Estimating the uncertainty of calculated out-of-field organ dose from a commercial treatment planning system

Calculation of Organ Dose Distribution (in-field and Out-of-field) in Breast Cancer Radiotherapy on RANDO Phantom Using GEANT4 Application for Tomographic Emission (Gate) Monte Carlo Simulation

Introduction

Organ dose distribution calculation in radiotherapy and knowledge about its side effects in cancer etiology is the most concern for medical physicists. Calculation of organ dose distribution for breast cancer treatment plans with Monte Carlo (MC) simulation is the main goal of this study.

Materials and Methods

Elekta Precise linear accelerator (LINAC) photon mode was simulated and verified using the GEANT4 application for tomographic emission. Eight different radiotherapy treatment plans on RANDO's phantom left breast were produced with the ISOgray treatment planning system (TPS). The simulated plans verified photon dose distribution in clinical tumor volume (CTV) with TPS dose volume histogram (DVH) and gamma index tools. To verify photon dose distribution in out-of-field organs, the point dose measurement results were compared with the same point doses in the MC simulation. Eventually, the DVHs for out-of-field organs that were extracted from the TPS and MC simulation were compared.

Results

Based on the implementation of gamma index tools with 2%/2 mm criteria, the simulated LINAC output demonstrated high agreement with the experimental measurements. Plan simulation for in-field and out-of-field organs had an acceptable agreement with TPS and experimental measurement, respectively. There was a difference between DVHs extracted from the TPS and MC simulation for out-of-field organs in low-dose parts. This difference is due to the inability of the TPS to calculate dose distribution in out-of-field organs.

Conclusion and Discussion

Based on the results, it was concluded that the treatment plans with the MC simulation have a high accuracy for the calculation of out-of-field dose distribution and could play a significant role in evaluating the important role of dose distribution for second primary cancer estimation.

Is deep learning-enabled real-time personalized CT dosimetry feasible using only patient images as input?

PURPOSE

To propose a novel deep-learning based dosimetry method that allows quick and accurate estimation of organ doses for individual patients, using only their computed tomography (CT) images as input.

METHODS

Despite recent advances in medical dosimetry, personalized CT dosimetry remains a labour-intensive process. Current state-of-the-art methods utilize time-consuming Monte Carlo (MC) based simulations for individual organ dose estimation in CT. The proposed method uses conditional generative adversarial networks (cGANs) to substitute MC simulations with fast dose image generation, based on image-to-image translation. The pix2pix architecture in conjunction with a regression model was utilized for the generation of the synthetic dose images. The lungs, heart, breast, bone and skin were manually segmented to estimate and compare organ doses calculated using both the original and synthetic dose images, respectively.

RESULTS

The average organ dose estimation error for the proposed method was 8.3% and did not exceed 20% for any of the organs considered. The performance of the method in the clinical environment was also assessed. Using segmentation tools developed in-house, an automatic organ dose calculation pipeline was set up. Calculation of organ doses for heart and lung for each CT slice took about 2 s.

CONCLUSIONS

This work shows that deep learning-enabled personalized CT dosimetry is feasible in real-time, using only patient CT images as input.

Comparison of 3DCRT and IMRT out-of-field doses in pediatric patients using Monte Carlo simulations with treatment planning system calculations and measurements

3DCRT and IMRT out-of-field doses in pediatric patients were compared using Monte Carlo simulations with treatment planning system calculations and measurements.

Purpose

Out-of-field doses are given to healthy tissues, which may allow the development of second tumors. The use of IMRT in pediatric patients has been discussed, as it leads to a "bath" of low doses to large volumes of out-of-field organs and tissues. This study aims to compare out-of-field doses in pediatric patients comparing IMRT and 3DCRT techniques using measurements, Monte Carlo (MC) simulations, and treatment planning system (TPS) calculations.

Materials and methods

A total dose of 54 Gy was prescribed to a PTV in the brain of a pediatric anthropomorphic phantom, for both techniques. To assess the out-of-field organ doses for both techniques, two treatment plans were performed with the 3DCRT and IMRT techniques in TPS. Measurements were carried out in a LINAC using a pediatric anthropomorphic phantom and thermoluminescent dosimeters to recreate the treatment plans, previously performed in the TPS. A computational model of a LINAC, the associated multileaf collimators, and a voxelized pediatric phantom implemented in the Monte Carlo N-Particle 6.1 computer program were also used to perform MC simulations of the out-of-field organ doses, for both techniques.

Results

The results obtained by measurements and MC simulations indicate a significant increase in dose using the IMRT technique when compared to the 3DCRT technique. More specifically, measurements show higher doses with IMRT, namely, in right eye (13,041 vs. 593 mGy), left eye (6,525 vs. 475 mGy), thyroid (79 vs. 70 mGy), right lung (37 vs. 28 mGy), left lung (27 vs. 20 mGy), and heart (31 vs. 25 mGy). The obtained results indicate that out-of-field doses can be seriously underestimated by TPS.

Discussion

This study presents, for the first time, out-of-field dose measurements in a realistic scenario and calculations for IMRT, centered on a voxelized pediatric phantom and an MC model of a medical LINAC, including MLC with log file-based simulations. The results pinpoint significant discrepancies in out-of-field doses for the two techniques and are a cause of concern because TPS calculations cannot accurately predict such doses. The obtained doses may presumably increase the risk of development of second tumors.

Out-of-field organ doses and associated risk of cancer development following radiation therapy with photons

Innovations in cancer treatment have contributed to the improved survival rate of these patients. Radiotherapy is one of the main options for cancer management nowadays. High doses of ionizing radiation are usually delivered to the tumor site with high energy photon beams. However, the therapeutic radiation exposure may lead to second cancer induction. Moreover, the introduction of intensity-modulated radiation therapy over the last decades has increased the radiation dose to out-of-field organs compared to that from conventional irradiation. The increased organ doses might result in elevated probabilities for developing secondary malignancies to critical organs outside the treatment volume. The organ-specific dosimetry is considered necessary for the theoretical second cancer risk assessment and the proper analysis of data derived from epidemiological reports. This study reviews the methods employed for the measurement and calculation of out-of-field organ doses from exposure to photons and/or neutrons. The strengths and weaknesses of these dosimetric approaches are described in detail. This is followed by a review of the epidemiological data associated with out-of-field cancer risks. Previously published theoretical cancer risk estimates for adult and pediatric patients undergoing radiotherapy with conventional and advanced techniques are presented. The methodology for the theoretical prediction of the probability of carcinogenesis to out-of-field sites and the limitations of this approach are discussed. The article also focuses on the factors affecting the magnitude of the probability for developing radiotherapy-induced malignancies. The restriction of out-of-field doses and risks through the use of different types of shielding equipment is presented.

Effects of flattening filter (FF) and flattening filter-free (FFF) beams on small-field and large-field dose distribution using the VMAT treatment plan

Introduction: The aim of the study was to evaluate the influence of flattening filter (FF) and flattening filter-free (FFF) beams on small-field and large-field dose distribution using the VMAT treatment plan.

Material and methods: Dose distribution calculations were performed for the VMAT technique in two locations: the larynx (small irradiation field; average 30.1 cm2) and gynecology (large irradiation field; average 173.1 cm2) using X-6MV flattening filter (FF) and flattening filter-free (FFF) beams. The following values were compared: the number of monitor units, minimum doses, average doses in PTV and maximum average doses in OaR (spinal cord – in larynx radiotherapy, bladder and rectum - in gynecological radiotherapy) and RPI (Radiation Planning Index) coefficient.

Results and Discussion: The performed statistical tests indicate that there is a significant difference (p <0.05) between the number of monitor units in the irradiation of large (gynecological) fields between the FF and FFF beams. The dose distributions show no statistically significant differences between the flattening filter and flattening-free filter beams (regardless of the field size).

Conclusions: Due to the smaller number of monitor units, it is recommended to use flattening filter beams (FF) for large-field radiotherapy.

Out-of-field doses from radiotherapy using photon beams: A comparative study for a pediatric renal treatment

PURPOSE

First, this experimental study aims at comparing out-of-field doses delivered by three radiotherapy techniques (3DCRT, VMAT (two different accelerators), and tomotherapy) for a pediatric renal treatment. Secondly, the accuracy of treatment planning systems (TPS) for out-of-field calculation is evaluated.

METHODS

EBT3 films were positioned in pediatric phantoms (5 and 10 yr old). They were irradiated according to four plans: 3DCRT (Clinac 2100CS, Varian), VMAT (Clinac 2100CS and Halcyon, Varian), and tomotherapy for a same target volume. 3D dose determination was performed with an in-house Matlab tool using linear interpolation of film measurements. 1D and 3D comparisons were made between techniques. Finally, measurements were compared to the Eclipse (Varian) and Tomotherapy (Accuray) TPS calculations.

RESULTS

Advanced radiotherapy techniques (VMATs and tomotherapy) deliver higher out-of-field doses compared to 3DCRT due to increased beam-on time triggered by intensity modulation. Differences increase with distance to target and reach a factor of 3 between VMAT and 3DCRT. Besides, tomotherapy delivers lower doses than VMAT: although tomotherapy beam-on time is higher than in VMAT, the additional shielding of the Hi-Art system reduces out-of-field doses. The latest generation Halcyon system proves to deliver lower peripheral doses than conventional accelerators. Regarding TPS calculation, tomotherapy proves to be suitable for out-of-field dose determination up to 30 cm from field edge whereas Eclipse (AAA and AXB) largely underestimates those doses.

CONCLUSION

This study shows that the high dose conformation allowed by advanced radiotherapy is done at the cost of higher peripheral doses. In the context of treatment-related risk estimation, the consequence of this increase might be significative. Modern systems require adapted head shielding and a particular attention has to be taken regarding on-board imaging dose. Finally, TPS advanced dose calculation algorithms do not certify dose accuracy beyond field edges, and thus, those doses are not suitable for risk assessment.

Phantom dosimetry and cancer risks estimation undergoing 6 MV photon beam by an Elekta SL-25 linac

The High-energy linear accelerator (linac) is a valuable tool and the most commonly used devices for external beam radiation treatments in subjects suffer from cancer. To estimate the dose deposited in several organs of a female patient due to pelvic irradiation by an Elekta SL-25 linac in 6 MV photon beam mode, the MCNPX code is used considering the most details of linac. The equivalent dose in different organs is computed according to the face down position (prone) of MIRD and UFRO phantoms. The data obtained using MCNPX show that the received dose in all commons organs of MIRD and UFRO phantoms is 535.73 and 433.09 mSv/Gy_x, respectively. The risks of second cancer incidence and mortality during radiotherapy treatment are compared between MIRD and UFRO phantoms. The results indicated that bladder has the maximum risk of secondary cancer incidence risk of 142.85 and 135.34 per 10⁵ persons based on MIRD and UFRO phantoms, respectively; while the total risk is about 1 in 163 and about 1 in 101 in these phantoms.

Part I: Out-of-field dose mapping for 6X and 6X-flattening-filter-free beams on the TrueBeam for extended distances

PURPOSE

With increasing cancer treatment success rates, many patients go on to live long, productive lives following recovery. Therefore, minimizing potential side effects due to dose outside the treated field is becoming a significant consideration in radiation therapy. With many potential treatment configurations available, it is important to quantify how out-of-field dose varies with common variables such as distance from isocenter, couch angle, jaw size, and flattening-filter setting. The accurate quantification of out-of-field dose at extended distances could also benefit researchers and detector developers. While data exist for out-of-field dose from older linear accelerator (Linac) models, the phenomenon has not been described for the latest generation of machines, such as the Varian TrueBeam. The purpose of this study was to comprehensively quantify out-of-field dose for the Varian TrueBeam Linac low energy photons in a wide range of positions and treatment geometries.

METHOD AND MATERIALS

Out-of-field doses were measured using two phantom setups: (a) A large volume ion chamber with a buildup sleeve to quantify head leakage and collimator scatter background dose; and (b) A farmer ion chamber in solid water to incorporate phantom scatter in addition to collimator scatter, and head leakage background dose. In both cases, the ion chamber was positioned with its length along the slowly varying transverse direction (perpendicular to the radial from isocenter). Doses were measured for four symmetric jaw settings (2 × 2 cm² , 4 × 4 cm² , 10 × 10 cm² , and 20 × 20 cm² ) for a range of distances from the isocenter (0-100 cm). The angular dependence of the out-of-field dose was measured using four different angles: 0°, 45°, 90°, and 135° with respect to the in-plane direction. All measurements were performed for both 6X and 6X-flattening-filter-free (FFF) beams.

RESULTS

The lowest out-of-field doses were observed at 60 cm away from isocenter in both in-plane and cross-plane directions for fields smaller than 10 × 10 cm² . Out-of-field dose decreased with decreasing jaw size (a factor of 4.7 for 6X-FFF and a factor of 3.1 for 6X going from 20 × 20 cm² to 2 × 2 cm² at 60 cm from isocenter in the in-plane direction). The 6X-FFF beam produced out-of-field doses as low as 64% of the 6X beam.

CONCLUSION

This study presents a comprehensive description of 6X and 6X-FFF out-of-field doses on a Varian TrueBeam Linac including measurements at a range of positions, angles, and jaw settings and with and without phantom scatter.