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The determination of output factors in small field dosimetry is a crucial point, especially when implementing stereotactic radiotherapy (SRT). Herein, a working group of the French medical physicist society (SFPM) was created to collect small field output factors. The objective was to gather and disseminate information on small field output factors based on different detectors for various clinical SRT equipment and measurement configurations. Method: Participants were surveyed for information about their SRT equipment, including the type of linear particle accelerator (linac), collimator settings, measurement conditions for the output factors and the detectors used. Participants had to report both the ratio of detector readings and the correction factors applied as described in the IAEA TRS-483 code of practice for nominal field sizes smaller or equal to 3 cm. Mean field output factors and their associated standard deviations were calculated when data from at least 3 linacs were available. Results: 23 centres were enrolled in the project. Standard deviations of the mean field output factors were systematically smaller than 1.5% for field sizes larger or equal to 1 cm and reached 5% for the smallest field size (0.5 cm). Deviations with published data were smaller than 2% except for the 0.5 cm circular fixed aperture collimator of the CyberKnife where it reached 3.5%. Conclusion:These field output factor values obtained via a large multicentre study can be considered as an external cross verification for any radiotherapy centre starting a SRT program and should help minimize systematic errors when determining small field output factors.

Background: The internal target volume (ITV) approach and the mid-ventilation (MidV) concept are the two main respiratory motion-management strategies under free breathing. The purpose of this work was to compare the actual in-treatment target coverage during volumetric modulated arctherapy (VMAT) delivered through both ITVbased and MidV-based planning target volume (PTV) and to provide knowledge in choosing the optimal PTV for stereotactic body radiotherapy (SBRT) for lung lesions. Methods and materials: Thirty-two lung cancer patients treated by a VMAT technique were included in the study. For each fraction, the mean time-weighted position of the target was localized by using a 4-dimensional conebeam CT (4D-CBCT)-based image guidance procedure. The respiratory-correlated location of the gross tumor volume (GTV) during treatment delivery was determined for each fraction by using in-treatment 4D-CBCT images acquired concurrently with VMAT delivery (4D-CBCT in-treat ). The GTV was delineated from each of the ten respiratory phase-sorted 4D-CBCT in-treat datasets for each fraction. We defined target coverage as the average percentage of the GTV included within the PTV during the patient's breathing cycle averaged over the treatment course. Target coverage and PTVs were reported for a MidV-based PTV (PTV MidV ) using dose-probabilistic margins and three ITVbased PTVs using isotropic margins of 5 mm (PTV ITV + 5mm ), 4 mm (PTV ITV + 4mm ) and 3 mm (PTV ITV + 3mm ). The intreatment baseline displacements and target motion amplitudes were reported to evaluate the impact of both parameters on target coverage. Results: Overall, 100 4D-CBCT in-treat images were analyzed. The mean target coverage was 98.6, 99.6, 98.9 and 97.2% for PTV MidV , PTV ITV + 5mm , PTV ITV + 4mm and PTV ITV + 3mm , respectively. All the PTV margins led to a target coverage per treatment higher than 95% in at least 90% of the evaluated cases. Compared to PTV ITV + 5mm , PTV MidV , PTV ITV + 4mm and PTV ITV + 3mm had mean PTV reductions of 16, 19 and 33%, respectively.Conclusion: When implementing VMAT with 4D-CBCT-based image guidance, an ITV-based approach with a tighter margin than the commonly used 5 mm margin remains an alternative to the MidV-based approach for reducing healthy tissue exposure in lung SBRT. Compared to PTV MidV , PTV ITV + 3mm significantly reduced the PTV while still maintaining an adequate in-treatment target coverage.

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BackgroundWe aimed to evaluate the toxicity, loco-regional control (LRC) and overall survival (OS) associated with accelerated intensity-modulated radiotherapy (IMRT) for locally advanced lung cancer.MethodsSeventy-three patients were consecutively treated with IMRT from November 2011 to August 2016. A total dose of 66 Gy was delivered using two different schedules of radiotherapy: simultaneous modulated accelerated radiotherapy (SMART) (30 × 2.2 Gy, across 6 weeks) with or without chemotherapy, or moderate hypofractionated radiotherapy (HRT) (24 × 2.75 Gy, across 4 weeks) in patients unfit to receive concomitant chemotherapy. Data on esophageal and pulmonary toxicities, LRC and OS were prospectively collected.ResultsThe median follow-up duration was 44 months. Severe pneumonitis and esophagitis (grade 3–4) were observed in 7% and 1% of patients respectively, with only one case of grade 4 (pneumonitis). Overall, the 1-year and 2-year LRCs were 76% [95 confidence interval (CI)%: 66–87%] and 62% [95 CI%: 49–77%] respectively. The 1 and 2-year OS rates were 72% [95% CI: 63–83%] and 54% [95 CI%: 43–68%] respectively. None parameters were correlated with LRC or OS. In particular, no difference was observed between patients treated with SMART and H-RT (p = 0.26 and 0.6 respectively), with a 1-year LRC of 74% [95 CI%: 62–86%] for SMART and 91% [95 CI%: 74–100%] for H-RT. No significant differences were observed in the toxicity rates associated with each of the RT schedules.ConclusionsAccelerated IMRT for locally advanced lung cancer is associated with low toxicities and high LRC. Moderate hypofractionated RT, by decreasing the total treatment time, may be promising in improving clinical outcomes.

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Objective: The aim of this study was to evaluate the potential of simultaneously modulated accelerated radiation therapy (SMART) to reduce the incidence of severe acute oesophagitis in the treatment of unresectable locally advanced non-small-cell lung cancer (LANSCLC). Methods: 21 patients were treated with SMART and concomitant platinum-based chemotherapy. The prescribed doses were limited to 54 Gy at 1.8 Gy per day to the zones of presumed microscopic extent while simultaneously maintaining doses of 66 Gy at 2.2 Gy per day to the macroscopic disease. The whole treatment was delivered over 30 fractions and 6 weeks. . With a median follow-up of 18 months (6-33 months), the 1-year local control rate was 70% and the disease-free survival rate was 47%. Conclusion: SMART reduces the incidence of severe oesophagitis and improves the whole dosimetric predictors of toxicity for the lung, heart and spine. Advances in knowledge: Our study shows that SMART optimizes the therapeutic ratio in the treatment of LANSCLC, opening a window for dose intensification. INTRODUCTIONConcurrent chemoradiotherapy is the standard of care for the treatment of unresectable locally advanced non-smallcell lung cancer (LANSCLC). 1 Acute oesophageal toxicity (AET) is the main acute limiting toxicity related to this approach, and is responsible for weight loss, insertion of tube feeding, hospital admissions and treatment discontinuation. A recent meta-analysis showed that hyperfractionated or accelerated radiotherapy (RT) schedules improve survival rates;3 however, when chemotherapy is associated concomitantly with these schedules, the incidence of severe oesophagitis of around 22% remains a subject of concern. 4 In the RT planning process, the delineation of target volumes to be treated is an obligatory step. The gross demonstrable disease is named gross tumour volume (GTV) and encompasses the primary tumour and the metastatic lymph nodes identified by the endoscopic and radiologic examinations. The microscopic tumour spread, outside of what can be visualized in a particular imaging modality, is named clinical target volume (CTV) and corresponds to a volume of tissue surrounding the primary tumour and the lymph node stations considered at risk of failure. 5 In the standard technique of RT, a homogeneous dose of 66 Gy at 2 or 1.8 Gy per fraction and per day is delivered to the GTV and CTV. In contrast to this standard technique, simultaneously modulated accelerated radiation therapy (SMART) is a technique delivering 66 Gy by using high fraction doses (2.2 Gy per fraction) to the GTV simultaneously with standard fraction doses (1.8 Gy per fraction) to regions of presumed microscopic extent (CTV), devised in such a manner that the biologically effective GTV dose is equivalent to 70 Gy delivered by conventional (2 Gy per fraction) fractionated RT. This regimen is delivered over 6 weeks, representing a moderate acceleration over a standard course. As proved in head and neck cancer, this technique lead to a moderate GTV dose acceleration without inc...