Second cancers following radiotherapy: a suggested common dosimetry framework for therapeutic and concomitant exposures

Risk of secondary malignancy following radiation therapy for prostate cancer

We investigated whether prostate cancer patients treated with external beam radiation therapy (EBRT) have a higher cumulative incidence of secondary cancer compared with patients treated with radical prostatectomy (RP). We used state-wide linked data from South Australia to follow men with prostate cancer diagnosed from 2002 to 2019. The cumulative incidence of overall and site-specific secondary cancers between 5 and 15 years after treatment was estimated. Fine-Gray competing risk analyses were performed with additional sensitivity analyses to test different scenarios. A total of 7625 patients were included (54% underwent RP and 46% EBRT). Characteristics of the two groups differed significantly, with the EBRT group being older (71 vs. 64 years), having higher comorbidity burden and being more likely to die during follow-up than the RP group. Fifteen-year cumulative incidence for all secondary cancers was 27.4% and 22.3% in EBRT and RP groups, respectively. In the adjusted models, patients in the EBRT group had a significantly higher risk of genitourinary (adjusted subhazard ratio (aSHR), 2.29; 95%CI 1.16-4.51) and lung (aSHR, 1.93; 95%CI 1.05-3.56) cancers compared with patients in the RP group. However, there was no statistically significant difference between the two groups for risk of any secondary cancer, gastro-intestinal, skin or haematologic cancers. No statistically significant differences in overall risk of secondary cancer were observed in any of the sensitivity analyses and patterns for risk at specific cancer sites were relatively consistent across different age restriction and latency/time-lag scenarios. In conclusion, the increased risk of genitourinary and lung cancers among men undergoing EBRT may relate partly to treatment effects and partly to unmeasured residual confounding.

Secondary bladder and rectal cancer risk estimates following standard fractionated and moderately hypofractionated VMAT for prostate carcinoma

PURPOSE

To estimate the risk for bladder and rectal cancer induction due to standard fractionated (SF) and moderately hypofractionated (HF) volumetric modulated arc therapy (VMAT) for prostate carcinoma.

METHODS

Twelve patients with low or intermediate-risk of prostate cancer referred for external-beam radiotherapy were included in this study. Three computed tomography-based VMAT plans were created for each study participant. The first plan was generated by assuming patient's irradiation with SF-VMAT (78 Gy in 39 fractions). The second and third plans were created on the basis of two different HF schedules (HF-VMAT₁ : 70 Gy in 30 fractions, HF:VMAT₂ : 60 Gy in 20 fractions). Data from differential dose-volume histograms obtained by the above treatment plans were employed to calculate the organ equivalent dose (OED) of the bladder and rectum with the aid of a nonlinear model accounting for fractionation and proliferation effects. The calculated OED values were used to estimate the average lifetime attributable risk (LAR_av ) for the appearance of radiotherapy-induced secondary bladder and rectal malignancies. The lifetime risk of radiation carcinogenesis was compared with the respective organ-, and age-dependent lifetime intrinsic risk (LIR) of cancer development for unexposed males.

RESULTS

The average OED of the rectum from SF-VMAT, HF-VMAT₁ and HF-VMAT₂ for prostate cancer was 972.0, 900.2, and 815.7 cGy, respectively. The corresponding values for bladder were 73.4, 72.3, and 71.0 cGy. The LAR_av for rectal cancer induction varied from 0.06% to 0.4% by the fractionation schedule used for irradiation and by the age of the patient at the time of treatment. The corresponding risk range related to the development of secondary bladder malignancies was 0.06-0.33%. The SF-VMAT, HF-VMAT₁ and HF-VMAT₂ led to an increase of the lifetime rectal cancer risk with respect to LIR by 2.2-9.8%, 2.0-9.1% and 1.8-8.2%, respectively, depending upon the patient's age. The corresponding elevation for bladder cancer induction was up to 8.0%, 7.9% and 7.7%.

CONCLUSIONS

The use of VMAT for prostate carcinoma leads to a noteworthy increase of the lifetime risk for bladder and rectal cancer induction compared to that of unexposed people irrespective of the patient's age at the time of treatment and the applied fractionation scheme. The cancer risk data presented in this study may be taken into account by radiation oncologists and medical physicists in the selection of the optimal radiation therapy plan.

Risk of developing radiogenic cancer following photon-beam radiotherapy for Graves' orbitopathy

PURPOSE

The objective of this study was to estimate the probability for cancer development due to radiotherapy for Graves' orbitopathy with 6 MV x rays.

METHODS

Orbital irradiation was simulated with the MCNP code. The radiation dose received by 10 out-of-field organs having a strong disposition for carcinogenesis was calculated with Monte Carlo methods. These dose calculations were used to estimate the organ-dependent lifetime attributable risk (LAR) for cancer induction in 30- and 50-yr-old males and females on the basis of the linear model suggested by the BEIR-VII report. Differential dose-volume histograms derived from patients' three-dimensional (3D) radiotherapy plans were employed to determine the organ equivalent dose (OED) of the brain which was partly exposed to primary radiation. The OED and the relevant LAR for brain cancer development were assessed with the plateau, bell-shaped and mechanistic models. The radiotherapy-induced cancer risks were compared with the lifetime intrinsic risk (LIR) values for unexposed population.

RESULTS

The radiation dose range to organs excluded from the treatment volume was 0.1-91.0 mGy for a target dose of 20 Gy. These peripheral organ doses increased the LIRs for cancer development of unexposed 30- and 50-yr-old males up to 1.0% and 0.2%, respectively. The corresponding elevations after radiotherapy of females were 2.0% and 0.4%. The use of nonlinear models gave an OED range of the brain of 482.0-562.5 mGy depending upon the model used for analysis and the patient's gender. The elevation of the LIR for developing brain malignancies after radiotherapy of 30-yr-old males and females reached to 13.3% and 16.6%, respectively. The corresponding increases after orbital irradiation at the age of 50 yr were 6.7% and 8.3%.

CONCLUSIONS

The level of the LIR increase attributable to radiation therapy for GO varied widely by the organ under examination and the age and gender of the exposed subject. This study provides the required data to quantify the elevation of these baseline cancer risks following orbital irradiation.

Cancer risk after radiotherapy for benign diseases

Radiotherapy with low to intermediate doses has been historically employed for the management of several benign diseases. The exposure to ionizing radiation may increase the probability for carcinogenesis. The knowledge of this probability is of value for weighting the benefits and risks of radiotherapy against different therapeutic approaches. This study initially reviews the epidemiologic data associated with the cancer induction due to radiotherapy for non-malignant conditions in previous decades. Most of these data were derived from patients irradiated with conventional techniques, which are no longer applied, for some benign diseases not treated with radiotherapy nowadays. The follow-up of a series of patients undergoing modern radiotherapy for benign disorders may be used for estimating the radiation-induced cancer risk. The realization of this process is often difficult due to the relatively small number of patients undergoing radiation therapy for such diseases in many countries and due to long latent period for the appearance of a malignancy after exposure. The combination of dosimetric data, which can be obtained by phantom measurements or treatment planning systems or Monte Carlo calculations, with the appropriate linear and non-linear risk models may lead to theoretical estimates of the radiotherapy-induced cancer risks. The limitations of the method providing a whole-body cancer risk based on the effective dose concept are presented. The theoretical organ-specific risks for carcinogenesis give useful information about the development of malignancies at any in-field, partially in-field and out-of-field critical site. The uncertainties of the organ-dependent cancer risk estimates are discussed.

3D calculation of radiation-induced second cancer risk including dose and tissue response heterogeneities

PURPOSE

Tools for comparing relative induced second cancer risk, to inform choice of radiotherapy treatment plan, are becoming increasingly necessary as the availability of new treatment modalities expands. Uncertainties, in both radiobiological models and model parameters, limit the confidence of such calculations. The aim of this study was to develop and demonstrate a software tool to produce a malignant induction probability (MIP) calculation which incorporates patient-specific dose and allows for the varying responses of different tissue types to radiation.

METHODS

The tool has been used to calculate relative MIPs for four different treatment plans targeting a subtotally resected meningioma: 3D conformal radiotherapy (3DCFRT), volumetric modulated arc therapy (VMAT), intensity-modulated x-ray therapy (IMRT), and scanned protons.

RESULTS

Two plausible MIP models, with considerably different dose-response relationships, were considered. A fractionated linear-quadratic induction and cell-kill model gave a mean relative cancer risk (normalized to 3DCFRT) of 113% for VMAT, 16% for protons, and 52% for IMRT. For a linear no-threshold model, these figures were 105%, 42%, and 78%, respectively. The relative MIP between plans was shown to be significantly more robust to radiobiological parameter uncertainties compared to absolute MIP. Both models resulted in the same ranking of modalities, in terms of MIP, for this clinical case.

CONCLUSIONS

The results demonstrate that relative MIP is a useful metric with which treatment plans can be ranked, regardless of parameter- and model-based uncertainties. With further validation, this metric could be used to discriminate between plans that are equivalent with respect to other planning priorities.

Cancer risk estimates from radiation therapy for heterotopic ossification prophylaxis after total hip arthroplasty

PURPOSE

Heterotopic ossification (HO) is a frequent complication following total hip arthroplasty. This study was conducted to calculate the radiation dose to organs-at-risk and estimate the probability of cancer induction from radiotherapy for HO prophylaxis.

METHODS

Hip irradiation for HO with a 6 MV photon beam was simulated with the aid of a Monte Carlo model. A realistic humanoid phantom representing an average adult patient was implemented in Monte Carlo environment for dosimetric calculations. The average out-of-field radiation dose to stomach, liver, lung, prostate, bladder, thyroid, breast, uterus, and ovary was calculated. The organ-equivalent-dose to colon, that was partly included within the treatment field, was also determined. Organ dose calculations were carried out using three different field sizes. The dependence of organ doses upon the block insertion into primary beam for shielding colon and prosthesis was investigated. The lifetime attributable risk for cancer development was estimated using organ, age, and gender-specific risk coefficients.

RESULTS

For a typical target dose of 7 Gy, organ doses varied from 1.0 to 741.1 mGy by the field dimensions and organ location relative to the field edge. Blocked field irradiations resulted in a dose range of 1.4-146.3 mGy. The most probable detriment from open field treatment of male patients was colon cancer with a high risk of 564.3 × 10(-5) to 837.4 × 10(-5) depending upon the organ dose magnitude and the patient's age. The corresponding colon cancer risk for female patients was (372.2-541.0) × 10(-5). The probability of bladder cancer development was more than 113.7 × 10(-5) and 110.3 × 10(-5) for males and females, respectively. The cancer risk range to other individual organs was reduced to (0.003-68.5) × 10(-5).

CONCLUSIONS

The risk for cancer induction from radiation therapy for HO prophylaxis after total hip arthroplasty varies considerably by the treatment parameters, organ site in respect to treatment volume and patient's gender and age. The presented risk estimates may be useful in the follow-up studies of irradiated patients.

Radiation dose and contralateral breast cancer risk associated with megavoltage cone-beam computed tomographic image verification in breast radiation therapy

PURPOSE

To measure and compare organ doses from a standard tangential breast radiation therapy treatment (50 Gy delivered in 25 fractions) and a megavoltage cone-beam computed tomography (MV-CBCT), taken for weekly image verification, and assess the risk of radiation-induced contralateral breast cancer.

METHODS AND MATERIALS

Organ doses were measured with thermoluminescent dosimeters placed strategically within a female anthropomorphic phantom. The risk of radiation-induced secondary cancer of the contralateral breast was estimated from these values using excess absolute risk and excess relative risk models.

RESULTS

The effective dose from a MV-CBCT (8-monitor units) was 35.9 ± 0.2 mSv. Weekly MV-CBCT imaging verification contributes 0.5% and 17% to the total ipsilateral and contralateral breast dose, respectively. For a woman irradiated at age 50 years, the 10-year postirradiation excess relative risk was estimated to be 0.8 and 0.9 for treatment alone and treatment plus weekly MV-CBCT imaging, respectively. The 10-year postirradiation excess absolute risk was estimated to be 4.7 and 5.6 per 10,000 women-years.

CONCLUSIONS

The increased dose and consequent radiation-induced second cancer risk as calculated by this study introduced by the imaging verification protocols utilizing MV-CBCT in breast radiation therapy must be weighed against the benefits of more accurate treatment. As additional image verification becomes more common, it is important that data be collected in regard to long-term malignancy risk.

Static jaw collimation settings to minimize radiation dose to normal brain tissue during stereotactic radiosurgery

At the University of Arkansas for Medical Sciences (UAMS) intracranial stereotactic radiosurgery (SRS) is performed by using a linear accelerator with an add-on micromultileaf collimator (mMLC). In our clinical setting, static jaws are automatically adapted to the furthest edge of the mMLC-defined segments with 2-mm (X jaw) and 5-mm (Y jaw) margin and the same jaw values are applied for all beam angles in the treatment planning system. This additional field gap between the static jaws and the mMLC allows additional radiation dose to normal brain tissue. Because a radiosurgery procedure consists of a single high dose to the planning target volume (PTV), reduction of unnecessary dose to normal brain tissue near the PTV is important, particularly for pediatric patients whose brains are still developing or when a critical organ, such as the optic chiasm, is near the PTV. The purpose of this study was to minimize dose to normal brain tissue by allowing minimal static jaw margin around the mMLC-defined fields and different static jaw values for each beam angle or arc. Dose output factors were measured with various static jaw margins and the results were compared with calculated doses in the treatment planning system. Ten patient plans were randomly selected and recalculated with zero static jaw margins without changing other parameters. Changes of PTV coverage, mean dose to predefined normal brain tissue volume adjacent to PTV, and monitor units were compared. It was found that the dose output percentage difference varied from 4.9-1.3% for the maximum static jaw opening vs. static jaw with zero margins. The mean dose to normal brain tissue at risk adjacent to the PTV was reduced by an average of 1.9%, with negligible PTV coverage loss. This dose reduction strategy may be meaningful in terms of late effects of radiation, particularly in pediatric patients. This study generated clinical knowledge and tools to consistently minimize dose to normal brain tissue.

Evaluation of the peripheral dose in stereotactic radiotherapy and radiosurgery treatments

PURPOSE

The main purpose of this work was to compare peripheral doses absorbed during stereotactic treatment of a brain lesion delivered using different devices. These data were used to estimate the risk of stochastic effects.

METHODS

Treatment plans were created for an anthropomorphic phantom and delivered using a LINAC with stereotactic cones and a multileaf collimator, a CyberKnife system (before and after a supplemental shielding was applied), a TomoTherapy system, and a Gamma Knife unit. For each treatment, 5 Gy were prescribed to the target. Measurements were performed with thermoluminescent dosimeters inserted roughly in the position of the thyroid, sternum, upper lung, lower lung, and gonads.

RESULTS

Mean doses ranged from of 4.1 (Gamma Knife) to 62.8 mGy (LINAC with cones) in the thyroid, from 2.3 (TomoTherapy) to 30 mGy (preshielding CyberKnife) in the sternum, from 1.7 (TomoTherapy) to 20 mGy (preshielding CyberKnife) in the upper part of the lungs, from 0.98 (Gamma Knife) to 15 mGy (preshielding CyberKnife) in the lower part of the lungs, and between 0.3 (Gamma Knife) and 10 mGy (preshielding CyberKnife) in the gonads.

CONCLUSIONS

The peripheral dose absorbed in the sites of interest with a 5 Gy fraction is low. Although the risk of adverse side effects calculated for 20 Gy delivered in 5 Gy fractions is negligible, in the interest of optimum patient radioprotection, further studies are needed to determine the weight of each contributor to the peripheral dose.

[IMRT combined to IGRT: increase of the irradiated volume. Consequences?]

Image-guided radiotherapy (IGRT) combined or not with intensity-modulated radiation therapy (IMRT) are new and very useful techniques. However, these new techniques are responsible of irradiation at low dose in large volumes. The control of alignment, realignment of the patient and target positioning in external beam radiotherapy are increasingly performed by radiological imaging devices. The management of this medical imaging depends on the practice of each radiotherapy centre. The physical doses due to the IGRT are however quantifiable and traceable. In one hand, these doses appear justified for a better targeting and could be considered negligible in the context of radiotherapy. On the other hand, the potential impact of these low doses should deserve the consideration of professionals. It appears important therefore to report and consider not only doses in target volumes and in "standard" organs at risk, but also the volume of all tissue receiving low doses of radiation. The recent development of IMRT launches the same issue concerning the effects of low doses of radiation. Indeed, IMRT increases the volume of healthy tissue exposed to radiation. At low dose (<100mGy), many parameters have to be considered for health risk estimations: the induction of genes and activation of proteins, bystander effect, radio-adaptation, the specific low-dose radio-hypersensitivity and individual radiation sensitivity. With the exception of the latter, the contribution of these parameters is generally protective in terms of carcinogenesis. An analysis of secondary cancers arising out of field appears to confirm such notion. The risk of secondary tumours is not well known in these conditions of treatment associating IMRT and IGRT. It is therefore recommended that the dose due to imaging during therapeutic irradiation be reported.

Assessment of patient organ doses and effective doses using the VIP-Man adult male phantom for selected cone-beam CT imaging procedures during image guided radiation therapy

A Monte Carlo based computational procedure for determining organ doses and effective doses has been described for two procedures used in newly developed image-guided radiation treatment: kilovoltage cone-beam computed tomography (kV CBCT) and mega-voltage computed tomography (MV CBCT). A whole-body patient computational phantom, VIP-Man phantom, is employed for Monte Carlo dose calculations. Results indicate that the thyroid dose is always the highest in head and neck (H&N) scan for both kV and MV CBCT, and the bladder dose is the highest in prostate scan for both kV and MV CBCT. For the VIP-Man phantom, it has been found that the effective dose for kV CBCT (assuming a total exposure of 1350 mAs) is approximately 9.5 mSv for the two anatomical sites, whereas the effective dose for MV CBCT (assuming a total of 6 monitor unit) ranges from 5.10 mSv for the H&N case to 8.39 mSv for the prostate scan. The estimated whole-body effective doses allow different imaging procedures to be compared and evaluated.

Doses to critical organs following radiotherapy and concomitant imaging of the larynx and breast

The development of conformal radiotherapy carries with it the implication of an increased number of imaging procedures at various stages throughout the overall treatment, principally for verification at some, or all, of the treatment fractions. This raises the issue of the balance between the benefit of these additional imaging exposures and the associated risk of radiocarcinogenesis arising from them. As such, it is necessary to appreciate the doses to critical organs for which individual carcinogenic risks have been estimated. In this study, doses to these organs have been measured with lithium fluoride thermoluminescence dosimetry loaded in anthropomorphic phantoms and subjected to realistic radiotherapy treatments of the larynx and breast, including concomitant CT and electronic portal imaging exposures associated with localization and verification of these treatments. Even for large numbers of concomitant images of either modality, arising from imaging at every fraction, the leakage and scatter from the radiotherapy itself is shown to dominate the overall organ dose, with imaging procedures generally contributing 5-20% of the total organ dose.

Cancer incidence after localized therapy for prostate cancer

BACKGROUND

Second cancers may occur in patients who have undergone radiation therapy. The risk for these adverse events after therapy is uncertain. In this study, the authors examined the size and significance of the observed association between occurrences of secondary cancers 5 years after radiotherapy in a large population of men with incident prostate cancer.

METHODS

Men with incident prostate cancer were identified from the Surveillance, Epidemiology, and End Results (SEER) registry and were distinguished by the type of treatment received, tumor stage, tumor grade, and age at diagnosis. SEER data also were used to identify occurrences of secondary cancer beginning 5 years after the date patients were diagnosed with prostate cancer. Multivariate logistic regression analysis was used to estimate the adjusted odds of the subsequent occurrence of other cancers associated with types of radiation therapy received and was adjusted for the type of surgery, tumor grade, stage, and patient age.

RESULTS

Compared with men who received no prostate cancer-directed radiation, men who received external beam radiation therapy (EBRT) as their only form of radiation therapy had statistically significant increased odds of developing secondary cancers at several sites potentially related to radiation therapy, including the bladder (odds ratio [OR], 1.63; 95% confidence interval [95% CI], 1.44-1.84) and rectum (OR, 1.60; 95% CI, 1.29-1.99). Men who received EBRT also had statistically significant higher odds of developing secondary cancers at sites in the upper body and other areas not potentially related to radiation therapy, including the cecum (OR, 1.63; 95% CI, 1.10-1.70), transverse colon (OR, 1.85; 95% CI, 1.30-2.63), brain (OR, 1.83; 95% CI, 1.22-2.75), stomach (OR, 1.38; 95% CI, 1.09-1.75), melanoma (OR, 1.29; 95% CI, 1.09-1.53), and lung and bronchus (OR, 1.25; 95% CI, 1.13-1.37) compared with the odds among men who received no radiation therapy. Men who received radiation therapy in the form of radioactive implants or isotopes, either in isolation or combined with beam radiation, did not have significantly different odds of secondary cancer occurring at any of the 20 most common sites.

CONCLUSIONS

Patients who received with EBRT had significantly higher odds of developing second cancers both overall and in the areas that were exposed to radiation. It is noteworthy that, to the authors' knowledge, this report shows for the first time that, despite the higher doses of radiation delivered, patients who received radioactive implants had the lowest odds of developing second cancers.

Organ doses from prostate radiotherapy and associated concomitant exposures

In addition to the therapeutic exposure, a course of radiotherapy will involve the additional (concomitant) irradiation of the patient using CT, simulator or portal imaging systems, for localization of the target volume and subsequent verification of treatment delivery. The number of concomitant exposures is likely to increase as the developing technical capabilities for conformal, image-guided radiotherapy make target and critical organ definition an increasingly important aspect of radiotherapy. Estimation of doses and risks to critical organs in the body from all sources is thus necessary to provide the basis for adequate justification of the exposures as required by ICRP. In this paper, doses to selected organs and tissues for which ICRP have identified fatal cancer probabilities have been measured using a realistic anthropomorphic phantom loaded with thermoluminescent dosemeters and irradiated using a treatment protocol for radical radiotherapy of the prostate. Independently, doses to the same organs and tissues have been measured from concomitant CT and portal imaging exposures given for localization and verification purposes. Although negligible in comparison with the target dose, realistic numbers of concomitant exposures give a small but significant contribution to the total dose to most organs and tissues outside the target volume. Generally, this is in the range 5-10% of the total organ dose, but can be as high as 20% for bone surfaces. These data may be used to estimate concomitant doses from any combination of CT and portal imaging and may help in the justification process, especially when additional verification exposures may be required during treatment.