Mid-position treatment strategy for locally advanced lung cancer: a dosimetric study.

OBJECTIVE: The internal target volume (ITV) strategy generates larger planning target volumes (PTVs) in locally advanced non-small cell lung cancer (LA-NSCLC) than the Mid-position (Mid-p) strategy. We investigated the benefit of the Mid-p strategy regarding PTV reduction and dose to the organs at risk (OARs). METHODS: 44 patients with LA-NSCLC were included in a randomized clinical study to compare ITV and Mid-p strategies. GTV were delineated by a physician on maximum intensity projection images and on Mid-p images from four-dimensional CTs. CTVs were obtained by adding 6 mm uniform margin for microscopic extension. CTV to PTV margins were calculated using the van Herk's recipe for setup and delineation errors. For the Mid-p strategy, the mean target motion amplitude was added as a random error. For both strategies, three-dimensional conformal plans delivering 60-66 Gy to PTV were performed. PTVs, dose-volume parameters for OARs (lung, esophagus, heart, spinal cord) were reported and compared. RESULTS: With the Mid-p strategy, the median of volume reduction was 23.5 cm3 (p = 0.012) and 8.8 cm3 (p = 0.0083) for PTVT and PTVN respectively; the median mean lung dose reduction was 0.51 Gy (p = 0.0057). For 37.1% of the patients, delineation errors led to smaller PTV with the ITV strategy than with the Mid-p strategy. CONCLUSION: PTV and mean lung dose were significantly reduced using the Mid-p strategy. Delineation uncertainty can unfavorably impact the advantage. ADVANCES IN KNOWLEDGE: To the best of our knowledge, this is the first dosimetric comparison study between ITV and Mid-p strategies for LA-NSCLC.

[Intensity-modulated radiation therapy in non-small cell lung cancers]. / Radiothérapie conformationnelle avec modulation d'intensité dans les carcinomes bronchiques non à petites cellules.

Intensity modulated radiotherapy is increasingly used in non-small-cell lung cancers despite a low level of evidence. A literature review was conducted. Several critical physical and dosimetric uncertainties are however unsolved. Methods to circumvent these limitations are being developed. In several retrospective studies, survival rates were at least similar with intensity-modulated radiotherapy as those reported with three-dimensional irradiation. To date, intensity modulated radiotherapy might be authorized in complex anatomical situations such as tumours close to the spinal cord (such as Pancoast Tobias, paraspinal and paracardiac tumours) or with limited motion amplitudes. Dosimetric benefits should also account for 4D dose distribution issues. The reduction of intermediate and high doses in the organs at risk with intensity modulated radiotherapy is advantageous. However, the effect of low doses in large volumes (lung, bone, unspecified tissues along beam paths) and the effect of increasing integral dose are still poorly known. In conclusion, dose-volume correlations need to be better documented and prospective randomized trials should be encouraged.

[Evaluation and implementation of in vivo transit dosimetry with an electronic portal imaging device]. / Évaluation et mise en œuvre de la dosimétrie in vivo de transmission par imageurs portals.

PURPOSE: In vivo dosimetry is now widely recommended to avoid major treatment error. Transit dosimetry using portal imagers allows fast and accurate in vivo dose verifications. Several teams have published clinical studies but no recommendation has been proposed to define tolerance levels and validation criteria. This study proposes a simple methodology to assess the overall standard deviation of transit dosimetry and was applied to our transit dosimetry method. MATERIAL AND METHODS: In a first step, the uncertainties due to the dose reconstruction method are evaluated. Their estimation is based on a set of geometries, representative of clinical situations for which 45 points of measurement have been defined. In a second step, we studied the variations of our method in clinical situations. During the treatment session of the patient, the dose was reconstructed and the differences between reconstructed dose and prescribed dose were used to define a realistic tolerance level, adapted to the clinical routine. Then, a methodology is proposed to determine if the transit dosimetry method, with the defined tolerance level allows detecting significant treatment errors (>5% of the prescribed dose). RESULTS -  CONCLUSION: Applying this methodology we concluded that a tolerance level of 6.5% (k=2) can be associated with our method. With this value, it is demonstrated that in many cases differences of 5% (or less) on the prescribed dose can be detected. This study demonstrates clearly that in vivo transit dosimetry is not able to detect all the treatment errors but remains an ultimate and efficient tool in many situations.