Quantification of Lung Tumor Motion and Optimization of Treatment.

Background: Mobility of lung tumors is induced by respiration and causes inadequate dose coverage. Objective: This study quantified lung tumor motion, velocity, and stability for small (≤5 cm) and large (>5 cm) tumors to adapt radiation therapy techniques for lung cancer patients. Material and Methods: In this retrospective study, 70 patients with lung cancer were included that 50 and 20 patients had a small and large gross tumor volume (GTV). To quantify the tumor motion and velocity in the upper lobe (UL) and lower lobe (LL) for the central region (CR) and a peripheral region (PR), the GTV was contoured in all ten respiratory phases, using 4D-CT. Results: The amplitude of tumor motion was greater in the LL, with motion in the superior-inferior (SI) direction compared to the UL, with an elliptical motion for small and large tumors. Tumor motion was greater in the CR, rather than in the PR, by 63% and 49% in the UL compared to 50% and 38% in the LL, for the left and right lung. The maximum tumor velocity for a small GTV was 44.1 mm/s in the LL (CR), decreased to 4 mm/s for both ULs (PR), and a large GTV ranged from 0.4 to 9.4 mm/s. Conclusion: The tumor motion and velocity depend on the tumor localization and the greater motion was in the CR for both lobes due to heart contribution. The tumor velocity and stability can help select the best technique for motion management during radiation therapy.

End-to-end test of respiratory gating radiation therapy for lung stereotactic body radiation therapy treatments.

In this paper, a dosimetric end-to-end test of respiratory gated radiation therapy (RGRT) applied in lung cancer stereotactic body radiation therapy (SBRT) was performed. The test was performed from treatment simulation to treatment delivery using a QUASAR phantom, for regular, slightly irregular and irregular breathing patterns in phase- and amplitude-gated modes. A mechanical and dosimetric verification was performed to evaluate all steps of the proposed treatment workflow. Dose measurements were performed using a PinPoint ion chamber and GafChromic EBT3 films. Mechanical verification confirmed good function of the chosen systems. Dosimetric verification showed good agreement between planned and measured doses, for the phase-gated versus amplitude-gated modes: 1.4 ± 0.4% versus 1.2 ± 0.2% for regular, 2.8 ± 0.3% versus 3.0 ± 0.3% for slightly irregular, and 6.2 ± 0.7% versus 7.4 ± 0.5% for irregular breathing patterns. The gamma passing rates for 3%/3 mm and 2%/2 mm criteria, comparing phase- versus amplitude-gated modes, were 99.0 ± 0.3% versus 99.5 ± 0.2% and 95.2 ± 0.2% versus 96.1 ± 0.2% for the regular, 97.4 ± 0.8% versus 98.0 ± 0.6% and 91.7 ± 0.5% versus 92.4 ± 0.4% for the slightly irregular, and 96.4 ± 0.5% versus 95.3 ± 0.7% and 86.4 ± 0.5% versus 84.6 ± 0.7% for the irregular breathing patterns, respectively. It is concluded that using equipment and workflow for the treatment of lung cancer by means of SBRT in RGRT mode is safe and efficient, for regular and slightly irregular breathing patterns.

Evaluation of thoracic surface motion during the free breathing and deep inspiration breath hold methods.

The aim of this study was to evaluate thoracic surface motion from chest wall expansion during free breathing (FB) and deep inspiration breath hold (DIBH) methods, measured with and without 4-dimensional computed tomography (4D-CT) simulation, using equipment developed in-house. The respiratory amplitude and chest wall expansion were evaluated at 5 levels of the thorax, (the sterno-clavicular joint (SCJ), the second level, the intermammary line (IML), the fourth level and the caudal end of the xiphoid process (XP)) using radiopaque wires and potentiometers, with a CT scan simultaneously. This study included 25 examinees (10 volunteers performed FB, 10 volunteers performed DIBH and 5 patients performed FB). For low and irregular respiration, coaching was used, and its impact was evaluated for both breathing methods, FB and DIBH. The breathing amplitude performed with FB between volunteers and patients was not detectable at the SCJ; increasing to the abdomen, 3 mm vs 2 mm (p = 0.326) at the second level; 6 mm vs 4 mm (p = 0.042) at the IML; 10 mm vs 8 mm (p < 0.01) at the fourth level; and 23 mm vs 19 mm (p < 0.001) at the XP. Contrary to the DIBH, where breathing amplitude was greater at 2 first levels 18 mm (SCJ) and 20 mm (second level), decreasing to the abdomen, 14 mm (IML); 11 mm (fourth level); and 10 mm (XP). Chest wall expansion was not detected at the SCJ, while at other levels measured from 1 to 7 mm. Coaching was improve breathing amplitude, for both methods, FB (3 mm) and DIBH (5 mm). The location of amplification is different depending on the breathing method and the in-house phantom was useful to check the amplification level.

Assessment of robustness of institutional applied clinical target volume (CTV) to planning target volume (PTV) margin in cervical cancer using biological models.

The aim of this study is to investigate the robustness of our institutionally applied clinical target volume (CTV)-to-planning target volume (PTV) margins in cervical cancer patients in terms of an equivalent uniform dose (EUD) based on tumor control probability (TCP). We simulated target motion using 25 IMRT cervical cancer plans to demonstrate the effect of geometrical uncertainties on the EUD and TCP. The different components of the total geometrical uncertainties budget were estimated. The biological effects were compared by calculating the EUDs from the trial DVHs. The impact of geometric uncertainties was calculated as a percentage of the difference between 〖EUD〗_static and 〖EUD〗_motion, where the 〖EUD〗_static is the EUD calculated from the target DVHs and 〖EUD〗_motion is averaged, over a 1000 calculated EUDs for each of the analyzed IMRT treatment plans. The multivariate nonlinear regression was used to find the predicted difference between the static and motion EUD. The estimate of the systematic and random motion errors were Σ_(total(SI,LR,AP)) (mm)=(2.6; 2.5; 1.8) and σ_(total(SI,LR,AP)) (mm)=(3.4; 1.4; 3.4). For average 〈EUD〉_motion=44.3 Gy (over 25 patients) we have found a TCP decrease of about 1%, %(ΔTCP)≈1% for predefined PTV margin. According to the calculated EUD motion-distributions, for particular patients, the CTV does receive the prescribed EUD of 45 Gy. The predicted difference in EUD showed that our isotropic margin of 10 mm is large enough to absorb geometric uncertainties and ensure dose coverage of the moving CTV in the cervical cancer patients.

Use of IAEA's phase-space files for virtual source model implementation: Extension to large fields.

In a previous work, phase-space data files (phsp) provided by the International Atomic Energy Agency (IAEA) were used to develop a hybrid virtual source model (VSM) for clinical photon beams. Very good agreement with dosimetric measurements performed on linear accelerators was obtained for field sizes up to 15×15cm(2). In the present work we extend the VSM to larger field sizes, for which phsp are not available. We incorporate a virtual flattening filter to our model, which can be determined from dose measurements for larger fields. In this way a fully functional VSM can be built, from publicly available IAEA's phsps and standard dose measurements, for fields of any size and tailored to a particular linac.

Use of IAEA's phase-space files for the implementation of a clinical accelerator virtual source model.

In the present work, phase-space data files (phsp) provided by the International Atomic Energy Agency (IAEA) for different accelerators were used in order to develop a Virtual Source Model (VSM) for clinical photon beams. Spectral energy distributions extracted from supplied phsp files were used to define the radiation pattern of a virtual extended source in a hybrid model which is completed with a virtual diaphragm used to simulate both electron contamination and the shape of the penumbra region. This simple virtual model was used as the radiation source for dosimetry calculations in a water phantom. The proposed model proved easy to build and test, and good agreement with clinical accelerators dosimetry measurements were obtained for different field sizes. Our results suggest this simple method could be useful for treatment planning systems (TPS) verification purposes.

Evaluation of set-up errors in head and neck radiotherapy using electronic portal imaging.

INTRODUCTION: The aim of this study was to evaluate three-dimensional (3D) set-up errors and propose optimum margins for planning target volume (PTV) coverage in head and neck radiotherapy. METHODS: Thirty-five patients were included in the study. The total number of portal images studied was 632. Population systematic (Σ) and random (σ) errors for the patients with head and neck cancer were evaluated based on the portal images in the caudocranial longitudinal (CC) and left-right lateral (LR) direction measured in the anterior-posterior (AP) field, as well as from the images in the caudocranial longitudinal (CC) and dorsoventral lateral (DV) direction measured in the lateral (LAT) field. The values for the clinical-to-planning target volume (CTV-PTV) margins were calculated using ICRU Report 62 recommendations, along with Stroom's and van Herk's formulae. RESULTS: The standard deviations of systematic set-up errors (Σ) ranged from 1.51 to 1.93 mm while the standard deviations of random set-up (σ) errors fell in between 1.77 and 1.86 mm. The mean 3D vector length of displacement was 2.66 mm. PTV margins calculated according to ICRU, Stroom's and van Herk's models were comprised between 1.95 and 6.16 mm in the three acquisition directions. DISCUSSION AND CONCLUSIONS: Based on our results we can conclude that a 6-mm extension of CTV to PTV margin, as the lower limit, is enough to ensure that 90% of the patients treated for head and neck cancer will receive a minimum cumulative CTV dose greater than or equal to 95% of the prescribed dose.