Estimation of organs doses and radiation-induced secondary cancer risk from scattered photons for conventional radiation therapy of nasopharynx: a Monte Carlo study

Assessment of secondary cancer risks within non-target organs during proton therapy for lung cancer: A Monte Carlo study

Proton therapy is a rapidly progressing modality with a significant impact on lung cancer treatment. However, there are concerns about the subsequent effects of secondary radiation in out-of-field organs. Thus, the present study aimed to evaluate the risk of subsequent secondary cancers within non-target organs during proton therapy for lung cancer. A Monte Carlo model of the International Commission on Radiological Protection (ICRP) 110 male phantom was employed to calculate the absorbed dose associated with secondary photons and neutrons within out-of-field organs for different tumor locations. The risk of induced secondary cancers was then estimated using the Biological Effects of Ionizing Radiation Committee (BEIR) VII and National Council on Radiation Protection and Measurements (NCRP) 116 risk models. Organs close to the tumor, such as the heart, esophagus, thymus, and liver, received the highest equivalent doses. The calculated equivalent doses increased as the tumor depth increased from 4-8 cm to 12-16 cm. The contribution of neutrons to the total equivalent dose was dominant (up to 90%) in most of the organs studied. The calculated risks of secondary cancers were higher in the liver and esophagus compared with other organs when using the BEIR risk model. The maximum risk value was obtained for the left lung when the NCRP 116 risk model was used. Furthermore, the estimated risks of secondary malignancies increased with the tumor depth using both risk models. The calculated risks of radiation-induced secondary cancers were relatively lower than the baseline cancer risks. However, extra attention is warranted to minimize subsequent secondary cancers after proton therapy for lung cancer.

Secondary cancer risk assessment in healthy organs following craniospinal irradiation

INTRODUCTION

Medulloblastoma is a central nerves tumor that often occurs in pediatrics. The main radiotherapy technique for this tumor type is craniospinal irradiation (CSI), through which the whole brain and spinal cord are exposed to radiation. Due to the immaturity of healthy organs in pediatrics, radiogenic side effects such as second cancer are more severe. Accordingly, the current study aimed to evaluate the risk of secondary cancer development in healthy organs following CSI.

MATERIALS AND METHODS

Seven organs at risk (OARs) including skin, eye lens, thyroid, lung, liver, stomach, bladder, colon, and gonads were considered and the dose received by each OAR during CSI was measured inside an anthropomorphic RANDO phantom by TLDs. Then, the mean obtained dose for each organ was used to estimate the probability of secondary malignancy development according to the recommended cancer risk coefficients for specific organs.

RESULTS

The results demonstrated that the stomach and colon are at high risk of secondary malignancy occurrence, while the skin has the lowest probability of secondary cancer development. The total received dose after the treatment course by all considered organs was lower than the corresponding tolerable dose levels.

CONCLUSIONS

From the results, it can be concluded that some OARs during CSI are highly at risk of secondary cancer development. This issue may be of concern due to organ immaturity in pediatrics which can intensify the radiogenic effects of radiation exposure. Accordingly, strict shielding the OARs during craniospinal radiotherapy and/or sparing them from the radiation field through modern techniques such as hadron therapy is highly recommended.

Risk factors of secondary cancer in laryngeal, oropharyngeal, or hypopharyngeal cancer after definitive therapy

BACKGROUND

Our previous research showed that a high rate of secondary carcinogenesis is observed during follow-up after transoral surgery in patients with early-stage laryngeal, oropharyngeal, and hypopharyngeal cancers. We speculate that the contributing factors are alcohol drinking, smoking, and aging; however, we could not provide clear evidence. In this study, we aimed to identify the risk factors for secondary carcinogenesis in patients with these cancers, particularly factors associated with drinking and/or smoking.

METHODS

The medical records of all-stage laryngeal, oropharyngeal, and hypopharyngeal cancer patients who had undergone definitive treatment were retrospectively analyzed. Assessments included visual and endoscopic observations of the primary site, enhanced cervical CT or US of the primary site and regional lymph nodes, PET-CT, and enhanced whole-body CT. Clinical characteristics were compared in patients with and without secondary carcinogenesis and in patients with hypopharyngeal cancer and patients with other cancers.

RESULTS

Hypopharyngeal cancer was an independent risk factor for secondary cancer. The 5-year incidence rate of secondary cancer was 25.5%, 28.6%, and 41.2% in laryngeal, oropharyngeal, and hypopharyngeal cancers, respectively. Radiotherapy was defined as an independent risk factor in hypopharyngeal cancer patients with secondary cancers. No direct correlation was found between secondary carcinogenesis and alcohol consumption, smoking, or aging.

CONCLUSIONS

Patients with hypopharyngeal cancer require close follow-up as they are at high risk of developing secondary cancer, possibly because out-of-field radiation exposure may induce systemic secondary carcinogenesis in hypopharyngeal cancer patients with genetic abnormality induced by alcohol consumption.

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.

Development of clinical application program for radiotherapy induced cancer risk calculation using Monte Carlo engine in volumetric-modulated arc therapy

Background

The purpose of this study is to develop a clinical application program that automatically calculates the effect for secondary cancer risk (SCR) of individual patient. The program was designed based on accurate dose calculations using patient computed tomography (CT) data and Monte Carlo engine. Automated patient-specific evaluation program was configured to calculate SCR.

Methods

The application program is designed to re-calculate the beam sequence of treatment plan using the Monte Carlo engine and patient CT data, so it is possible to accurately calculate and evaluate scatter and leakage radiation, difficult to calculate in TPS. The Monte Carlo dose calculation system was performed through stoichiometric calibration using patient CT data. The automatic SCR evaluation program in application program created with a MATLAB was set to analyze the results to calculate SCR. The SCR for organ of patient was calculated based on Biological Effects of Ionizing Radiation (BEIR) VII models. The program is designed to sequentially calculate organ equivalent dose (OED), excess absolute risk (EAR), excess relative risk (ERR), and the lifetime attributable risk (LAR) in consideration of 3D dose distribution analysis. In order to confirm the usefulness of the developed clinical application program, the result values from clinical application program were compared with the manual calculation method used in the previous study.

Results

The OED values calculated in program were calculated to be at most approximately 13.3% higher than results in TPS. The SCR result calculated by the developed clinical application program showed a maximum difference of 1.24% compared to the result of the conventional manual calculation method. And it was confirmed that EAR, ERR and LAR values can be easily calculated by changing the biological parameters.

Conclusions

We have developed a patient-specific SCR evaluation program that can be used conveniently in the clinic. The program consists of a Monte Carlo dose calculation system for accurate calculation of scatter and leakage radiation and a patient-specific automatic SCR evaluation program using 3D dose distribution. The clinical application program that improved the disadvantages of the existing process can be used as an index for evaluating a patient treatment plan.

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.

Design and fabrication of a Nano-based neutron shield for fast neutrons from medical linear accelerators in radiation therapy

BACKGROUND

Photo-neutrons are produced at the head of the medical linear accelerators (linac) by the interaction of high-energy photons, and patients receive a whole-body-absorbed dose from these neutrons. The current study aimed to find an efficient shielding material for fast neutrons.

METHODS

Nanoparticles (NPs) of Fe3O4 and B4C were applied in a matrix of silicone resin to design a proper shield against fast neutrons produced by the 18 MeV photon beam of a Varian 2100 C/D linac. Neutron macroscopic cross-sections for three types of samples were calculated by the Monte Carlo (MC) method and experimentally measured for neutrons of an Am-Be source. The designed shields in different concentrations were tested by MCNPX MC code, and the proper concentration was chosen for the experimental test. A shield was designed with two layers, including nano-iron oxide and a layer of nano-boron carbide for eliminating fast neutrons.

RESULTS

MC simulation results with uncertainty less than 1% showed that for discrete energies and 50% nanomaterial concentration, the macroscopic cross-sections for iron oxide and boron carbide at the energy of 1 MeV were 0.36 cm- 1 and 0.32 cm- 1, respectively. For 30% nanomaterial concentration, the calculated macroscopic cross-sections for iron oxide and boron carbide shields for Am-Be spectrum equaled 0.12 cm- 1 and 0.15 cm- 1, respectively, while they are 0.15 cm- 1 and 0.18 cm- 1 for the linac spectrum. In the experiment with the Am-Be spectrum, the macroscopic cross-sections for 30% nanomaterial concentration were 0.17 ± 0.01 cm- 1 for iron oxide and 0.21 ± 0.02 cm- 1 for boron carbide. The measured transmission factors for 30% nanomaterial concentration with the Am-Be spectrum were 0.71 ± 0.01, 0.66 ± 0.02, and 0.62 ± 0.01 for the iron oxide, boron carbide, and double-layer shields, respectively. In addition, these values were 0.74, 0.69, and 0.67, respectively, for MC simulation for the linac spectrum at the same concentration and thickness of 2 cm.

CONCLUSION

Results achieved from MC simulation and experimental tests were in a satisfactory agreement. The difference between MC and measurements was in the range of 10%. Our results demonstrated that the designed double-layer shield has a superior macroscopic cross-section compared with two single-layer nanoshields and more efficiently eliminates fast photo-neutrons.

MONTE CARLO SIMULATION OF OUT-OF-FIELD ORGAN DOSES AND CANCER RISK IN TANZANIA FOR RADIATION THERAPY OF UNILATERAL RETINOBLASTOMA USING A 60Co UNIT

The use of 60Co teletherapy unit for the treatment of unilateral retinoblastoma (Rb) patients is a very common procedure in many developing countries including Tanzania. The aim of this study was to estimate organ-specific absorbed doses from an external beam radiation therapy 60Co unit for unilateral Rb and to assess the risks of the patients developing a secondary primary cancer. The absorbed dose estimations were based on a Monte Carlo method and a set of age-dependent computational male phantoms. The estimated doses were used to calculate the secondary cancer risks in out-of-field organs using the Biological Effects of Ionising Radiation VII risk models. The survival information and baseline cancer risks were based on relevant statistics for the Tanzanian population. The resulting out-of-field organ doses data showed that organs which are close to the target volume, such as the brain, salivary glands and thyroid glands, received the highest absorbed dose from scattered photons during the treatment of Rb. It was also found that the resulting photons dose to specific organs depends on the patient's age. Younger patients are more sensitive to radiation and also received higher dose contributions from the treatment head due to a larger part of the body exposed to the photon radiation. In all sites considered, the overall risks associated with radiation-induced secondary cancer were relatively lower than the baseline risks. Thus, the results in this article can help to provide good estimations of radiation-induced secondary cancer after radiation treatment of unilateral Rb using 60Co teletherapy unit in Tanzania and other developing countries.

Technical Note: Comparison of peripheral patient dose from MR-guided 60 Co therapy and 6 MV linear accelerator IGRT

PURPOSE

The use of X-ray imaging in radiation therapy can give a substantial dose to the patient. A Cobalt machine combined with an magnetic resonance imaging (MRI) was recently introduced to clinical work. One positive aspect of using non-ionizing imaging devices is the reduction of the patient exposure. The purpose of this study was to quantify the difference in out-of-field dose to the patient between the image guided radiation therapy (IGRT) treatment applied with a linear accelerator with cone beam CT (CBCT) equipment and a Cobalt machine combined with an MRI.

METHODS

The treatment of a rhabdomyosarcoma in the prostate was planned and irradiated using different modalities and radiation therapy machines. The whole-body dose was measured for a 3D-conformal radiation therapy (3DCRT), an intensity-modulated radiation therapy (IMRT), and a volumetric-modulated arc therapy plan applied with a conventional linear accelerator operated at 6 MV beam energy. Additionally, the dose of an IMRT plan applied with a ⁶⁰ Co machine combined with an MRI was measured. Furthermore, the dose of one CBCT scan using the linear accelerator's on-board imaging system was determined. The 3D dose measurements were performed in an anthropomorphic phantom containing 168 slots for thermoluminescence dosimeters (TLDs). A combination of LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD100H) was used to accurately determine the in- and out-of-field dose. The plans were rescaled to different fractionation schemes (2 Gy, 3 Gy, and 5 Gy fraction dose) and the dose of one CBCT scan was additionally added to the treatment dose per fraction applied with the linear accelerator. The resulting absorbed doses per fraction of the two machines were compared.

RESULTS

In the target region, all measured treatment plans presented the same magnitude of dose, while the CBCT dose was a factor of 100 smaller. Close to the planned target volume (PTV), the dose from the ⁶⁰ Co machine was a factor of two higher compared with the 3DCRT + CBCT dose. Up to 45 cm from the PTV, the treatment applied with the ⁶⁰ Co-sources showed an increased out-of-field dose compared to the linear accelerator + CBCT IGRT treatments. Further away from the PTV in the region where leakage from the gantry head is dominating, the out-of-field dose of the Cobalt machine was smaller compared to the linear accelerator + CBCT.

CONCLUSION

The peripheral dose of the ⁶⁰ Co machine combined with an MRI is larger up to 45 cm from the PTV and further away, it is lower than the dose from a linear accelerator + CBCT treatment. The presented fractionation schemes had a marginal impact on the results.

Conversion coefficients for determination of dispersed photon dose during radiotherapy: NRUrad input code for MCNP

Radiotherapy is a common cancer treatment module, where a certain amount of dose will be delivered to the targeted organ. This is achieved usually by photons generated by linear accelerator units. However, radiation scattering within the patient’s body and the surrounding environment will lead to dose dispersion to healthy tissues which are not targets of the primary radiation. Determination of the dispersed dose would be important for assessing the risk and biological consequences in different organs or tissues. In the present work, the concept of conversion coefficient (F) of the dispersed dose was developed, in which F = (D_d/D_t), where D_d was the dispersed dose in a non-targeted tissue and D_t is the absorbed dose in the targeted tissue. To quantify D_d and D_t, a comprehensive model was developed using the Monte Carlo N-Particle (MCNP) package to simulate the linear accelerator head, the human phantom, the treatment couch and the radiotherapy treatment room. The present work also demonstrated the feasibility and power of parallel computing through the use of the Message Passing Interface (MPI) version of MCNP5.

Use of PET/CT instead of CT-only when planning for radiation therapy does not notably increase life years lost in children being treated for cancer

BACKGROUND

PET/CT may be more helpful than CT alone for radiation therapy planning, but the added risk due to higher doses of ionizing radiation is unknown.

OBJECTIVE

To estimate the risk of cancer induction and mortality attributable to the [F-18]2-fluoro-2-deoxyglucose (FDG) PET and CT scans used for radiation therapy planning in children with cancer, and compare to the risks attributable to the cancer treatment.

MATERIALS AND METHODS

Organ doses and effective doses were estimated for 40 children (2-18 years old) who had been scanned using PET/CT as part of radiation therapy planning. The risk of inducing secondary cancer was estimated using the models in BEIR VII. The prognosis of an induced cancer was taken into account and the reduction in life expectancy, in terms of life years lost, was estimated for the diagnostics and compared to the life years lost attributable to the therapy. Multivariate linear regression was performed to find predictors for a high contribution to life years lost from the radiation therapy planning diagnostics.

RESULTS

The mean contribution from PET to the effective dose from one PET/CT scan was 24% (range: 7-64%). The average proportion of life years lost attributable to the nuclear medicine dose component from one PET/CT scan was 15% (range: 3-41%). The ratio of life years lost from the radiation therapy planning PET/CT scans and that of the cancer treatment was on average 0.02 (range: 0.01-0.09). Female gender was associated with increased life years lost from the scans (P < 0.001).

CONCLUSION

Using FDG-PET/CT instead of CT only when defining the target volumes for radiation therapy of children with cancer does not notably increase the number of life years lost attributable to diagnostic examinations.