Calculation of cranial nerve complication probability for acoustic neuroma radiosurgery

Obtaining organ-specific radiobiological parameters from clinical data for radiation therapy planning of head and neck cancers

Objective.Different radiation therapy (RT) strategies, e.g. conventional fractionation RT (CFRT), hypofractionation RT (HFRT), stereotactic body RT (SBRT), adaptive RT, and re-irradiation are often used to treat head and neck (HN) cancers. Combining and/or comparing these strategies requires calculating biological effective dose (BED). The purpose of this study is to develop a practical process to estimate organ-specific radiobiologic model parameters that may be used for BED calculations in individualized RT planning for HN cancers.Approach.Clinical dose constraint data for CFRT, HFRT and SBRT for 5 organs at risk (OARs) namely spinal cord, brainstem, brachial plexus, optic pathway, and esophagus obtained from literature were analyzed. These clinical data correspond to a particular endpoint. The linear-quadratic (LQ) and linear-quadratic-linear (LQ-L) models were used to fit these clinical data and extract relevant model parameters (alpha/beta ratio, gamma/alpha,dTand BED) from the iso-effective curve. The dose constraints in terms of equivalent physical dose in 2 Gy-fraction (EQD2) were calculated using the obtained parameters.Main results.The LQ-L and LQ models fitted clinical data well from the CFRT to SBRT with the LQ-L representing a better fit for most of the OARs. The alpha/beta values for LQ-L (LQ) were found to be 2.72 (2.11) Gy, 0.55 (0.30) Gy, 2.82 (2.90) Gy, 6.57 (3.86) Gy, 5.38 (4.71) Gy, and the dose constraint EQD2 were 55.91 (54.90) Gy, 57.35 (56.79) Gy, 57.54 (56.35) Gy, 60.13 (59.72) Gy and 65.66 (64.50) Gy for spinal cord, optic pathway, brainstem, brachial plexus, and esophagus, respectively. Additional two LQ-L parametersdTwere 5.24 Gy, 5.09 Gy, 7.00 Gy, 5.23 Gy, and 6.16 Gy, and gamma/alpha were 7.91, 34.02, 8.67, 5.62 and 4.95.Significance.A practical process was developed to extract organ-specific radiobiological model parameters from clinical data. The obtained parameters can be used for biologically based radiation planning such as calculating dose constraints of different fractionation regimens.

Double scattering and pencil beam scanning Monte Carlo workflows for proton therapy retrospective studies on radiation-induced toxicities

PURPOSE

Monte Carlo (MC) simulations can be used to accurately simulate dose and linear energy transfers (LET) distributions, thereby allowing for the calculation of the relative biological effectiveness (RBE) of protons. We present hereby the validation and implementation of a workflow for the Monte Carlo modelling of the double scattered and pencil beam scanning proton beamlines at our institution.

METHODS

The TOPAS/Geant4 MC model of the clinical nozzle has been comprehensively validated against measurements. The validation also included a comparison between simulated clinical treatment plans for four representative patients and the clinical treatment planning system (TPS). Moreover, an in-house tool implemented in Python was tested to assess the variable RBE-weighted dose in proton plans, which was illustrated for a patient case with a developing radiation-induced toxicity.

RESULTS

The simulated range and modulation width closely matches the measurements. Gamma-indexes (3%/3mm 3D), which compare the TPS and MC computations, showed a passing rate superior to 98%. The calculated RBE-weighted dose presented a slight increase at the necrosis location, within the PTV margins. This indicates the need for reporting on the physical and biological effects of irradiation in high dose regions, especially at the healthy tissues and increased LET distributions location.

CONCLUSION

The results demonstrate that the Monte Carlo method can be used to independently validate a TPS calculation, and to estimate LET distributions. The features of the in-house tool can be used to correlate LET and RBE-weighted dose distributions with the incidence of radiation-induced toxicities following proton therapy treatments.

A case-control study of linear energy transfer and relative biological effectiveness related to symptomatic brainstem toxicity following pediatric proton therapy

BACKGROUND AND PURPOSE

A fixed relative biological effectiveness (RBE) of 1.1 (RBE_1.1) is used clinically in proton therapy even though the RBE varies with properties such as dose level and linear energy transfer (LET). We therefore investigated if symptomatic brainstem toxicity in pediatric brain tumor patients treated with proton therapy could be associated with a variable LET and RBE.

MATERIALS AND METHODS

36 patients treated with passive scattering proton therapy were selected for a case-control study from a cohort of 954 pediatric brain tumor patients. Nine children with symptomatic brainstem toxicity were each matched to three controls based on age, diagnosis, adjuvant therapy, and brainstem RBE_1.1 dose characteristics. Differences across cases and controls related to the dose-averaged LET (LET_d) and variable RBE-weighted dose from two RBE models were analyzed in the high-dose region.

RESULTS

LET_d metrics were marginally higher for cases vs. controls for the majority of dose levels and brainstem substructures. Considering areas with doses above 54 Gy(RBE_1.1), we found a moderate trend of 13% higher median LET_d in the brainstem for cases compared to controls (P = .08), while the difference in the median variable RBE-weighted dose for the same structure was only 2% (P = .6).

CONCLUSION

Trends towards higher LET_d for cases compared to controls were noticeable across structures and LET_d metrics for this patient cohort. While case-control differences were minor, an association with the observed symptomatic brainstem toxicity cannot be ruled out.

Dosimetric analysis of radiation-induced brainstem necrosis for nasopharyngeal carcinoma treated with IMRT

Background

Radiation-induced brainstem necrosis (RIBN) is a late life-threatening complication that can appear after treatment in patients with nasopharyngeal carcinoma (NPC). However, the relationship between RIBN and radiation dose is not still well-defined.

Methods

During January 2013 and December 2017, a total of 1063 patients with NPC were treated at Sichuan cancer hospital with IMRT. A total of 479 patients were eligible for dosimetric analysis. Dosimetric parameters of the RIBN, D_max(the maximum dose), D_0.1c (maximum average dose delivered to a 0.1-cc volume), D_1cc, D_2cc, D_3cc, D_5cc, D_10cc and D_mean (mean does) were evaluated and recorded. ROC curve was used to analyze the area under curve (AUC) and cutoff points. Logistic regression for screening dose-volume parameter and logistic dose response model were used to predict the incidence of brainstem necrosis.

Results

Among the 479 patients with NPC, 6 patients were diagnosed with RIBN, the incidence of RIBN was 1.25% (6/479), and the median time to RIBN after treatment was 28.5 months (range 18–48 months). The dose of the brainstem in patients with RIBN were higher than that in patients without necrosis. ROC curve showed that the area under the curve (AUC) of D_max was the largest (0.987). Moreover, logistic stepwise regression indicated that D_max was the most important dose factor. The RIBN incidence at 5% over 5 years (TD_5/5) and 50% incidence over 5 years (TD_50/5) was 69.59 Gy and76.45 Gy, respectively.

Conclusions

Brainstem necrosis is associated with high dose irritation. D_max is the most significant predictive dosimetric factor for RIBN. D_max of brainstem should be considered as the dose limitation parameter. We suggest that the limitation dose for brainstem was D_max < 69.59 Gy.

Normal tissue complication probability modeling to guide individual treatment planning in pediatric cranial proton and photon radiotherapy

PURPOSE

Proton therapy (PT) is broadly accepted as the gold standard of care for pediatric patients with cranial cancer. The superior dose distribution of PT compared to photon radiotherapy reduces normal tissue complication probability (NTCP) for organs at risk. As NTCPs for pediatric organs are not well understood, clinics generally base radiation response on adult data. However, there is evidence that radiation response strongly depends on the age and even sex of a patient. Furthermore, questions surround the influence of individual intrinsic radiosensitivity (α/β ratio) on pediatric NTCP. While the clinical pediatric NTCP data is scarce, radiobiological modeling and sensitivity analyses can be used to investigate the NTCP trends and its dependence on individual modeling parameters. The purpose of this study was to perform sensitivity analyses of NTCP models to ascertain the dependence of radiosensitivity, sex, and age of a child and predict cranial side-effects following intensity-modulated proton therapy (IMPT) and intensity-modulated radiotherapy (IMRT).

METHODS

Previously, six sex-matched pediatric cranial datasets (5, 9, and 13 years old) were planned in Varian Eclipse treatment planning system (13.7). Up to 108 scanning beam IMPT plans and 108 IMRT plans were retrospectively optimized for a range of simulated target volumes and locations. In this work, dose-volume histograms were extracted and imported into BioSuite Software for radiobiological modeling. Relative-Seriality and Lyman-Kutcher-Burman models were used to calculate NTCP values for toxicity endpoints, where TD50, (based on reported adult clinical data) was varied to simulate sex dependence of NTCP. Plausible parameter ranges, based on published literature for adults, were used in modeling. In addition to sensitivity analyses, a 20% difference in TD50 was used to represent the radiosensitivity between the sexes (with females considered more radiosensitive) for ease of data comparison as a function of parameters such as α/β ratio.

RESULTS

IMPT plans resulted in lower NTCP compared to IMRT across all models (p < 0.0001). For medulloblastoma treatment, the risk of brainstem necrosis (> 10%) and cochlea tinnitus (> 20%) among females could potentially be underestimated considering a lower TD50 value for females. Sensitivity analyses show that the difference in NTCP between sexes was significant (p < 0.0001). Similarly, both brainstem necrosis and cochlea tinnitus NTCP varied significantly (p < 0.0001) across tested α/β as a function of TD50 values (assumption being that TD50 values are 20% lower in females). If the true α/β of these pediatric tissues is higher than expected (α/β ∼ 3), the risk of tinnitus for IMRT can significantly increase (p < 0.0001).

CONCLUSION

Due to the scarcity of pediatric NTCP data available, sensitivity analyses were performed using plausible ranges based on published adult data. In the clinical scenario where, if female pediatric patients were 20% more radiosensitive (lower TD50 value), they could be up to twice as likely to experience side-effects of brainstem necrosis and cochlea tinnitus compared to males, highlighting the need for considering the sex in NTCP models. Based on our sensitivity analyses, age and sex of a pediatric patient could significantly affect the resultant NTCP from cranial radiotherapy, especially at higher α/β values.

The Organ Sparing Potential of Different Biological Optimization Strategies in Proton Therapy

Purpose

Variable relative biological effectiveness (RBE) models allow for differences in linear energy transfer (LET), physical dose, and tissue type to be accounted for when quantifying and optimizing the biological damage of protons. These models are complex and fraught with uncertainties, and therefore, simpler RBE optimization strategies have also been suggested. Our aim was to compare several biological optimization strategies for proton therapy by evaluating their performance in different clinical cases.

Methods and Materials

Two different optimization strategies were compared: full variable RBE optimization and differential RBE optimization, which involve applying fixed RBE for the planning target volume (PTV) and variable RBE in organs at risk (OARs). The optimization strategies were coupled to 2 variable RBE models and 1 LET-weighted dose model, with performance demonstrated on 3 different clinical cases: brain, head and neck, and prostate tumors.

Results

In cases with low (α/β)x in the tumor, the full RBE optimization strategies had a large effect, with up to 10% reduction in RBE-weighted dose to the PTV and OARs compared with the reference plan, whereas smaller variations (<5%) were obtained with differential optimization. For tumors with high (α/β)x, the differential RBE optimization strategy showed a greater reduction in RBE-weighted dose to the OARs compared with the reference plan and the full RBE optimization strategy.

Conclusions

Differences between the optimization strategies varied across the studied cases, influenced by both biological and physical parameters. Whereas full RBE optimization showed greater OAR sparing, awareness of underdosage to the target must be carefully considered.

Identification of patient benefit from proton beam therapy in brain tumour patients based on dosimetric and NTCP analyses

BACKGROUND

The limited availability of proton beam therapy (PBT) requires individual treatment selection strategies, such as the model-based approach. In this study, we assessed the dosimetric benefit of PBT compared to photon therapy (XRT), analysed the corresponding changes in normal tissue complication probability (NTCP) on a variety of available models, and illustrated model-based patient selection in an in-silico study for patients with brain tumours.

METHODS

For 92 patients treated at two PBT centres, volumetric modulated arc therapy treatment plans were retrospectively created for comparison with the clinically applied PBT plans. Several dosimetric parameters for the brain excluding tumour and margins, cerebellum, brain stem, frontal and temporal lobes, hippocampi, cochleae, chiasm, optic nerves, lacrimal glands, lenses, pituitary gland, and skin were compared between both modalities using Wilcoxon signed-rank tests. NTCP differences (ΔNTCP) were calculated for 11 models predicting brain necrosis, delayed recall, temporal lobe injury, hearing loss, tinnitus, blindness, ocular toxicity, cataract, endocrine dysfunction, alopecia, and erythema. A patient was assumed to be selected for PBT if ΔNTCP exceeded a threshold of 10 percentage points for at least one of the side-effects.

RESULTS

PBT substantially reduced the dose in almost all investigated OARs, especially in the low and intermediate dose ranges and for contralateral organs. In general, NTCP predictions were significantly lower for PBT compared to XRT, in particular in ipsilateral organs. Considering ΔNTCP of all models, 80 patients (87.0%) would have been selected for PBT in this in-silico study, mainly due to predictions of a model on delayed recall (51 patients).

CONCLUSION

In this study, substantial dose reductions for PBT were observed, mainly in contralateral organs. However, due to the sigmoidal dose response, NTCP was particularly reduced in ipsilateral organs. This underlines that physical dose-volume parameters alone may not be sufficient to describe the clinical relevance between different treatment techniques and highlights potential benefits of NTCP models. Further NTCP models for different modern treatment techniques are mandatory and existing models have to be externally validated in order to implement the model-based approach in clinical practice for cranial radiotherapy.

Spatial Agreement of Brainstem Dose Distributions Depending on Biological Model in Proton Therapy for Pediatric Brain Tumors

Purpose

During radiation therapy for pediatric brain tumors, the brainstem is a critical organ at risk, possibly with different radio-sensitivity across its substructures. In proton therapy, treatment planning is currently performed using a constant relative biological effectiveness (RBE) of 1.1 (RBE_1.1), whereas preclinical studies point toward spatial variability of this factor. To shed light on this biological uncertainty, we investigated the spatial agreement between isodose maps produced by different RBE models, with emphasis on (smaller) substructures of the brainstem.

Methods and Materials

Proton plans were recalculated using Monte Carlo simulations in 3 anonymized pediatric patients with brain tumors (a craniopharyngioma, a low-grade glioma, and a posterior fossa ependymoma) to obtain dose and linear energy transfer distributions. Doses and volume metrics for the brainstem and its substructures were calculated using a constant RBE_1.1, 4 phenomenological RBE models with varying (α/β)_x parameters, and with a simpler linear energy transfer-dependent model. The spatial agreement between the dose distributions of constant RBE_1.1 versus the variable RBE models was compared using the Dice similarity coefficient.

Results

The spatial agreement between the variable RBE dose distributions and RBE_1.1 decreased with increasing isodose levels in all patient cases. The patient with ependymoma showed the greatest variation in dose and dose volumes, where V_50Gy(RBE) in the brainstem increased from 32% (RBE_1.1) to 35% to 49% depending on the applied model, corresponding to a spatial agreement (Dice similarity coefficient) between 0.79 and 0.95. The remaining patients showed similar trends, however, with lower absolute values due to lower brainstem doses.

Conclusions

All phenomenological RBE models fully enclosed the isodose volumes of the constant RBE_1.1, and the volumes based on variable RBE spatially agreed. The spatial agreement was dependent on the isodose level, where higher isodose levels showed larger expansions and less agreement between the variable RBE models and RBE_1.1.

Implementation of a double scattering nozzle for Monte Carlo recalculation of proton plans with variable relative biological effectiveness

A constant relative biological effectiveness (RBE) of 1.1 is currently used in clinical proton therapy. However, theRBEvaries with factors such as dose level, linear energy transfer (LET) and tissue type. MultipleRBEmodels have been developed to account for this biological variation. To enable recalculation of patients treated with double scattering (DS) proton therapy, includingLETand variableRBE, we implemented and commissioned a Monte Carlo (MC) model of a DS treatment nozzle. The main components from the IBA nozzle were implemented in the FLUKA MC code. We calibrated and verified the following entities to experimental measurements: range of pristine Bragg peaks (PBPs) and spread-out Bragg peaks (SOBPs), energy spread, lateral profiles, compensator range degradation, and absolute dose. We recalculated two patients with different field setups, comparing FLUKA vs. treatment planning system (TPS) dose, also obtainingLETand variableRBEdoses. We achieved good agreement between FLUKA and measurements. The range differences between FLUKA and measurements were for the PBPs within ±0.9 mm (83% ⩽ 0.5 mm), and for SOBPs ±1.6 mm (82% ⩽ 0.5 mm). The differences in modulation widths were below 5 mm (79% ⩽ 2 mm). The differences in the distal dose fall off (D80%-D20%) were below 0.5 mm for all PBPs and the lateral penumbras diverged from measurements by less than 1 mm. The mean dose difference (RBE= 1.1) in the target between the TPS and FLUKA were below 0.4% in a three-field plan and below 1.4% in a four-field plan. A dose increase of 9.9% and 7.2% occurred when using variableRBEfor the two patients, respectively. We presented a method to recalculate DS proton plans in the FLUKA MC code. The implementation was used to obtainLETand variableRBEdose and can be used for investigating variableRBEfor previously treated patients.

Inter-patient variations in relative biological effectiveness for cranio-spinal irradiation with protons

Cranio-spinal irradiation (CSI) using protons has dosimetric advantages compared to photons and is expected to reduce risk of adverse effects. The proton relative biological effectiveness (RBE) varies with linear energy transfer (LET), tissue type and dose, but a variable RBE has not replaced the constant RBE of 1.1 in clinical treatment planning. We examined inter-patient variations in RBE for ten proton CSI patients. Variable RBE models were used to obtain RBE and RBE-weighted doses. RBE was quantified in terms of dose weighted organ-mean RBE ([Formula: see text] = mean RBE-weighted dose/mean physical dose) and effective RBE of the near maximum dose (D2%), i.e. RBED2% = [Formula: see text], where subscripts RBE and phys indicate that the D2% is calculated based on an RBE model and the physical dose, respectively. Compared to the median [Formula: see text] of the patient population, differences up to 15% were observed for the individual [Formula: see text] values found for the thyroid, while more modest variations were seen for the heart (6%), lungs (2%) and brainstem (<1%). Large inter-patient variation in RBE could be correlated to large spread in LET and dose for these organs at risk (OARs). For OARs with small inter-patient variations, the results show that applying a population based RBE in treatment planning may be a step forward compared to using RBE of 1.1. OARs with large inter-patient RBE variations should ideally be selected for patient-specific biological or RBE robustness analysis if the physical doses are close to known dose thresholds.

Clinical implementation in proton therapy of multi-field optimization by a hybrid method combining conventional PTV with robust optimization

To implement a robust multi-field optimization (MFO) technique compatible with the application of a Monte Carlo (MC) algorithm and to evaluate its robustness. Nine patients (three brain, five head-and-neck, one spine) underwent proton treatment generated by a novel robust MFO technique. A hybrid (hMFO) approach was implemented, planning dose coverage on isotropic PTV compensating for setup errors, whereas range calibration uncertainties are incorporated into PTV robust optimization process. hMFO was compared with single-field optimization (SFO) and full robust multi-field optimization (fMFO), both on the nominal plan and the worst-case scenarios assessed by robustness analysis. The SFO and the fMFO plans were normalized to hMFO on CTV to obtain iso-D95 coverage, and then the organs at risk (OARs) doses were compared. On the same OARs, in the normalized nominal plans the potential impact of variable relative biological effectiveness (RBE) was investigated. hMFO reduces the number of scenarios computed for robust optimization (from twenty-one in fMFO to three), making it practicable with the application of a MC algorithm. After normalizing on D95 CTV coverage, nominal hMFO plans were superior compared to SFO in terms of OARs sparing (p   <  0.01), without significant differences compared to fMFO. The improvement in OAR sparing with hMFO with respect to SFO was preserved in worst-case scenarios (p   <  0.01), confirming that hMFO is as robust as SFO to physical uncertainties, with no significant differences when compared to the worst case scenarios obtained by fMFO. The dose increase on OARs due to variable RBE was comparable to the increase due to physical uncertainties (i.e. 4-5 Gy(RBE)), but without significant differences between these techniques. hMFO allows improving plan quality with respect to SFO, with no significant differences with fMFO and without affecting robustness to setup, range and RBE uncertainties, making clinically feasible the application of MC-based robust optimization.

Biophysical modeling and experimental validation of relative biological effectiveness (RBE) for 4He ion beam therapy

BACKGROUND

Helium (4He) ion beam therapy provides favorable biophysical characteristics compared to currently administered particle therapies, i.e., reduced lateral scattering and enhanced biological damage to deep-seated tumors like heavier ions, while simultaneously lessened particle fragmentation in distal healthy tissues as observed with lighter protons. Despite these biophysical advantages, raster-scanning 4He ion therapy remains poorly explored e.g., clinical translational is hampered by the lack of reliable and robust estimation of physical and radiobiological uncertainties. Therefore, prior to the upcoming 4He ion therapy program at the Heidelberg Ion-beam Therapy Center (HIT), we aimed to characterize the biophysical phenomena of 4He ion beams and various aspects of the associated models for clinical integration.

METHODS

Characterization of biological effect for 4He ion beams was performed in both homogenous and patient-like treatment scenarios using innovative models for estimation of relative biological effectiveness (RBE) in silico and their experimental validation using clonogenic cell survival as the gold-standard surrogate. Towards translation of RBE models in patients, the first GPU-based treatment planning system (non-commercial) for raster-scanning 4He ion beams was devised in-house (FRoG).

RESULTS

Our data indicate clinically relevant uncertainty of ±5-10% across different model simulations, highlighting their distinct biological and computational methodologies. The in vitro surrogate for highly radio-resistant tissues presented large RBE variability and uncertainty within the clinical dose range.

CONCLUSIONS

Existing phenomenological and mechanistic/biophysical models were successfully integrated and validated in both Monte Carlo and GPU-accelerated analytical platforms against in vitro experiments, and tested using pristine peaks and clinical fields in highly radio-resistant tissues where models exhibit the greatest RBE uncertainty. Together, these efforts mark an important step towards clinical translation of raster-scanning 4He ion beam therapy to the clinic.

Management of nonfunctioning pituitary tumors: radiotherapy

External beam radiotherapy (RT) is an essential part of the management of intracranial tumors and has been used in treating pituitary adenomas for more than five decades. It has been demonstrated that conventional RT for postoperative residual or progressive nonfunctioning pituitary adenomas (NFAs) present an excellent long-term local tumor control, although its use has been limited because of the potential late toxicity related to radiation treatments. Recent advances in radiation techniques have led to more accurate treatments, rendering obsolete many commonly held views of the "old" radiotherapy. New techniques include intensity modulated radiotherapy, volumetric-modulated arc therapy, and stereotactic techniques, either stereotactic radiosurgery or fractionated stereotactic radiotherapy. New techniques allow the delivering of higher radiation doses to the target with rapid dose fall-off in the surrounding normal tissues, and potentially limiting the long term toxicity of radiation. In this review, we present a critical analysis of the most recent available literature on the use of radiation in patients with NFAs, focusing particularly on the efficacy and safety of radiation stereotactic techniques.

Relating the proton relative biological effectiveness to tumor control and normal tissue complication probabilities assuming interpatient variability in α/β

BACKGROUND

Proton therapy uses a constant relative biological effectiveness (RBE) of 1.1. The use of variable RBE values has been suggested but is currently not feasible due to uncertainties. The impact of variable RBE has solely been studied using dosimetric indices. This work elucidates the impact of RBE variations on tumor control and normal tissue complication probabilities (TCP/NTCP).

METHODS

Models to estimate TCP and NTCP were used in combination with an empirical proton RBE model. Variations in outcome as a function of linear-quadratic model parameters for cellular radiosensitivity were determined for TCP in prostate and ependymoma. In addition, NTCP analysis was done for brainstem necrosis.

RESULTS

Considering a variable proton RBE as a dose-modifying factor for prescription doses and dose constraints is misleading, as TCP/NTCP do not simply scale with RBE. The dependency of RBE on α/β cannot be interpreted independent of TCP/NTCP because variations in radiosensitivity affect both photon and proton treatments. Assuming interpatient variability in radiosensitivity results in lower TCP for patients with low α/β. In proton therapy, the magnitude of TCP variations is reduced due to an RBE increase as α/β decreases. The TCP in proton therapy is less affected by interpatient variability in α/β. On the other hand, patients with a lower α/β would have a lower complication probability, which is counteracted by an increase in RBE as α/β decreases. Toxicities in proton therapy would be more affected by α/β variations compared to photon therapy.

CONCLUSIONS

Assessment of variable RBE in proton therapy should be based on TCP and NTCP. Potential interpatient variability in radiosensitivity causes a smaller variance in TCP but a larger variance in NTCP for proton patients. The relative TCP as a function of α/β was found to be higher than the RBE, whereas the relative NTCP was lower than a calculated RBE.

CyberKnife Stereotactic Radiosurgery and Hypofractionated Stereotactic Radiotherapy As First-line Treatments for Imaging-diagnosed Intracranial Meningiomas

Definitive radiotherapy is an important alternative treatment for meningioma patients who are inoperable or refuse surgery. We evaluated the efficacy and toxicity of CyberKnife-based stereotactic radiosurgery (SRS) and hypofractionated stereotactic radiotherapy (hSRT) as first-line treatments for intracranial meningiomas that were diagnosed using magnetic resonance imaging (MRI) and/or computed tomography (CT). Between February 2005 and September 2015, 41 patients with intracranial meningiomas were treated with CyberKnife-based SRS or hSRT. Eleven of those tumors were located in the skull base. The median tumor volume was 10.4 ml (range, 1.4–56.9 ml). The median prescribed radiation dose was 17 Gy (range, 13–20 Gy to the 61–88% isodose line) for SRS (n = 9) and 25 Gy (range, 14–38 Gy to the 44–83% isodose line) for hSRT (n = 32). The hSRT doses were delivered in 2 to 10 daily fractions. The median follow-up period was 49 months (range, 7–138). The 5-year progression-free survival rate (PFS) for all 41 patients was 86%. The 3-year PFS was 69% for the 14 patients with tumor volumes of ≥13.5 ml (30 mm in diameter) and 100% for the 27 patients with tumor volumes of <13.5 ml (P = 0.031). Grade >2 toxicities were observed in 5 patients (all of them had tumor volumes of ≥13.5 ml). SRS and hSRT are safe and effective against relatively small (<13.5 ml) meningiomas.

Target delineation and optimal radiosurgical dose for pituitary tumors

Stereotactic radiosurgery (SRS) delivered as either single-fraction or multi-fraction SRS (2–5 fractions) is frequently employed in patients with residual or recurrent pituitary adenoma. The most common delivery systems used for SRS include the cobalt-60 system Gamma Knife, the CyberKnife (CK) robotic radiosurgery system, or a modified conventional radiotherapy machine (linear accelerator, LINAC). Tumor control and normalization of hormone hypersecretion have been reported in 75–100 % and 25–80 % of patients, respectively. Hypopituitarism is the most commonly reported late complication of radiation treatment, whereas other toxicities occur less frequently. We have provided an overview of the recent available literature on SRS in patients with a pituitary adenoma. Critical aspects of pituitary irradiation, including target delineation and doses to organs at risk, optimal radiation dose, as well as the long-term efficacy and toxicity of SRS for either nonfunctioning or secreting pituitary adenomas are discussed. Single-fraction SRS represents an effective treatment for patients with a pituitary adenoma; however, caution should be used for lesions > 2.5–3 cm in size and/or involving the anterior optic pathway. Future studies will be necessary to optimize target doses and critical organ dose constrains in order to reduce the long-term toxicity of treatments while maintaining high efficacy.

Proton Treatment Techniques for Posterior Fossa Tumors: Consequences for Linear Energy Transfer and Dose-Volume Parameters for the Brainstem and Organs at Risk

PURPOSE

In proton therapy of posterior fossa tumors, at least partial inclusion of the brainstem in the target is necessary because of its proximity to the tumor and required margins. Additionally, the preferred beam geometry results in directing the field distal edge toward this critical structure, raising concerns for brainstem toxicity. Some treatment techniques place the beam's distal edge within the brainstem (dose-sparing techniques), and others avoid elevated linear energy transfer (LET) of the proton field by placing the distal edge beyond it (LET-sparing techniques). Hybrid approaches are also being used. We examine the dosimetric efficacy of these techniques, accounting for LET-dependent and dose-dependent variable relative biologic effectiveness (RBE) distributions.

METHODS

Six techniques were applied in ependymoma cases: (a) 3-field dose-sparing; (b) 3-field LET-sparing; (c) 2-field dose-sparing, wide angles; (d) 2-field LET-sparing, wide angles; (e) 2-field LET-sparing, steep angles; and (f) 2-field LET-sparing with feathered distal end. Monte Carlo calculated dose, LET, and RBE-weighted dose distributions were compared.

RESULTS

Decreased LET values in the brainstem by LET-sparing techniques were accompanied by higher, not statistically significant, median dose: 53.6 Gy(RBE), 53.4 Gy(RBE), and 54.3 Gy(RBE) for techniques (b), (d), and (e) versus 52.1 Gy(RBE) for technique (a). Accounting for variable RBE distributions, the brainstem volume receiving at least 55 Gy(RBE) increased from 72.5% for technique (a) to 80.3% for (b) (P<.01) and from 70.7% for technique (c) to 77.6% for (d) (P<.01). Less than 2%, but statistically significant, decrease in maximum variable RBE-weighted brainstem dose was observed for the LET-sparing techniques compared with the corresponding dose-sparing (P=.03 and .004).

CONCLUSIONS

Extending the proton range beyond the brainstem to reduce LET results in clinically comparable maximum radiobiologic effective dose to this sensitive structure. However this method significantly increasing the brainstem volume receiving RBE-weighted dose higher than 55 Gy(RBE) with possible consequences based on known dose-volume parameters for increased toxicity.

A complication probability planning study to predict the safety of a new protocol for intracranial tumour radiotherapy in dogs

Technical advances make it possible to deliver radiation therapy for canine intracranial tumours in fewer fractions, under the assumption of equivalent tumour control. With the aim of estimating the late toxicity risk profile for various tumour sizes and locations, the present paper evaluates the normal tissue complication probability (NTCP) values for the intracranial organs at risk. By making isoeffect calculations, a new 10-fraction radiation protocol was developed with the same tumour control probability (TCP) as a currently used 20-fraction standard protocol, and complication risk profiles for brain, brainstem and optic chiasm were modelled using a representative population of 64 dogs with brain tumours. For >59% of cases, the new 10-fraction protocol yielded an acceptable, low risk estimate of late toxicity (<10%). Our calculations suggest that it may be safe to treat small to intermediate-sized tumours that are neither located near the optic chiasm nor at the brainstem with 10 daily fractions of 4.35 Gy.

Implementation of Talairach Atlas Based Automated Brain Segmentation for Radiation Therapy Dosimetry

Radiotherapy for brain cancer inevitably results in irradiation of uninvolved brain. While it has been demonstrated that irradiation of the brain can result in cognitive deficits, dose-volume relationships are not well established. There is little work correlating a particular cognitive deficit with dose received by the region of the brain responsible for the specific cognitive function. One obstacle to such studies is that identification of brain anatomy is both labor intensive and dependent on the individual performing the segmentation. Automatic segmentation has the potential to be both efficient and consistent. Brains2 is a software package developed by the University of Iowa for MRI volumetric studies. It utilizes MR images, the Talairach atlas, and an artificial neural network (ANN) to segment brain images into substructures in a standardized manner. We have developed a software package, Brains2DICOM, that converts the regions of interest identified by Brains2 into a DICOM radiotherapy structure set. The structure set can be imported into a treatment planning system for dosimetry. We demonstrated the utility of Brains2DICOM using a test case, a 34-year-old man with diffuse astrocytoma treated with three-dimensional conformal radiotherapy. Brains2 successfully applied the Talairach atlas to identify the right and left frontal, parietal, temporal, occipital, subcortical, and cerebellum regions. Brains2 was not successful in applying the ANN to identify small structures, such as the hippocampus and caudate. Further work is necessary to revise the ANN or to develop new methods for identification of small structures in the presence of disease and radiation induced changes. The segmented regions-of-interest were transferred to our commercial treatment planning system using DICOM and dose-volume histograms were constructed. This method will facilitate the acquisition of data necessary for the development of normal tissue complication probability (NTCP) models that assess the probability of cognitive complications secondary to radiotherapy for intracranial and head and neck neoplasms.

Optimal radiotherapy dose schedules under parametric uncertainty

We consider the effects of parameter uncertainty on the optimal radiation schedule in the context of the linear-quadratic model. Our interest arises from the observation that if inter-patient variability in normal and tumor tissue radiosensitivity or sparing factor of the organs-at-risk (OAR) are not accounted for during radiation scheduling, the performance of the therapy may be strongly degraded or the OAR may receive a substantially larger dose than the allowable threshold. This paper proposes a stochastic radiation scheduling concept to incorporate inter-patient variability into the scheduling optimization problem. Our method is based on a probabilistic approach, where the model parameters are given by a set of random variables. Our probabilistic formulation ensures that our constraints are satisfied with a given probability, and that our objective function achieves a desired level with a stated probability. We used a variable transformation to reduce the resulting optimization problem to two dimensions. We showed that the optimal solution lies on the boundary of the feasible region and we implemented a branch and bound algorithm to find the global optimal solution. We demonstrated how the configuration of optimal schedules in the presence of uncertainty compares to optimal schedules in the absence of uncertainty (conventional schedule). We observed that in order to protect against the possibility of the model parameters falling into a region where the conventional schedule is no longer feasible, it is required to avoid extremal solutions, i.e. a single large dose or very large total dose delivered over a long period. Finally, we performed numerical experiments in the setting of head and neck tumors including several normal tissues to reveal the effect of parameter uncertainty on optimal schedules and to evaluate the sensitivity of the solutions to the choice of key model parameters.

A phenomenological relative biological effectiveness (RBE) model for proton therapy based on all published in vitro cell survival data

Proton therapy treatments are currently planned and delivered using the assumption that the proton relative biological effectiveness (RBE) relative to photons is 1.1. This assumption ignores strong experimental evidence that suggests the RBE varies along the treatment field, i.e. with linear energy transfer (LET) and with tissue type. A recent review study collected over 70 experimental reports on proton RBE, providing a comprehensive dataset for predicting RBE for cell survival. Using this dataset we developed a model to predict proton RBE based on dose, dose average LET (LETd) and the ratio of the linear-quadratic model parameters for the reference radiation (α/β)x, as the tissue specific parameter. The proposed RBE model is based on the linear quadratic model and was derived from a nonlinear regression fit to 287 experimental data points. The proposed model predicts that the RBE increases with increasing LETd and decreases with increasing (α/β)x. This agrees with previous theoretical predictions on the relationship between RBE, LETd and (α/β)x. The model additionally predicts a decrease in RBE with increasing dose and shows a relationship between both α and β with LETd. Our proposed phenomenological RBE model is derived using the most comprehensive collection of proton RBE experimental data to date. Previously published phenomenological models, based on a limited data set, may have to be revised.

Incidence of CNS Injury for a Cohort of 111 Patients Treated With Proton Therapy for Medulloblastoma: LET and RBE Associations for Areas of Injury

BACKGROUND

Central nervous system (CNS) injury is a rare complication of radiation therapy for pediatric brain tumors, but its incidence with proton radiation therapy (PRT) is less well defined. Increased linear energy transfer (LET) and relative biological effectiveness (RBE) at the distal end of proton beams may influence this risk. We report the incidence of CNS injury in medulloblastoma patients treated with PRT and investigate correlations with LET and RBE values.

METHODS AND MATERIALS

We reviewed 111 consecutive patients treated with PRT for medulloblastoma between 2002 and 2011 and selected patients with clinical symptoms of CNS injury. Magnetic resonance imaging (MRI) findings for all patients were contoured on original planning scans (treatment change areas [TCA]). Dose and LET distributions were calculated for the treated plans using Monte Carlo system. RBE values were estimated based on LET-based published models.

RESULTS

At a median follow-up of 4.2 years, the 5-year cumulative incidence of CNS injury was 3.6% for any grade and 2.7% for grade 3+. Three of 4 symptomatic patients were treated with a whole posterior fossa boost. Eight of 10 defined TCAs had higher LET values than the target but statistically nonsignificant differences in RBE values (P=.12).

CONCLUSIONS

Central nervous system and brainstem injury incidence for PRT in this series is similar to that reported for photon radiation therapy. The risk of CNS injury was higher for whole posterior fossa boost than for involved field. Although no clear correlation with RBE values was found, numbers were small and additional investigation is warranted to better determine the relationship between injury and LET.

Radiation associated brainstem injury

Publications relating brainstem radiation toxicity to quantitative dose and dose-volume measures derived from three-dimensional treatment planning were reviewed. Despite the clinical importance of brainstem toxicity, most studies reporting brainstem effects after irradiation have fewer than 100 patients. There is limited evidence relating toxicity to small volumes receiving doses above 60-64 Gy using conventional fractionation and no definitive criteria regarding more subtle dose-volume effects or effects after hypofractionated treatment. On the basis of the available data, the entire brainstem may be treated to 54 Gy using conventional fractionation using photons with limited risk of severe or permanent neurological effects. Smaller volumes of the brainstem (1-10 mL) may be irradiated to maximum doses of 59 Gy for dose fractions <or=2 Gy; however, the risk appears to increase markedly at doses >64 Gy.

MULTISESSION CYBERKNIFE STEREOTACTIC RADIOSURGERY OF LARGE, BENIGN CRANIAL BASE TUMORS

OBJECTIVE

Although radiosurgery plays an important role in managing benign cranial base lesions, the potential for increased toxicity with single-session treatment of large tumors is a concern. In this retrospective study, we report the intermediate-term rate of local control, morbidity, and clinical outcomes of patients with large cranial base tumors treated with multisession stereotactic radiosurgery with the CyberKnife (Accuray, Inc., Sunnyvale, CA).

METHODS

Between 1999 and 2008, 34 consecutive patients with large (&gt;15 cm3), benign cranial base tumors (21 meningiomas, 9 schwannomas, 4 glomus jugulare tumors) underwent primary or postoperative radiosurgical treatment using a multisession approach at Stanford University and were considered in this retrospective study. Forty-four percent of these patients had undergone previous subtotal surgical resection or radiotherapy. CyberKnife radiosurgery was delivered in 2 to 5 sessions (median, 3 sessions) to a median tumor volume of 19.3 cm3 (range, 15.8–69.3 cm3). The median marginal dose was 24 Gy (range, 18–25 Gy) prescribed to a median 78% isodose line.

RESULTS

After a median clinical follow-up of 31 months (range, 12–77 months), 21% of patients experienced clinical improvement of neurological symptoms, whereas neurological status remained unchanged among the rest. Four patients experienced prolonged use of glucocorticoids owing to transient neurological worsening and radiographic signs of radiation injury. No permanent neurotoxicity was seen. To date, all tumors remain locally controlled.

CONCLUSION

Over our modest length of follow-up, multisession radiosurgery appears to be a safe and effective option for selected large, benign brain and cranial base lesions.

Delayed toxicity from gamma knife radiosurgery to lesions in and adjacent to the brainstem

The aims of this study were to assess the incidence of, and risk factors for, delayed toxicity following gamma knife stereotactic radiosurgery (GKRS) to lesions in and adjacent to the brainstem. We retrospectively evaluated the delayed toxicity of GKRS following the treatment of 114 lesions in and adjacent to the brainstem in 107 patients. The median tumor volume was 6.2 cm(3) and the median dose to the tumor margin was 16Gy. The mean follow-up was 40 months. Thirteen patients (12%) demonstrated clinical evidence of delayed toxicity, with a median latency to the development of toxicity of 6 months. The actuarial incidence of toxicity at 1 year and 5 years was 10.2% and 13.8%. Larger tumor volume (p=0.02) and larger treatment volume (p=0.04) were associated with an increased incidence of delayed toxicity. Large lesions adjacent to the brainstem have a higher than previously suspected rate of delayed toxicity.

Boron distribution in the normal rat brain after intravenous injection of boronophenylalanine-fructose

Boron neutron capture therapy (BNCT) is an experimental form of radiation therapy for malignant brain tumors and peripheral melanoma. The micro-distribution of the boron compound is critical to determine the radiation effects for both tumors and normal tissue. In the current dose calculation of BNCT, normal brain tissue is considered to have a homogeneous boron concentration. The purpose of this study was to examine the structure-specific boron concentration in normal rat neural tissue. At 10, 30 and 60 min after intravenous injection of 300 mg/kg boronophenylalanine-fructose to 10-week-old CD Fisher rats, neural tissue and blood were collected. Various neural structures were anatomically and histologically identified and specific boron concentrations were analyzed using high-resolution quantitative autoradiography. At 60 min after the injection, only the pituitary gland showed a higher boron concentration than that in blood, with the former being threefold higher. All other neural structures showed lower boron concentrations than that in blood. The present study thus demonstrated an extremely high boron concentration in the pituitary gland following intravenous injection of boronophenylalanine-fructose. In clinical trials of BNCT using an epithermal neutron beam, the radiation dose to the pituitary gland should be carefully evaluated.

Radiosurgery in patients with bilateral vestibular schwannomas

BACKGROUND

Patients with bilateral vestibular schwannomas offer a unique opportunity to determine the effectiveness of radiosurgery. By using the untreated tumor as an internal control, one can determine whether radiosurgery was able to interrupt the natural history of the treated tumor.

METHODS

From September 1998 to November 2004, 13 patients with neurofibromatosis type 2 had 14 tumors treated with radiosurgery at the University of Florida. A retrospective analysis was performed on these patients. Actuarial statistics were used to analyze local control in both the treated and untreated tumor.

RESULTS

The average follow-up length was 38 months. One patient failed to send a follow-up MRI. Actuarial local control in the treated tumors was 100% at 1 year and 92% at 2 and 5 years. Only 1 of the treated tumors continued to grow. In the untreated tumors, actuarial local control was 100% at 1 year, 78% at 2 years and 21% at 5 years. None of the untreated tumors decreased in size.

CONCLUSION

In all but 1 patient with follow-up data, radiosurgery successfully prevented or reversed tumor growth. Additionally, half of the untreated tumors continued to grow. This study shows that radiosurgery alters the natural history of vestibular schwannomas.

Management of acoustic neuromas with fractionated stereotactic radiotherapy (FSRT): Long-term results in 106 patients treated in a single institution

PURPOSE

To assess the long-term outcome and toxicity of fractionated stereotactic radiotherapy for acoustic neuromas in 106 patients treated in a single institution.

PATIENTS AND METHODS

Between October 1989 and January 2004, fractionated stereotactic radiotherapy (FSRT) was performed in 106 patients with acoustic neuroma (AN). The median total dose applied was 57.6 Gy in median single fractions of 1.8 Gy in five fractions per week. The median irradiated tumor volume was 3.9 mL (range, 2.7-30.7 mL). The median follow-up time was 48.5 months (range, 3-172 months).

RESULTS

Fractionated stereotactic radiotherapy was well tolerated in all patients. Actuarial local tumor control rates at 3- and 5- years after FSRT were 94.3% and 93%, respectively. Actuarial useful hearing preservation was 94% at 5 years. The presence of neurofibromatosis (NF-2) significantly adversely influenced hearing preservation in patients that presented with useful hearing at the initiation of RT (p = 0.00062). Actuarial hearing preservation without the diagnosis of NF-2 was 98%. In cases with NF-2, the hearing preservation rate was 64%. Cranial nerve toxicity other than hearing impairment was rare. The rate of radiation induced toxicity to the trigeminal and facial nerve was 3.4% and 2.3%, respectively.

CONCLUSION

Fractionated stereotactic radiotherapy is safe and efficacious for the treatment of AN, with mild toxicity with regard to hearing loss and cranial nerve function. FSRT might be considered as an equieffective treatment modality compared to neurosurgery and therefore represents an interesting alternative therapy for patients with AN.

Low-dose radiosurgery for large vestibular schwannomas: long-term results of functional preservation

Object.The author conducted a study to assess the long-term results obtained in patients who underwent GKS for large vestibular schwannomas (> cochlear nerve functions were evaluated.

Methods.Twenty consecutive large tumors in 18 patients (including two cases of neurofibromatosus Type 2 [NF2]) were followed for more than 6 years. There were eight tumors that were more than 4 cm in maximum diameter. Microsurgery had already been performed prior to GKS in 11 patients (nine recurrent and two residual tumors).

Four patients (including one with NF2) died during the follow-up period of other diseases or by accident. Fourteen of 15 tumors were stable or decreased in size. Microsurgery was performed in one patient 2 years after radiosurgery. Facial nerve function was preserved in all patients and hearing preserved in four of five patients with cochlear nerve function prior to radiosurgery. No adverse effects of radiosurgery have been observed to date.

Conclusions.Gamma knife surgery seems to have a place in the low-dose treatment of selected large vestibular schwannoma in patients with a reasonable chance of retaining facial function and pretreatment hearing level. Patients with severe brainstem compression should first be undergo microsurgery.

Low-dose radiosurgery for large vestibular schwannomas: long-term results of functional preservation

Object. The author conducted a study to assess the long-term results obtained in patients who underwent GKS for large vestibular schwannomas (> cochlear nerve functions were evaluated.

Methods. Twenty consecutive large tumors in 18 patients (including two cases of neurofibromatosus Type 2 [NF2]) were followed for more than 6 years. There were eight tumors that were more than 4 cm in maximum diameter. Microsurgery had already been performed prior to GKS in 11 patients (nine recurrent and two residual tumors).

Four patients (including one with NF2) died during the follow-up period of other diseases or by accident. Fourteen of 15 tumors were stable or decreased in size. Microsurgery was performed in one patient 2 years after radiosurgery. Facial nerve function was preserved in all patients and hearing preserved in four of five patients with cochlear nerve function prior to radiosurgery. No adverse effects of radiosurgery have been observed to date.

Conclusions. Gamma knife surgery seems to have a place in the low-dose treatment of selected large vestibular schwannoma in patients with a reasonable chance of retaining facial function and pretreatment hearing level. Patients with severe brainstem compression should first be undergo microsurgery.

Radiosurgical Planning of Meningiomas: Compromises with Conformity

The radiosurgical planning of meningiomas frequently necessitates compromises between irradiating tumour and risking damage to adjacent structures. In selected cases, we resolved this by excluding part of the tumour from the prescription isodose volume. Most of these compromises or 'suboptimal' plans achieved growth control. Growth control could not be related to conformity indices or to various measures of the radiation dose received by the meningioma. Examining recurrences, 75% arose from dura outside the original treatment field. These findings are discussed in terms of dose prescription protocols and the use of conformity indices in planning. The importance of the dural origin of meningiomas is well established in surgical practice, as reflected by Simpson's grades, but may be equally significant in radiosurgical practice.

Radiosurgery for paraganglioma of the temporal bone

PURPOSE

To report the outcome of patients with paraganglioma of the temporal bone treated with stereotactic radiosurgery at the University of Florida.

METHODS AND MATERIALS

Between January 1997 and June 1999, five patients with paraganglioma of the temporal bone were treated with Linac-based stereotactic radiosurgery at the University of Florida. The ages of the three female and two male patients were between 40 and 88 years (median, 49 years). Four patients were treated at initial presentation, and one had recurrent disease. Treatment volumes ranged from 4.9 cm3 to 18.4 cm3, with a mean of 10.84 cm3. The dose applied to the margin of the tumor varied from 12.5 to 15 Gy (median, 15 Gy). The treatment dose was specified to the 80% isodose shell in two cases and to the 70% isodose shell in three cases. The median follow-up time was 27 months, ranging from 14 to 50 months.

RESULTS

One of four previously untreated patients had a relapse at the primary tumor site. Treatment failure occurred at the field margin 6 months after radiosurgery; the patient was subsequently treated with fractionated stereotactic radiotherapy and at the time of analysis had no evidence of disease, 21 months after initiation of salvage therapy. The patient treated at the time of recurrence after conventional radiotherapy had a local recurrence 40 months after radiosurgery. At the time of this recurrence, the patient had biopsy-proven metastatic disease in two cervical lymph nodes, and no salvage therapy was performed. All patients were alive at the time of the analysis, one with disease present. Presenting symptoms improved in two patients and stabilized in one. The two patients who had local recurrence develop had worsening of their symptoms. One patient had a cranial nerve V palsy develop 6 months after treatment, which resolved after a few months.

CONCLUSIONS

In this series, the results with stereotactic radiosurgery are discouraging compared with our results with conventional fractionated radiotherapy in patients with paraganglioma of the temporal bone.