Lung stereotactic body radiation therapy: personalized PTV margins according to tumor location and number of four-dimensional CT scans

Objectives To characterise the motion of pulmonary tumours during stereotactic body radiation therapy (SBRT) and to evaluate different margins when creating the planning target volume (PTV) on a single 4D CT scan (4DCT). Methods We conducted a retrospective single-site analysis on 30 patients undergoing lung SBRT. Two 4DCTs (4DCT1 and 4DCT2) were performed on all patients. First, motion was recorded for each 4DCT in anterior–posterior (AP), superior-inferior (SI) and rightleft (RL) directions. Then, we used 3 different margins (3,4 and 5 mm) to create the PTV, from the internal target volume (ITV) of 4DCT1 only (PTV D1 + 3, PTV D1 + 4, PTV D1 + 5). We compared, using the Dice coefficient, the volumes of these 3 PTVs, to the PTV actually used for the treatment (PTVttt). Finally, new treatment plans were calculated using only these 3 PTVs. We studied the ratio of the D2%, D50% and D98% between each new plan and the plan actually used for the treatment (D2% PTVttt, D50% PTVttt, D50% ITVttt D98% PTVttt). Results 30 lesions were studied. The greatest motion was observed in the SI axis (8.8 ± 6.6 [0.4–25.8] mm). The Dice index was higher when comparing PTVttt to PTV D1 + 4 mm (0.89 ± 0.04 [0.82–0.98]). Large differences were observed when comparing plans relative to PTVttt and PTV D1 + 3 for D98% PTVttt (0.85 ± 0.24 [0.19–1.00]). and also for D98% ITVttt (0.93 ± 0.12 [0.4–1.0]).D98% PTVttt (0.85 ± 0.24 [0.19–1.00], p value = 0.003) was statistically different when comparing plans relative to PTVttt and PTV D1 + 3. No stastistically differences were observed when comparing plans relative to PTVttt and PTV D1 + 4. A difference greater than 10% relative to D98% PTVttt was found for only in one UL lesion, located under the carina. Conclusion A single 4DCT appears feasible for upper lobe lesions located above the carina, using a 4-mm margin to generate the PTV. Advance in knowledge Propostion of a personalized SBRT treatment (number of 4DCT, margins) according to tumor location (above or under the carina).

Lung Stereotactic Radiation Therapy: Intercomparison of Several Irradiation Devices in Terms of Outcome and Predictive Factors

Background: Stereotactic body radiotherapy (SBRT) is recommended for the treatment of inoperable early stage non-small-cell lung cancer and lung oligometastases. The radiation oncology department of the Lausanne University Hospital (CHUV) gathers three different radiotherapy devices able to treat pulmonary lesions in SBRT conditions: CyberKnife® (CK), Helical Tomotherapy® (HT), and volumetric modulated arc therapy (VMAT). The aim of this study is to define the patients’ outcome in terms of irradiation efficacy and toxicities after lung SBRT depending of the choice of the SBRT technique.Methods: We retrospectively analyzed the clinical, radiological, and dosimetric data of patients with primary lung tumor or pulmonary oligometastases treated with SBRT between January 2016 and February 2020. We analyzed descriptive data using the Chi-2 test for proportions and the T-test for means comparisons, survival data by the Kaplan-Meier method and comparisons between groups by the Log-rank test and Cox-regression.Results: We identified 111 patients mostly in good condition (82.9% PS 0-1) with a median age of 71.4 years. One hundred forty-two lesions were treated with a typical fractionation of 55 Gy in 5 fractions, delivered by CK (59.9%), VMAT (38.0%), or HT (2.1%). Compared to other techniques, CK technique allowed to treat comparable gross tumor volume (GTV; 2.1 vs 1.4cc, p = 0.84) with smaller planning treatment volume (PTV; 12.3 vs 21.9 cc, p = 0.013), and was associated with a lower mean lung dose (MLD; 2.6 vs 4.1 Gy, p < 0.001), a lower V5 (13.5 vs 19.9 cc, p = 0.002) and a lower V20 (2.3 vs 5.4 cc, p < 0.001). Local control rates at 2 years were not different depending on the irradiation device, respectively of 96.2% (range, 90.8-100) and 98.1% (range, 94.4-100), p = 0.68. Toxicity incidence was significantly increased with V5 value > 17.2% (56.0 vs 77.4%, p = 0.021). Conclusions: Compared to other SBRT techniques, CK treatments permitted to treat comparable GTV with reduced PTV, MLD, V20, and V5. The dosimetric benefit of CK SBRT was not associated with a clear clinical benefit, with comparable outcome in terms of control rates and toxicity. Toxicity incidence was less frequent when reducing the V5. The use of CK is particularly attractive in case of multiple courses of lung SBRT or in case of local relapse requiring lung re-irradiation.Trial registration: Registered on February 24th 2021, ID 2021-00267, with the authorization of the CER-VD ethics committee (Switzerland).

How Does the Gradient Measure of the Lung SBRT Treatment Plan Depend on the Tumor Volume and Shape?

PurposeGradient measure (GM) is a critical index related to normal tissue sparing in radiosurgery. This study aims to describe the dependence of GM on target volume and target shape for lung stereotactic body radiation therapy (SBRT) treatment plans.MethodsA total of 307 peripheral and 119 central lung SBRT treatment plans were enrolled for this study. A least-squares regression was used for data analysis. First, the equations with different functional forms were established to determine the dependence of GM on a univariaty (VP or Sp) and bivariaty (VP and Sp), respectively. Then, the correlation coefficients and p-values of variables for all equations were compared and analyzed to determine the dependence of GM on PTV volume (VP) and sphericity (Sp).ResultsThe power equations had the highest coefficient of determination (R2) in the dependence results of GM on univariate VP. The equations were GM=0.674VP0.178 and GM=0.660VP0.185 for peripheral and central lesions, respectively. On the other hand, the R2 of all functional forms were less than 0.25 when the relationship of GM versus univariate Sp was analyzed. Similarly, the power equation also obtained the highest R2 in bivariaty VP and Sp analysis, whether for central or peripheral. However, the R2 of the bivariate equations were not improved compared with those of univariate equations. Moreover, the p-values of the variable Sp were greater than 0.05.ConclusionsThe GM of the lung SBRT plan is shape-independent and volume-dependent. The dependence of GM on PTV volume for peripheral and central lung cancer can be described by two different power equations. The results of this study can be used as a potential tool to assist dosimetric quality control during the radiosurgery process.

Can the PTV Volume Be Used to Predict the Better Solution in Manual and Automatic Planning Using the Pinnacle Treatment Planning System for Lung Stereotactic Body Radiation Therapy?

Purpose: The plan quality of the stereotactic body radiation therapy (SBRT) plan is affected by the patient’s planning target volume (PTV). The predictability of PTV volume and cut-off points were investigated to judge the suitability of manual and automatic plans for lung SBRT patients.Methods: The manual and automatic SBRT plans were retrospectively designed using the Pinnacle 16.2 treatment planning system (TPS) for 98 lung cancer patients. the suitability of manual and automatic plans for each patient is comprehensively evaluated. Receiver operating characteristic (ROC) analysis was used to investigate the predictability of PTV volume and determine the cut-off point. Once the cut-off point exists, all patients were divided into two groups according to this cut-off point. The Wilcoxon signed-rank test was performed for the dosimetric comparisons between the two groups. Results: ROC analysis showed that PTV volume (AUC [Area under curve]: 0.918, p= 0.005) has diagnostic power to predict the suitability of manual and automatic plans for lung SBRT patients. The cut-off points of 22.675cc were selected for PTV volume. Regardless of some comparable results, the CI, GI, V10, and V20 of automatic plans were found to be better than manual plans below the cut-off points, and the manual plan is superior to the automatic plan in HI, GI, heart d15cc, V10, V20 above the cut-off points.Conclusion: The PTV volume of cut-off points (22.675cc) are predictive of the suitability of manual and automatic plan using Pinnacle TPS for lung SBRT patients. Automatic plans were recommended for patients with PTV volumes less than 22.675cc, and manual plans can be tried for patients with larger PTV volumes.