Compatibility of the Linear-Quadratic Formalism and Biologically Effective Dose Concept to High-Dose-Per-Fraction Irradiation in a Murine Tumor

A generalised method for calculating repopulation-corrected tumour EQD2 values in a wide range of clinical situations, including interrupted treatments

Any radiotherapy schedule can be characterised by its 2 Gy per fraction equivalent dose (EQD2). EQD2s are easily calculated for late-responding normal tissues but for tumours significant errors may arise if no allowance is made for any repopulation which occurs in the reference and/or the derived EQD2 schedule. This article presents a systematic approach to calculating tumour EQD2 values utilising the concept of biologically effective dose (BED) with inclusion of repopulation effects. A factor (f) is introduced which allows the inter-dependence between EQD2 and its delivery time (and, hence, the amount of repopulation involved) to be embedded within the formulation without any additional assumptions. There exists a transitional BED below which simple methods of calculating tumour EQD2 remain valid. In cases where simpler approaches are inadequate, the correct EQD2 may be determined from the reference schedule BED (BEDref) by the relationship: EQD2 = A × BEDref - B, where A and B are constants which involve the same radiobiological parameters as are conventionally used in deriving tumour BED values. Some Worked Examples illustrate application of the method to fractionated radiotherapy and indicate that there can be substantial differences with results obtained from using over-simplified approaches. Since reference BEDs are calculable for other types of radiotherapy (brachytherapy, permanent implants, high-LET applications, etc) the methodology allows estimation of tumour EQD2 values in a wide range of clinical circumstances, including cases which involve interrupted treatments.

The impact of local control on widespread progression and survival in oligometastasis-directed SBRT: Results from a large international database

PURPOSE

We investigated the impact of local control (LC) on widespread progression (WSP) and overall survival (OS) in patients treated to all extracranial oligometastases (OMs) at presentation to SBRT in this retrospective review across 6 international centers.

MATERIALS/METHODS

Relationships between LC status of SBRT-directed OMs and OS and WSP (>5 new active/untreated lesions) were explored using Cox and Fine-Gray regression models, adjusting for radioresistant histology and pre-SBRT systemic therapy receipt. The association between LC and dosimetric predictors was analyzed with competing risk regression using death as a competing risk and across a wide range of simulated α/βratios.

RESULTS

In total, 1700 OMs in 1033 patients were analyzed, with 25.2% NSCLC, 22.7% colorectal, 12.8% prostate, and 8.1% breast histology. Patients who failed locally in any SBRT-directed OM within 6 mo were at 3.6-fold higher risk of death and 2.7-fold higher risk of WSP compared to those who remained locally-controlled (p < 0.001). Similar associations existed for each duration of LC investigated through 3 yrs post-SBRT. There was no significant difference in risk of WSP or death between patients who failed in a subset of SBRT-treated lesions vs. patients who failed in all lesions. Minimum dose (Dmin) to the GTV/ITV was most predictive of LC when compared to prescription dose, PTV Dmin, and PTV Dmax. Sensitivity analysis for achieving 1-yr LC > 95% found thresholds of 41.2 Gy and 55.2 Gy in 5 fractions for smaller (< 27.7 cc) and larger radioresistant lesions, respectively.

CONCLUSION

This large multinational cohort suggests that the duration of LC following OM-directed SBRT strongly correlates with WSP and OS.

Translational study for stereotactic body radiotherapy against non-small cell lung cancer, including oligometastases, considering cancer stem-like cells enable predicting clinical outcome from in vitro data

BACKGROUND

Curative effects of stereotactic body radiotherapy (SBRT) for non-small cell lung cancer (NSCLC) have been evaluated using various biophysical models. Because such model parameters are empirically determined based on clinical experience, there is a large gap between in vitro and clinical studies. In this study, considering the heterogeneous cell population, we performed a translational study to realize the possible linkage based on a modeling approach.

METHODS

We modeled cell-killing and tumor control probability (TCP) considering two populations: progeny and cancer stem-like cells. The model parameters were determined from in vitro survival data of A549 and EBC-1 cells. Based on the cellular parameters, we predicted TCP and compared it with the corresponding clinical data from 553 patients collected at Hirosaki University Hospital.

RESULTS

Using an all-in-one developed model, the so-called integrated microdosimetric-kinetic (IMK) model, we successfully reproduced both in vitro survival after acute irradiation and the 3-year TCP with various fractionation schemes (6-10 Gy per fraction). From the conventional prediction without considering cancer stem cells (CSCs), this study revealed that radioresistant CSCs play a key role in the linkage between in vitro and clinical outcomes.

CONCLUSIONS

This modeling study provides a possible generalized biophysical model that enables precise estimation of SBRT worldwide.

The Effect of Atm Loss on Radiosensitivity of a Primary Mouse Model of Pten-Deleted Brainstem Glioma

Diffuse midline gliomas arise in the brainstem and other midline brain structures and cause a large proportion of childhood brain tumor deaths. Radiation therapy is the most effective treatment option, but these tumors ultimately progress. Inhibition of the phosphoinositide-3-kinase (PI3K)-like kinase, ataxia-telangiectasia mutated (ATM), which orchestrates the cellular response to radiation-induced DNA damage, may enhance the efficacy of radiation therapy. Diffuse midline gliomas in the brainstem contain loss-of-function mutations in the tumor suppressor PTEN, or functionally similar alterations in the phosphoinositide-3-kinase (PI3K) pathway, at moderate frequency. Here, we sought to determine if ATM inactivation could radiosensitize a primary mouse model of brainstem glioma driven by Pten loss. Using Cre/loxP recombinase technology and the RCAS/TVA retroviral gene delivery system, we established a mouse model of brainstem glioma driven by Pten deletion. We find that Pten-null brainstem gliomas are relatively radiosensitive at baseline. In addition, we show that deletion of Atm in the tumor cells does not extend survival of mice bearing Pten-null brainstem gliomas after focal brain irradiation. These results characterize a novel primary mouse model of PTEN-mutated brainstem glioma and provide insights into the mechanism of radiosensitization by ATM deletion, which may guide the design of future clinical trials.

Cellular bases of hypofractionated radiotherapy protocols for lung cancer

The extreme demand on health systems due to the COVID-19 pandemic has led to reconsider hypofractionation. Although the best clinical efficacy of these schemes is being demonstrated, the biological bases have not been established. Thus, after validating basic clinical parameters, through complementary in vitro models, we characterized the cellular and molecular mechanisms of hypofractionation protocols. Cell cultures of human lung cancer cell line A549 were irradiated with 0, 2, 4, 8, 12, 16 and 20 Gy. The clastogenic, cytotoxic, proliferative and clonogenic capacities and bystander effect were evaluated. In addition, we assessed survival and toxicity in a retrospective study of 49 patients with lung cancer. Our findings showed that the greater efficacy of ablative regimens should not only be attributed to events of direct cell death induced by genotoxic damage, but also to a lower cell repopulation and the indirect action of clastogenic factors secreted. These treatments were optimal in terms of 1- and 2-year overall survival (74 and 65%, respectively), and progression-free survival at 1 and 2 years (71 and 61%, respectively). The greater efficacy of high doses per fraction could be attributed to a multifactorial mechanism that goes beyond the 4Rs of conventional radiotherapy.

Variability of α/β ratios for prostate cancer with the fractionation schedule: caution against using the linear-quadratic model for hypofractionated radiotherapy

BACKGROUND

Prostate cancer (PCa) is known to be suitable for hypofractionated radiotherapy due to the very low α/β ratio (about 1.5-3 Gy). However, several randomized controlled trials have not shown the superiority of hypofractionated radiotherapy over conventionally fractionated radiotherapy. Besides, in vivo and in vitro experimental results show that the linear-quadratic (LQ) model may not be appropriate for hypofractionated radiotherapy, and we guess it may be due to the influence of fractionation schedules on the α/β ratio. Therefore, this study attempted to estimate the α/β ratio in different fractionation schedules and evaluate the applicability of the LQ model in hypofractionated radiotherapy.

METHODS

The maximum likelihood principle in mathematical statistics was used to fit the parameters: α and β values in the tumor control probability (TCP) formula derived from the LQ model. In addition, the fitting results were substituted into the original TCP formula to calculate 5-year biochemical relapse-free survival for further verification.

RESULTS

Information necessary for fitting could be extracted from a total of 23,281 PCa patients. A total of 16,442 PCa patients were grouped according to fractionation schedules. We found that, for patients who received conventionally fractionated radiotherapy, moderately hypofractionated radiotherapy, and stereotactic body radiotherapy, the average α/β ratios were 1.78 Gy (95% CI 1.59-1.98), 3.46 Gy (95% CI 3.27-3.65), and 4.24 Gy (95% CI 4.10-4.39), respectively. Hence, the calculated α/β ratios for PCa tended to become higher when the dose per fraction increased. Among all PCa patients, 14,641 could be grouped according to the risks of PCa in patients receiving radiotherapy with different fractionation schedules. The results showed that as the risk increased, the k (natural logarithm of an effective target cell number) and α values decreased, indicating that the number of effective target cells decreased and the radioresistance increased.

CONCLUSIONS

The LQ model appeared to be inappropriate for high doses per fraction owing to α/β ratios tending to become higher when the dose per fraction increased. Therefore, to convert the conventionally fractionated radiation doses to equivalent high doses per fraction using the standard LQ model, a higher α/β ratio should be used for calculation.

Radiobiology of stereotactic ablative radiotherapy (SABR): perspectives of clinical oncologists

Stereotactic ablative radiotherapy (SABR) is a novel radiation treatment method that delivers an intense dose of radiation to the treatment targets with high accuracy. The excellent local control and tolerance profile of SABR have made it become an important modality in cancer treatment. The radiobiology of SABR is a key factor in understanding and further optimizing the benefits of SABR. In this review, we have addressed several issues in the radiobiology of SABR from the perspective of clinical oncologists. The appropriateness of the linear-quadratic (LQ) model for SABR is controversial based on preclinical data, but it is a reliable tool from the perspective of clinical application because the biological effective dose (BED) calculated with it can represent the tumor control probability (TCP). Hypoxia is a common phenomenon in SABR in spite of the relatively small tumor size and has a negative effect on the efficacy of SABR. Preliminary studies indicate that a hypoxic radiosensitizer combined with SABR may be a feasible strategy, but so far there is not adequate evidence to support its application in routine practice. The vascular change of endothelial apoptosis and blood perfusion reduction in SABR may enhance the response of tumor cells to radiation. Combination of SABR with anti-angiogenesis therapy has shown promising efficacy and good tolerance in advanced cancers. SABR is more powerful in enhancing antitumor immunity and works better with immune checkpoint inhibitors (ICIs) than conventional fractionation radiotherapy. Combination of SABR with ICIs has become a practical option for cancer patients with metastases.

Stereotactic Radiotherapy with Fractionation for the Lesions in and Around the Brainstem and Optic Nerve

Purpose

Among the components of the central nervous system, the optic nerve and the brainstem are considered to be the eloquent structures that are sensitive to stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT). SRS or SRT with fractionation in areas adjacent to these tissues is both promising and challenging.

Materials and methods

To clarify the precise dose distribution achievable with fractionation in and around the optic nerve and brainstem, theoretical simulations were performed, based on the biological effective dose (BED).

Results

These simulations clearly showed that the doses to the optic nerve and brainstem can be adjusted using fractionation, meaning that the prescribed doses to the surrounding brain tissue can be reduced. Conversely doses to the lesions themselves can be increased by fractionation, while maintaining a stable dose to normal optic nerve and brainstem tissue.

Comparison of treatment results between 3- and 2-stage Gamma Knife radiosurgery for large brain metastases: a retrospective multi-institutional study

OBJECTIVE

In order to obtain better local tumor control for large (i.e., > 3 cm in diameter or > 10 cm3 in volume) brain metastases (BMs), 3-stage and 2-stage Gamma Knife surgery (GKS) procedures, rather than a palliative dose of stereotactic radiosurgery, have been proposed. Here, authors conducted a retrospective multi-institutional study to compare treatment results between 3-stage and 2-stage GKS for large BMs.

METHODS

This retrospective multi-institutional study involved 335 patients from 19 Gamma Knife facilities in Japan. Major inclusion criteria were 1) newly diagnosed BMs, 2) largest tumor volume of 10.0-33.5 cm3, 3) cumulative intracranial tumor volume ≤ 50 cm3, 4) no leptomeningeal dissemination, 5) no more than 10 tumors, and 6) Karnofsky Performance Status 70% or better. Prescription doses were restricted to between 9.0 and 11.0 Gy in 3-stage GKS and between 11.8 and 14.2 Gy in 2-stage GKS. The total treatment interval had to be within 6 weeks, with at least 12 days between procedures. There were 114 cases in the 3-stage group and 221 in the 2-stage group. Because of the disproportion in patient numbers and the pre-GKS clinical factors between these two GKS groups, a case-matched study was performed using the propensity score matching method. Ultimately, 212 patients (106 from each group) were selected for the case-matched study. Overall survival, tumor progression, neurological death, and radiation-related adverse events were analyzed.

RESULTS

In the case-matched cohort, post-GKS median survival time tended to be longer in the 3-stage group (15.9 months) than in the 2-stage group (11.7 months), but the difference was not statistically significant (p = 0.65). The cumulative incidences of tumor progression (21.6% vs 16.7% at 1 year, p = 0.31), neurological death (5.1% vs 6.0% at 1 year, p = 0.58), or serious radiation-related adverse events (3.0% vs 4.0% at 1 year, p = 0.49) did not differ significantly.

CONCLUSIONS

This retrospective multi-institutional study showed no differences between 3-stage and 2-stage GKS in terms of overall survival, tumor progression, neurological death, and radiation-related adverse events. Both 3-stage and 2-stage GKS performed according to the aforementioned protocols are good treatment options in selected patients with large BMs.

Dose to the dominant intraprostatic lesion using HDR vs. LDR monotherapy: A Phase II randomized trial

PURPOSE

To present the dosimetric results of a Phase II randomized trial comparing dose escalation to the MRI-defined dominant intraprostatic lesion (DIL) using either low-dose-rate (LDR) or high-dose-rate (HDR) prostate brachytherapy.

MATERIAL AND METHODS

Patients receiving prostate brachytherapy as monotherapy were randomized to LDR or HDR brachytherapy. Prostate and DILs were contoured on preoperative multiparametric MRI. These images were registered with transrectal ultrasound for treatment planning. LDR brachytherapy was preplanned using I-125 seeds. HDR brachytherapy used intraoperative transrectal ultrasound-based planning to deliver 27 Gy/2 fractions in separate implants. DIL location was classified as peripheral, central, or anterior. A student t-test compared DIL D₉₀ between modalities and DIL locations.

RESULTS

Of 60 patients, 31 underwent LDR and 29 HDR brachytherapy. Up to three DILs were identified per patient (100 total) with 74 peripheral, six central, and 20 anterior DILs. Mean DIL volume was 1.9 cc (SD: 1.7 cc) for LDR and 1.6 cc (SD 1.3 cc) for HDR (p = 0.279). Mean DIL D₉₀ was 151% (SD 30%) for LDR and 132% (SD 13%) for HDR. For LDR, mean peripheral DIL D₉₀ was 159% (SD 27%) and central or anterior 127% (SD 13%). HDR peripheral DILs received 137% (SD 12%) and central or anterior 119% (SD 7%). DIL D₉₀ for peripheral lesions was higher than anterior and central (p < 0.001).

CONCLUSIONS

DIL location affects dose escalation, particularly because of urethral proximity, such as for anterior and central DILs. HDR brachytherapy may dose escalate better when target DIL is close to critical organs.

The Tethered Effect of Vestibular Schwannoma Tumor Shrinkage Following Stereotactic Radiosurgery in Secondary Trigeminal Neuralgia

BACKGROUND

Compression of the trigeminal nerve by vessels and tumors causes trigeminal neuralgia. However, a tethering effect, provoking an abnormal root-stretching force, has been previously reported to play a role in trigeminal nerve hyperexcitability. We report 2 patients with vestibular schwannomas treated by stereotactic radiosurgery (SRS) who presented with typical manifestations of trigeminal neuralgia after tumor shrinkage. Furthermore, we discuss the mechanisms of trigeminal neuralgia.

CASE DESCRIPTION

Two patients without a history of trigeminal dysfunction, including trigeminal neuralgia, underwent SRS for vestibular schwannomas. Both patients demonstrated tumor shrinkage after transient tumor expansion following SRS. Neither patient presented with facial pain or dysesthesia at the time of peak tumor volume. However, trigeminal neuralgia occurred after tumor shrinkage. One patient underwent surgery, as the neuralgia was refractory to medical treatment; although the trigeminal nerve was adhered and tethered to the tumor, no neurovascular conflict was identified between the tumor and the nerve. We removed the tumor partially, dissecting between the nerve and the tumor, and relieved the tethered effect. Trigeminal neuralgia was relieved without medication after surgery.

CONCLUSIONS

The present cases demonstrate a tethered effect of tumor shrinkage after SRS, which was considered to play a role in trigeminal neuralgia. Surgical dissection surrounding the nerve root is effective for medically resistant neuralgia, even if the tumor shrinks. Partial tumor removal is adequate in such cases, as the tumor has been controlled by radiosurgery.

The impact of different stereotactic radiation therapy regimens for brain metastases on local control and toxicity

Purpose

Stereotactic radiation therapy (SRT) enables focused, short course, high dose per fraction radiation delivery to brain tumors that are less ideal for single fraction treatment because of size, shape, or close proximity to sensitive structures. We sought to identify optimal SRT treatment regimens for maximizing local control while minimizing morbidity.

Methods and materials

We performed a retrospective review of patients treated with SRT for solid brain metastases using variable dose schedules between 2001 and 2011 at 3 academic hospitals. Endpoints included (1) local control, (2) acute toxicity (Common Toxicity Criteria for Adverse Events v3.0), and (3) symptomatic radionecrosis. Kaplan-Meier and a competing risks methodology were used to estimate the actuarial rate of local failure and assess the association of clinical and treatment covariates with time to local failure.

Results

A total of 156 patients was identified. Common tumor histologies included breast (21%), non-small cell lung (32%), melanoma (22%), small cell lung (9%), and renal cell carcinoma (6%). The majority of lesions were supratentorial (57%). Median target volume was 3.99 mL (range, 0.04-58.42). Median total SRT dose was 25 Gy (range, 12-36), median fractional dose was 5 Gy (range, 2.5-11), and median number of fractions was 5 (range, 2-10). Cumulative incidence of local progression at 3, 6, 12, 18, and 24 months was 11%, 22%, 29%, 34%, and 36%. Total prescription dose was the only factor significantly associated with time to local progression on univariate (P = .02) and multivariable analysis (P = .01, adjusted hazards ratio, 0.87). Five patients experienced seizures within 10 days of SRT and 5 patients developed radionecrosis. All patients with documented radionecrosis received prior radiation to the index lesion.

Conclusions

Our series of SRT for brain metastases found total prescription dose to be the only factor associated with local control. Both acute and long-term toxicity events from SRT were modest.

Dosimetric and radiobiologic comparison of 103Pd COMS plaque brachytherapy and Gamma Knife radiosurgery for choroidal melanoma

PURPOSE

Plaque brachytherapy (BT) and Gamma Knife radiosurgery (GKRS) are highly conformal treatment options for choroidal melanoma. This study objectively compares physical dose and biologically effective dose (BED) distributions for these two modalities.

METHODS AND MATERIALS

Tumor and organ-at-risk (OAR) dose distributions from a CT-defined reference right eye were compared between ¹⁰³Pd COMS (Collaborative Ocular Melanoma Study Group) plaques delivering 70 Gy (plaque heterogeneity corrected) over 120 h to the tumor apex and GKRS plans delivering 22 Gy to the 40% isodose line for a representative sample of clinically relevant choroidal melanoma locations and sizes. Tumor and OAR biologically effective dose-volume histograms were generated using consensus radiobiologic parameters and modality-specific BED equations.

RESULTS

Published institutional prescriptive practices generally lead to larger tumor and OAR physical doses from COMS BT vs. GKRS. Radiobiologic dose conversions, however, revealed variable BEDs. Medium and large tumors receive >1.3 times higher BEDs with COMS BT vs. GKRS. OAR BEDs have even greater dependence on tumor size, location, and treatment modality. For example, COMS BT maximum BEDs to the optic nerve are lower than from GKRS for large anterior and all posterior tumors but are higher for anterior small and medium tumors.

CONCLUSIONS

BT and GKRS for choroidal melanoma have different physical dose and BED distributions with potentially unique clinical consequences. Using published institutional prescriptive practices, neither modality is uniformly favored, although COMS BT delivers higher physical doses and BEDs to tumors. These results suggest that lowering the physical prescription dose for COMS BT to more closely match the BED of GKRS might maintain equivalent tumor control with less potential morbidity.

Radiobiology of hypofractionated stereotactic radiotherapy: what are the optimal fractionation schedules?

In hypofractionated stereotactic radiotherapy (SRT), high doses per fraction are usually used and the dose delivery pattern is different from that of conventional radiation. The daily dose is usually given intermittently over a longer time compared with conventional radiotherapy. During prolonged radiation delivery, sublethal damage repair takes place, leading to the decreased effect of radiation. In in vivo tumors, however, this decrease in effect may be counterbalanced by rapid reoxygenation. Another issue related to hypofractionated SRT is the mathematical model for dose evaluation and conversion. The linear-quadratic (LQ) model and biologically effective dose (BED) have been suggested to be incorrect when used for hypofractionation. The LQ model overestimates the effect of high fractional doses of radiation. BED is particularly incorrect when used for tumor responses in vivo, since it does not take reoxygenation into account. Correction of the errors, estimated at 5-20%, associated with the use of BED is necessary when it is used for SRT. High fractional doses have been reported to exhibit effects against tumor vasculature and enhance host immunity, leading to increased antitumor effects. This may be an interesting topic that should be further investigated. Radioresistance of hypoxic tumor cells is more problematic in hypofractionated SRT, so trials of hypoxia-targeted agents are encouraged in the future. In this review, the radiobiological characteristics of hypofractionated SRT are summarized, and based on the considerations, we would like to recommend 60 Gy in eight fractions delivered three times a week for lung tumors larger than 2 cm in diameter.

Hypofractionated stereotactic radiotherapy for brain metastases from lung cancer : Evaluation of indications and predictors of local control

AIM

To evaluate the efficacy and toxicity of hypofractionated stereotactic radiotherapy (HSRT) for brain metastases (BMs) from lung cancer, and to explore prognostic factors associated with local control (LC) and indication.

PATIENTS AND METHODS

We evaluated patients who were treated with linac-based HSRT for BMs from lung cancer. Lesions treated with stereotactic radiosurgery (SRS) in the same patients during the same periods were analysed and compared with HSRT in terms of LC or toxicity. There were 53 patients with 214 lesions selected for this analysis (HSRT: 76 lesions, SRS: 138 lesions). For HSRT, the median prescribed dose was 35 Gy in 5 fractions.

RESULTS

The 1‑year LC rate was 83.6 % in HSRT; on multivariate analysis, a planning target volume (PTV) of <4 cm(3), biologically effective dose (BED10) of ≥51 Gy, and adenocarcinoma were significantly associated with better LC. Moreover, in PTVs ≥ 4 cm(3), there was a significant difference in LC between BED10 < 51 Gy and ≥ 51 Gy (p = 0.024). On the other hand, in PTVs < 4 cm(3), both HSRT and SRS had good LC with no significant difference (p = 0.195). Radiation necrosis emerged in 5 of 76 lesions (6.6 %) treated with HSRT and 21 of 138 (15.2 %) lesions treated with SRS (p = 0.064).

CONCLUSION

Linac-based HSRT was safe and effective for BMs from lung cancer, and hence might be particularly useful in or near an eloquent area. PTV, BED10, and pathological type were significant prognostic factors. Furthermore, in BMs ≥ 4 cm(3), a dose of BED ≥ 51 Gy should be considered.

Treatment of Breast and Prostate Cancer by Hypofractionated Radiotherapy: Potential Risks and Benefits

Breast cancer and prostate cancer are the most common cancers diagnosed in women and men, respectively, in the UK, and radiotherapy is used extensively in the treatment of both. In vitro data suggest that tumours in the breast and prostate have unique properties that make a hypofractionated radiotherapy treatment schedule advantageous in terms of therapeutic index. Many clinical trials of hypofractionated radiotherapy treatment schedules have been completed to establish the extent to which hypofractionation can improve patient outcome. Here we present a concise description of hypofractionation, the mathematical description of converting between conventional and hypofractionated schedules, and the motivation for using hypofractionation in the treatment of breast and prostate cancer. Furthermore, we summarise the results of important recent hypofractionation trials and highlight the limitations of a hypofractionated treatment regimen.

A matter of timing: identifying significant multi-dose radiotherapy improvements by numerical simulation and genetic algorithm search

Multi-dose radiotherapy protocols (fraction dose and timing) currently used in the clinic are the product of human selection based on habit, received wisdom, physician experience and intra-day patient timetabling. However, due to combinatorial considerations, the potential treatment protocol space for a given total dose or treatment length is enormous, even for relatively coarse search; well beyond the capacity of traditional in-vitro methods. In constrast, high fidelity numerical simulation of tumor development is well suited to the challenge. Building on our previous single-dose numerical simulation model of EMT6/Ro spheroids, a multi-dose irradiation response module is added and calibrated to the effective dose arising from 18 independent multi-dose treatment programs available in the experimental literature. With the developed model a constrained, non-linear, search for better performing cadidate protocols is conducted within the vicinity of two benchmarks by genetic algorithm (GA) techniques. After evaluating less than 0.01% of the potential benchmark protocol space, candidate protocols were identified by the GA which conferred an average of 9.4% (max benefit 16.5%) and 7.1% (13.3%) improvement (reduction) on tumour cell count compared to the two benchmarks, respectively. Noticing that a convergent phenomenon of the top performing protocols was their temporal synchronicity, a further series of numerical experiments was conducted with periodic time-gap protocols (10 h to 23 h), leading to the discovery that the performance of the GA search candidates could be replicated by 17-18 h periodic candidates. Further dynamic irradiation-response cell-phase analysis revealed that such periodicity cohered with latent EMT6/Ro cell-phase temporal patterning. Taken together, this study provides powerful evidence towards the hypothesis that even simple inter-fraction timing variations for a given fractional dose program may present a facile, and highly cost-effecitive means of significantly improving clinical efficacy.

[Stereotactic body radiation therapy in the management of liver tumours]

Stereotactic radiotherapy is a high-precision technique based on the administration of high doses to a limited target volume. This treatment constitutes a therapeutic progress in the management of many tumours, especially hepatic ones. If surgery remains the standard local therapy, stereotactic radiotherapy is first dedicated to inoperable patients or unresectable tumours. Patients with moderately altered general status, preserved liver function and tumour lesions limited in number as in size are eligible to this technique. Results in terms of local control are satisfying, regarding primary tumours (notably hepatocellular carcinomas) as metastases stemming from various origins. If treatment protocols and follow-up modalities are not standardized to this day, iconographic acquisition using four-dimensional computed tomography, target volumes delineation based on morphological and/or metabolic data, and image-guided radiotherapy contribute to an oncologic efficacy and an improved sparing of the functional liver. The purpose of this literature review is to report the results of the main works having assessed stereotactic radiotherapy in the management of primary and secondary liver tumours. Technical particularities of this radiation modality will also be described.

Applicability of the linear-quadratic model to single and fractionated radiotherapy schedules: an experimental study

The aim of this study was to examine the applicability of the linear-quadratic (LQ) model to single and fractionated irradiation in EMT6 cells. First, the α/β ratio of the cells was determined from single-dose experiments, and a biologically effective dose (BED) for 20 Gy in 10 fractions (fr) was calculated. Fractional doses yielding the same BED were calculated for 1-, 2-, 3-, 4-, 5-, 7-, 15- and 20-fraction irradiation using LQ formalism, and then irradiation with these schedules was actually given. Cell survival was determined by a standard colony assay. Differences in cell survival between pairs of groups were compared by t-test. The α/β ratio of the cells was 3.18 Gy, and 20 Gy in 10 fr corresponded to a BED3.18 of 32.6 Gy. The effects of 7-, 15- and 20-fraction irradiation with a BED3.18 of 32.6 Gy were similar to those of the 10-fraction irradiation, while the effects of 1- to 5-fraction irradiation were lower. In this cell line, the LQ model was considered applicable to 7- to 20-fraction irradiation or doses per fraction of 2.57 Gy or smaller. The LQ model might be applicable in the dose range below the α/β ratio.

Five-fraction CyberKnife radiotherapy for large brain metastases in critical areas: impact on the surrounding brain volumes circumscribed with a single dose equivalent of 14 Gy (V14) to avoid radiation necrosis

The efficacy and toxicity of five-fraction CyberKnife radiotherapy were evaluated in patients with large brain metastases in critical areas. A total of 85 metastases in 78 patients, including tumors >30 cm(3) (4 cm in diameter) were treated with five-fraction CyberKnife radiotherapy with a median marginal dose of 31 Gy at a median prescribed isodose of 58%. Changes in the neurological manifestations, local tumor control, and adverse effects were investigated after treatment. The surrounding brain volumes circumscribed with 28.8 Gy (single dose equivalent to 14 Gy: V14) were measured to evaluate the risk of radiation necrosis. Neurological manifestations, such as motor weakness, visual disturbances and aphasia improved in 28 of 55 patients (50.9%). Local tumor control was obtained in 79 of 85 metastases (92.9%) during a median follow-up of eight months. Symptomatic edema occurred in 10 patients, and two of them (2.6%) required surgical resection because of radiation necrosis. The V14 of these patients was 3.0-19.7 cm(3). There were 16 lesions with a V14 of ≥7.0 cm(3), and two of these lesions developed extensive brain edema due to radiation necrosis. None of the patients with a V14 of <7.0 cm(3) exhibited edema requiring surgical intervention. We therefore conclude that a high rate of local tumor control and low rates of complications can be obtained after five-fraction CyberKnife radiotherapy for large metastases in critical areas. The V14 of the surrounding brain is therefore a useful indicator for the risk of radiation necrosis in patients with large metastases.

Radiotherapy and the tumor stroma: the importance of dose and fractionation

Ionizing radiation is a non-specific but highly effective way to kill malignant cells. However, tumor recurrence sustained by a minor fraction of surviving tumor cells is a commonplace phenomenon caused by the activation of both cancer cell intrinsic resistance mechanisms, and also extrinsic intermediaries of therapy resistance, represented by non-malignant cells and structural components of the tumor stroma. The improved accuracy offered by advanced radiotherapy (RT)-technology permits reduced volume of healthy tissue in the irradiated field, and has been triggering an increase in the prescription of high-dose oligo-fractionated regimens in the clinics. Given the remarkable clinical success of high-dose RT and the current therapeutic shift occurring in the field, in this review we revise the existing knowledge on the effects that different radiation regimens exert on the different compartments of the tumor microenvironment, and highlight the importance of anti-tumor immunity and other tumor cell extrinsic mechanisms influencing therapeutic responses to high-dose radiation.

Radiobiology of radiosurgery for the central nervous system

According to Leksell radiosurgery is defined as "the delivery of a single, high dose of irradiation to a small and critically located intracranial volume through the intact skull." Before its birth in the early 60s and its introduction in clinical therapeutic protocols in late the 80s dose application in radiation therapy of the brain for benign and malignant lesions was based on the administration of cumulative dose into a variable number of fractions. The rationale of dose fractionation is to lessen the risk of injury of normal tissue surrounding the target volume. Radiobiological studies of cell culture lines of malignant tumors and clinical experience with patients treated with conventional fractionated radiotherapy helped establishing this radiobiological principle. Radiosurgery provides a single high dose of radiation which translates into a specific toxic radiobiological response. Radiobiological investigations to study the effect of high dose focused radiation on the central nervous system began in late the 50s. It is well known currently that radiobiological principles applied for dose fractionation are not reproducible when single high dose of ionizing radiation is delivered. A review of the literature about radiobiology of radiosurgery for the central nervous system is presented.

Fractionated stereotactic radiotherapy using CyberKnife for the treatment of large brain metastases: a dose escalation study

AIMS

To evaluate the toxicity and efficacy of fractionated stereotactic radiotherapy (FSRT) with doses of 18-30 Gy in three fractions and 21-35 Gy in five fractions against large brain metastases.

MATERIALS AND METHODS

Between 2005 and 2012, 61 large brain metastases (≥ 2.5 cm in maximum diameter) of a total of 102 in 54 patients were treated with FSRT as a first-line therapy. Neurological symptoms were observed in 47 of the 54 patients before FSRT. Three fractions were applied to tumours with a maximum diameter ≥ 2.5 cm and <4 cm, and five fractions were used for brain metastases ≥ 4 cm. After ensuring that the toxicities were acceptable (≤ grade 2), doses were escalated in steps. Doses to the large brain metastases were as follows: level I, 18-22 Gy/three fractions or 21-25 Gy/five fractions; level II, 22-27 Gy/three fractions or 25-31 Gy/five fractions; level III, 27-30 Gy/three fractions or 31-35 Gy/five fractions. Level III was the target dose level.

RESULTS

Overall survival rates were 52 and 31% at 6 and 12 months, respectively. Local tumour control rates of the 102 total brain metastases were 84 and 78% at 6 and 12 months, respectively. Local tumour control rates of the 61 large brain metastases were 77 and 69% at 6 and 12 months, respectively. Grade 3 or higher toxicities were not observed.

CONCLUSIONS

The highest dose levels of 27-30 Gy/three fractions and 31-35 Gy/five fractions seemed to be tolerable and effective in controlling large brain metastases. These doses can be used in future studies on FSRT for large brain metastases.

Fractionated stereotactic radiosurgery using the Novalis system for the management of pituitary adenomas close to the optic apparatus

Radiosurgery has been proven to be an effective treatment for residual or recurrent pituitary adenomas after surgery. However, it causes severe complications when the optic apparatus is irradiated over the tolerance dose. In this study, we analyzed the feasibility of fractionated stereotactic radiosurgery to treat pituitary tumors close to the optic apparatus. Thirty-four patients from June 2006 to June 2011 with recurrent or residual pituitary adenomas close to (<3 mm) the optic apparatus were treated with fractionated stereotactic radiosurgery. Three fractions with a total dose of 2100 cGy were applied to the tumors. Imaging, examination of vision, and estimation of hormone level were regularly performed before and after radiosurgery. The mean tumor volume before fractioned stereotactic radiosurgery was 5.06±3.08 cm3 (range: 0.82-12.69 cm3). After a mean follow up of 36.8±15.7 months (range: 16-72 months), tumor size was reduced in seven (20.6%) patients and remained the same in the other 27 (79.4%) patients. Vision was improved in one patient and remained stable in the rest. Only one patient developed transient post-treatment diplopia. This study suggests that fractionated stereotactic radiosurgery is safe for treating pituitary adenomas close to the optic apparatus. Studies with more patients and longer follow-up are required to draw definite conclusions.

Three-fraction CyberKnife radiotherapy for brain metastases in critical areas: referring to the risk evaluating radiation necrosis and the surrounding brain volumes circumscribed with a single dose equivalence of 14 Gy (V14)

The efficacy and toxicity of three-fraction CyberKnife radiotherapy were evaluated in patients with brain metastases in critical areas. One hundred and fifty-nine metastases in 145 patients including tumors >10 cm(3) were treated with three-fraction CyberKnife radiotherapy with a median marginal dose of 27 Gy at a median prescribed isodose of 60%. Changes in the neurological manifestations, local tumor control and adverse effects were investigated after treatment. The surrounding brain volumes circumscribed with 23.1 Gy (single dose equivalence of 14 Gy: V14) were measured to evaluate the risk of adverse effects. Neurological manifestations, such as motor weakness, visual disturbances and aphasia improved in 26 of 97 patients (26.8%). Local tumor control was obtained in 137 of 143 metastases (95.8%) during a median follow-up of 7 months. Nine patients had symptomatic edema and three of them (2.1%) required surgical resection because of radiation necrosis. The V14 of these patients was 4.6-31.5 cm(3). There were 35 lesions with a V14 of 7 cm(3) or more and three of them developed extensive brain edema due to radiation necrosis. None of the patients with a V14 of <7 cm(3) exhibited edema requiring an operation. We therefore conclude that a high rate of local tumor control and low rates of complications are obtained after three-fraction CyberKnife radiotherapy for metastases in critical areas. The V14 of the surrounding brain therefore seems to be a useful indicator for the risk evaluation of radiation necrosis in patients with larger metastases.

The impact of fractionation in SBRT: analysis with the linear quadratic model and the universal survival curve model

BACKGROUND

Since the impact of fractionation in SBRT has not been systematically investigated, this modelling study was performed to see how the therapeutic window is affected for a range of fractions at target doses commonly administered in SBRT.

MATERIAL AND METHODS

Isoeffective tumour-doses (constant cell survival) were calculated with the linear quadratic (LQ) and the universal survival curve (USC) models for 2-20 fractions. The isoeffective tumour-regimes (with α/β = 10 Gy) were used to calculate the sparing of normal tissues (with α/β = 3 Gy) for an increasing number of fractions. Sparing was calculated as an increase in cell survival and decrease in normal tissue complication probability (NTCP) as compared to a common scheme with 3 fractions of 22 Gy to the centre of the target [(15 Gy to the periphery of the planning target volume (PTV)].

RESULTS

At a high dose per fraction, above about 15 Gy, the USC model predicted much lower fractionation sensitivity than the LQ model. This holds true for both tumour and normal tissues. The USC model also predicted greater sparing of normal tissues outside the PTV as compared to the LQ model. Especially at dose levels of the order of 30-50% to that in the centre of the target. The decrease in NTCP predicted by the USC model was of the order of 30% for 10 fractions as compared to the NTCP for 3 fractions. With the LQ model the corresponding decrease was of the order of 10%.

CONCLUSION

The USC model generally predicts a larger therapeutic window than the LQ model for an increasing number of fractions than today's practice in SBRT.

Once-weekly stereotactic radiotherapy for patients with oligometastases: compliance and preliminary efficacy

AIMS AND BACKGROUND

This retrospective analysis reports the outcomes obtained with an original once-weekly stereotactic radiotherapy fractionation given to patients affected by evolving oligometastases from different solid malignancies.

METHODS

From 2009 to 2011, patients with symptomatic and/or evolving oligometastases were submitted to a median 5-fraction cycle of stereotactic radiotherapy of one fraction per week in order to exploit a radiobiological rationale designed to increase the therapeutic index. Individual fractionation was mainly planned according to patient performance status, oligometastasis size and site, and record of previous irradiation in the same site.

RESULTS

Thirty-six patients in stage IV UICC-TNM affected by oligometastases were treated with image-guided intensity-modulated stereotactic tomotherapy with a single weekly radiation. Median age was 70 years (range, 34-89). The median weekly single dose, number of fractions and overall total radiation dose were 7 Gy, 5 fractions and 35 Gy, respectively. Thirty-five (97%) patients completed the treatment schedule. No patient suffered mild or severe radiation-related side effects. Twenty-one (87%) of 24 patients with local pain had complete symptomatic response within 30 days following the end of radiotherapy. Local control assessed at imaging after stereotactic radiotherapy was evidenced in 30 (83%) patients. Median time to response after the end of radiotherapy was 40 days.

CONCLUSIONS

The original radiotherapy regimen delivering only a single stereotactic dose per week seems to be highly feasible with an interesting high efficacy rate in patients with oligometastases from different solid tumors. Overall, the once-weekly treatment was very compliant in an advanced cancer stage especially for elderly and frail patients.

Compatibility of the repairable-conditionally repairable, multi-target and linear-quadratic models in converting hypofractionated radiation doses to single doses

We investigated the applicability of the repairable-conditionally repairable (RCR) model and the multi-target (MT) model to dose conversion in high-dose-per-fraction radiotherapy in comparison with the linear-quadratic (LQ) model. Cell survival data of V79 and EMT6 single cells receiving single doses of 2-12 Gy or 2 or 3 fractions of 4 or 5 Gy each, and that of V79 spheroids receiving single doses of 5-26 Gy or 2-5 fractions of 5-12 Gy, were analyzed. Single and fractionated doses to actually reduce cell survival to the same level were determined by a colony assay. Single doses used in the experiments and surviving fractions at the doses were substituted into equations of the RCR, MT and LQ models in the calculation software Mathematica, and each parameter coefficient was computed. Thereafter, using the coefficients and the three models, equivalent single doses for the hypofractionated doses were calculated. They were then compared with actually-determined equivalent single doses for the hypofractionated doses. The equivalent single doses calculated using the RCR, MT and LQ models tended to be lower than the actually determined equivalent single doses. The LQ model seemed to fit relatively well at doses of 5 Gy or less. At 6 Gy or higher doses, the RCR and MT models seemed to be more reliable than the LQ model. In hypofractionated stereotactic radiotherapy, the LQ model should not be used, and conversion models incorporating the concept of the RCR or MT models, such as the generalized linear-quadratic models, appear to be more suitable.

Radiobiological evaluation of the radiation dose as used in high-precision radiotherapy: effect of prolonged delivery time and applicability of the linear-quadratic model

Since the dose delivery pattern in high-precision radiotherapy is different from that in conventional radiation, radiobiological assessment of the physical dose used in stereotactic irradiation and intensity-modulated radiotherapy has become necessary. In these treatments, the daily dose is usually given intermittently over a time longer than that used in conventional radiotherapy. During prolonged radiation delivery, sublethal damage repair takes place, leading to the decreased effect of radiation. This phenomenon is almost universarily observed in vitro. In in vivo tumors, however, this decrease in effect can be counterbalanced by rapid reoxygenation, which has been demonstrated in a laboratory study. Studies on reoxygenation in human tumors are warranted to better evaluate the influence of prolonged radiation delivery. Another issue related to radiosurgery and hypofractionated stereotactic radiotherapy is the mathematical model for dose evaluation and conversion. Many clinicians use the linear-quadratic (LQ) model and biologically effective dose (BED) to estimate the effects of various radiation schedules, but it has been suggested that the LQ model is not applicable to high doses per fraction. Recent experimental studies verified the inadequacy of the LQ model in converting hypofractionated doses into single doses. The LQ model overestimates the effect of high fractional doses of radiation. BED is particularly incorrect when it is used for tumor responses in vivo, since it does not take reoxygenation into account. For normal tissue responses, improved models have been proposed, but, for in vivo tumor responses, the currently available models are not satisfactory, and better ones should be proposed in future studies.