S. Graydon

Clinical application of flattening filter free beams for extracranial stereotactic radiotherapy.

PURPOSE To investigate the clinical application of flattening filter free (FFF) beams at maximum dose rate for stereotactic body radiotherapy (SBRT). METHODS AND MATERIALS Patients with tumors in the lung or abdomen were subjected to SBRT using 6 MV FFF or 10 MV FFF beams. For each patient, three plans were calculated using 6 MV flattened, 6 MV FFF, and 10 MV FFF beams. Treatment times were recorded and analyzed, and tumor displacements were assessed by pre- and post-treatment cone beam computed tomography (CBCT). RESULTS Altogether, 26 patients (16 lung, 10 abdominal tumors) were treated. The average dose rate per patient ranged from 442 to 1860 MU/min. Beam-on time was on average 1.6 min (1SD=0.6 min), with the total treatment times recorded at 18.5 min (1SD=3.5 min). The time advantage of using FFF beams was dose-dependent and started at 4 Gy for 6 MV FFF and at 10 Gy for 10 MV FFF beams. The average of the tumor displacements during treatment was 2.0mm (1SD = 1.0mm). CONCLUSIONS SBRT using FFF beams is time efficient and associated with excellent patient stability. According to Van Herks formula, ITV-PTV margins of 6mm are sufficient in our patient cohort. Further studies are necessary to assess clinical outcome and toxicity.

Clinical evaluation of an anatomy-based patient specific quality assurance system

The Delta4DVH Anatomy 3D quality assurance (QA) system (ScandiDos), which converts the measured detector dose into the dose distribution in the patient geometry was evaluated. It allows a direct comparison of the calculated 3D dose with the measured back‐projected dose. In total, 16 static and 16 volumetric‐modulated arc therapy (VMAT) fields were planned using four different energies. Isocenter dose was measured with a pinpoint chamber in homogeneous phantoms to investigate the dose prediction by the Delta4DVH Anatomy algorithm for static fields. Dose distributions of VMAT fields were measured using GAFCHROMIC film. Gravitational gantry errors up to 10° were introduced into all VMAT plans to study the potential of detecting errors. Additionally, 20 clinical treatment plans were verified. For static fields, the Delta4DVH Anatomy predicted the isocenter dose accurately, with a deviation to the measured phantom dose of 1.1%±0.6%. For VMAT fields the predicted Delta4DVH Anatomy dose in the isocenter plane corresponded to the measured dose in the phantom, with an average gamma agreement index (GAI) (3 mm/3%) of 96.9±0.4%. The Delta4DVH Anatomy detected the induced systematic gantry error of 10° with a relative GAI (3 mm/3%) change of 5.8%±1.6%. The conventional Delta4PT QA system detected a GAI change of 4.2%±2.0%. The conventional Delta4PT GAI (3 mm/3%) was 99.8%±0.4% for the clinical treatment plans. The mean body and PTV‐GAI (3 mm/5%) for the Delta4DVH Anatomy were 96.4%±2.0% and 97.7%±1.8%; however, this dropped to 90.8%±3.4% and 87.1%±4.1% for passing criteria of 3 mm/3%. The anatomy‐based patient specific quality assurance system predicts the dose distribution correctly for a homogeneous case. The limiting factor for the error detection is the large variability in the error‐free plans. The dose calculation algorithm is inferior to that used in the TPS (Eclipse). PACS numbers: 87.56.Fc, 87.56.‐v

Intensity-modulated radiotherapy and volumetric-modulated arc therapy for malignant pleural mesothelioma after extrapleural pleuropneumonectomy

Radiotherapy reduces the local relapse rate after pleuropneumonectomy of malignant pleural mesothelioma (MPM). The optimal treatment technique with photons remains undefined. Comparative planning for intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT) was performed. Six MPM patients with significant postoperative intrathoracic air cavities were planned with IMRT and VMAT. A dose comparison for the targets and organ at risks (OAR) was performed. Robustness was assessed in respect to the variation of target dose with change in volume of air cavities. VMAT reduced the dose to the contralateral lung by reducing the volume covered by 13 Gy and 20 Gy by a factor 1.8 and 2.8, in respect to IMRT (p=0.02). Dose distribution with VMAT was the most stable technique in regard to postsurgical air cavity variation. For IMRT, V90,V95, and the minimal target dose decreased by 40%, 64%, and 12% compared to 29%, 47%, and 7% with VMAT when air cavity decreased. Two arcs compared to one arc decreased the dose to all the organs at risk (OAR) while leaving PTV dose coverage unchanged. Increasing the number of arcs from two to three did not reduce the dose to the OAR further, but increased the beam‐on time by 50%. Using partial arcs decreased the beam‐on time by 43%. VMAT allows a lower lung dose and is less affected by the air cavity variation than IMRT. The best VMAT plans were obtained with two partial arcs. VMAT seems currently the most suitable technique for the treatment of MPM patients when air cavities are remaining and no adaptive radiotherapy is performed. PACS number: 87.55.D‐

Safety of high-dose-rate stereotactic body radiotherapy

Background and purposeFlattening filter free (FFF) beams with high dose rate are increasingly used for stereotactic body radiotherapy (SBRT), because they substantially shorten beam-on time. The physical properties of these beams together with potentially unknown radiobiological effects might affect patient safety. Therefore here we analyzed the clinical outcome of our patients.Material and methodsBetween 3/2010 and 2/2014 84 patients with 100 lesions (lung 75, liver 10, adrenal 6, lymph nodes 5, others 4) were treated with SBRT using 6 MV FFF or 10 MV FFF beams at our institution. Clinical efficacy endpoints and toxicity were assessed by Kaplan-Meier analysis and CTCAE criteria version 4.0.ResultsMedian follow-up was 11 months (range: 3–41). No severe acute toxicity was observed. There has been one case of severe late toxicity (1%), a grade 3 bile duct stricture that was possibly related to SBRT. For all patients, the 1-year local control rate, progression free survival and overall survival were 94%, 38% and 80% respectively, and for patients with lung lesions 94%, 48% and 83%, respectively.ConclusionsNo unexpected toxicity occurred. Toxicity and treatment efficacy are perfectly in range with studies investigating SBRT with flattened beams. The use of FFF beams at maximum dose rate for SBRT is time efficient and appears to be safe.

Dosimetric influence of pitch in patient positioning for radiotherapy of long treatment volumes; the usefulness of six degree of freedom couch

OBJECTIVE: Pitch, the rotation around the transversal axis of the patient during radiotherapy has little impact on the dose distribution of small spherical treatment volumes; however it might affect treatment of long volumes requiring a correction with a six degree of freedom couch. METHODS: We included 10 patients each with nasopharyngeal carcinoma (NPC) and esophageal cancer, treated with volumetric modulated arc therapy. Pitch was simulated by tilting the planning CT in ventral and dorsal direction by ± 1.5° and ± 3°. Verification plans were calculated on the tilted datasets and were compared to the original plan and the dose constraints of the organs at risk (OAR). RESULTS: The deviation in dose to the planning target volume is increasing with the degree of pitch with mean changes of up to 2% for NPC and 1% for esophageal cancer. The most affected OAR in NPC patients are brainstem (max. dose +6.0%) and spinal cord (max. dose +10.0%) when tilted by 3° dorsally and lenses (max. dose +3.3%), oral mucosa (mean dose +2.6%) and parotid glands (mean dose +4.3%) when tilted by 3° ventrally. For esophageal cancer patients, there was no significant change in dose to any OAR. Whereas for esophageal cancer, all tilted treatment plans were still clinically acceptable regarding OAR, 5 NPC plans would no longer be acceptable with a pitch of 1.5° ventral (N = 1), 3° ventral (N = 2) and 3° dorsal (N = 2). CONCLUSION: Planning target volume coverage in both tumor entities was only slightly affected, but pitch errors could be relevant for OAR in NPC patients. ADVANCES IN KNOWLEDGE: A correction with a six degree of freedom couch is recommended for NPC patients with a pitch mismatch of more than 1.5° to avoid exceeded doses to the OAR.