Peripheral doses in CyberKnife radiosurgery

Modulated Arc Therapy for hippocampal-avoidance whole brain radiation therapy: planning comparison with intensity modulated Radiation Therapy

This study aimed to evaluate the modulated arc therapy (mARC) technique as a planning and treatment option for hippocampal sparing whole brain radiotherapy (HS-WBRT) following the Radiation Therapy Oncology Group (RTOG) 0933 dosimetric criteria. Computed tomography (CT) and magnetic resonance imaging (MRI) were selected retrospectively for 15 patients. Two types of plans were created for each patient, namely an intensity-modulated radiation therapy (IMRT) and a mARC plan. IMRT and mARC plans were compared in terms of plan quality indices, absorbed dose to organs at risk (OARs), number of monitor units (MUs), and treatment time. All plans in both techniques were considered clinically acceptable for treatment. However, IMRT plans presented a higher conformity (p = 0.01) as well as a higher homogeneity as compared to mARC plans, but this difference was not statistically significant (p > 0.05). In terms of the preservation of the hippocampus, it was observed that the IMRT plans achieved significantly lower doses for both 100% of its volume and for its maximum dose (p < 0.001). The evaluation of the remaining OARs showed that the IMRT technique resulted in lower doses, and significant differences were observed for the following organs: left cochlea (p < 0.001), left eye (p < 0.001), right eye (p = 0.03), both lenses of the eye (p < 0.001), and right optic nerve (p = 0.02). Despite these differences, the absolute differences in all dosimetric parameters were low enough to bear any clinical relevance. A drastic (close to 65%) and significant (p < 0.001) decrease was observed in the number of MUs for the mARC plans. This resulted in a substantial decrease in treatment time (60.45%, p < 0.001). It is concluded that the mARC technique is a feasible planning and treatment solution for HS-WBRT that meets the RTOG 0933 criteria. The main advantage of using mARC over IMRT for HS-WBRT is the considerable reduction in MUs and treatment time.

Comparison of the accuracy of Monte Carlo and Ray Tracing dose calculation algorithms for multiple target brain treatments on CyberKnife

Single plan multiple brain targets (MBT) stereotactic radiosurgery dose difference between Monte Carlo (MC) and Ray Tracing (RT) algorithms has not been studied. A retrospective study and dose measurements were performed to access factors influencing dose differences. Fifty-three RT treatment plans with a total of 209 brain metastases were extracted from Precision Treatment Planning System (TPS). These plans were generated using fixed cones and were delivered using the CyberKnife M6 system. The same treatment plans were recalculated using MC algorithm and keeping the beam parameters unchanged. MC calculated plan parameters were extracted and dose differences were normalised to MC calculated dose. Correlations were investigated. RT and MC calculated off-centre-ratio (OCR) and tissue-phantom-ratio (TPRs) were exported from the TPS and compared with measured. Plans with 5 gross tumour volumes (GTVs) were created on a phantom and dose measured using a CC04 ionisation chamber and microdiamond detector for comparison with calculated doses. Calculated and measured TPR agreed within ± 1% beyond depth of maximum dose. The OCR showed differences up to 4.3% in the penumbra and out-of-field (OOF) regions. Largest RT and MC calculated GTV mean dose difference was - 5.7%. An increase in the number of GTVs and reduction in the geometric separation of metastases were associated with increased differences between RT and MC calculated doses. In conclusion, calculated dose disagreement in MBT depends on the number of GTVs per plan, number of GTVs within a certain separation distance and plan complexity. MC dose calculation is recommended for complex CyberKnife SRS of MBT.

Influence of Specific Treatment Parameters on Nontarget and Out-of-Field Doses in a Phantom Model of Prostate SBRT with CyberKnife and TrueBeam

The aim of the study was to determine the influence of a key treatment plan and beam parameters on overall dose distribution and on doses in organs laying in further distance from the target during prostate SBRT. Multiple representative treatment plans (n = 12) for TrueBeam and CyberKnife were prepared and evaluated. Nontarget doses were measured with anionization chamber, in a quasi-humanoid phantom at four sites corresponding to the intestines, right lung, thyroid, and head. The following parameters were modified: radiotherapy technique, presence or not of a flattening filter, degree of modulation, and use or not of jaw tracking function for TrueBeam and beam orientation set-up, optimization techniques, and number of MUs for CyberKnife. After usual optimization doses in intestines (near the target) were 0.73% and 0.76%, in head (farthest from target) 0.05% and 0.19% for TrueBeam and CyberKnife, respectively. For TrueBeam the highest peripheral (head, thyroid, lung) doses occurred for the VMAT with the flattening filter while the lowest for 3DCRT. For CyberKnife the highest doses were for gantry with caudal direction beams blocked (gantry close to OARs) while the lowest was the low modulated VOLO optimization technique. The easiest method to reduce peripheral doses was to combine FFF with jaw tracking and reducing monitor units at TrueBeam and to avoid gantry position close to OARs together with reduction of monitor units at CyberKnife, respectively. The presented strategies allowed to significantly reduce out-of-field and nontarget doses during prostate radiotherapy delivered with TrueBeam and CyberKnife. A different approach was required to reduce peripheral doses because of the difference in dose delivery techniques: non-coplanar using CyberKnife and coplanar using TrueBeam, respectively.

Utilization of brachytherapy in Quebec, Canada

BACKGROUND AND PURPOSE

Despite the excellent clinical outcomes from brachytherapy treatments compared with other modalities and the low associated costs, there have been reports of a decline in utilization of brachytherapy. The aim of this study was to investigate in detail the trend in utilization of brachytherapy in the province of Québec, Canada, from 2011 to 2019.

MATERIALS AND METHODS

All radiotherapy clinics in the province of Quebec, and among these the clinics that provide brachytherapy treatments, were identified. This observational retrospective cohort study involved analysis of data compiled by the Ministère de la Santé et des Services Sociaux du Québec for the period of 2011 to end of 2019 on all brachytherapy procedures performed in the province of Quebec. Time series graphs were used to describe the number of high dose rate (HDR) and low dose rate (LDR) brachytherapy treatments during the studied time period. Statistical analysis was conducted using R statistical software.

RESULTS

Between 2011 and 2019, 12 hospitals in the province of Québec provided radiotherapy treatments, and all of them offered brachytherapy services. The median annual number of brachytherapy sessions was 4413 (range 3930-4829). HDR brachytherapy represented over 90% of all brachytherapy treatments throughout the study period. Significant changes over time were observed in the number of treatments: at least 5% change was seen only for the two most common subtypes of brachytherapy, HDR interstitial and HDR intracavitary, with an increase of 9.6% and a decrease of 9.2%, respectively. The use of other subtypes of brachytherapy (HDR-plesiotherapy, LDR-interstitial, LDR-intracavitary, LDR-eye plaque) was stable between 2011 and 2019, with ≤ 2.5% variation.

CONCLUSION

This study demonstrates an overall steady use of brachytherapy between 2011 and 2019 in Quebec. Brachytherapy offers numerous advantages for the treatment of diverse cancer sites. Although more sophisticated external beam radiotherapy treatments have emerged in the last decades, the precision and cost-effectiveness of brachytherapy remain unbeaten. To ensure the continued use and availability of brachytherapy, governments must put in place policies and regulations to that effect. Training and exposure of future health care professionals to brachytherapy within Quebec and Canada is essential to provide all patients the same access to this life saving modality.

Brain Damage and Patterns of Neurovascular Disorder after Ionizing Irradiation. Complications in Radiotherapy and Radiation Combined Injury

Exposure to ionizing radiation, mechanical trauma, toxic chemicals or infections, or combinations thereof (i.e., combined injury) can induce organic injury to brain tissues, the structural disarrangement of interactive networks of neurovascular and glial cells, as well as on arrays of the paracrine and systemic destruction. This leads to subsequent decline in cognitive capacity and decompensation of mental health. There is an ongoing need for improvement in mitigating and treating radiation- or combined injury-induced brain injury. Cranial irradiation per se can cause a multifactorial encephalopathy that occurs in a radiation dose- and time-dependent manner due to differences in radiosensitivity among the various constituents of brain parenchyma and vasculature. Of particular concern are the radiosensitivity and inflammation susceptibility of: 1. the neurogenic and oligodendrogenic niches in the subependymal and hippocampal domains; and 2. the microvascular endothelium. Thus, cranial or total-body irradiation can cause a plethora of biochemical and cellular disorders in brain tissues, including: 1. decline in neurogenesis and oligodendrogenesis; 2. impairment of the blood-brain barrier; and 3. ablation of vascular capillary. These changes, along with cerebrovascular inflammation, underlie different stages of encephalopathy, from the early protracted stage to the late delayed stage. It is evident that ionizing radiation combined with other traumatic insults such as penetrating wound, burn, blast, systemic infection and chemotherapy, among others, can exacerbate the radiation sequelae (and vice versa) with increasing severity of neurogenic and microvascular patterns of radiation brain damage.

Extracranial dose and the risk of radiation-induced malignancy after intracranial stereotactic radiosurgery: is it time to establish a therapeutic reference level?

BACKGROUND

To measure extracranial doses from Gamma Knife Perfexion (GKP) intracranial stereotactic radiosurgery (SRS) and model the risk of malignancy after SRS for different treatment platforms.

METHODS

Doses were measured for 20 patients undergoing SRS on a GKP at distances of 18, 43 and 75 cm from the target, corresponding to the approximate positions of the thyroid, breast and gonads respectively. A literature review was conducted to collect comparative data from other radiosurgery platforms. All data was used to calculate the dose to body organs. The National Cancer Institute (NCI) RadRAT calculator was used to estimate excess lifetime cancer risk from this exposure. Five different age groups covering childhood and younger adults were modelled for both sexes.

RESULTS

Extracranial doses delivered during SRS with the GKP were a median 0.04%, 0.008% and 0.002% of prescription dose at 18 cm, 43 cm and 70 cm from the isocentre respectively. Comparison with the literature revealed that the extracranial dose was lowest from GKP, then linacs equipped with micro-multileaf collimators (mMLC), then linacs equipped with circular collimators (cones), and highest from Cyberknife (CK). Estimated lifetime risks of radiation-induced malignancy in the body for patients treated with SRS aged 5-45 years were 0.03-0.88%, 0.36-11%, 0.61-18% and 2.2-39% for GKP, mMLC, cones and CK respectively.

CONCLUSIONS

We have compared typical extracranial doses from different platforms and quantified the lifetime risk of radiation-induced malignancy. The risk varies with platform. This should be taken into account when treating children and young adults with SRS. The concept of a therapeutic reference level (TRL), similar to the diagnostic reference level (DRL) established in radiology, is proposed.

Dosimetric comparison of coplanar and non-coplanar volumetric-modulated arc therapy in head and neck cancer treated with radiotherapy

Purpose

To evaluate the dosimetric variations in patients of head and neck cancer treated with definitive or adjuvant radiotherapy using optimized non-coplanar (ncVMAT) beams with coplanar (cVMAT) beams using volumetric arc therapy.

Materials and Methods

Twenty-two patients of head and neck cancer that had received radiotherapy using VMAT in our department were retrospectively analyzed. Each of the patients was planned using coplanar and non-coplanar orientations using an optimized couch angle and fluences. We analyzed the Conformity Index (CI_RTOG), Dose Homogeneity Index (DHI), Heterogeneity Index (HI_RTOG), low dose volume, target and organs-at-risk coverage in both the plans without changing planning optimization parameters.

Results

The prescription dose ranged from 60 Gy to 70 Gy. Using ncVMAT, CI_RTOG, DHI and HI_RTOG, and tumor coverage (ID_95%) had improved, low dose spillage volume in the body V_5Gy was increased and V_10Gy was reduced. Integral dose and intensity-modulated radiation therapy factor had increased in ncVMAT. In the case of non-coplanar beam arrangements, maximum dose (D_max) of right and left humeral head were reduced significantly whereas apex of the right and left lung mean dose were increased.

Conclusion

The use of ncVMAT produced better target coverage and sparing of the shoulder and soft tissue of the neck as well as the critical organ compared with the cVMAT in patients of head and neck malignancy.

Margin-Free Fractionated Stereotactic Radiation Therapy for Pediatric Brain Tumors

PURPOSE

Conventional radiation therapy (RT) to pediatric brain tumors exposes a large volume of normal brain to unwarranted radiation causing late toxicity. We hypothesized that in well demarcated pediatric tumors lacking microscopic extensions, fractionated stereotactic RT (SRT), without target volume expansions, can reduce high dose normal tissue irradiation without affecting local control.

METHODS AND MATERIALS

Between 2008 and 2017, 52 pediatric patients with brain tumors were treated using the CyberKnife (CK) with SRT in 180 to 200 cGy per fraction. Thirty representative cases were retrospectively planned for intensity modulated RT (IMRT) with 4-mm PTV expansion. We calculated the volume of normal tissue within the high or intermediate dose region adjacent to the target. Plan quality and radiation dose-volume dosimetry parameters were compared between CK and IMRT plans. We also reported overall survival, progression-free survival (PFS), and local control.

RESULTS

Tumors included low-grade gliomas (n = 28), craniopharyngiomas (n = 16), and ependymomas (n = 8). The volumes of normal tissue receiving high (≥80% of prescription dose or ≥40 Gy) or intermediate (80% > dose ≥50% of the prescription dose or 40 Gy > dose ≥25 Gy) dose were significantly smaller with CK versus IMRT plans (P < .0001 for all comparisons). With a median follow-up of 3.7 years (range, 0.1-9.0), 3-year local control was 92% for all patients. Eight failures occurred: 1 craniopharyngioma (marginal), 2 ependymomas (both in-field), and 5 low-grade gliomas (2 in-field, 1 marginal, and 2 distant).

CONCLUSIONS

Fractionated SRT using CK without target volume expansion appears to reduce the volume of irradiated tissue without majorly compromising local control in pediatric demarcated brain tumors. These results are hypothesis generating and should be tested and validated in prospective studies.

Influence of CyberKnife Prescription Isodose Line on the Discrepancy of Dose Results Calculated by the Ray Tracing and Monte Carlo Algorithms for Head and Lung Plans: A Phantom Study

Incorporation of the Monte Carlo (MC) algorithm in optimizing CyberKnife (CK) plans is cumbersome, and early models unconfgured MC calculations, therefore, this study investigated algorithm-based dose calculation discrepancies by selecting different prescription isodose lines (PIDLs) in head and lung CK plans. CK plans were based on anthropomorphic phantoms. Four shells were set at 2-60 mm from the target, and the constraint doses were adjusted according to the design strategy. After optimization, 30%-90% PIDL plans were generated by ray tracing (RT). In the evaluation module, CK plans were recalculated using the MC algorithm. Therefore, the dosimetric parameters of different PIDL plans based on the RT and MC algorithms were obtained and analyzed. The discrepancies (mean±SD) were 3.72%±0.31%, 3.40%±0.11%, 3.47%±0.32%, 0.17%±0.11%, 0.64%±3.60%, 7.73%±1.60%, 14.62%±3.21% and 10.10%±1.57% for D_1%, D_(mean), D_98% and coverage of the PTV, DGI, V₅, V₃ and V1 in the head plans and -6.32%±1.15%, -13.46%±0.98%, -20.63%±2.25%, -34.78%±25.03%, 122.48%±175.60%, -12.92%±5.41%, 3.19%±4.67% and 7.13%±1.56% in the lung plans, respectively. The following parameters were signifcantly correlated with PIDL: d_D98% at the 0.05 level and d_DGI, d_V5 and d_V3 at the 0.01 level for the head plans; d_D98% at the 0.05 level and d_D1%, d_D(mean), d_Coverage, d_DGI, d_V5 and d_V3 at the 0.01 level for the lung plans. RT may be used to calculate the dose in CK head plans, but when the dose of organs at risk is close to the limit, it is necessary to refer to the MC results or to further optimize the CK plan to reduce the dose. For lung plans, the MC algorithm is recommended. For early models without the MC algorithm, a lower PIDL plan is recommended; otherwise, a large PIDL plan risks serious underdosage in the target area.

10-MV SBRT FFF IRRADIATION TECHNIQUE IS ASSOCIATED TO THE LOWEST PERIPHERAL DOSE: THE OUTCOME OF 142 TREATMENT PLANS FOR THE 10 MOST COMMON TUMOUR LOCATIONS

There is a growing interest in the combined use of Stereotactic Body Radiation Therapy (SBRT) with Flattening Filter Free (FFF) due to the high local control rates and reduced treatment times, compared to conventionally fractionated treatments. It has been suggested that they may also provide a better radiation protection to radiotherapy patients as a consequence of the expected decrease in peripheral doses. This work aims to determine this reduction in unattended out-of-field regions, where no CT information is available but an important percentage of second primary cancers occur. For that purpose, ten different cases suitable for SBRT were chosen. Thus, 142 different treatment plans including SBRT, as well as 3D-CRT, IMRT and VMAT (with standard fractionation) in low and high energies for Varian (FF and FFF), Siemens and Elekta machines were created. Then, photon and neutron peripheral dose in 14 organs were assessed and compared using two analytical models. For the prostate case, uncomplicated and cancer free control probability estimation was also carried out. As a general behavior, SBRT plans led to the lowest peripheral doses followed by 3D-CRT, VMAT and IMRT, in this order. Unflattened beams proved to be the most effective in reducing peripheral doses, especially for 10 MV. The obtained results suggest that FFF beams for SBRT with 10 MV represent the best compromise between dose delivery efficiency and peripheral dose reduction.

A new Monte Carlo model of a Cyberknife® system for the precise determination of out-of-field doses

In a previous work, a PENELOPE Monte Carlo model of a Cyberknife system equipped with fixed collimator was developed and validated for in-field dose evaluation. The aim of this work is to extend it to evaluate peripheral doses and to determine the precision of the treatment planning system (TPS) Multiplan in evaluating the off-axis doses. The Cyberknife^® head model was completed with surrounding components based on manufacturer drawings. The contribution of the different head parts on the out-of-field dose was studied. To model the attenuation and the modification of particle energy caused by components not modelled, the photon transport was modified in one of the added components. The model was iteratively adjusted to fit dose profiles measured with EBT3 films and an ionization chamber for several collimator sizes. Finally, dose profiles were calculated using the two Multiplan TPS algorithms and were compared to our simulations. The contributions to out-of-field dose were identified as scattered radiation from the phantom and head leakage and scatter originating at the secondary collimator level. Particle transport in the additional pieces was modified to model this radiation. The maximum differences between simulated and measured doses are of 20.4%. Regarding the detector responses away from axis, EBT3 films and the Farmer chamber give similar response (less than 20% difference). The TPS Monte Carlo algorithm underestimates the doses away from axis more importantly for the smaller field sizes (up to 98%). Besides, RayTracing simplifies peripheral dose to a constant value with no inclusion of particle transport. A Monte Carlo model of a Cyberknife system for the determination of out-of-field doses up to 14 cm off-axis was successfully developed and validated for different depths and field sizes in comparison with measurements. This study also confirms that TPS algorithms do not model peripheral dose properly.

Evaluation of Healthy Tissue Dose at Different Regions between Volumetric-Modulated Arc Therapy and Intensity-Modulated Radiation Therapy Plans in the Treatment of Various Cancers

Background:

Radiotherapy plays an important role in the management of cancer. Although the improved technologies increase therapeutic index, different delivery techniques deliver different dose pattern to the healthy tissue within and outside treatment volume.

Objective:

The objective of this study was to evaluate the low, intermediate, and high dose to healthy tissue within and outside the treatment volume and to find the relation between tumor volume and various doses received healthy tissue volume.

Materials and Methods:

A total of 150 patients were included. For all patients, planning computed tomography images were acquired. Tumors, critical structures, and healthy tissue volumes at different regions were delineated. Two sets of plans, one with volumetric-modulated arc therapy and another with intensity-modulated radiation therapy (IMRT) were created, optimized for 6 MV photons and dose was calculated. Dosimetry results for tumor, organs at risks (OARs), and healthy tissue from both the techniques were evaluated and compared.

Results:

Tumor coverage and dose to OARs was significantly better with volumetric-modulated arc therapy (VMAT). Volume of healthy tissue received high-dose within the treatment volume as well as volume of healthy tissue received low and intermediate-dose out of treatment volume were significantly (P < 0.002) lesser with VMAT. Besides, the results showed that as the tumor volume increased, the various dose received healthy tissue volume also increased.

Conclusions:

VMAT plan can reduce the risk of secondary malignancy while treating different sites of cancer. VMAT is the most appropriate technique than IMRT, especially in the treatment of large tumor volume. Special attention has to be given, especially while treating women and children.

Investigating the surface dose contribution of intrafractional kV imaging in CyberKnife-based stereotactic radiosurgery

CyberKnife treatment consists of hundreds of noncoplanar beams and numerous intrafractional images that can be taken during a single treatment fraction; thus, doses because of imaging should be considered in this technique. The aim of this study is to investigate the in-field and out-of-field surface doses induced from kV imaging system during stereotactic radiosurgery (SRS) treatment. The imaging-induced surface doses were measured at the center of the imaging field and within ±15-cm distance from the center in both craniocaudal and lateral directions. TLD100H thermoluminescence dosimeters and EBT2 gafchromic films were used to take the measurements at the locations of 0, ±5, ±10, and ±15 cm in the 2 orthogonal directions on abdominal region of a Rando phantom. The surface dose contributions of imaging system for the 4 most commonly used energy options of 90, 100, 110, and 120 kVp with 3 mAs options of 10, 30, and 90 mAs were measured and compared. Imaging dose values have a positive correlation with both parameters of energy and mAs. The energy options of 100, 110, and 120 kVp, in average, induced 60%, 101%, and 141% more doses per mAs than 90 kVp energy in the imaging field center. A threefold increase in mAs values, i.e., from 10 mAs to 30 mAs and from 30 mAs to 90 mAs, caused higher dose in field center with a factor of 2.53 ± 0.08 when the energy value was kept constant. The in-field dose distributions within ±10 cm in both directions showed a flat pattern with a standard deviation lower than 5%, whereas the out-of-field doses at ±15-cm distance from the field center suddenly dropped to almost half of the central doses. Although a single imaging attempt causes a very low dose compared with the therapeutic dose level, one should be aware of the cumulative surface dose increase with higher imaging number. Proper patient setup, fiducial usage, and reduction of both the mAs values and the imaging numbers should be, therefore, considered to keep the cumulative surface dose in a lower level.

CyberKnife® M6™: Peripheral dose evaluation for brain treatments

PURPOSE

This study evaluates the peripheral dose (PD) delivered to healthy tissues for brain stereotactic radiotherapy treatments (SRT) performed with a CyberKnife M6™ Robotic Radiosurgery System and proposes a model to estimate PD before treatment.

METHOD

PD was measured with thermoluminescent dosimeters. Measurements were performed to evaluate the influence of distance, collimator type (fixed or Iris™) and aperture size on PD for typical brain treatment plans simulated on an anthropomorphic phantom. A model to estimate PD was defined by fitting functions to these measurements. In vivo measurements were subsequently performed on 30 patients and compared to the model-predicted PD.

RESULTS

PD (in cGy) was about 0.06% of MU at 15cm for a 20mm fixed collimator and 0.04% of MU for the same aperture with Iris™ collimator. In vivo measurements showed an average thyroid dose of 55mGy (σ=18.8mGy). Computed dose for thyroid, breast, umbilicus and gonads showed on average a relative difference of 3.4% with the in vivo dose (σ=12.4%).

CONCLUSION

PD at the thyroid with Iris™ was about a third lower than with a fixed collimator in case of brain SRT. Despite uncertainties (use of anthropomorphic PD to estimate patient specific PD, surface PD to estimate OAR PD) the model allows PD to be estimated without in vivo measurements. This method could be used to optimise PD with different planning strategies.

Risk of second cancers in the era of modern radiation therapy: does the risk/benefit analysis overcome theoretical models?

In the era of modern radiation therapy, the compromise between the reductions in deterministic radiation-induced toxicities through highly conformal devices may be impacting the stochastic risk of second malignancies. We reviewed the clinical literature and evolving theoretical models evaluating the impact of intensity-modulated radiation therapy (IMRT) on the risk of second cancers, as a consequence of the increase in volumes of normal tissues receiving low doses. The risk increase (if any) is not as high as theoretical models have predicted in adults. Moreover, the increase in out-of-field radiation doses with IMRT could be counterbalanced by the decrease in volumes receiving high doses. Clinical studies with short follow-up have not corroborated the hypothesis that IMRT would drastically increase the incidence of second cancers. In children, the risk of radiation-induced carcinogenesis increases from low doses and consequently the relative risk of second cancers after IMRT could be higher than in adults, justifying current developments of proton therapy with priority given to this population. Although only longer follow-up will allow a true assessment of the real impact of these modern techniques on radiation-induced carcinogenesis, a comprehensive risk-adapted strategy will help minimize the probability of second cancers.

EURADOS strategic research agenda: vision for dosimetry of ionising radiation

Since autumn 2012, the European Radiation Dosimetry Group (EURADOS) has been developing its Strategic Research Agenda (SRA), which is intended to contribute to the identification of future research needs in radiation dosimetry in Europe. The present article summarises-based on input from EURADOS Working Groups (WGs) and Voting Members-five visions in dosimetry and defines key issues in dosimetry research that are considered important for the next decades. The five visions include scientific developments required towards (a) updated fundamental dose concepts and quantities, (b) improved radiation risk estimates deduced from epidemiological cohorts, (c) efficient dose assessment for radiological emergencies, (d) integrated personalised dosimetry in medical applications and (e) improved radiation protection of workers and the public. The SRA of EURADOS will be used as a guideline for future activities of the EURADOS WGs. A detailed version of the SRA can be downloaded as a EURADOS report from the EURADOS website (www.eurados.org).

[The issue of low doses in radiation therapy and impact on radiation-induced secondary malignancies]

Several studies have well documented that the risk of secondary neoplasms is increasing among patients having received radiation therapy as part of their primary anticancer treatment. Most frequently, radiation-induced neoplasms occur in volume exposed to high doses. However, the impact of "low" doses (<5 Gy) in radiation-induced carcinogenesis should be clinically considered because modern techniques of intensity-modulated radiation therapy (IMRT) or stereotactic irradiation significantly increase tissue volumes receiving low doses. The risk inherent to these technologies remains uncertain and estimates closely depend on the chosen risk model. According to the (debated) linear no-threshold model, the risk of secondary neoplasms could be twice higher with IMRT, as compared to conformal radiation therapy. It seems that only proton therapy could decrease both high and low doses delivered to non-target volumes. Except for pediatric tumors, for which the unequivocal risk of second malignancies (much higher than in adults) should be taken into account, epidemiological data suggest that the risk of secondary cancer related to low doses could be very low, even negligible in some cases. However, clinical follow-up remains insufficient and a marginal increase in secondary tumors could counterbalance the benefit of a highly sophisticated irradiation technique. It therefore remains necessary to integrate the potential risk of new irradiation modalities in a risk-adapted strategy taking into account therapeutic objectives but also associated risk factors, such as age (essentially), chemotherapy, or life style.

Determination of gonad doses during robotic stereotactic radiosurgery for various tumor sites

PURPOSE

The authors evaluated the absorbed dose received by the gonads during robotic stereotactic radiosurgery (SRS) for the treatment of different tumor localizations.

METHODS

The authors measured the gonad doses during the treatment of head and neck, thoracic, abdominal, or pelvic tumors in both RANDO phantom and actual patients. The computerized tomography images were transferred to the treatment planning system. The contours of tumor and critical organs were delineated on each slice, and treatment plans were generated. Measurements for gonad doses were taken from the geometric projection of the ovary onto the skin for female patients, and from the scrotal skin for male patients by attaching films and Thermoluminescent dosimeters (TLDs). SRS was delivered with CyberKnife (Accuray Inc., Sunnyvale, CA).

RESULTS

The median gonadal doses with TLD and film dosimeter in actual patients were 0.19 Gy (range, 0.035-2.71 Gy) and 0.34 Gy (range, 0.066-3.18 Gy), respectively. In the RANDO phantom, the median ovarian doses with TLD and film dosimeter were 0.08 Gy (range, 0.03-0.159 Gy) and 0.05 Gy (range, 0.015-0.13 Gy), respectively. In the RANDO phantom, the median testicular doses with TLD and film dosimeter were 0.134 Gy (range 0.056-1.97 Gy) and 0.306 Gy (range, 0.065-2.25 Gy).

CONCLUSIONS

Gonad doses are below sterility threshold in robotic SRS for different tumor localizations. However, particular attention should be given to gonads during robotic SRS for pelvic tumors.

Integral dose and radiation-induced secondary malignancies: comparison between stereotactic body radiation therapy and three-dimensional conformal radiotherapy

The aim of the present paper is to compare the integral dose received by non-tumor tissue (NTID) in stereotactic body radiation therapy (SBRT) with modified LINAC with that received by three-dimensional conformal radiotherapy (3D-CRT), estimating possible correlations between NTID and radiation-induced secondary malignancy risk. Eight patients with intrathoracic lesions were treated with SBRT, 23 Gy × 1 fraction. All patients were then replanned for 3D-CRT, maintaining the same target coverage and applying a dose scheme of 2 Gy × 32 fractions. The dose equivalence between the different treatment modalities was achieved assuming α/β = 10Gy for tumor tissue and imposing the same biological effective dose (BED) on the target (BED = 76Gy₁₀). Total NTIDs for both techniques was calculated considering α/β = 3Gy for healthy tissue. Excess absolute cancer risk (EAR) was calculated for various organs using a mechanistic model that includes fractionation effects. A paired two-tailed Student t-test was performed to determine statistically significant differences between the data (p ≤ 0.05). Our study indicates that despite the fact that for all patients integral dose is higher for SBRT treatments than 3D-CRT (p = 0.002), secondary cancer risk associated to SBRT patients is significantly smaller than that calculated for 3D-CRT (p = 0.001). This suggests that integral dose is not a good estimator for quantifying cancer induction. Indeed, for the model and parameters used, hypofractionated radiotherapy has the potential for secondary cancer reduction. The development of reliable secondary cancer risk models seems to be a key issue in fractionated radiotherapy. Further assessments of integral doses received with 3D-CRT and other special techniques are also strongly encouraged.

Peripheral doses in patients undergoing Cyberknife treatment for intracranial lesions. A single centre experience

Background

Stereotactic radiosurgery/radiotherapy procedures are known to deliver a very high dose per fraction, and thus, the corresponding peripheral dose could be a limiting factor for the long term surviving patients. The aim of this clinical study was to measure the peripheral dose delivered to patients undergoing intracranial Cyberknife treatment, using the MOSFET dosimeters. The influence of the supplemental shielding, the number of monitor units and the collimator size to the peripheral dose were investigated.

Methods

MOSFET dosimeters were placed in preselected anatomical regions of the patient undergoing Cyberknife treatment, namely the thyroid gland, the nipple, the umbilicus and the pubic symphysis.

Results

The mean peripheral doses before the supplemental shielding was added to the Cyberknife unit were 51.79 cGy, 13.31 cGy and 10.07 cGy while after the shielding upgrade they were 38.40 cGy, 10.94 cGy, and 8.69 cGy, in the thyroid gland, the umbilicus and the pubic symphysis, respectively. The increase of the collimator size corresponds to an increase of the PD and becomes less significant at larger distances, indicating that at these distances the PD is predominate due to the head leakage and collimator scatter.

Conclusion

Weighting the effect of the number of monitor units and the collimator size can be effectively used during the optimization procedure in order to choose the most suitable treatment plan that will deliver the maximum dose to the tumor, while being compatible with the dose constraints for the surrounding organs at risk. Attention is required in defining the thyroid gland as a structure of avoidance in the treatment plan especially in patients with benign diseases.

A contemporary review of stereotactic radiotherapy: inherent dosimetric complexities and the potential for detriment

OBJECTIVE

The advantages of highly localised, conformal treatments achievable with stereotactic radiotherapy (SRT) are increasingly being extended to extracranial sites as stereotactic body radiotherapy with advancements in imaging and beam collimation. One of the challenges in stereotactic treatment lies in the significant complexities associated with small field dosimetry and dose calculation. This review provides a comprehensive overview of the complexities associated with stereotactic radiotherapy and the potential for detriment.

METHODS

This study is based on a comprehensive review of literature accessible via PubMed and other sources, covering stereotactic radiotherapy, small-field dosimetry and dose calculation.

FINDINGS

Several key issues were identified in the literature. They pertain to dose prescription, dose measurement and dose calculation within and beyond the treatment field. Field-edge regions and penumbrae occupy a significant portion of the total field size. Spectral and dosimetric characteristics are difficult to determine and are compounded by effects of tissue inhomogeneity. Measurement of small-fields is made difficult by detector volume averaging and energy response. Available dosimeters are compared, and emphasis is given to gel dosimetry which offers the greatest potential for three-dimensional small-field dosimetry. The limitations of treatment planning system algorithms as applied to small-fields (particularly in the presence of heterogeneities) is explained, and a review of Monte Carlo dose calculation is provided, including simplified treatment planning implementations. Not incorporated into treatment planning, there is evidence that far from the primary field, doses to patients (and corresponding risks of radiocarcinogenesis) from leakage/scatter in SRT are similar to large fields.

CONCLUSIONS

Improved knowledge of dosimetric issues is essential to the accurate measurement and calculation of dose as well as the interpretation and assessment of planned and delivered treatments. This review highlights such issues and the potential benefit that may be gained from Monte Carlo dose calculation and verification via three-dimensional dosimetric methods (such as gel dosimetry) being introduced into routine clinical practice.

Consideration of the radiation dose delivered away from the treatment field to patients in radiotherapy

Radiation delivery to cancer patients for radiotherapy is invariably accompanied by unwanted radiation to other parts of the patient's body. Traditionally, considerable effort has been made to calculate and measure the radiation dose to the target as well as to nearby critical structures. Only recently has attention been focused also on the relatively low doses that exist far from the primary radiation beams. In several clinical scenarios, such doses have been associated with cardiac toxicity as well as an increased risk of secondary cancer induction. Out-of-field dose is a result of leakage and scatter and generally difficult to predict accurately. The present review aims to present existing data, from measurements and calculations, and discuss its implications for radiotherapy.

Evaluation of MatriXX for IMRT and VMAT dose verifications in peripheral dose regions

PURPOSE

MatriXX is a two-dimensional ion chamber array designed for IMRT/VMAT (RapidArc, IMAT, etc.) dose verifications. Its dosimetric properties have been characterized for megavoltage beams in a number of studies; however, to the best of the authors' knowledge, there is still a lack of an investigation into its performance in the peripheral or low dose regions. In this work, the authors have carried out a systematic study on this issue.

METHODS

The authors compare the performance of MatriXX with a cylindrical ion chamber in solid water phantoms in the peripheral dose regions. The comparisons are performed for a number of typical irradiation conditions that involve different gantry and/or MLC motions, field sizes, and distances to the target including static gantry/open fields, static gantry/sweeping MLC gap (mimicking an IMRT delivery), dynamic gantry/oscillating sweeping MLC gap (mimicking a VMAT delivery), as well as clinical IMRT and VMAT plans.

RESULTS

MatriXX, when used according to the manufacturer's recommendations, is found to disagree with an ion chamber in peripheral dose regions. This disagreement has been attributed to four types of MatriXX errors, namely, positive bias, over-response to scattered doses, round-off error, and angular dependence, all of which contribute to dose inaccuracies in the peripheral regions. The positive bias, which is independent of the dose level, is cumulative when MatriXX operates in the movie mode. The accumulation is proportional to the number of movie frames (snaps) when the sampling time is greater than 500 ms and is proportional to the overall movie time for a sampling time shorter than 500 ms. This behavior suggests multiple sources of the bias. MatriXX is also found to over-respond to peripheral doses by about 2.0% for the regions investigated in this work (3-15 cm from the field edge), where phantom scatter and collimator scatter dominate. Round-off error is determined to be due to insufficient precision in conversion of the raw signals to MatriXX software data for low doses. Angular dependence is defined as the dose response of MatriXX at different gantry angles. Up to 8% difference in detector response has been observed between 0 degree and 180 degrees. Possible sources of these errors are discussed and a correction method is suggested. With corrections, MatriXX shows good agreement with the ion chamber in all cases involving different gantry and/or MLC dynamics, as well as the clinical plans. For both primary and peripheral doses, MatriXX shows dose linearity down to 2 cGy with an accuracy of within 1% of the local dose.

CONCLUSIONS

The performance of MatriXX has been systematically evaluated in the peripheral dose regions. Major sources of error associated with MatriXX are identified and a correction method is suggested. This method has been successfully tested using both experimental and clinical plans. In all cases, good agreements between MatriXX and an ion chamber are achieved after corrections. The authors conclude that with proper corrections, MatriXX can be reliably used for peripheral dose measurements within the ranges studied.

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.

A review of the impact of photon and proton external beam radiotherapy treatment modalities on the dose distribution in field and out-of-field; implications for the long-term morbidity of cancer survivors

The use of untraditional treatment modalities for external beam radiotherapy such as intensity modulated radiation therapy (IMRT) and proton beam therapy is increasing. This review focuses on the changes in the dose distribution and the impact on radiation related risks for long-term cancer survivors. We compare conventional radiotherapy, IMRT, and proton beam therapy based on published treatment planning studies as well as published measurements and Monte Carlo simulations of out-of-field dose distributions. Physical dose parameters describing the dose distribution in the target volume, the conformity index, the dose distribution in organs at risk, and the dose distribution in non-target tissue, respectively, are extracted from the treatment planning studies. Measured out-of-field dose distributions are presented as the dose equivalent as a function of distance from the treatment field. Data in the literature clearly shows that, compared with conventional radiotherapy, IMRT improves the dose distribution in the target volume, which may increase the probability of tumor control. IMRT also seems to increase the out-of-field dose distribution, as well as the irradiated non-target volume, although the data is not consistent, leading to a potentially increased risk of radiation induced secondary malignancies, while decreasing the dose to normal tissues close to the target volume, reducing the normal tissue complication probability. Protons show no or only minor advantage on the dose distribution in the target volume and the conformity index compared to IMRT. However, the data consistently shows that proton beam therapy substantially decreases the OAR average dose compared to the other two techniques. It is also clear that protons provide an improved dose distribution in non-target tissues compared to conventional radiotherapy and IMRT. IMRT and proton beam therapy may significantly improve tumor control for cancer patients and quality of life for long-term cancer survivors.

Dose to normal tissues outside the radiation therapy patient's treated volume: a review of different radiation therapy techniques

Radiation therapy treatment planning and delivery capabilities have changed dramatically since the introduction of three-dimensional treatment planning and are continuing to change relatively rapidly in response to the implementation of new advanced technologies. Three-dimensional conformal radiation therapy (3DCRT) is now firmly in place as the standard of practice in clinics around the world. Medical accelerator manufacturers have employed advanced computer technology to produce treatment planning/delivery systems capable of precise shaping of dose distributions via computer-controlled multileaf collimator (MLC) systems, by which the beam fluence is varied optimally to achieve the desired dose distribution. This mode of conformal therapy is referred to as intensity modulated radiation therapy (IMRT), and is capable of generating dose distributions (including concave isodose volumes) that closely conform the prescription dose to the target volume and/or avoid specific sensitive normal structures. The increasing use of IMRT has focused attention on the need to better account for the intra- and inter-fraction spatial uncertainties in the dose delivery process. This has helped spur the development of treatment machines with integrated planar and volumetric advanced imaging capabilities, providing a new treatment modality referred to as image-guided IMRT (IG-IMRT), or simply image-guided radiation therapy (IGRT). In addition, there is a growing interest in replacing x rays with protons because of the physical characteristics of the depth dose curve, which peaks at the end of particle range, and eventually with even heavier charged particles to take advantage of the greater density of energy deposition close to the Bragg peak and hence larger relative biological effectiveness (RBE). Three-dimensional CRT, IMRT and proton beam therapy all provide improved target coverage and lower doses to surrounding normal tissues as compared to the previously used two-dimensional radiation therapy techniques. However, this is achieved at the expense of a greater volume of normal tissue in the irradiated volume receiving some dose and a higher whole body dose (or peripheral dose) to distant normal tissues. The higher whole body dose is a result of the increased x-ray leakage radiation to the patient due to the longer beam-on times associated with IMRT, and also from neutron leakage radiation associated with high energy x-ray beams (>10 MV) and proton beams. Dose distributions for the various CRT techniques and the current status of available data for normal tissues, and whole body dose are reviewed.

A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction

It has been long known that patients treated with ionizing radiation carry a risk of developing a second cancer in their lifetimes. Factors contributing to the recently renewed concern about the second cancer include improved cancer survival rate, younger patient population as well as emerging treatment modalities such as intensity-modulated radiation treatment (IMRT) and proton therapy that can potentially elevate secondary exposures to healthy tissues distant from the target volume. In the past 30 years, external-beam treatment technologies have evolved significantly, and a large amount of data exist but appear to be difficult to comprehend and compare. This review article aims to provide readers with an understanding of the principles and methods related to scattered doses in radiation therapy by summarizing a large collection of dosimetry and clinical studies. Basic concepts and terminology are introduced at the beginning. That is followed by a comprehensive review of dosimetry studies for external-beam treatment modalities including classical radiation therapy, 3D-conformal x-ray therapy, intensity-modulated x-ray therapy (IMRT and tomotherapy) and proton therapy. Selected clinical data on second cancer induction among radiotherapy patients are also covered. Problems in past studies and controversial issues are discussed. The needs for future studies are presented at the end.

Peripheral dose in ocular treatments with CyberKnife and Gamma Knife radiosurgery compared to proton radiotherapy

Peripheral radiation can have deleterious effects on normal tissues throughout the body, including secondary cancer induction and cataractogenesis. The aim of this study is to evaluate the peripheral dose received by various regions of the body after ocular treatment delivered with the Model C Gamma Knife, proton radiotherapy with a dedicated ocular beam employing no passive-scattering system, or a CyberKnife unit before and after supplemental shielding was introduced. TLDs were used for stray gamma and x-ray dosimetry, whereas CR-39 dosimeters were used to measure neutron contamination in the proton experiments. Doses to the contralateral eye, neck, thorax and abdomen were measured on our anthropomorphic phantom for a 56 Gy treatment to a 588 mm(3) posterior ocular lesion. Gamma Knife (without collimator blocking) delivered the highest dose in the contralateral eye, with 402-2380 mSv, as compared with 118-234 mSv for CyberKnife pre-shielding, 46-255 mSv for CyberKnife post-shielding and 9-12 mSv for proton radiotherapy. Gamma Knife and post-shielding CyberKnife delivered comparable doses proximal to the treatment site, with 190 versus 196 mSv at the thyroid, whereas protons doses at these locations were less than 10 mSv. Gamma Knife doses decreased dramatically with distance from the treatment site, delivering only 13 mSv at the lower pelvis, comparable to the proton result of 4 to 7 mSv in this region. In contrast, CyberKnife delivered between 117 and 132 mSv to the lower pelvis. In conclusion, for ocular melanoma treatments, a proton beam employing no double scattering system delivers the lowest peripheral doses proximally to the contralateral eye and thyroid when compared to radiosurgery with the Model C Gamma Knife or CyberKnife. At distal locations in the pelvis, peripheral doses delivered with proton and Gamma Knife are of an order of magnitude smaller than those delivered with CyberKnife.

The Leksell Gamma Knife Perfexion and Comparisons with its Predecessors

Objective:

The objective of this study is to introduce a new radiosurgical device, the Leksell Gamma Knife Perfexion (Elekta Instruments AB, Stockholm, Sweden). Design and performance characteristics are compared with previous models of the gamma knife in a clinical setting.

Methods:

Performance-related features in the design of the new radiosurgical system are described, and the ability to create complex shapes of isodose volumes even with a single isocenter is demonstrated. The concept of “dynamic shaping” of dose distribution is introduced as a means of decreasing the exposure to structures outside the target. Dose plans for the Leksell Gamma Knife Models B, 4C, and Perfexion have been created for target pathologies to illustrate how the software and hardware of the new radiosurgical system can increase treatment quality, decrease treatment time, and increase patient comfort. Radiation doses to critical organs in the rest of the human body are compared.

Results:

The automated collimator arrangement in the Perfexion makes it possible to produce more complex treatment volumes than with previous models of the gamma knife. This results from the enhanced ability to shape isodose volumes, even for single isocenters. The collimator arrangement and the patient positioning system also allow shorter patient transit times, reducing unwanted radiation exposure during movement between isocenters.

Conclusion:

The Perfexion exceeds the capabilities of previous gamma knife models in terms of treatment efficiency, conformity, and radiation protection. These qualities enable treatment of larger target volumes, especially close to eloquent areas. Pathologies previously inaccessible in the head and neck are now treatable due to the increased volume of the radiation cavity.

Intensity Modulating and Other Radiation Therapy Devices for Dose Painting

The introduction of intensity-modulated radiation therapy (IMRT) in the early 1990s created the possibility of generating dramatically improved dose distributions that could be tailored to fit a complex geometric arrangement of targets that push against or even surround healthy critical structures. IMRT is a new treatment paradigm that goes beyond the capabilities of the earlier technology called three-dimensional radiation therapy (3DCRT). IMRT took the older approach of using fields that conformed to the silhouette of the target to deliver a relatively homogeneous intensity of radiation and separated the conformal fields into many subfields so that intensity could be varied to better control the final dose distribution. This technique makes it possible to generate radiation dose clouds that have indentations in their surface. Initially, this technology was mainly used to avoid and thus control the dose delivered to critical structures so that they are not seriously damaged in the process of irradiating nearby targets to an appropriately high dose. Avoidance of critical structures allowed homogeneous dose escalation that led to improved local control for small tumors. However, the normal tissue component of large tumors often prohibits homogeneous dose escalation. A newer concept of dose-painting IMRT is aimed at exploiting inhomogeneous dose distributions adapted to tumor heterogeneity. Tumor regions of increased radiation resistance receive escalated dose levels, whereas radiation-sensitive regions receive conventional or even de-escalated dose levels. Dose painting relies on biologic imaging such as positron emission tomography, functional magnetic resonance imaging, and magnetic resonance spectroscopy. This review will describe the competing techologies for dose painting with an emphasis on their commonalities.