively. Compared to the MidV, the ITV-based strategy using a 4 mm PTV margin resulted to a non-statistically significant difference neither for target coverage (p = 0.401) nor for PTV volume (p = 0.186). Conclusions With a fast VMAT delivery technique and a 4D image guidance protocol, a 4 mm PTV margin can be safely applied when using the ITV-based strategy for frameless lung SBRT. This ITV-based PTV setting provides equivalent irradiated volumes and tumor coverage than the MidV approach.

Introduction The objective of this study is to compare the dosimetric and geometric performances of the CyberKnife (Accuray) and VersaHD (Elekta) with the ExacTrac system (BrainLab) in stereotactic radiotherapy for brain metastases. Material and method This study was conducted on 5 patients for a prescribed dose of 27 Gy in 3 fractions with a margin of 1 mm between CTV and PTV for the Cyberknife and 2 mm for the VersaHD in accordance with our clinical practices. The dosimetric study was also conducted with a 2 mm margin for the Cyberknife to assess the dosimetric impact of the margins. Plans have been computed for the CyberKnife M6 v10.6 with non-isocentric multi-beams generated by inverse optimization on Multiplan v5.3 (Accuray). For the VersaHD equipped with ExacTrac v6.1 and used with the Frameless system (BrainLab), 4 non-coplanar arcs (VMAT) has been generated by inverse optimization on Pinnacle v9.10 (Philips). The dosimetric evaluation focused on the coverage of PTV and volume of brain outside PTV receiving 23.1 Gy corresponding to the main criteria of medical approval. The volume of isodoses 6 Gy, 2.7 Gy and 1 Gy and the ratio of the volume of isodose 13.5 Gy and 27 Gy have been reported to characterize the spread of low doses and the dose gradient around the PTV. Quality controls have been performed with radiochromic films (EBT3/Ashland) and with an ionization chamber (Pinpoint 31014/PTW) in an anthropomorphic phantom (STEEV/CIRS). The dose measured by film has been compared to the calculated dose according to the method of the gamma index with 3% (local)/2 mm criteria (analytical threshold: 30% of the maximum dose). The geometric difference between the measured and calculated dose distribution has been also reported. Results Table 1 shows that validation criteria were reached for both modalities. For an equivalent PTV coverage, dose gradients obtained with the Cyberknife were greater than those obtained with the VersaHD and lower volumes of healthy tissue received doses lower than 6 Gy. The mean differences between the dose measured by ionization chamber and the calculated dose were 2.3% and − 0.5% with Cyberknife and VersaHD respectively. The mean value of the gamma index was 0.41 for the Cyberknife and 0.35 for the VersaHD. The mean geometric differences between the measured and calculated dose distributions were 0.86 mm and 0.50 mm Cyberknife to VersaHD. Conclusion Both treatment modalities, Cyberknife and VersaHD with ExacTrac, allow compliance to dosimetric criteria in the stereotactic treatment of brain metastases. The Cyberknife provides higher dose gradients than the VersaHD and limit irradiation of healthy tissues at low doses. The agreement between calculated dose and measured dose was acceptable for both modalities with mean gamma values lower than 0.5. An investigation will be performed to evaluate low margins (1 mm) with the VersaHD/ ExacTrac due to the very low geometric deviations.

Introduction The CyberKnife® (Accuray) is a stereotactic robotic system using historically circular collimators to define the beam apertures. A new multileaf collimator (MLC) is now available on the Cyberknife® M6 version. The purpose of this study was to evaluate the quality of treatment plans using MLC compared to treatment plans using a variable circular aperture collimator IRIS™ for radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Methods The study was performed on a Cyberknife® M6™ v10.6. A total of 17 cases including cranial, liver, spinal and prostatic cases were investigated. For each case, two treatment plans were generated with Multiplan® v5.3 treatment planning system: one plan using the Incise2™ MLC associated with the Finite Size Pencil Beam (FSPB) dose calculation algorithm and one using the IRIS™ collimator associated with Ray Tracing or Monte Carlo dose calculation algorithm. The dose was prescribed near the 80% isodose level relative to max dose. Plans were normalized to have the same target coverage at ±0.5%. The quality of the MLC and IRIS™ treatment plans was assessed using two criteria: - Dose distribution by reporting the conformal index (COIN), Homogeneity Index (HI) and dose gradient index (GI). - Efficiency by reporting the number of delivered monitor units (MUs) and the treatment time. To evaluate the accuracy of dose calculation algorithm within heterogeneity, a typical lung treatment plan for each collimator was created on a QUASAR™ Respiratory phantom (Modus Medical Inc) with a lung target insert. Calculated planar dose was compared with measurements by radiochromic EBT3 films (Ashland). The analysis was done using gamma analysis 3% (local) / 2 mm (threshold: 30% of the max dose) Results Results are summarized in Table 1. Dose conformality and homogeneity were equivalent for both collimator systems (mean differences less than 3%) except for targets with volumes smaller than 2 cm3 for which dose conformality was better of 14% on average with the IRIS collimator. The dose gradient index was improved by 10.5% on average with the MLC and up to 26% on average for volume larger than 100 cm3. The number of MUs decreased by 25% on average (up to 70%) and the treatment times were systematically reduced with a mean reduction of 28% (max 54%) with the MLC. The comparison between calculated and measured dose in lung phantom showed a gamma passing rate of 51.6 %, 45.5 % and 98.7% for FSPB, Ray Tracing and Monte Carlo respectively. Conclusion The MLC Incise2™ on Cyberknife® allows a significant reduction of MUs and treatment time compared to circular collimators while maintaining a high degree of conformality and a steep dose gradient. For treatment of target volumes smaller than 2 cm3, circular collimators should be still used. The use of MLC for SBRT of lung targets should not be recommended currently due to the absence of type B dose calculation algorithm.

Introduction: We investigated the influence of several parameters: i) the angular control points spacing, j) the minimum opposing leaf separation, k) the number of arcs, on dosimetric agreement between calculated and delivered doses for Volumetric Modulated Arc Therapy (VMAT). Materials and methods: VMAT plans were calculated with Pinnacle/ Philips TPS V9.0 (P3IMRT/SmartArc). The plans were delivered on Synergy/Elekta linear accelerator (MLCi2). We investigated plans with two levels of complexity: 5 Head and Neck cases with simultaneous integrated boost (SIB-H&N) and 4 prostate cases. The influence of control points spacing was analyzed for 2 , 4 and 6 , the influence of the minimum opposing leaf separation for 0.5, 1 and 2 cm and the influence of the arcs number for one and two arcs (360 each). Measurements were acquired with ionization chambers matrix (2Darray-Seven29) with Octavius phantom and Verisoft V4.1 analyze software (PTW). Dosimetric agreement between calculated and delivered doses was analyzed with gamma index test (g-index) calculated from 2%/2mm to 5%/5mm. Studied parameters were percentage of accepted points, mean, median and maximum gamma. Results: The percentage of accepted points with 3%/3mm g-index (threshold 30%) decreased when control points spacing increased: it decreased of 2% for 4 and 12% for 6 regarding to 2 spacing for prostate cases, and respectively of 17% and 47% for SIB-H&N cases. For SIB-H&N cases, the percentage of accepted points with 3%/ 3mm g-index (threshold 30%) increased of 6% and of 11% when minimum opposing leaf separation increased from 0.5cm to respectively 1cm and 2cm. The percentage of accepted points with 3%/3mm g-index (threshold 30%) increased of 3% for prostate cases and of 20% for SIB-H&N cases when the number of arc increased from 1 to 2. The parameters influence was analyzed too on quality of the dose distributions calculated by the TPS. Conclusion: For VMAT, higher the degree of modulation was, more important the parameters influence was: when the control points spacing increased from 2 to 4 , the percentage of accepted points with 3%/3mm g-index (threshold 30%) decreased respectively from 98% to 96% for prostate cases and it decreased respectively from 97% to 80% for SIB-H&N cases. In VMAT, the arc parameters had to be selected first on dosimetric agreement between calculated and delivered doses and then on the quality of the dose distributions calculated by the TPS.