Dosimetric advantage of using 6 MV over 15 MV photons in conformal therapy of lung cancer: Monte Carlo studies in patient geometries

Technological quality requirements for stereotactic radiotherapy : Expert review group consensus from the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy

This review details and discusses the technological quality requirements to ensure the desired quality for stereotactic radiotherapy using photon external beam radiotherapy as defined by the DEGRO Working Group Radiosurgery and Stereotactic Radiotherapy and the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The covered aspects of this review are 1) imaging for target volume definition, 2) patient positioning and target volume localization, 3) motion management, 4) collimation of the irradiation and beam directions, 5) dose calculation, 6) treatment unit accuracy, and 7) dedicated quality assurance measures. For each part, an expert review for current state-of-the-art techniques and their particular technological quality requirement to reach the necessary accuracy for stereotactic radiotherapy divided into intracranial stereotactic radiosurgery in one single fraction (SRS), intracranial fractionated stereotactic radiotherapy (FSRT), and extracranial stereotactic body radiotherapy (SBRT) is presented. All recommendations and suggestions for all mentioned aspects of stereotactic radiotherapy are formulated and related uncertainties and potential sources of error discussed. Additionally, further research and development needs in terms of insufficient data and unsolved problems for stereotactic radiotherapy are identified, which will serve as a basis for the future assignments of the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The review was group peer-reviewed, and consensus was obtained through multiple working group meetings.

Beam modeling and beam model commissioning for Monte Carlo dose calculation-based radiation therapy treatment planning: Report of AAPM Task Group 157

Dose calculation plays an important role in the accuracy of radiotherapy treatment planning and beam delivery. The Monte Carlo (MC) method is capable of achieving the highest accuracy in radiotherapy dose calculation and has been implemented in many commercial systems for radiotherapy treatment planning. The objective of this task group was to assist clinical physicists with the potentially complex task of acceptance testing and commissioning MC-based treatment planning systems (TPS) for photon and electron beam dose calculations. This report provides an overview on the general approach of clinical implementation and testing of MC-based TPS with a specific focus on models of clinical photon and electron beams. Different types of beam models are described including those that utilize MC simulation of the treatment head and those that rely on analytical methods and measurements. The trade-off between accuracy and efficiency in the various source-modeling approaches is discussed together with guidelines for acceptance testing of MC-based TPS from the clinical standpoint. Specific recommendations are given on methods and practical procedures to commission clinical beam models for MC-based TPS.

Selection of gamma analysis acceptance criteria in IMRT QA using Gafchromic EBT3 film dosimetry

Background and purpose

This study reported the justification and selection of acceptable γ criteria with respect to low (6 MV) and high (15 MV) photon beams for intensity-modulated radiation therapy quality assurance (IMRT QA) using the Gafchromic external beam therapy 3 (EBT3) film.

Materials and methods

Five-field step-and-shoot IMRT was used to treat 16 brain IMRT patients using the dual-energy DHX-S linear accelerator (Varian Medical System, Palo Alto, CA, USA). Dose comparisons between computed values of the treatment planning system (TPS) and Gafchromic EBT3 film were evaluated based on γ analysis using the Film QA Pro software. The dose distribution was analysed with gamma area histograms (GAHs) generated using different γ criteria (3%/2 mm, 3%/3 mm and 5%/3 mm) for the 6 and 15 MV photon beams, to optimise the best distance-to-agreement (DTA) criteria with respect to the beam energy.

Results

From the comparison between the dose distributions acquired from the TPS and EBT3 film, a DTA criterion of 3%/2 mm showed less dose differences (DDs) with passing rates up to 93% for the 6 MV photon beams, while for the 15 MV a relaxed DTA criterion of 5%/3 mm was consistent with the DD acceptability criteria with a 95% passing rate.

Conclusions

Our results suggested that high-energy photon beams required relaxed DTA criteria for the brain IMRT QA, while low-energy photon beams showed better results even with tight DTA criteria.

Performance Evaluation of Algorithms in Lung IMRT: A comparison of Monte Carlo, Pencil Beam, Superposition, Fast Superposition and Convolution Algorithms

BACKGROUND

Inclusion of inhomogeneity corrections in intensity modulated small fields always makes conformal irradiation of lung tumor very complicated in accurate dose delivery.

OBJECTIVE

In the present study, the performance of five algorithms via Monte Carlo, Pencil Beam, Convolution, Fast Superposition and Superposition were evaluated in lung cancer Intensity Modulated Radiotherapy planning.

MATERIALS AND METHODS

Treatment plans for ten lung cancer patients previously planned on Monte Carlo algorithm were re-planned using same treatment planning indices (gantry angel, rank, power etc.) in other four algorithms.

RESULTS

The values of radiotherapy planning parameters such as Mean dose, volume of 95% isodose line, Conformity Index, Homogeneity Index for target, Maximum dose, Mean dose; %Volume receiving 20Gy or more by contralateral lung; % volume receiving 30 Gy or more; % volume receiving 25 Gy or more, Mean dose received by heart; %volume receiving 35Gy or more; %volume receiving 50Gy or more, Mean dose to Easophagous; % Volume receiving 45Gy or more, Maximum dose received by Spinal cord and Total monitor unit, Volume of 50 % isodose lines were recorded for all ten patients. Performance of different algorithms was also evaluated statistically.

CONCLUSION

MC and PB algorithms found better as for tumor coverage, dose distribution homogeneity in Planning Target Volume and minimal dose to organ at risks are concerned. Superposition algorithms found to be better than convolution and fast superposition. In the case of tumors located centrally, it is recommended to use Monte Carlo algorithms for the optimal use of radiotherapy.

Quantitative comparison of dose distribution in radiotherapy plans using 2D gamma maps and X-ray computed tomography

BACKGROUND

The advanced dose calculation algorithms implemented in treatment planning system (TPS) have remarkably improved the accuracy of dose calculation especially the modeling of electrons transport in the low density medium. The purpose of this study is to evaluate the use of 2D gamma (γ) index to quantify and evaluate the impact of the calculation of electrons transport on dose distribution for lung radiotherapy.

METHODS

X-ray computed tomography images were used to calculate the dose for twelve radiotherapy treatment plans. The doses were originally calculated with Modified Batho (MB) 1D density correction method, and recalculated with anisotropic analytical algorithm (AAA), using the same prescribed dose. Dose parameters derived from dose volume histograms (DVH) and target coverage indices were compared. To compare dose distribution, 2D γ-index was applied, ranging from 1%/1 mm to 6%/6 mm. The results were displayed using γ-maps in 2D. Correlation between DVH metrics and γ passing rates was tested using Spearman's rank test and Wilcoxon paired test to calculate P values.

RESULTS

the plans generated with AAA predicted more heterogeneous dose distribution inside the target, with P<0.05. However, MB overestimated the dose predicting more coverage of the target by the prescribed dose. The γ analysis showed that the difference between MB and AAA could reach up to ±10%. The 2D γ-maps illustrated that AAA predicted more dose to organs at risks, as well as lower dose to the target compared to MB.

CONCLUSIONS

Taking into account of the electrons transport on radiotherapy plans showed a significant impact on delivered dose and dose distribution. When considering the AAA represent the true cumulative dose, a readjusting of the prescribed dose and an optimization to protect the organs at risks should be taken in consideration in order to obtain the better clinical outcome.

Lung Cancer Radiation Therapy: Monte Carlo Investigation of “Under Dose” by High Energy Photons

Loss of electronic equilibrium in lung tissue causes a build-up region in the tumor. Increasing the photon energy increases the depth at which electronic equilibrium is reestablished within the lung tumor. This study uses the Monte Carlo code PENELOPE for simulations of radiation treatment of tumor surrounded by lung. Six MV photons were compared to 15 MV photons using four beam arrangements in both homogeneous and heterogeneous media. The experimental results demonstrate that for every beam arrangement in heterogeneous media 15 MV photons delivered 5% to 10% lower dose to the tumor periphery than 6 MV photons. The simulations also show that in axial coplanar treatment plans, the loss of electronic equilibrium was greatest in the coronal plane. In conclusion there is a tumor sparing effect at the tumor-lung interface that is a function of beam energy. As an alternative to increasing beam energy, the addition of multiple beam angles with lower energy photons improved target coverage. If higher energy beams are required for patients with large separation, then adding multiple beam angles does offer some improved target coverage. The non-coplanar technique with the lower energy photons covered the tumor with a greatest isodose at the tumor periphery without tangential sparing in the coronal plane.

Lung dose analysis in loco-regional hypofractionated radiotherapy of breast cancer

OBJECTIVES

To report the ipsilateral lung dosimetry data of breast cancer (BC) patients treated with loco-regional hypofractionated radiotherapy (HFRT). 

METHODS

Treatment plans of 150 patients treated in the Radiotherapy Unit, King Abdulaziz University Hospital, Jeddah, Kingdom of Saudi Arabia between January 2012 and March 2015 by HFRT for BC were retrospectively reviewed. All patients received 42.4 Gy in 16 fractions by tangential and supra-clavicular fields with 6 MV, 18 MV, or mixed energies. Ipsilateral lung dosimetric data V20Gy and mean lung dose (MLD) were recorded. Correlations between lung dose, patient characteristics, and treatment delivery parameters were assessed by a logistic regression test.

RESULTS

The mean ipsilateral lung V20Gy was 24.6% and mean MLD was 11.9 Gy. A weak, but statistically significant correlation was found between lung dose and lung volume (p=0.043). The lung dose was significantly decreasing with patient separation and depth of axillary lymph node (ALN) and supra-claviculary lymph nodes (SCLN) (p less than 0.0001), and increasing with ALN (p=0.001) and SCLN (p=0.003) dose coverage. Lung dose significantly decreased with beam energy (p less than 0.0001): mean V20Gy was 27.8%, 25.4% for 6 MV, mixed energy, and 21.2% for 18 MV. The use of a low breast-board angle correlates with low lung dose.

CONCLUSION

Our data suggest that the use of high energy photon beams and low breast-board angulation can reduce the lung dose.

Is pre-trial quality assurance necessary? Experiences of the CONVERT Phase III randomized trial for good performance status patients with limited-stage small-cell lung cancer

OBJECTIVE

This study is an analysis of the pre-trial quality assurance (QA) exercises submitted by clinicians from radiotherapy (RT) centres across Europe and Canada to qualify for participation in the CONVERT trial.

METHODS

QA exercises submitted by 64 clinicians at 64 RT centres were included in this analysis. The exercises included the completion of a trial-specific questionnaire and submission of a treatment plan, for both trial arms, for a patient fitting the eligibility criteria of the trial. This article describes the QA programme set up for the CONVERT trial and identifies deviations from the trial protocol. Patient eligibility, disease and critical structure outlining and treatment planning technique were assessed.

RESULTS

Results from QA trial-specific questionnaires received between February 2008 and September 2011, returned as part of the QA exercise, indicated that the majority of centres (70.3%) were using 6-MV photons and type B treatment planning system algorithms (57.8%). 90.6% of clinicians assessed submitted data for patients who fitted the eligibility criteria for the trial. There were inconsistencies in outlining of gross tumour volume (GTV) and organs at risk, mainly heart and oesophagus, and in the use of margins around the GTV.

CONCLUSION

Such a QA programme helps to ensure that centres conform to trial protocol and should reduce inconsistencies in RT planning that may confound the results of the CONVERT trial.

ADVANCES IN KNOWLEDGE

Few studies reporting pre-trial QA have been published to date. This article outlines the importance of such a QA programme in the context of multicentre Phase III studies.

A dosimetric phantom study of dose accuracy and build-up effects using IMRT and RapidArc in stereotactic irradiation of lung tumours

Background and purpose

Stereotactic lung radiotherapy (SLRT) has emerged as a curative treatment for medically inoperable patients with early-stage non-small cell lung cancer (NSCLC) and the use of intensity-modulated radiotherapy (IMRT) and volumetric modulated arc treatments (VMAT) have been proposed as the best practical approaches for the delivery of SLRT. However, a large number of narrow field shapes are needed in the dose delivery of intensity-modulated techniques and the probability of underdosing the tumour periphery increases as the effective field size is decreased. The purpose of this study was to evaluate small lung tumour doses irradiated by intensity-modulated techniques to understand the risk for dose calculation errors in precision radiotherapy such as SLRT.

Materials and methods

The study was executed with two heterogeneous phantoms with targets of Ø1.5 and Ø4.0 cm. Dose distributions in the simulated tumours delivered by small sliding window apertures (SWAs), IMRT and RapidArc treatment plans were measured with radiochromic film. Calculation algorithms of pencil beam convolution (PBC) and anisotropic analytic algorithm (AAA) were used to calculate the corresponding dose distributions.

Results

Peripheral doses of the tumours were decreased as SWA decreased, which was not modelled by the calculation algorithms. The smallest SWA studied was 2 mm, which reduced the 90% isodose line width by 4.2 mm with the Ø4.0 cm tumour as compared to open field irradiation. PBC was not able to predict the dose accurately as the gamma evaluation failed to meet the criteria of ±3%/±1 mm on average in 61% of the defined volume with the smaller tumour. With AAA the corresponding value was 16%. The dosimetric inaccuracy of AAA was within ±3% with the optimized treatment plans of IMRT and RapidArc. The exception was the clinical RapidArc plan with dose overestimation of 4%.

Conclusions

Overall, the peripheral doses of the simulated lung tumours were decreased by decreasing the SWA. To achieve adequate surface dose coverage to small lung tumours with a difference less than 1 mm in the isodose line radius between the open and modulated field, a larger than 6 mm SWA should be used in the dose delivery of SLRT.

Optimization of photon beam energy in aperture-based inverse planning

Optimal choice of beam energy in radiation therapy is easy in many well‐documented cases, but less obvious in some others. Low‐energy beams may provide better conformity around the target than their high‐energy counterparts due to reduced lateral scatter, but they also contribute to overdosage of peripheral normal tissue. Beam energy was added as an optimization parameter in an automatic aperture‐based inverse planning system. We have investigated a total of six cases for two sites (prostate and lung), representative of deep‐seated and moderately deep‐seated tumors. For one case for each site, different numbers of beam incidences were considered. The other cases for each site were optimized using a fixed number of incidences. Four types of plans were optimized: 6 MV, 23 MV, and mixed energy plans with one or two energies per incidence. Each plan was scored with a dose‐volume cost function. Cost function values, number of segments, monitor units, dose‐volume parameters, and isodose distributions were compared. For the prostate and lung cases, energy mixing improved plans in terms of cost function values, with a more important reduction for a small number of beam incidences. Use of high energy allowed better peripheral tissue sparing, while keeping similar target coverage and sensitive structures avoidance. Low energy contribution to monitor units usually increased with the number of beam incidences. Thus, for deep‐seated and moderately deep‐seated tumors, energy optimization can produce interesting plans with less peripheral dose and monitor units than for low energy alone.

PACS numbers: 87.55.de, 87.55.dk, 87.56.N‐