Optimizing dose prescription in stereotactic body radiotherapy for lung tumours using Monte Carlo dose calculation

doi:10.1016/j.radonc.2009.11.008

Radiotherapy and Oncology

Volume 94, Issue 1, January 2010, Pages 42-46

https://doi.org/10.1016/j.radonc.2009.11.008 Get rights and content

Abstract

Purpose

To define a method of dose prescription employing Monte Carlo (MC) dose calculation in stereotactic body radiotherapy (SBRT) for lung tumours aiming at a dose as low as possible outside of the PTV.

Methods and materials

Six typical T1 lung tumours – three small, three large – were constructed centrally, peripherally in the lung, and nearby the thoracic wall, respectively. For each of these, five treatment plans employing dynamic conformal arc technique were made in which the dose was prescribed to encompass the PTV with the prescription isodose level (PIL) set in a range between 50% and 80% of the isocenter dose. Three shells of respectively 10 mm thickness around the PTV were constructed to assess the dose in the tissues directly adjacent to the PTV.

Results

The PTV was nicely covered (mean 98.8% ± 0.9%) with favourable conformity indices (mean 1.09 ± 0.1). Mean doses around the PTVs were 73% (±1.3%), 76% (±3.5%), and 85% (±5.1%) of the prescribed dose in shell 1 for PIL50%, PIL65%, and PIL80%, respectively; 40% (±2.6%), 44% (±5.1%), 54% (±9.3%) in shell 2; and 24% (±1.9%), 26% (±3.6%), 33% (±6.8%) in shell 3. All normal tissue doses including the integral dose were also consistently worst for PIL80%. Monitor units were 30% higher for PIL65%, and 70% higher for PIL50%, compared with PIL80%.

Conclusions

To improve normal tissue sparing the dose should be prescribed at an isodose lower than 80% of the isocenter dose in SBRT when using conformal arc technique with MC dose calculation.

Section snippets

Methods and materials

The four-dimensional planning CT acquired with free un-coached breathing of a patient treated with SBRT for a NSCLC stage T1N0M0 was entered into the planning software (i-plan-RT-dose 4.0, Brainlab, Feldkirchen, Germany). The patient had been positioned frameless in a vacuum-mattress with the arms raised above the head. In the planning CT, GTVs, ITVs en PTVs for two typical T1-tumours (small: diameter 15 mm, and large: diameter 30 mm, respectively) were constructed at three localizations: near

PTV coverage

The numeric planning parameters are displayed in Supplementary Table 1. For small and large lesions planned, coverage of the PTV by the PI (prescription isodose) was 96.5% or higher, and for 16 out of 30 of the plans, PTV coverage was >99%. Conformity indices (CI) were very low at <1.07 in 15 out of 30 plans, the worst three plans yielding conformity indices of 1.22–1.24 (all for PILs of 80%; Supplementary Table 1). In concordance with the steepness of the dose gradient (more rapid dose falloff

Discussion

Stereotactic body radiotherapy is an increasingly used treatment modality for indications including early-stage primary lung cancer and lung metastases. The essence of SBRT consists in administering very high doses per fraction to small target volumes, which in general only comprise gross tumour tissue with tight margins [7]. The tightness of the margin to the PTV in the millimetre range is made possible, firstly, because special methods such as 4D-CT for treatment planning [8], respiratory

Conclusion

In stereotactic radiotherapy for lung-lesions using MC dose calculation and dynamic conformal arc setup, dose prescription at a PIL (prescription isodose level) between 50% and 70% of the dose at the isocenter results in lower dose to surrounding tissues and lungs compared with employing a PIL of 80%. Except for large lesions adjacent to the thoracic wall and for lesions, where normal tissue lies within the PTV, dose should be prescribed at an isodose level considerably lower than 80% when

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2017, Physica Medica
Emerging data are showing the safety and the efficacy of Stereotactic Body Radiation therapy (SBRT) in lung cancer management. In this context, the very high doses delivered to the Planning Target Volume, make the planning phase essential for achieving high dose levels conformed to the shape of the target in order to have a good prognosis for tumor control and to avoid an overdose in relevant healthy adjacent tissue. In this non-systematic review we analyzed the technological and the physics aspects of SBRT planning for lung cancer. In particular, the aims of the study were: (i) to evaluate prescription strategies (homogeneous or inhomogeneous), (ii) to outline possible geometrical solutions by comparing the dosimetric results (iii) to describe the technological possibilities for a safe and effective treatment, (iv) to present the issues concerning radiobiological planning and the automation of the planning process.
Lung stereotactic ablative body radiotherapy: A large scale multi-institutional planning comparison for interpreting results of multi-institutional studies
2016, Physica Medica
Citation Excerpt :
One of the main problems when comparing the SABR plans from different institutions is the varied PTV dose prescription. There are two main approaches regarding dose distribution within the target in SABR: (i) to maintain dose homogeneity within the target, which is usually prescribed at the PTV mean value, and (ii) to prescribe dose at PTV boundaries, without considering the dose heterogeneity inside the target [16]. In conventional radiotherapy, the International Commission on Radiation Units and Measurements (ICRU) Report 62 [17] recommended a uniform dose distribution within the target volume with dose prescribed at a reference point (generally the isocenter).
A large-scale multi-institutional planning comparison on lung cancer SABR is presented with the aim of investigating possible criticism in carrying out retrospective multicentre data analysis from a dosimetric perspective.
Five CT series were sent to the participants. The dose prescription to PTV was 54 Gy in 3 fractions of 18 Gy. The plans were compared in terms of PTV-gEUD₂ (generalized Equivalent Uniform Dose equivalent to 2 Gy), mean dose to PTV, Homogeneity Index (PTV-HI), Conformity Index (PTV-CI) and Gradient Index (PTV-GI). We calculated the maximum dose for each OAR (organ at risk) considered as well as the MLD₂ (mean lung dose equivalent to 2 Gy). The data were stratified according to expertise and technology.
Twenty-six centers equipped with Linacs, 3DCRT (4% – 1 center), static IMRT (8% – 2 centers), VMAT (76% – 20 centers), CyberKnife (4% – 1 center), and Tomotherapy (8% – 2 centers) collaborated. Significant PTV-gEUD₂ differences were observed (range: 105–161 Gy); mean-PTV dose, PTV-HI, PTV-CI, and PTV-GI were, respectively, 56.8 ± 3.4 Gy, 14.2 ± 10.1%, 0.70 ± 0.15, and 4.9 ± 1.9. Significant correlations for PTV-gEUD₂ versus PTV-HI, and MLD₂ versus PTV-GI, were observed.
The differences in terms of PTV-gEUD₂ may suggest the inclusion of PTV-gEUD₂ calculation for retrospective data inter-comparison.
Dose-volume-response analysis in stereotactic radiotherapy for early lung cancer
2014, Radiotherapy and Oncology
Citation Excerpt :
A margin of 5 mm was added to arrive at PTV. Further treatment details were described previously [17,18]. Doses were prescribed and calculated differently.
Japanese and Western approaches to stereotactic ablative radiotherapy (SABR) are considerably different, particularly with respect to dose prescription and reporting, which makes comparisons of Japanese versus European or American results challenging. Using individual patient data, the aim of this study was to analyze the dose-local-control relationship and its impact on survival.
Patients receiving SABR for single-lesion early stage NSCLC in Osaka (OM) or Groningen (GN) were analyzed. Doses were recalculated using state-of-the-art dose calculation algorithms and expressed as biologically effective dose (BED) at PTV margin. Survival, local control (LC), and effect of treatment failure in operable and inoperable patients on survival were analyzed.
Between 2006 and 2010, 383 patients were included. The BED at PTV periphery was 102 Gy₁₀ (±21) in GN and 83 Gy₁₀ (±5) in OM. Unadjusted overall survival (OS) was better in OM (72% vs 52%; p < 0.001), but GTVs and performance status (PS) were also significantly more favorable in OM. Adjusted for GTV and PS, OS was not different between institutions (HR 0.88; p = 0.47). LC was better in GN (93% vs 84%; p < 0.05). Local control predicted survival in operable patients: Adjusted for GTV and PS, the HR of local failure for OS was 7.5 (2–27; p = 0.003) for operable, and 1.1 (0.7–1.9; p = 0.6) for inoperable patients.
Sufficient dose is crucial for local control, which was a significant factor for survival for operable patients.
Monte Carlo simulation approaches to dose distributions for 6 MV photon beams in clinical linear accelerator
2014, Biocybernetics and Biomedical Engineering
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Therefore, the best method is Monte Carlo method used for the dose calculations in case of very difficult experimental measurements as well as many applications. Various Monte Carlo simulation programs were often utilized on the determination of dose distributions [2,3], on effective dose estimation [4] and on calculating the scattering of photon beams [5,6]. In order to evaluate the dosimetric properties of 6 MV photon beams, Monte Carlo code systems as OMEGA [7], MCNP4C [8,9], EGS4 [10,11] and GEANT3 [12] were used.
Monte Carlo method is often used in radiation therapy as utilized in all the branches of science. For this purpose, various preset codes are used for the dose calculations in radiotherapy. In this study, a new Monte Carlo Simulation Program (MCSP) was developed for the dose distributions of a clinical linear accelerator (LINAC) in water phantom. MCSP was carried out by taking into account the interactions of photons with matter in MATLAB (The Mathworks, Inc.). In the study, 6 MeV (6 MV photon mode) energies of photons are examined. In order to validate the performance and accuracy of the simulation, the experimental measurements and MCSP calculations were compared for both percentage depth dose curves and beam profiles. The Monte Carlo results show good agreement with experimental results.
Stereotactic ablative body radiation therapy with dynamic conformal multiple arc therapy for liver tumors: Optimal isodose line fitting to the planning target volume
2014, Practical Radiation Oncology
To assess optimal relative prescribed dose values in stereotactic ablative body radiation therapy (SABR) using dynamic conformal multiple arc therapy (DCMAT) for liver tumors.
We generated SABR plans for 8 typical liver tumors that received SABR with 50 Gy in 5 fractions. The prescribed dose had previously been defined as 80% of the maximal dose (“80% isodose plan”). Alternatively, 20%-90% isodose plans were created to compare dosimetric factors.
The mean liver volume values (%) that received > 20 Gy (V20) and the mean liver dose were both the lowest with a 70% isodose plan and were the second lowest with a 60% isodose plan. The V20 dose was 5.19% lower (11.14%) with a 70% isodose plan and 4.51% lower (11.22%) with a 60% isodose plan compared with the value with an 80% isodose plan (11.75%). Mean planning target volume (PTV) dose increased as the % isodose decreased. The mean PTV dose was 10% higher (62.4 Gy) with a 70% isodose plan and 21% higher (68.9 Gy) with a 60% isodose plan compared with the value with an 80% isodose plan (56.8 Gy).
During SABR treatment planning using DCMAT for liver tumors, target doses increased as the percentage isodose value decreased, which could result in better outcomes. In contrast, a 70% isodose plan had the lowest normal liver dose and a 60% isodose plan had the second lowest. An optimal percentage isodose level might be adjusted depending on tumor radiation sensitivity and liver function reserve. Further investigation is warranted to determine whether these dosimetric advantages result in improved outcomes.
Survival and quality of life after stereotactic or 3D-conformal radiotherapy for inoperable early-stage lung cancer
2011, International Journal of Radiation Oncology Biology Physics
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The total dose of 60 Gy was prescribed at the margin of the PTV, constituting 80% of the dose at the isocenter and following the dose-conformity guidelines as used in the Radiation Therapy Oncology Group 0236 trial. Treatment was delivered using four noncoplanar dynamic arcs as previously described (15). The PTV comprising the tumor as seen on a slow 3D-planning CT with a margin of 20 mm (15 mm to CTV; 5 mm to PTV) was treated to 46 Gy, thereafter portals were reduced to tumor plus 5 mm as PTV to the total dose of 70 Gy, which results in a BED of 84 Gy10 for tumor effects.
To investigate survival and local recurrence after stereotactic ablative radiotherapy (SABR) or three-dimensional conformal radiotherapy (3D-CRT) administered for early-stage primary lung cancer and to investigate longitudinal changes of health-related quality of life (HRQOL) parameters after either treatment.
Two prospective cohorts of inoperable patients with T1-2N0M0 primary lung tumors were analyzed. Patients received 70 Gy in 35 fractions with 3D-CRT or 60 Gy in three to eight fractions with SABR. Global quality of life (GQOL), physical functioning (PF), and patient-rated dyspnea were assessed using the respective dimensions of European Organization for Research and Treatment of Cancer Core Questionnaire-C30 and LC13. HRQOL was analyzed using multivariate linear mixed-effects modeling, survival and local control (LC) using the Kaplan-Meier method, Cox proportional hazards analysis, and Fine and Gray multivariate competing risk analysis as appropriate.
Overall survival (OS) was better after SABR compared with 3D-CRT with a HR of 2.6 (95% confidence interval [CI]: 1.5–4.8; p < 0.01). 3D-CRT conferred a subhazard ratio for LC of 5.0 (95% CI: 1.7–14.7; p < 0.01) compared with SABR. GQOL and PF were stable after SABR (p = 0.21 and p = 0.62, respectively). Dyspnea increased after SABR by 3.2 out of 100 points (95% CI: 1.0–5.3; p < 0.01), which is clinically insignificant. At 1 year, PF decreased by an excess of 8.7 out of 100 points (95% CI: 2.8–14.7; p < 0.01) after 3D-CRT compared with SABR.
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SBRT of lung cancerOptimizing dose prescription in stereotactic body radiotherapy for lung tumours using Monte Carlo dose calculation

Abstract

Purpose

Methods and materials

Results

Conclusions

Section snippets

Methods and materials

PTV coverage

Discussion

Conclusion

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Clin Oncol (R Coll Radiol)

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Chest

Radiother Oncol

SBRT of lung cancer
Optimizing dose prescription in stereotactic body radiotherapy for lung tumours using Monte Carlo dose calculation