Danis Blais

Assessing lung function using contrast‐enhanced dual‐energy computed tomography for potential applications in radiation therapy

Purpose: There is an increasing interest in the evaluation of lung function from physiological images in radiation therapy treatment planning to reduce the extent of postradiation toxicities. The purpose of this work was to retrieve reliable functional information from contrast‐enhanced dual‐energy computed tomography (DECT) for new applications in radiation therapy. The functional information obtained by DECT is also compared with other methods using single‐energy CT (SECT) and single‐photon emission computed tomography (SPECT) with CT. The differential function between left and right lung, as well as between lobes is computed for all methods. Methods: Five lung cancer patients were retrospectively selected for this study; each underwent a SPECT/CT scan and a contrast‐injected DECT scan, using 100 and 140 Sn kVp. The DECT images are postprocessed into iodine concentration maps, which are further used to determine the perfused blood volume. These maps are calculated in two steps: (a) a DECT stoichiometric calibration adapted to the presence of iodine and followed by (b) a two‐material decomposition technique. The functional information from SECT is assumed proportional to the HU numbers from a mixed CT image. The functional data from SPECT/CT are considered proportional to the number of counts. A radiation oncologist segmented the entire lung volume into five lobes on both mixed CT images and low‐dose CT images from SPECT/CT to allow a regional comparison. The differential function for each subvolume is computed relative to the entire lung volume. Results: The differential function per lobe derived from SPECT/CT correlates strongly with DECT (Pearsons coefficient r = 0.91) and moderately with SECT (r = 0.46). The differential function for the left lung shows a mean difference of 7% between SPECT/CT and DECT; and 17% between SPECT/CT and SECT. The presence of nonfunctional areas, such as localized emphysema or a lung tumor, is reflected by an intensity drop in the iodine concentration maps. Functional dose volume histograms (fDVH) are also generated for two patients as a proof of concept. Conclusion: The extraction of iodine concentration maps from a contrast‐enhanced DECT scan is achieved to compute the differential function for each lung subvolume and good agreement is found in respect to SPECT/CT. One promising avenue in radiation therapy is to include such functional information during treatment planning dose optimization to spare functional lung tissues.

Larynx motion considerations in partial larynx volumetric modulated arc therapy for early glottic cancer

To assess laryngeal motion in early glottic cancer in order to determine safe margins for partial larynx volumetric modulated arc therapy (PL‐VMAT), and to quantify dosimetric advantages of PL‐VMAT.

SU-F-J-91: Sparing Lung Function in Treatment Planning Using Dual Energy Tomography

PURPOSE To propose an alternate treatment plan that minimizes the dose to the functional lung tissues. In clinical situation, the evaluation of the lung functionality is typically derived from perfusion scintigraphy. However, such technique has spatial and temporal resolutions generally inferior to those of a CT scan. Alternatively, it is possible to evaluate pulmonary function by analysing the iodine concentration determined via contrast-enhanced dual energy CT (DECT) scan. METHODS Five lung cancer patients underwent a scintigraphy and a contrast-enhanced DECT scan (SOMATOM Definition Flash, Siemens). The iodine concentration was evaluated using the two-material decomposition method to produce a functional map of the lung. The validation of the approach is realized by comparison between the differential function computed by DECT and scintigraphy. The functional map is then used to redefine the V5 (volume of the organ that received more than 5 Gy during a radiotherapy treatment) to a novel functional parameter, the V5f. The V5f, that uses a volume weighted by its function level, can assist in evaluating optimal beam entry points for a specific treatment plan. RESULTS The results show that the differential functions obtained by scintigraphy and DECT are in good agreement with a mean difference of 6%. In specific cases, we are able to visually correlate low iodine concentration with abnormal pulmonary lung or cancerous tumors. The comparison between V5f and V5 has shown that some entry points can be better exploited and that new ones are now accessible, 2.34 times more in average, without increasing the V5f -- thus allowing easier optimization of other planning objectives. CONCLUSION In addition to the high-resolution DECT images, the iodine map provides local information used to detect potential functional heterogeneities in the 3D space. We propose that this information be used to calculate new functional dose parameters such as the V5f. The presenting author, Andreanne Lapointe, received a canadian scholarship from MITACS. Part of the funding is from the compagny Siemens.

SU‐FF‐J‐104: Performance of the Philips Gemini GXL PET/CT Simulator

Purpose: To present results obtained during the acceptance of the Gemini GXL PET/CT Simulator system. Method and Materials: A flat radiotherapy tabletop is permanently installed on our system. CT Simulator acceptance was performed using the TG‐66 Tables II and III as templates. Two 60‐cm long CT Sim phantoms were used. The relative electron density (RED) to HU relationship was measured with the RMI 465 and Catphan 600 phantoms and analysed using the stoichiometric method of Schneider et al. HU uniformity was measured with a 20‐cm water phantom and contrast‐to‐noise (CNR) performance measured with the Catphan. PET performance was measured using the NEMA 2‐2001 standard at the factory before shipment and after installation. Results: Table sag was measured to be +/− 4 mm over a 1200 mm scan range with loads of 0 to 282 lbs. During longitudinal table displacements, a 2–3 mm shift occurs over a 300 mm range and makes it impractical to use the head‐end of the table in some applications. The slice sensitivity profiles for the 0.75 and 1.5 mm nominal slice thickness were measured to be 1.1 and 1.8 mm. From the RMI 465 and Catphan measurements, a RED‐HU relationship applicable to biological tissues at 120 and 140 kVp was developed. HU uniformity for water was measured to be +10 HU to −12 HU for clinical protocols. Image quality is evaluated by quantifying the CNR for a 15 mm object. PET peak true count rate and peak NEC rate performance measured at the factory and at installation are below manufacturer published specifications: (191, 182 vs 203 kcps) and (61, 56 vs 70 kcps). Conclusion: Future work will involve evaluating PETspatial resolution at 20 cm off‐axis positions, quantifying the impact of table load on PET/CT fusion accuracy and developing tools to permit clinical PET/CT simulation.