In silico assessment of the dosimetric quality of a novel, automated radiation treatment planning strategy for linac-based radiosurgery of multiple brain metastases and a comparison with robotic methods

Background

To appraise the dosimetric features and the quality of the treatment plan for radiosurgery of multiple brain metastases optimized with a novel automated engine and to compare with plans optimized for robotic-based delivery.

Methods

A set of 15 patients with multiple brain metastases was selected for this in silico study. The technique under investigation is the recently introduced HyperArc. For all patients, three treatment plans were computed and compared: i: a HyperArc; ii: a standard VMAT; iii) a CyberKnife. Dosimetric features were computed for the clinical target volumes as well as for the healthy brain tissue and the organs at risk.

Results

The data showed that the best dose homogeneity was achieved with the VMAT technique. HyperArc allowed to minimize the volume of brain receiving 4Gy (as well as for the mean dose and the volume receiving 12Gy, although not statistically significant). The smallest dose on 1 cm³ volume for all organs at risk is for CK techniques, and the biggest for VMAT (p < 0.05). The Radiation Planning Index coefficient indicates that, there are no significant differences among the techniques investigated, suggesting an equivalence among these.

Conclusion

At treatment planning level, the study demonstrates that the use of HyperArc technique can significantly improve the sparing of the healthy brain while maintaining a full coverage of the target volumes.

FDXR is a biomarker of radiation exposure in vivo

Previous investigations in gene expression changes in blood after radiation exposure have highlighted its potential to provide biomarkers of exposure. Here, FDXR transcriptional changes in blood were investigated in humans undergoing a range of external radiation exposure procedures covering several orders of magnitude (cardiac fluoroscopy, diagnostic computed tomography (CT)) and treatments (total body and local radiotherapy). Moreover, a method was developed to assess the dose to the blood using physical exposure parameters. FDXR expression was significantly up-regulated 24 hr after radiotherapy in most patients and continuously during the fractionated treatment. Significance was reached even after diagnostic CT 2 hours post-exposure. We further showed that no significant differences in expression were found between ex vivo and in vivo samples from the same patients. Moreover, potential confounding factors such as gender, infection status and anti-oxidants only affect moderately FDXR transcription. Finally, we provided a first in vivo dose-response showing dose-dependency even for very low doses or partial body exposure showing good correlation between physically and biologically assessed doses. In conclusion, we report the remarkable responsiveness of FDXR to ionising radiation at the transcriptional level which, when measured in the right time window, provides accurate in vivo dose estimates.

Clinical examples of 3D dose distribution reconstruction, based on the actual MLC leaves movement, for dynamic treatment techniques

AIM

To present practical examples of our new algorithm for reconstruction of 3D dose distribution, based on the actual MLC leaf movement.

BACKGROUND

DynaLog and RTplan files were used by DDcon software to prepare a new RTplan file for dose distribution reconstruction.

MATERIALS AND METHODS

FOUR DIFFERENT CLINICALLY RELEVANT SCENARIOS WERE USED TO ASSESS THE FEASIBILITY OF THE PROPOSED NEW APPROACH: (1) Reconstruction of whole treatment sessions for prostate cancer; (2) Reconstruction of IMRT verification treatment plan; (3) Dose reconstruction in breast cancer; (4) Reconstruction of interrupted arc and complementary plan for an interrupted VMAT treatment session of prostate cancer. The applied reconstruction method was validated by comparing reconstructed and measured fluence maps. For all statistical analysis, the U Mann-Whitney test was used.

RESULTS

In the first two and the fourth cases, there were no statistically significant differences between the planned and reconstructed dose distribution (p = 0.910, p = 0.975, p = 0.893, respectively). In the third case the differences were statistically significant (p = 0.015). Treatment plan had to be reconstructed.

CONCLUSION

Developed dose distribution reconstruction algorithm presents a very useful QA tool. It provides means for 3D dose distribution verification in patient volume and allows to evaluate the influence of actual MLC leaf motion on the dose distribution.

Beam rate influence on dose distribution and fluence map in IMRT dynamic technique

AIM

To examine the impact of beam rate on dose distribution in IMRT plans and then to evaluate agreement of calculated and measured dose distributions for various beam rate values.

BACKGROUND

Accelerators used in radiotherapy utilize some beam rate modes which can shorten irradiation time and thus reduce ability of patient movement during a treatment session. This aspect should be considered in high conformal dynamic techniques.

MATERIALS AND METHODS

Dose calculation was done for two different beam rates (100 MU/min and 600 MU/min) in an IMRT plan. For both, a comparison of Radiation Planning Index (RPI) and MU was conducted. Secondly, the comparison of optimal fluence maps and corresponding actual fluence maps was done. Next, actual fluence maps were measured and compared with the calculated ones. Gamma index was used for that assessment. Additionally, positions of each leaf of the MLC were controlled by home made software.

RESULTS

Dose distribution obtained for lower beam rates was slightly better than for higher beam rates in terms of target coverage and risk structure protection. Lower numbers of MUs were achieved in 100 MU/min plans than in 600 MU/min plans. Actual fluence maps converted from optimal ones demonstrated more similarity in 100 MU/min plans. Better conformity of the measured maps to the calculated ones was obtained when a lower beam rate was applied. However, these differences were small. No correlation was found between quality of fluence map conversion and leaf motion accuracy.

CONCLUSION

Execution of dynamic techniques is dependent on beam rate. However, these differences are minor. Analysis shows a slight superiority of a lower beam rate. It does not significantly affect treatment accuracy.

Feasibility of reducing the irradiation dose in regions of active neurogenesis for prophylactic cranial irradiation in patients with small-cell lung cancer

Prophylactic cranial irradiation (PCI) is performed on patients with limited or extensive small-cell lung cancer to reduce incidence of brain metastases and prolong survival. PCI may induce neurocognitive impairment. Decreasing irradiation of neural stem cells (NSC) might reduce PCI-induced toxicity. We tested the feasibility of reducing irradiation doses to neural stem cell (NSC) regions while maintaining prescribed doses to the planned target volume (PTV). Irradiation plans utilizing intensity-modulated radiotherapy (IMRT), helical TomoTherapy, and RapidArc for 10 consecutive lung cancer patients were evaluated. The dose distribution, dose-volume histograms, and dose homogeneity indexes were analyzed. Planned and actual dose distributions were compared by dosimetric analysis. Both helical tomotherapy and LINAC-based IMRT reduced the radiation dose to the NSC regions by approximately 45% while maintaining the full dose to the rest of brain. Measured dose distributions matched the planned dose distributions.Protecting the regions of active neurogenesis is technically feasible. Whether reducing the dose by 35% to 45% is sufficient to reduce treatment toxicity, however, can only be addressed in a randomized study. Further reducing the dose within the NSC region might also significantly decrease the dosage to the PTV.

Bystander effects induced by direct and scattered radiation generated during penetration of medium inside a water phantom

BACKGROUND

The biological effects of ionizing radiation have long been thought to results from direct targeting of the nucleus leading to DNA damage. Over the years, a number of non-targeted or epigenetic effects of radiation exposure have been reported where genetic damage occurs in cells that are not directly irradiated but respond to signals transmitted from irradiated cells, a phenomenon termed the "bystander effects".

AIM

We compared the direct and bystander responses of human A 549, BEAS-2-B and NHDF cell lines exposed to both photon (6 MV) and electron (22 MeV) radiation inside a water phantom. The cultures were directly irradiated or exposed to scattered radiation 4 cm outside the field. In parallel, non-irradiated cells (termed bystander cells) were incubated in ICM (irradiation conditioned medium) collected from another pool of irradiated cells (termed donor cells).

MATERIALS AND METHODS

In directly irradiated cells as well as ICM-treated cells, the frequency of micronuclei and condensation of chromatin characteristic for the apoptotic process were estimated using the cytokinesis-block micronucleus test.

RESULTS

In all tested cell lines, radiation induced apoptosis and formation of micronuclei. A549 and BEAS-2B cells cultured in ICM showed increased levels of micronuclei and apoptosis, whereas normal human fibroblasts (NHDF line) were resistant to bystander response. In A549 and BEAS-2B cells placed outside the radiation field and exposed to scattered radiation the formation of micronuclei and induction of apoptosis were similar to that after ICM-treatment.

CONCLUSION

Results suggest that the genetic damage in cells exposed to scattered radiation is caused by factors released by irradiated cells into the medium rather than by DNA damage induced directly by X rays. It seems that bystander effects may have important clinical implications for health risk after low level radiation exposure of cells lying outside the radiation field during clinical treatment.

Direct and bystander effects induced by scattered radiation generated during penetration of radiation inside a water-phantom

In this study, the dose distribution of photon (6 MV) and electron (22 MeV) radiation in a water-phantom was compared with the frequency of apoptotic and micronucleated cells of two human cell lines (BEAS-2B normal bronchial epithelial cells and A549 lung cancer epithelial cells). Formation of micronuclei and apoptotic-like bodies was evaluated by the cytokinesis-block micronucleus test. Measurements were performed for five different phantom depths (3-20 cm). Irradiated cells were placed in a water-phantom in three variants: directly on the axis in the beam, under shielding (only in photon radiation) and outside the beam field. The results reveal a discrepancy between the distribution of physical dose at different depths of the water-phantom and biological effects. This discrepancy is of special significance in case of cells irradiated at a greater depth or placed outside the field and under shield during the exposure to radiation. The frequency of cytogenetic damage was higher than the expected value based on the physical dose received at different depths. Cells placed outside the beam axis were exposed to scattered radiation at very low doses, so we tested if bystander effects could have had a role in the observed discrepancy between physical radiation dose and biological response. We explored this question by use of a medium-transfer technique in which medium (ICM-irradiation conditioned medium) from irradiated cells was transferred to non-irradiated (bystander) cells. The results indicate that when cells were incubated in ICM transferred from cells irradiated at bigger depths or from cells exposed outside the radiation field, the number of apoptotic and micronucleated cells was similar to that after direct irradiation. This suggests that these damages are caused by factors released by irradiated cells into the medium rather than being induced directly in DNA by X-rays. Evaluation of biological effects of scattered radiation appears useful for clinical practice.

Anatomy-corresponding method of IMRT verification

BACKGROUND

During a proper execution of dMLC plans, there occurs an undesired but frequent effect of the dose locally accumulated by tissue being significantly different than expected. The conventional dosimetric QA procedures give only a partial picture of the quality of IMRT treatment, because their solely quantitative outcomes usually correspond more to the total area of the detector than the actually irradiated volume.

AIM

The aim of this investigation was to develop a procedure of dynamic plans verification which would be able to visualize the potential anomalies of dose distribution and specify which tissue they exactly refer to.

MATERIALS & METHODS

The paper presents a method discovered and clinically examined in our department. It is based on a Gamma Evaluation concept and allows accurate localization of deviations between predicted and acquired dose distributions, which were registered by portal as well as film dosimetry. All the calculations were performed on the self-made software GammaEval, the γ-images (2-dimensional distribution of γ-values) and γ-histograms were created as quantitative outcomes of verification.

RESULTS

Over 150 maps of dose distribution have been analyzed and the cross-examination of the gamma images with DRRs was performed.

CONCLUSIONS

It seems, that the complex monitoring of treatment would be possible owing to the images obtained as a cross-examination of γ-images and corresponding DRRs.

EPID in vivo dosimetry in RapidArc technique

AIM

The aim of the study was to estimate the dose at the reference point applying an aSi-EPID device in the course of patient treatment.

MATERIALS AND METHODS

The method assumes direct proportionality between EPID signal and dose delivered to the patient reference point during the treatment session. The procedure consists of treatment plan calculation for the actual patient in the arc technique. The plan was realized with an elliptic water-equivalent phantom. An ionization chamber inside the phantom measured the dose delivered to the reference point. Simultaneously, the EPID matrix measured the CU distribution. EPID signal was also registered during patient irradiation with the same treatment plan. The formula for in vivo dose calculation was based on the CU(g) function, EPID signal registered during therapy and the relation between the dose and EPID signal level measured for the phantom. In vivo dose was compared with dose planned with the treatment planning system. Irradiation was performed with a Clinac accelerator by Varian Medical Systems in the RapidArc technique. The Clinac was equipped with an EPID matrix (electronic portal image device) of aSi-1000. Treatment plans were calculated with the Eclipse/Helios system. The phantom was a Scanditronix/Wellhöfer Slab phantom, and the ionization chamber was a 0.6 ccm PTW chamber.

RESULTS

In vivo dose calculations were performed for five patients. Planned dose at the reference point was 2 Gy for each treatment plan. Mean in vivo dose was in the range of 1.96-2.09.

CONCLUSIONS

Our method was shown to be appropriate for in vivo dose evaluation in the RapidArc technique.

Two-dimensional imaging of tumour control probabilities and normal tissue complication probabilities

AIM

To create a presentation method of TCP and NTCP distributions calculated based on dose distribution for a selected CT slice.

MATERIALS AND METHODS

Three 24-bit colour maps - of dose distribution, delineated structures and CT information - were converted into m-by-n-by-3 data arrays, containing intensities of red, green, and blue colour components for each pixel. All calculations were performed with Matlab v.6.5. The transformation function, which consists of five linear functions, was prepared to translate the colour map into a one-dimensional data array of dose values. A menu-driven application based on the transformation function and mathematical models of complication risk (NTCP) and treatment control probability (TCP) was designed to allow pixel-by-pixel translation of colour maps into one-dimensional arrays of TCP and NTCP values.

RESULTS

The result of this work is an application created to visualize the TCP and NTCP distribution for a single CT scan based on the spatial dose distribution calculated in the treatment planning system. The application allows 10 targets (PTV) and 10 organs at risks (OaR) to be defined. The interface allows alpha/beta values to be inserted for each delineated structure. The application computes TCP and NTCP matrices, which are presented as colour maps superimposed on the corresponding CT slice. There is a set of parameters used for TCP/NTCP calculations which can be defined by the user.

CONCLUSION

Our application is a prototype of an evaluation tool. Although limited to a single plane of the treatment plan, it is believed to be a starting point for further development.

Continuous accelerated 7-days-a-week radiotherapy for head-and-neck cancer: long-term results of phase III clinical trial

PURPOSE

To update 5-year results of a previously published study on special 7-days-a-week fractionation continuous accelerated irradiation (CAIR) for head-and-neck cancer patients.

METHODS AND MATERIALS

One hundred patients with squamous cell carcinoma of head and neck in Stage T(2-4)N(0-1)M(0) were randomized between two definitive radiation treatments: accelerated fractionation 7 days a week including weekends (CAIR) and conventional 5 days a week (control). Hence the overall treatment time was 2 weeks shorter in CAIR.

RESULTS

Five-year local tumor control was 75% in the CAIR group and 33% in the control arm (p < 0.00004). Tumor-cure benefit corresponded with significant improvement in disease-free survival and overall survival rates. Confluent mucositis was the main acute toxicity, with the incidence significantly higher in CAIR patients than in control (respectively, 94% vs. 53%). When 2.0-Gy fractions were used, radiation necrosis developed in 5 patients (22%) in the CAIR group as a consequential late effect (CLE), but when fraction size was reduced to 1.8 Gy no more CLE occurred. Actuarial 5-year morbidity-free survival rate was similar for both treatments.

CONCLUSIONS

Selected head-and-neck cancer patients could be treated very effectively with 7-days-a-week radiation schedule with no compromise of total dose and with slight 10% reduction of fraction dose (2 Gy-1.8 Gy), which article gives 1 week reduction of overall treatment time compared with standard 70 Gy in 35 fractions over 47-49 days. Although this report is based on the relatively small group of patients, its results have encouraged us to use CAIR fractionation in a standard radiation treatment for moderately advanced head-and-neck cancer patients.

Pitfalls in IMRT treatment planning with the CadPlan-Helios system

The CadPlan-Helios treatment planning system for intensity-modulated radiation therapy (IMRT) includes some physical parameters that are specified by the planner, and thus can be subjectively interpreted. The choice of the value of these parameters strongly depends on the practical experience of the planner. This paper presents some extremes of the optimization constraints, to illustrate some traps of IMRT planning. The examples used in this paper are not directly related to clinical situations and some problems were deliberately exaggerated. Treatment fields were defined as recommended by Memorial Sloan-Kettering Cancer Center procedures for prostate IMRT. Prostate plans were arranged using a 5-field technique and 20-MV photon beam. The target and critical structures were defined and contoured for CadPlan-Helios. All computer calculations were performed for the same field arrangement. The following optimization parameters and factors are presented and analyzed in examples to visualize the importance of each parameter: dose-volume constraints and priority factors, scatter distance, maximum number of iterations and termination tolerance. The presented analysis strongly suggests that all constraints and parameters should be recorded in individual planning charts because without their nominal values, any reanalysis or dose recalculation may be difficult.

Endovascular gamma irradiation of the iliac arteries: 1-year results from a clinical safety and feasibility study

PURPOSE

To estimate safety and feasibility of vascular brachytherapy in iliac arteries.

METHODS

Fourteen patients (11 men; mean age 56.7+/-9.9 years, range 44-81) with occlusive disease in 15 iliac arteries (7 external, 8 common) were treated with balloon dilation followed by irradiation from an iridium 192 source (15 Gy at 3 mm from the inner surface of the artery wall) applied with a PARIS centering catheter and bilateral 10-mm margins. Patients receiving stents for suboptimal angioplasty were prescribed a 6-month course of antiplatelet treatment with ticlopidine. Angiography was routinely scheduled for 6 months after intervention.

RESULTS

There were no complications of the angioplasty procedure or EVBT treatment; 7 patients received stents for dissection or residual stenosis. Mean follow-up was 12.4+/-6.0 months. At 6-month angiography, 1 (6.7%) restenosis in a common iliac artery stent was found. Another patient with a common iliac artery stent developed transient lower limb ischemia at 4 months, probably due to temporary suspension of antiplatelet treatment and distal disease.

CONCLUSIONS

Brachytherapy in the iliac arteries appears to be feasible and safe; longer follow-up in more patients is needed to determine its clinical utility in the prevention of restenosis.

Clinical radiobiology of glottic T1 squamous cell carcinoma

PURPOSE

Radiation therapy is the treatment of choice for early glottic squamous cell cancer in many institutions over the world. Despite a relatively homogenous clinical model of T1 glottic tumors for the fractionation studies, the relationships between dose-time parameters remain unclear. To analyze the influence of fractionation parameters and hemoglobin level on tumor cure, this study has been performed.

MATERIALS AND METHODS

This is a retrospective review of 235 patients with T1N0M0 glottic cancer treated by radiation therapy alone given in a conventional schedule with 5 fractions each week. The individual total dose, dose per fraction, and overall treatment time (OTT) ranged from 51-70 Gy, 1.5-3.0 Gy, and 24-79 days, respectively. The median follow-up was 48 months. Patient data--total dose, dose per fraction, OTT, and hemoglobin level (Hb) measured before the radiation treatment--were fitted by the mixed LQ/log-logistic model.

RESULTS

The 5-year local relapse-free survival rate was 84%. All parameters included in the mixed LQ/log-logistic model improved the fit significantly. The dose-response curve for 235 patients with T1 glottic cancer was well defined and steep, and showed significant decrease in tumor control probability (TCP) when total doses were below 61 Gy. The 10-day prolongation of OTT, from 45 to 55 days, decreased the TCP by 13%. The dose of 0.35 Gy/day, compensated repopulation during the 1 day of prolongation, which indicates a potential doubling time (Tpot) for glottic T1 tumor clonogens of 5.5 days. The drop of Hb level of 1 g/dl (from 13.8 g/dl to 12.8 g/dl) gave a 6% decrease of TCP, provided that OTT was 45 days.

CONCLUSION

The significant correlation between the total dose, overall treatment time, hemoglobin concentration, and tumor control probability has been found for T1 glottic cancer.

[Efficiency evaluation of radiotherapy of recurrences after surgery in head and neck cancer]

An analysis of 200 consecutive patients with head and neck squamous cell cancer treated after surgery with radiation at the Centre of Oncology, Maria Sklodowska-Curie Memorial Institute in Gliwice was performed. At the beginning of radiotherapy recurrences were found in 67 (33.5%) patients. Recurrences were diagnosed at the primary site exclusively in 30 patients, in lymph nodes only in 21 patients, in both localizations in 14 patients, and in 2 patients in the tracheostomy. Conventional 60Co radiation treatment to the mean total dose of 62.5 Gy was applied. Three year disease-free survival probability was 22% for patients with recurrences comparing to 54% for patients with no relapse. In the group of patients with recurrences better prognosis was found for patients with preoperatively early stages, and with early recurrences, located in the primary tumour site, which completely responded to radiotherapy. High rate of recurrences after surgery for head and neck cancer and low probability of recurrence control with radiotherapy indicate the necessity of more careful qualification for surgical treatment.

Dose-rate distribution under partially shielded beams

We are presenting a method of calculating the dose-rate distributions in partially shielded field. An attenuator was placed in central beam axis and also in an off-axis geometry. The study was made with a Co-60 radiation and with photons generated by 10 and 23 MV Linac at depths, in phantom, from 5 to 30 cm. To obtain the calculated values of dose-rates; the contributions of the three components to the total dose-rate were measured. The calculated dose-rate distributions agree well with the measured data in case of 10 MV and 23 MV radiation.

[Use of the linear-square model for evaluation of the distribution of equivalent doses of electron irradiation]

The differences between physical absorbed dose of the electron radiation and normalized up to the total dose, fractionated by daily dose equal to 2 Gy, corresponding to "equivalent biologic dose isobio Gy 2" have been analysed. The effect of the fraction dose on the distribution of equivalent total dose in relation to the localization of the neoplastic tumor in the irradiated area has been assessed. On account of the calculations it has been found that alike as in the case of photon radiation the application of fractionated doses equal or lower than 2 Gy is more safe for healthy tissues as compared with higher fractionated doses. No distinct differences have been present between the distribution of physical and biological doses for isodoses below 80%. This fact suggests that the choice of the energy of electron radiation should be decided on account of the depth of the neoplastic tumor and the distribution of physical dose--whereas the distribution of biologic isodoses (isobio Gy 2) of electron radiation is of significant importance in the estimation of the tolerance of critical organs situated in the area between skin and the irradiated tumor.