The clinical radiobiology of brachytherapy

Future Treatment Strategies for Cancer Patients Combining Targeted Alpha Therapy with Pillars of Cancer Treatment: External Beam Radiation Therapy, Checkpoint Inhibition Immunotherapy, Cytostatic Chemotherapy, and Brachytherapy

Cancer is one of the most complex and challenging human diseases, with rising incidences and cancer-related deaths despite improved diagnosis and personalized treatment options. Targeted alpha therapy (TαT) offers an exciting strategy emerging for cancer treatment which has proven effective even in patients with advanced metastatic disease that has become resistant to other treatments. Yet, in many cases, more sophisticated strategies are needed to stall disease progression and overcome resistance to TαT. The combination of two or more therapies which have historically been used as stand-alone treatments is an approach that has been pursued in recent years. This review aims to provide an overview on TαT and the four main pillars of therapeutic strategies in cancer management, namely external beam radiation therapy (EBRT), immunotherapy with checkpoint inhibitors (ICI), cytostatic chemotherapy (CCT), and brachytherapy (BT), and to discuss their potential use in combination with TαT. A brief description of each therapy is followed by a review of known biological aspects and state-of-the-art treatment practices. The emphasis, however, is given to the motivation for combination with TαT as well as the pre-clinical and clinical studies conducted to date.

AAPM Task Group Report 267: A joint AAPM GEC-ESTRO report on biophysical models and tools for the planning and evaluation of brachytherapy

Brachytherapy utilizes a multitude of radioactive sources and treatment techniques that often exhibit widely different spatial and temporal dose delivery patterns. Biophysical models, capable of modeling the key interacting effects of dose delivery patterns with the underlying cellular processes of the irradiated tissues, can be a potentially useful tool for elucidating the radiobiological effects of complex brachytherapy dose delivery patterns and for comparing their relative clinical effectiveness. While the biophysical models have been used largely in research settings by experts, it has also been used increasingly by clinical medical physicists over the last two decades. A good understanding of the potentials and limitations of the biophysical models and their intended use is critically important in the widespread use of these models. To facilitate meaningful and consistent use of biophysical models in brachytherapy, Task Group 267 (TG-267) was formed jointly with the American Association of Physics in Medicine (AAPM) and The Groupe Européen de Curiethérapie and the European Society for Radiotherapy & Oncology (GEC-ESTRO) to review the existing biophysical models, model parameters, and their use in selected brachytherapy modalities and to develop practice guidelines for clinical medical physicists regarding the selection, use, and interpretation of biophysical models. The report provides an overview of the clinical background and the rationale for the development of biophysical models in radiation oncology and, particularly, in brachytherapy; a summary of the results of literature review of the existing biophysical models that have been used in brachytherapy; a focused discussion of the applications of relevant biophysical models for five selected brachytherapy modalities; and the task group recommendations on the use, reporting, and implementation of biophysical models for brachytherapy treatment planning and evaluation. The report concludes with discussions on the challenges and opportunities in using biophysical models for brachytherapy and with an outlook for future developments.

Equivalent uniform RBE-weighted dose in eye plaque brachytherapy

BACKGROUND

Brachytherapy for ocular melanoma is based on the application of eye plaques with different spatial dose nonuniformity, time-dependent dose rates and relative biological effectiveness (RBE).

PURPOSE

We propose a parameter called the equivalent uniform RBE-weighted dose (EUDRBE) that can be used for quantitative characterization of integrated cell survival in radiotherapy modalities with the variable RBE, dose nonuniformity and dose rate. The EUDRBE is applied to brachytherapy with 125I eye plaques designed by the Collaborative Ocular Melanoma Study (COMS).

METHODS

The EUDRBE is defined as the uniform dose distribution with RBE = 1 that causes equal cell survival for a given nonuniform dose distribution with the variable RBE > 1. The EUDRBE can be used for comparison of cell survival for nonuniform dose distributions with different RBE, because they are compared to the reference dose with RBE = 1. The EUDRBE is applied to brachytherapy with 125I COMS eye plaques that are characterized by a steep dose gradient in tumor base-apex direction, protracted irradiation during time intervals of 3-8 days, and variable dose-rate dependent RBE with a maximum of about 1.4. The simulations are based on dose of 85 Gy prescribed to the farthest intraocular extent of the tumor (tumor apex). To compute the EUDRBE in eye plaque brachytherapy and correct for protracted irradiation, the distributions of physical dose have been converted to non-uniform distributions of biologically effective dose (BED) to include the biological effects of sublethal cellular repair, Our radiobiological analysis considers the combined effects of different time-dependent dose rates, spatial dose non-uniformity, dose fractionation and different RBE and can be used to derive optimized dose regimens brachytherapy.

RESULTS

Our simulations show that the EUDRBE increases with the prescription depths and the maximum increase may achieve 6% for the tumor height of 12 mm. This effect stems from a steep dose gradient within the tumor that increases with the prescription depth. The simulations also show that the EUDRBE increase may achieve 12% with increasing the dose rate when implant duration decreases. The combined effect of dose nonuniformity and dose rate may change the EUDRBE up to 18% for the same dose prescription of 85 Gy to tumor apex. The absolute dose range of 48-61 Gy (RBE) for the EUDRBE computed using 4 or 5 fractions is comparable to the dose prescriptions used in stereotactic body radiation therapy (SBRT) with megavoltage X-rays (RBE = 1) for different cancers. The tumor control probabilities in SBRT and eye plaque brachytherapy are very similar at the level of 80% or higher that support the hypothesis that the selected approximations for the EUDRBE are valid.

CONCLUSIONS

The computed range of the EUDRBE in 125I COMS eye plaque brachytherapy suggests that the selected models and hypotheses are acceptable. The EUDRBE can be useful for analysis of treatment outcomes and comparison of different dose regimens in eye plaque brachytherapy.

Reirradiation with advanced brachytherapy techniques in recurrent GYN cancers

PURPOSE

To evaluate clinical outcomes of recurrent gynaecological cancers treated with reirradiation (reRT) using advanced brachytherapy (BT) technique.

METHODS AND MATERIALS

Seventy-six women who underwent reRT with BT for gynaecological cancers at our institute between January 2000 and December 2019 were analysed to determine patient, disease and treatment characteristics and clinical outcomes. Descriptive analysis was used for demographics, and the Kaplan Meir method was used for survival analysis.

RESULTS

Median age at recurrence was 55 years (Range: 35-73). Forty-three patients had recurrent cervical cancer with intact uterus, and 33 had recurrent vault/vaginal cancers post adjuvant RT. Eight patients received EBRT prior to BT (Range: 30-50Gy). Twenty-two patients (28.9%) received salvage chemotherapy before consideration of brachytherapy. Brachytherapy application was done using MUPIT in 38, Vienna applicator in 20, Syed Neblett in 8, central vaginal cylinder in 3, multicatheter intravaginal applicator in 2, tandem-ovoids in 4 and Houdek applicator in 1 patient. Median cumulative EQD2 for all courses of radiation was 108 Gy (IQR 92-123 Gy). At median follow up of 39 months, local control (LC), progression-free survival (PFS) and Overall survival (OS) at 2-years was 60%, 56.3%, and 72.9 respectively. Patients who had recurrences beyond 2 years had significantly better OS compared to early recurrences. Patients who received BT doses >40 Gy had a higher LC and PFS. Grade 3 to 4 late rectal toxicity was seen in 10 (13%), bladder toxicity in 6 (8%) and vaginal fibrosis in 24 (31%) patients.

CONCLUSION

The use of advanced BT approach in reirradiation setting is a feasible and safe option in treatment of post-treatment recurrent cervical, endometrial, and vaginal cancers.

Improving the Accuracy of Biologically Effective Dose Estimates, from a Previously Published Study, After Radiosurgery for Acoustic Neuromas

OBJECTIVE

To recalculate biological effective dose values (BED) for radio-surgical treatments of acoustic neuroma from a previous study. BEDs values were previously overestimated by only using beam-on times in calculations, so excluding the important beam-off-times (when deoxyribonucleic acid repair continues) which contribute to the overall treatment time. Simple BED estimations using a mono-exponential approximation may not always be appropriate but if used should include overall treatment time.

METHODS

Time intervals between isocenters were estimated. These were especially important for the Gamma Knife Model 4C cases since manual changes significantly increase overall treatment times. Individual treatment parameters, such as iso-center number, beam-on-time, and beam-off-time, were then used to calculate BED values using a more appropriate bi-exponential model that includes fast and slow components of DNA damage repair over a wider time range.

RESULTS

The revised BED estimates differed significantly from previously published values. The overestimates of BED, obtained using beam-on-time only, varied from 0%-40.3%. BED subclasses, each with a BED range of 5 Gy2.47, indicated that revised values were consistently reduced when compared with originally quoted values, especially for 4C compared with Perfexion cases. Furthermore, subdivision of 4C cases by collimator number further emphasized the impact of scheduled gap times on BED. Further analysis demonstrated important limitations of the mono-exponential model. Target volume was a major confounding factor in the interpretation of the results of this study.

CONCLUSIONS

BED values should be estimated by including beam-on and beam-off times. Suggestions are provided for more accurate BED estimations in future studies.

Salvage image-guided freehand interstitial brachytherapy for pelvic sidewall recurrence after hysterectomy for uterine malignancies

PURPOSE

Pelvic sidewall recurrence after hysterectomy for uterine malignances has a poor prognosis, and the salvage therapy for this type of recurrence is still challenging. The purpose of this study was to investigate the efficacy of freehand high-dose-rate interstitial brachytherapy (HDR-ISBT) through the perineum using transrectal ultrasonography for this disease.

METHODS AND MATERIALS

We retrospectively evaluated 42 patients with pelvic sidewall recurrence after hysterectomy for uterine cervical and endometrial cancers. We investigated patients' characteristics, the 2-year local control and survival rates, and late adverse events of the rectum and bladder.

RESULTS

The 2-year overall survival, local control, and progression-free survival rates were 73.7% (95% confidence interval [CI], 60.8-89.3%), 69.4% (95% CI, 55.4-80.1%), and 37.3% (95% CI, 24.6-56.5%), respectively. In Cox multivariate analysis, tumor size at recurrence (<45 mm vs. ≥45 mm) (p = 0.04) and disease-free periods after hysterectomy (<10 months vs. ≥10 months) (p < 0.01) were significant prognostic factors for overall survival. Lymph node metastasis at recurrence (p < 0.01) was also a significant prognostic factor for progression-free survival. Three patients experienced Grade 3-4 late proctitis (7%).

CONCLUSIONS

Transperineal freehand salvage HDR-ISBT using transrectal ultrasonography was demonstrated to be a curative treatment option for patients with pelvic sidewall recurrence following hysterectomy. Based on the findings of this study, we emphasize the importance of HDR-ISBT for pelvic sidewall recurrence.

Biological effective dose in analysis of rectal dose in prostate cancer patients who underwent a combination therapy of VMAT and LDR with hydrogel spacer insertion

This study aimed to evaluate rectal dose reduction in prostate cancer patients who underwent a combination of volumetric modulated arc therapy (VMAT) and low‐dose‐rate (LDR) brachytherapy with insertion of hydrogel spacer (SpaceOAR). For this study, 35 patients receiving hydrogel spacer and 30 patients receiving no spacer were retrospectively enrolled. Patient was treated to doses of 45 Gy to the primary tumor site and nodal regions over 25 fractions using VMAT and 100 Gy to the prostate using prostate seed implant (PSI). In VMAT plans of patients with no spacer, mean doses of rectal wall were 43.6, 42.4, 40.1, and 28.8 Gy to the volume of 0.5, 1, 2, and 5 cm³, respectively. In patients with SpaceOAR, average rectal wall doses decreased to 39.0, 36.9, 33.5, and 23.9 Gy to the volume of 0.5, 1, 2, and 5 cm³, respectively (p < 0.01). In PSI plans, rectal wall doses were on average 78.5, 60.9, 41.8, and 14.8 Gy to the volume of 0.5, 1, 2, and 5 cm³, respectively, in patients without spacer. In contrast, the doses decreased to 34.5, 28.4, 20.6 (p < 0.01), and 8.5 Gy (p < 0.05) to rectal wall volume of 0.5, 1, 2, and 5 cm³, respectively, in patient with SpaceOAR. To demonstrate rectal sum dose sparing, dose‐biological effective dose (BED) calculation was accomplished in those patients who showed >60% overlap of rectal volumetric doses between VMAT and PSI. In patients with SpaceOAR, average BED_sum was decreased up to 34%, which was 90.1, 78.9, 65.9, and 40.8 Gy to rectal volume of 0.5, 1, 2, and 5 cm³, respectively, in comparison to 137.4, 116.7, 93.0, and 50.2 Gy to the volume of 0.5, 1, 2, and 5 cm³, respectively, in those with no spacer. Our result suggested a significant reduction of rectal doses in those patients who underwent a combination of VMAT and LDR with hydrogel spacer placement.

Voxel-Level BED Corrected Dosimetric and Radiobiological Assessment of 2 Kinds of Hybrid Radiotherapy Planning Methods for Stage III NSCLC

Background/purpose: To access the comparative dosimetric and radiobiological advantages of two methods of intensity-modulated radiation therapy (IMRT)-based hybrid radiotherapy planning for stage III nonsmall cell lung cancer (NSCLC). Methods: Two hybrid planning methods were respectively characterized by conventional fraction radiotherapy (CFRT) and stereotactic body radiotherapy (SBRT) and CFRT and simultaneous integrated boost (SIB) planning. All plans were retrospectively completed using the 2 methods for 20 patients with stage III NSCLC. CFRT and SBRT dose regimes 2 Gy  ×  30 f and 12.5 Gy  ×  4 f were, respectively, used for planning target volume of lymph node (PTV_LN) and planning target volume of the primary tumor (PTV_PT), while dose regimes 2 Gy  ×  26 f for PTV_LN and sequential 2 Gy  ×  4 f for PTV_LN combined with 12.5 Gy  ×  4 f for PTV_PT were adopted for CFRT and SIB plans. SBRT and SIB EQD₂ dose were calculated voxel by voxel, and then, respectively, superimposed with 30-fraction and 26-fraction CFRT plan dose to achieve biological equivalent dose (BED) dosimetric parameters of CFRT and SBRT and CFRT and SIB plans. Tumor control probability (TCP)/normal tissue complication probability (NTCP) was, respectively, calculated by equivalent uniform dose/Lyman–Kutcher–Burman models. BED plan parameters and TCP/NTCP were analyzed between 2 methods of hybrid planning. Primary tumor/lymph node (LN)/total TCP values were, respectively, evaluated as a function of the radiation dose needed to control 50% of tumor (TCD₅₀) for 20 patients. Dosimetric errors were analyzed by nontransit electronic portal imaging device dosimetry measurement during hybrid plan delivery. Results: Statistically lower BED plan parameters of PTV_LN D₂ and homogeneity index resulted in slightly lower averaged LN/total TCP curves by CFRT and SIB planning. The gaps between Max and Min LN/total TCP curves were significantly closer for CFRT and SIB planning, which indicated better robustness of LN/total TCPs. A lower esophagus dose resulted in a lower esophagus NTCP by CFRT and SIB planning, which may be compromised by 1 week shorter overall treatment time by CFRT and SIB irradiation. Spinal cord D_max was significantly reduced by CFRT and SIB plans. The dose verification results of the subplans involved in hybrid plans were acceptable, which showed that the 2 methods of hybrid planning could be delivered accurately in our center. Conclusion: CFRT and SIB plannings have more advantages on BED plan parameters and TCP/NTCP than CFRT and SBRT planning, and both methods of IMRT-based hybrid planning could be executed accurately for stage III NSCLC. The effectiveness of the results needs to be validated in the hybrid trial.

Radiobiological evaluation of organs at risk for electronic high-dose-rate brachytherapy in uveal melanoma: a radiobiological modeling study

Purpose

The objective of this study was to examine feasibility of single- or hypo-fraction of high-dose-rate (HDR) electronic brachytherapy (eBT) in uveal melanoma treatment.

Material and methods

Biologically effective doses (BED) of organs at risk (OARs) were compared to those of iodine-125-based eye plaque low-dose-rate brachytherapy (¹²⁵I LDR-BT) with vitreous replacement (VR). Single- or hypo-fractionated equivalent physical doses (SFEDs or HFEDs) for tumor were calculated from tumor BED of ¹²⁵I LDR-BT using linear-quadratic (LQ) and universal survival curve (USC) models. BED OARs doses to retina opposite the implant, macula, optic disc, and lens were calculated and compared among SFED, HFED, and ¹²⁵I LDR-BT. Electronic BT of 50 kVp was considered assuming dose fall-off as clinically equivalent to ¹²⁵I LDR-BT. All OARs BEDs were analyzed with and without silicone oil VR.

Results

For a single-fraction incorporating VR, the median/interquartile range of LQ (USC)-based BED doses of the retina opposite the implant, macula, optic disc, and lens were 16%/1.2% (33%/4%), 35%/19.5% (64%/17.7%), 37%/19% (75%/17.8%), and 27%/7.9% (68%/23.2%) of those for ¹²⁵I LDR-BT, respectively. SFED tumor values were 29.8/0.2 Gy and 51.7/0.5 Gy when using LQ and USC models, respectively, which could be delivered within 1 hour. SFED can be delivered within 1 hour using a high-dose-rate eBT. Even four-fraction delivery of HFED without VR resulted in higher OARs doses in the macula, optic disc, and lens (135 ~ 159%) than when using ¹²⁵I LDR-BT technique. A maximum p-value of 0.005 was observed for these distributions.

Conclusions

The simulation of single-fraction eBT, including vitreous replacement, resulted in significantly reduced OARs doses (16 ~ 75%) of that achieved with ¹²⁵I LDR-BT.

Multifunctional Hybrid Hydrogel Enhanced Antitumor Therapy through Multiple Destroying DNA Functions by a Triple-Combination Synergistic Therapy

Brachytherapy, as an effective setting for precise cancer therapy in clinic, can lead to serious DNA damage. However, its therapeutic efficacy is always limited by the DNA self-repair property, tumor hypoxia-associated radiation resistance as well as inhomogeneous distribution of the radioactive material. Herein, a multifunctional hybrid hydrogel (¹³¹ I-hydrogel/DOX/GNPs aggregates) is developed by loading gold nanoparticle aggregates (GNPs aggregates) and DOX into a radionuclide iodine-131 (¹³¹ I) labelled polymeric hydrogels (¹³¹ I-PEG-P(Tyr)₈ ) for tumor destruction by completely damaging DNA self-repair functions. This hybrid hydrogel exhibits excellent photothermal/radiolabel stability, biocompatibility, and fluorescence/photothermal /SPECT imaging properties. After local injection, the sustained releasing DOX within tumor greatly inhibits the DNA replication. Meanwhile, GNPs aggregates as a radiosensitizer and photosensitizer show a significant improvement of brachytherapeutic efficacy and cause serious DNA damage. Simultaneously, GNPs aggregates induce mild photothermal therapy under 808 nm laser irradiation, which not only inhibits self-repair of the damaged DNA but also effectively relieves tumor hypoxic condition to enhance the therapeutic effects of brachytherapy, leading to a triple-synergistic destruction of DNA functions. Therefore, this study provides a highly efficient tumor synergistic therapy platform and insight into the synergistic antitumor mechanism in DNA level.

Radioactive Iodine-125 in Tumor Therapy: Advances and Future Directions

Radioactive iodine-125 (I-125) is the most widely used radioactive sealed source for interstitial permanent brachytherapy (BT). BT has the exceptional ability to deliver extremely high doses that external beam radiotherapy (EBRT) could never achieve within treated lesions, with the added benefit that doses drop off rapidly outside the target lesion by minimizing the exposure of uninvolved surrounding normal tissue. Spurred by multiple biological and technological advances, BT application has experienced substantial alteration over the past few decades. The procedure of I-125 radioactive seed implantation evolved from ultrasound guidance to computed tomography guidance. Compellingly, the creative introduction of 3D-printed individual templates, BT treatment planning systems, and artificial intelligence navigator systems remarkably increased the accuracy of I-125 BT and individualized I-125 ablative radiotherapy. Of note, utilizing I-125 to treat carcinoma in hollow cavity organs was enabled by the utility of self-expandable metal stents (SEMSs). Initially, I-125 BT was only used in the treatment of rare tumors. However, an increasing number of clinical trials upheld the efficacy and safety of I-125 BT in almost all tumors. Therefore, this study aims to summarize the recent advances of I-125 BT in cancer therapy, which cover experimental research to clinical investigations, including the development of novel techniques. This review also raises unanswered questions that may prompt future clinical trials and experimental work.

Sublobar resection with intraoperative brachytherapy versus sublobar resection alone for early-stage non-small-cell lung cancer: a meta-analysis

OBJECTIVES

The purpose of this study was to compare the clinical outcomes for sublobar resection (SR) or SR plus intraoperative brachytherapy (SRB) for clinical stage I non-small-cell lung cancer.

METHODS

A systematic search was performed in the EMBASE, PubMed and Cochrane Library databases to identify related studies comparing SR to SRB. Data were collected on local recurrence (LR) as a primary outcome and regional or distant recurrence, overall survival and disease-free survival (DFS) as secondary outcomes. Meta-analysis was carried out using Stata 12.0.

RESULTS

A total of 476 patients received SRB, and 617 received SR across 5 studies. Meta-analysis of LR, regional or distant recurrence, overall survival and disease-free survival rates showed no significant difference between SRB and SR groups. However, when biologically effective dose (BED) was >100 Gy, LR rate was lower in the SRB group than in the SR group (Relative risk [RR] = 0.143, 95% confidence interval [CI]: 0.051-0.397) (p < 0.001). When BED was <100 Gy, no significant difference was found in LR rate between SRB and SR groups (SRB versus SR: RR = 1.132, 95%CI: 0.704-1.821) (p = 0.608).

CONCLUSIONS

Intraoperative brachytherapy was not associated with reduced risk of regional or distant metastasis or improved outcomes for patients with clinical stage I non-small-cell lung cancer; however, it might reduce the LR rate when BED was >100 Gy.

The influence of the α/β ratio on treatment time iso-effect relationships in the central nervous system

Purpose: To investigate the influence of changes in α/β ratio (range 1.5-3 Gy) on iso-effective doses, with varying treatment time, in spinal cord and central nervous system tissues with comparable radio-sensitivity. It is important to establish if an α/β ratio of 2 Gy, the accepted norm for neuro-oncology iso-effect estimations, can be used.Methods: The rat spinal cord irradiation data of Pop et al. provided ED₅₀ values for radiation myelopathy for treatment times that varied from minutes to ∼6 days. Analysis using biphasic repair kinetics, allowing for variable dose-rates, provided the best fit with repair half-times of 0.19 and 2.16 hr, each providing ∼50% of overall repair; with an α/β ratio 2.47 Gy (CI 1.5-3.95 Gy). Using the above data set, graphical methods were used to investigate changes in the repair parameters for differing fixed α/β ratios between 1.5 and 3.0 Gy. Two different intermittent dose delivery equations were used to evaluate the implications in a radiosurgery setting.Results: Changes in the α/β ratio (1.5-3.0 Gy) have a minor effect on equivalent doses for radiation myelopathy for treatment durations of a few hours. Changing the α/β value from 2 Gy to 2.47 Gy, modified equivalent single doses by < 1% when overall treatment times ranged from 0.1 to 5.0 hr. Significant changes were only found for treatment times longer than 5-10 hr. These two α/β ratios were also compared in a practical radiosurgery situation, using two different models for estimating BED, again there was no significant loss of accuracy.Conclusions: It is reasonable to use an α/β ratio of 2 Gy for CNS tissue, with the same repair half-times as published in the original publication by Pop et al., in situations where the assessment of the BED in radiosurgery is used with other form of radiotherapy. In radiosurgery, the variation in BED with treatment duration (for a fixed physical dose) is very similar, but absolute BED values depend on the α/β value. In radiosurgery, clinical recommendations obtained using BED calculations using the originally proposed α/β ratio of 2.47 Gy are still appropriate. For calculations involving a combination of radiosurgery and other modalities, such as fractionated radiotherapy, it would be appropriate in all cases to apply a value of 2 Gy, the accepted norm in neuro-oncology, without significant loss of accuracy in the radio-surgical component. This may have important applications in retreatment situations.

IMRT and brachytherapy comparison in gynaecological cancer treatment: thinking over dosimetry and radiobiology

Background

The role of radiotherapy and brachytherapy in the management of locally advanced cervical and endometrial cancer is well established. However, in some cases, intracavitary brachytherapy (ICBRT) is not recommended or cannot be carried out. We aimed to investigate whether external-beam irradiation delivered by means of intensity-modulated radiation therapy (IMRT) might replace ICBRT in gynaecological cancer when the standard ICBRT boost delivering cannot be administered for technical or clinical reasons.

Materials and methods

Fifteen already delivered treatments for gynaecological cancer patients were analysed. The treatments were performed through 3-dimensional conformal radiotherapy (3D-CRT) to the whole-pelvis up to the dose of 45–50.4 Gy followed by a boost dose administered with ICBRT in high-dose-rate or pulsed-dose-rate modality. For each patient, IMRT plans were elaborated to mimic the ICBRT. We analysed the ICBRT boost versus IMRT boost in terms of dosimetric and radiobiological aspects.

Results

Mean conformity index value calculated on boost volume was 0.73 for ICBRT and 0.97 for IMRT. Mean conformation number was 0.24 for ICBRT boost and 0.78 for IMRT boost. Mean normal tissue complication probability (NTCP) values for 3D-CRT plus ICBRT and for IMRT (pelvis plus boost) were, respectively, 28% and 5% for rectum; 1.5% and 0.1% for urinary bladder and 8.9% and 6.1% for bowel.

Conclusions

Our findings suggest that IMRT may represent a viable alternative in delivering the boost in patients diagnosed with gynaecological cancer not amenable to ICBRT.

Dosimetric and radiobiological investigation of permanent implant prostate brachytherapy based on Monte Carlo calculations

PURPOSE

Permanent implant prostate brachytherapy plays an important role in prostate cancer treatment, but dose evaluations typically follow the water-based TG-43 formalism, ignoring patient anatomy and interseed attenuation. The purpose of this study is to investigate advanced TG-186 model-based dose calculations via retrospective dosimetric and radiobiological analysis for a new patient cohort.

METHODS AND MATERIALS

A cohort of 155 patients treated with permanent implant prostate brachytherapy from The Ottawa Hospital Cancer Centre is considered. Monte Carlo (MC) dose calculations are performed using tissue-based virtual patient models. Dose-volume histogram (DVH) metrics (target, organs at risk) are extracted from 3D dose distributions and compared with those from calculations under TG-43 assumptions (TG43). Equivalent uniform biologically effective dose and tumor control probability are calculated.

RESULTS

For the target, D₉₀ (V₁₀₀) is 136.7 ± 20.6 Gy (85.8% ± 7.8%) for TG43 and 132.8 ± 20.1 Gy (84.1% ± 8.2%) for MC; D₉₀ is 3.0% ± 1.1% lower for MC than TG43. For organs at risk, MC D_1cc = 104.4 ± 27.4 Gy (TG43: 106.3 ± 28.3 Gy) for rectum and 80.8 ± 29.7 Gy (TG43: 78.4 ± 28.4 Gy) for bladder; D_1cc = 185.9 ± 30.2 Gy (TG43: 191.1 ± 32.0 Gy) for urethra. Equivalent uniform biologically effective dose and tumor control probability are generally lower when evaluated using MC doses. The largest dosimetric and radiobiological discrepancies between TG43 and MC are for patients with intraprostatic calcifications, for whom there are low doses (cold spots) in the vicinity of calcifications within the target, identified with MC but not TG43.

CONCLUSIONS

DVH metrics and radiobiological indices evaluated with TG43 are systematically inaccurate by upward of several percent compared with MC patient-specific models. Mean cohort DVH metrics and their MC:TG43 variances are sensitive to patient cohort and clinical practice, underlining the importance of further retrospective MC studies toward widespread clinical adoption of advanced model-based dose calculations.

Radiobiological dose calculation parameters for cervix cancer brachytherapy: A systematic review

The GEC-ESTRO recommendation in cervical cancer treatment planning, including external beam radiotherapy and brachytherapy boosts, is to use radiobiological dose calculations. Such calculations utilize the linear-quadratic model to estimate the effect of multiple cellular response factors and dose delivery parameters. The radiobiological parameters utilized in these calculations are literature values estimated based on clinical and experimental results. However, the impact of the uncertainties associated with these parameters is often not fully appreciated. This review includes a summary of the radiobiological dose calculation (for both high-dose-rate and pulsed-dose-rate brachytherapy boost treatments) for cervical cancer and a compilation of the reported values of the associated parameters. As discrepancies exist between conventionally recommended and published values, equivalencies between current brachytherapy boosts may be imprecise and could create underappreciated uncertainties in the radiobiological dose calculations. This review highlights these uncertainties by calculating the radiobiological dose delivered by the brachytherapy boost when assuming different radiobiological parameter values (within the range reported by previous research). Furthermore, conventional treatment planning does not consider the effects of proliferation of the tumor over the treatment time, which can significantly decrease its radiobiological dose and can introduce an additional variance of over 7 Gy₁₀. Further investigation of uncertainties in parameter values and modifications of current dose models could improve the accuracy of radiobiological dose calculation.

Comparison of radiobiological parameters for 90Y radionuclide therapy (RNT) and external beam radiotherapy (EBRT) in vitro

Background

Dose rate variation is a critical factor affecting radionuclide therapy (RNT) efficacy. Relatively few studies to date have investigated the dose rate effect in RNT. Therefore, the aim of this study was to benchmark ⁹⁰Y RNT (at different dose rates) against external beam radiotherapy (EBRT) in vitro and compare cell kill responses between the two irradiation processes.

Results

Three human colorectal carcinoma (CRC) cell lines (HT29, HCT116, SW48) were exposed to ⁹⁰Y doses in the ranges 1–10.4 and 6.2–62.3 Gy with initial dose rates of 0.013–0.13 Gy/hr (low dose rate, LDR) and 0.077–0.77 Gy/hr (high dose rate, HDR), respectively. Results were compared to a 6-MV photon beam doses in the range from 1–9 Gy with constant dose rate of 277 Gy/hr. The cell survival parameters from the linear quadratic (LQ) model were determined. Additionally, Monte Carlo simulations were performed to calculate the average dose, dose rate and the number of hits in the cell nucleus.

For the HT29 cell line, which was the most radioresistant, the α/β ratio was found to be ≈ 31 for HDR–⁹⁰Y and ≈ 3.5 for EBRT. LDR–⁹⁰Y resulting in insignificant cell death compared to HDR–⁹⁰Y and EBRT. Simulation results also showed for LDR–⁹⁰Y, for doses ≲ 3 Gy, the average number of hits per cell nucleus is ≲ 2 indicating insufficiently delivered lethal dose. For ⁹⁰Y doses \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\gtrsim $\end{document}≳ 3 Gy the number of hits per nucleus decreases rapidly and falls below ≈ 2 after ≈ 5 days of incubation time. Therefore, our results demonstrate that LDR–⁹⁰Y is radiobiologically less effective than EBRT. However, HDR–⁹⁰Y at ≈ 56 Gy was found to be radiobiologically as effective as acute ≈ 8 Gy EBRT.

Conclusion

These results demonstrate that the efficacy of RNT is dependent on the initial dose rate at which radiation is delivered. Therefore, for a relatively long half-life radionuclide such as ⁹⁰Y, a higher initial activity is required to achieve an outcome as effective as EBRT.

Modelling the influence of treatment time on the biological effectiveness of single radiosurgery treatments: derivation of "protective" dose modification factors

OBJECTIVE

To provide simpler models for adjusting total dose to compensate for significant variations in central nervous system radiosurgical treatment times, which vary and will influence treatment bioeffectiveness. At present, no allowance is made for time variations. A framework of simpler equations would allow radiosurgical outcomes to be analysed with respect to treatment time, and a system for dose adjustments between radioisotope and linac-based techniques with different treatment durations.

METHODS

The standard biological effective dose (BED) equations for fractionated and protracted radiations have been combined, using biexponential DNA repair kinetics, to provide the following equation:BED=x.nd(1+(ndk-dk)f(μ1T)+dkf(μ1t))+(1-x). nd(1+(ndk-dk)f(μ2T)+dkf(μ2t))for "n" isocentres (or subfractions), each treated to a variable dose "d" in time "t", the overall time-being, T, µ1, µ2, are fast and slow repair rate coefficients, with partition factors of x and (1-x), respectively and k is the alpha/beta ratio, with f(μT) being the function that summates sublethal damage repair. Thus, repair during the period of irradiation and in the time interval between each isocentre can be taken into account. Simpler monoexponential and linear models are also used.

RESULTS

The results obtained using simpler models are compared with those obtained using more complex retrospective Gamma Knife BED treatment planning by Millar et al. (2015) in a group of 23 patients on a 13 Gy physical isodose surface. The above equation provides a BED value around 3% above their minimum values, 4% below their average value and 10% below their maximum BED values. Changes in isocentre numbers used, due to treatment plan complexity, can influence total treatment time, producing variations in the BED-time data: instead of a unique curve for each "n" value, in aggregate form the data (ranging from around 20 to 140 min treatment times) can be fitted by monoexponential time functions and further approximated to a linear function for more rapid estimations. Worked examples show how dose can then be tailored to the expected treatment times in order to obtain isoeffective treatments for central nervous system tissues.

CONCLUSION

The models allow better analysis of radiosurgical treatment time data and guidance to the choice of dose to match the overall time. Although this study is based on Gamma Knife treatments, in principle the methods will also apply to any radiosurgical technique, so that dose-time compensations can be made between differing techniques.

ADVANCES IN KNOWLEDGE

The new BED equation-based framework is relevant to analyse and optimise radiosurgical treatments.

Radiobiological doses, tumor, and treatment features influence on local control, enucleation rates, and survival after epiescleral brachytherapy. A 20-year retrospective analysis from a single-institution: part I

PURPOSE

To assess influence of the radiobiological doses, tumor, and treatment features on local control, enucleation rates, overall and disease-specific survival rates after brachytherapy for posterior uveal melanoma.

MATERIAL AND METHODS

Local control, enucleation, overall and disease-specific survival rates were evaluated on the base of 243 patients from 1996 through 2016, using plaques loaded with iodine sources. Clinical and radiotherapy data were extracted from a dedicated prospective database. Biologically effective dose (BED) was included in survival analysis using Kaplan-Meier and Cox regressions. The 3-, 5-, 10-, and 15-year relative survival rates were estimated, and univariate/multivariate regression models were constructed for predictive factors of each item. Hazard ratio (HR) and confidence interval at 95% (CI) were determined.

RESULTS

The median follow-up was 73.9 months (range, 3-202 months). Cumulative probabilities of survival by Kaplan-Meier analysis at 3, 5, 10 and 15 years were respectively: 96%, 94%, 93%, and 87%, for local control; 93%, 88%, 81%, and 73% for globe preservation; 98%, 93%, 84%, and 73% for overall survival, and 98%, 96%, 92%, and 87% for disease-specific survival. By multivariate analysis, we concluded variables as significant: for local control failure - the longest basal diameter and the juxtapapillary location; for globe preservation failure - the longest basal dimension, the mushroom shape, the location in ciliary body, and the dose to the foveola; for disease-specific survival - the longest basal dimension. Some radiobiological doses were significant in univariate models but not in multivariate ones for the items studied.

CONCLUSIONS

The results show as predictive factors of local control, enucleation, and disease-specific survival rates those related with the features of the tumor, specifically the longest basal dimension. There is no clear relation between radiobiological doses or treatment parameters in patients after brachytherapy.

Visual outcome after posterior uveal melanoma episcleral brachytherapy including radiobiological doses

Purpose

To assess the long-term influence of radiobiological doses in the evolution of visual acuity (VA) in patients with uveal melanoma treated by episcleral brachytherapy.

Material and methods

Visual acuity was evaluated prospectively from a case series of 243 patients in 2016 treated with ¹²⁵I. Data analysis was applied to trend VA outcome and find the accurate best-fit line. Biologically effective dose (BED) was included in survival analysis with the use of Kaplan-Meier and Cox regressions. Hazard ratio (HR) and confidence interval at 95% (CI) were determined. Variables statistically significant were analyzed and compared by log-rank tests.

Results

The median follow-up was 74.2 months (range, 3-223). Exponential regression shows a 25% reduction and 50% in visual acuity score (VAS) scale for 5 and 27.8 months, respectively. Cumulative probabilities of survival analysis were 57%, 42%, 27%, and 23% at 3, 5, 10, and 15 years, respectively. Multivariable analysis found tumor height (HR = 1.18, 95% CI: 1.07-1.29), applicator size (HR = 1.22, 95% CI: 1.08-1.36), juxtapapillary localization (HR = 1.70, 95% CI: 1.01-2.84), and dose to foveola (HR = 1.01, 95% CI: 1.00-1.01) significantly associated with VA loss. Log-rank tests were significant for all those variables. BED has a strong influence in univariate model, but not statistically significant in the multivariate one.

Conclusions

Visual acuity changes can be modeled by an exponential function for the first 5 years after treatment. No relation between VA loss and BED has been found; nevertheless, apical height, plaque size, juxtapapillary localization, and dose to fovea were found as statistical significant variables.

High-Dose-Rate Brachytherapy in the Management of Operable Rectal Cancer: A Systematic Review

PURPOSE

To evaluate the role of high-dose-rate endorectal brachytherapy (HDREBT) in the preoperative and definitive management of operable rectal cancer in terms of clinical outcomes and toxicities using a systematic review.

METHODS AND MATERIALS

A review of published articles from January 1990 to December 2016 was conducted using the PubMed, Embase, and Scopus databases using the search terms "rectal" or "rectum" in combination with "brachytherapy," "high dose rate," "HDR," and "endorectal." Additional publications were identified by scanning references. Only studies published in English reporting clinical outcomes with ≥30 patients treated with HDREBT were included.

RESULTS

The search identified 1688 articles, of which 22 met our inclusion criteria. Twelve studies were included in this systematic review. Following preoperative HDREBT with chemoradiation therapy (CRT), the pathologic complete response (pCR) rate ranged between 18% and 31% (weighted mean rate, 22.2%); R0 resection rate, between 80% and 99% (weighted mean rate, 95.5%); and sphincter-preservation rate, between 29% and 54% (weighted mean rate, 46.4%). The weighted mean 2-year progression-free survival and overall survival (OS) rates were 68.1% and 81.5%, respectively. After preoperative HDREBT alone, the pCR rate ranged between 10.4% and 27% (weighted mean rate, 23.8%), the R0 rate was 96.5% (1 study), and the sphincter-preservation rate ranged between 53.8% and 75.8% (weighted mean rate, 59.4%). The weighted mean 5-year progression-free survival and OS rates were 66.6% and 70.8%, respectively. There was only 1 study of HDREBT for nonsurgical management of rectal cancer, which reported a 2-year OS rate of 100%.

CONCLUSIONS

Preoperative HDREBT either alone or in combination with CRT may result in a better pCR but may not necessarily translate into better survival, which is similar to outcomes seen following preoperative CRT alone. There were significant variations across studies in terms of patient selection, treatment approaches, and evaluation of clinical outcomes, suggesting the need for an international consensus on the dosimetric parameters and techniques of HDREBT, timing and methods of response assessment, definitions and assessment of toxicities, and optimal timing of surgery before further prospective studies. Future studies should include evaluation of the role of HDREBT in the nonsurgical curative treatment of screen-detected early cancers and organ preservation in lower rectal cancers.

Iridium-Knife: Another knife in radiation oncology

PURPOSE

Intratarget dose escalation with superior conformity is a defining feature of three-dimensional (3D) iridium-192 (¹⁹²Ir) high-dose-rate (HDR) brachytherapy (BRT). In this study, we analyzed the dosimetric characteristics of interstitial ¹⁹²Ir HDR BRT for intrathoracic and cerebral malignancies. We examined the dose gradient sharpness of HDR BRT compared with that of linear accelerator-based stereotactic radiosurgery and stereotactic body radiation therapy, usually called X-Knife, to demonstrate that it may as well be called a Knife.

METHODS AND MATERIALS

Treatment plans for 10 patients with recurrent glioblastoma multiforme or intrathoracic malignancies, five of each entity, treated with X-Knife (stereotactic radiosurgery for glioblastoma multiforme and stereotactic body radiation therapy for intrathoracic malignancies) were replanned for simulated HDR BRT. For 3D BRT planning, we used identical structure sets and dose prescription as for the X-Knife planning. The indices for qualitative treatment plan analysis encompassed planning target volume coverage, conformity, dose falloff gradient, and the maximum dose-volume limits to different organs at risk.

RESULTS

Volume coverage in HDR plans was comparable to that calculated for X-Knife plans with no statistically significant difference in terms of conformity. The dose falloff gradient-sharpness-of the HDR plans was considerably steeper compared with the X-Knife plans.

CONCLUSIONS

Both 3D ¹⁹²Ir HDR BRT and X-Knife are effective means for intratarget dose escalation with HDR BRT achieving at least equal conformity and a steeper dose falloff at the target volume margin. In this sense, it can reasonably be argued that 3D ¹⁹²Ir HDR BRT deserves also to be called a Knife, namely Iridium-Knife.

Guidelines by the AAPM and GEC-ESTRO on the use of innovative brachytherapy devices and applications: Report of Task Group 167

Although a multicenter, Phase III, prospective, randomized trial is the gold standard for evidence-based medicine, it is rarely used in the evaluation of innovative devices because of many practical and ethical reasons. It is usually sufficient to compare the dose distributions and dose rates for determining the equivalence of the innovative treatment modality to an existing one. Thus, quantitative evaluation of the dosimetric characteristics of innovative radiotherapy devices or applications is a critical part in which physicists should be actively involved. The physicist's role, along with physician colleagues, in this process is highlighted for innovative brachytherapy devices and applications and includes evaluation of (1) dosimetric considerations for clinical implementation (including calibrations, dose calculations, and radiobiological aspects) to comply with existing societal dosimetric prerequisites for sources in routine clinical use, (2) risks and benefits from a regulatory and safety perspective, and (3) resource assessment and preparedness. Further, it is suggested that any developed calibration methods be traceable to a primary standards dosimetry laboratory (PSDL) such as the National Institute of Standards and Technology in the U.S. or to other PSDLs located elsewhere such as in Europe. Clinical users should follow standards as approved by their country's regulatory agencies that approved such a brachytherapy device. Integration of this system into the medical source calibration infrastructure of secondary standard dosimetry laboratories such as the Accredited Dosimetry Calibration Laboratories in the U.S. is encouraged before a source is introduced into widespread routine clinical use. The American Association of Physicists in Medicine and the Groupe Européen de Curiethérapie-European Society for Radiotherapy and Oncology (GEC-ESTRO) have developed guidelines for the safe and consistent application of brachytherapy using innovative devices and applications. The current report covers regulatory approvals, calibration, dose calculations, radiobiological issues, and overall safety concerns that should be addressed during the commissioning stage preceding clinical use. These guidelines are based on review of requirements of the U.S. Nuclear Regulatory Commission, U.S. Department of Transportation, International Electrotechnical Commission Medical Electrical Equipment Standard 60601, U.S. Food and Drug Administration, European Commission for CE Marking (Conformité Européenne), and institutional review boards and radiation safety committees.

Bladder accumulated dose in image-guided high-dose-rate brachytherapy for locally advanced cervical cancer and its relation to urinary toxicity

The purpose of this study was to estimate locally accumulated dose to the bladder in multi-fraction high-dose-date (HDR) image-guided intracavitary brachytherapy (IG-ICBT) for cervical cancer, and study the locally-accumulated dose parameters as predictors of late urinary toxicity. A retrospective study of 60 cervical cancer patients who received five HDR IG-ICBT sessions was performed. The bladder outer and inner surfaces were segmented for all sessions and a bladder-wall contour point-set was created in MATLAB. The bladder-wall point-sets for each patient were registered using a deformable point-set registration toolbox called coherent point drift (CPD), and the fraction doses were accumulated. Various dosimetric and volumetric parameters were calculated using the registered doses, including [Formula: see text] (minimum dose to the most exposed n-cm³ volume of bladder wall), r V _{n Gy} (wall volume receiving at least m Gy), and [Formula: see text] (minimum equivalent biologically weighted dose to the most exposed n-cm³ of bladder wall), where n  =  1/2/5/10 and m  =  3/5/10. Minimum dose to contiguous 1 and 2 cm³ hot-spot volumes was also calculated. The unregistered dose volume histogram (DVH)-summed equivalent of [Formula: see text] and [Formula: see text] parameters (i.e. [Formula: see text] and [Formula: see text]) were determined for comparison. Late urinary toxicity was assessed using the LENT-SOMA scale, with toxicity Grade 0-1 categorized as Controls and Grade 2-4 as Cases. A two-sample t-test was used to identify the differences between the means of Control and Case groups for all parameters. A binomial logistic regression was also performed between the registered dose parameters and toxicity grouping. Seventeen patients were in the Case and 43 patients in the Control group. Contiguous values were on average 16 and 18% smaller than parameters for 1 and 2 cm³ volumes, respectively. Contiguous values were on average 26 and 27% smaller than parameters. The only statistically significant finding for Case versus Control based on both methods of analysis was observed for r V_{3 Gy} (p  =  0.01). DVH-summed parameters based on unregistered structure volumes overestimated the bladder dose in our patients, particularly when contiguous high dose volumes were considered. The bladder-wall volume receiving at least 3 Gy of accumulated dose may be a parameter of interest in further investigations of Grade 2+  urinary toxicity.

Increased Biological Effective Dose of Radiation Correlates with Prolonged Survival of Patients with Limited-Stage Small Cell Lung Cancer: A Systematic Review

OBJECTIVE

Thoracic radiotherapy (TRT) is a critical component of the treatment of limited-stage small cell lung cancer (LS-SCLC). However, the optimal radiation dose/fractionation remains elusive. This study reviewed current evidence and explored the dose-response relationship in patients with LS-SCLC who were treated with radiochemotherapy.

MATERIALS AND METHODS

A quantitative analysis was performed through a systematic search of PubMed, Web of Science, and the Cochrane Library. The correlations between the biological effective dose (BED) and median overall survival (mOS), median progression-free survival (mPFS), 1-, 3-, and 5-year overall survival (OS) as well as local relapse (LR) were evaluated.

RESULTS

In all, 2389 patients in 19 trials were included in this study. Among these 19 trials, seven were conducted in Europe, eight were conducted in Asia and four were conducted in the United States. The 19 trials that were included consisted of 29 arms with 24 concurrent and 5 sequential TRT arms. For all included studies, the results showed that a higher BED prolonged the mOS (R2 = 0.198, p<0.001) and the mPFS (R2 = 0.045, p<0.001). The results also showed that increased BED improved the 1-, 3-, and 5-year OS. A 10-Gy increment added a 6.3%, a 5.1% and a 3.7% benefit for the 1-, 3-, and 5-year OS, respectively. Additionally, BED was negatively correlated with LR (R2 = 0.09, p<0.001). A subgroup analysis of concurrent TRT showed that a high BED prolonged the mOS (p<0.001) and the mPFS (p<0.001), improved the 1-, 3-, and 5-year OS (p<0.001) and decreased the rate of LR (p<0.001).

CONCLUSION

This study showed that an increased BED was associated with improved OS, PFS and decreased LR in patients with LS-SCLC who were treated with combined chemoradiotherapy, which indicates that the strategy of radiation dose escalation over a limited time frame is worth exploring in a prospective clinical trial.

American Brachytherapy Society consensus guidelines for thoracic brachytherapy for lung cancer

PURPOSE

To update brachytherapy recommendations for pretreatment evaluation, treatment, and dosimetric issues for thoracic brachytherapy for lung cancer.

METHODS AND MATERIALS

Members of the American Brachytherapy Society with expertise in thoracic brachytherapy updated recommendations for thoracic brachytherapy based on literature review and clinical experience.

RESULTS

The American Brachytherapy Society consensus guidelines recommend the use of endobronchial brachytherapy for disease palliation in patients with central obstructing lesions, particularly in patients who have previously received external beam radiotherapy. The use of interstitial implants after incomplete resection may improve outcomes and provide enhanced palliation. Early reports support the use of CT-guided intratumoral volume implants within clinical studies. The use of brachytherapy routinely after sublobar resection is not generally recommended, unless within the confines of a clinical trial or a registry.

CONCLUSIONS

American Brachytherapy Society recommendations for thoracic brachytherapy are provided. Practitioners are encouraged to follow these guidelines and to develop further clinical trials to examine this treatment modality to increase the evidence base for its use.

Inter-fraction variation in interstitial high-dose-rate brachytherapy

Aim

To evaluate the inter-fraction variation in interstitial high-dose-rate (HDR) brachytherapy. To assess the positional displacement of catheters during the fractions and the resultant impact on dosimetry.

Background

Although brachytherapy continues to be a key cornerstone of cancer care, it is clear that treatment innovations are needed to build on this success and ensure that brachytherapy continues to provide quality care for patients. The dosimetric advantages offered by HDR brachytherapy to the tumour volume rely on catheter positions being accurately reproduced for all fractions of treatment.

Materials and methods

A total of 66 patients treated over a period of 22 months were considered for this study. All the patients underwent computer tomography (CT) scan and three-dimensional treatment planning was carried out. Brachytherapy treatment was delivered by the HDR afterloading system. On completing the last fraction, CT scan was repeated and treatment re-planning was done. The variation in position of the implanted applicators and their impact on dosimetric parameters were analysed using both the plans.

Results

For all breast-implant patients, the catheter displacement and D90dose to clinical target volume were <3 mm and 3%, respectively. The displacement for carcinoma of the tongue, carcinoma of the buccal mucosa, carcinoma of the floor of mouth, carcinoma of the cervix, soft-tissue sarcoma and carcinoma of the lip were comparatively high.

Conclusion

Inter-fraction errors occur frequently in interstitial HDR brachytherapy. If no action is taken, it will result in a significant risk of geometrical miss and overdose to the organs at risk. It is not recommended to use a single plan to deliver all the fractions. Imaging is recommended before each fraction and decision on re-planning must be taken.

On the sensitivity of α/β prediction to dose calculation methodology in prostate brachytherapy

PURPOSE

To study the relationship between the accuracy of the dose calculation in brachytherapy and the estimations of the radiosensitivity parameter, α/β, for prostate cancer.

METHODS AND MATERIALS

In this study, Monte Carlo methods and more specifically the code ALGEBRA was used to produce accurate dose calculations in the case of prostate brachytherapy. Equivalent uniform biologically effective dose was calculated for these dose distributions and was used in an iso-effectiveness relationship with external beam radiation therapy.

RESULTS

By considering different levels of detail in the calculations, the estimation for the α/β parameter varied from 1.9 to 6.3 Gy, compared with a value of 3.0 Gy suggested by the American Association of Physicists in Medicine Task Group 137.

CONCLUSIONS

Large variations of the α/β show the sensitivity of this parameter to dose calculation modality. The use of accurate dose calculation engines is critical for better evaluating the biological outcomes of treatments.

Combined effects of C225 and 125-iodine seed radiation on colorectal cancer cells

Background

To characterize the effect of combined treatment of the anti-epidermal growth factor receptor (EGFR) monoclonal antibody C225 and 125-iodine (¹²⁵I) seed radiation in human colorectal cancer.

Methods

We treated LS180 cells with ¹²⁵I continuous low dose rate radiation in the presence and absence of 100 nM C225. The clonogenic capacity, cellular proliferation, cell cycle distribution, apoptosis, and molecular pathways of the cells following the treatments were analyzed in vitro.

Results

The sensitizer enhancement ratio of C225 was approximately 1.4. Treatment with C225 and radiation alone produced significant inhibition of cell growth, but combination therapy produced greater inhibition than either treatment administered alone. C225 increased the radiation-induced apoptosis and the fraction of γ-H2AX foci positive cells at 48 h after treatment. The Akt phosphorylation level was lower in the cells receiving the combination treatment than in the cells treated with radiation or C225 alone.

Conclusions

These findings indicate that C225 sensitizes LS180 cells to ¹²⁵I seed radiation. Growth inhibition is mediated by inducing apoptosis and not cell cycle arrest. Additionally, we confirmed that C225 impairs DNA repair by reducing the cellular level of the DNA-PKcs and Ku70 proteins. Furthermore, the inhibition of Akt signaling activation may be responsible for the C225-mediated radiosensitization.

Pulsed dose rate brachytherapy of lip cancer

Purpose

Purpose: To present our experience with pulsed dose rate brachytherapy (PDR BT) of lip cancer.

Material and methods

The study group included 32 T1-4N0M0 lip cancer patients with a median age of 71 years (ranged 41-87 years), treated with interstitial PDR BT to the planned total dose of 60-70 Gy; 1 Gy/pulse, pulses repeated every hour a day. There were 26 untreated patients, and six patients after previous surgery including five implanted at the time of cancer relapse.

Results

PDR BT was delivered over few days and was well tolerated. After therapy, all patients experienced temporary, usually mild, acute mucositis. Late severe (Grade 3) mucositis of oral vestibule mucosa occurred in one case. Among 31 patients who completed the therapy, local control was achieved in 29 (93.5%). One patient with reccurrent upper lip T2 tumor was susccessfully salvaged surgically, another one died due to persisted T3 lip tumor with lymph node metastases. Overall, four patients developed neck nodal cancer relapse and two – distant metastases. The 5-year local control, and all-cause overall survival probabilities are 94% and 73%, respectively. Good/excellent cosmetic and functional outcome was obtained in all but two patients.

Conclusions

PDR at the dose of 1 Gy/pulse is effective and well tolerated BT technique in treating lip cancer patients.

Patient dosimetry for 90Y selective internal radiation treatment based on 90Y PET imaging

Until recently, the radiation dose to patients undergoing the 90Y selective internal radiation treatment (SIRT) procedure is determined by applying the partition model to 99mTc MAA pretreatment scan. There can be great uncertainty in radiation dose calculated from this approach and we presented a method to compute the 3D dose distributions resulting from 90Y SIRT based on 90Y positron emission tomography (PET) imaging. Five 90Y SIRT treatments were retrospectively analyzed. After 90Y SIRT, patients had 90Y PET/CT imaging within 6 hours of the procedure. To obtain the 3D dose distribution of the patients, their respective 90Y PET images were convolved with a Monte Carlo generated voxel dose kernel. The sensitivity of the PET/CT scanner for 90Y was determined through phantom studies. The 3D dose distributions were then presented in DICOM RT dose format. By applying the linear quadratic model to the dose data, we derived the biologically effective dose and dose equivalent to 2 Gy/fraction delivery, taking into account the spatial and temporal dose rate variations specific for SIRT. Based on this data, we intend to infer tumor control probability and risk of radiation induced liver injury from SIRT by comparison with established dose limits. For the five cases, the mean dose to target ranged from 51.7 ± 28.6 Gy to 163 ± 53.7 Gy. Due to the inhomogeneous nature of the dose distribution, the GTVs were not covered adequately, leading to very low values of tumor control probability. The mean dose to the normal liver ranged from 21.4 ± 30.7 to 36.7 ± 25.9 Gy. According to QUANTEC recommendation, a patient with primary liver cancer and a patient with metastatic liver cancer has more than 5% risk of radiotherapy-induced liver disease (RILD).

[Radiobiology in brachytherapy]

Low-dose rate brachytherapy has some radiobiological advantages compared to external beam radiotherapy: subletal damages repair during irradiation leading to a relative protection of healthy tissues; no tumor cell repopulation, cell cycle redistribution and a low oxygen enhancement ratio. High dose rate and pulsed dose rate modalities allow an optimization of dose distribution by varying the dwell times over the different dwell positions. Because of the use of afterloaders, they also offer a better radioprotection of the staff. High dose rate and pulsed dose rate treatments seem to offer the same results as low-dose rate brachytherapy, particularly in cervix carcinoma. For high dose rate brachytherapy, schedules must be designed according to the linear-quadratic model. In pulsed dose rate brachytherapy, pulse dose and time intervals must also be derived from the linear-quadratic model, but half-time repair must be taken into account.

Endometrial recurrence in the proximal vagina: brachytherapy volume delineation with 18FDG PET-CT

Background: Endometrial cancer vaginal recurrence in a patient deemed unsuitable for EBRT. Brachytherapy proposed although standard image-guidance insufficient.

Proposed Solution: PET-CT image-guidance.

Results: PET-CT acquired with brachytherapy applicator in situ. BTV delineated by the Nuclear Medicine physician. All subsequent volumes delineated by the Clinical Oncologist. 2 phase plan delivered with minimal toxicity.

Investigating the dosimetric and tumor control consequences of prostate seed loss and migration

PURPOSE

Low dose-rate brachytherapy is commonly used to treat prostate cancer. However, once implanted, the seeds are vulnerable to loss and movement. The goal of this work is to investigate the dosimetric and radiobiological effects of the types of seed loss and migration commonly seen in prostate brachytherapy.

METHODS

Five patients were used in this study. For each patient three treatment plans were created using Iodine-125, Palladium-103, and Cesium-131 seeds. The three seeds that were closest to the urethra were identified and modeled as the seeds lost through the urethra. The three seeds closest to the exterior of prostatic capsule were identified and modeled as those lost from the prostate periphery. The seed locations and organ contours were exported from Prowess and used by in-house software to perform the dosimetric and radiobiological evaluation. Seed loss was simulated by simultaneously removing 1, 2, or 3 seeds near the urethra 0, 2, or 4 days after the implant or removing seeds near the exterior of the prostate 14, 21, or 28 days after the implant.

RESULTS

Loss of one, two or three seeds through the urethra results in a D(90) reduction of 2%, 5%, and 7% loss, respectively. Due to delayed loss of peripheral seeds, the dosimetric effects are less severe than for loss through the urethra. However, while the dose reduction is modest for multiple lost seeds, the reduction in tumor control probability was minimal.

CONCLUSIONS

The goal of this work was to investigate the dosimetric and radiobiological effects of the types of seed loss and migration commonly seen in prostate brachytherapy. The results presented show that loss of multiple seeds can cause a substantial reduction of D(90) coverage. However, for the patients in this study the dose reduction was not seen to reduce tumor control probability.

Dose gradient impact on equivalent dose at 2 Gy for high dose rate interstitial brachytherapy

PURPOSE

To evaluate a new calculation model estimating the equivalent dose at 2 Gy (EQD2) taking into account dose gradient in high dose rate interstitial brachytherapy (HDRIB).

MATERIAL AND METHODS

Forty dose-volume histograms (DVHs) of breast (20 pts) and prostate (20 pts) cancer dose distributions were reviewed. Physical prescribed doses (PPD) were 34 Gy (10f/5d) and 18 Gy (6f/2d) for breast (partial irradiation protocol) and prostate (boost after external irradiation) treatment, respectively. For each DVH, clinical target volume (CTV), V100, V150, V200, D90 and D100 were determined. Based on DVH segmentation, elementary doses (d) delivered to elementary volumes were determined, then multiplied by C (% of CTV receiving d). According to the linear quadratic model, EQD2 was calculated for different α/β ratios.

RESULTS

For breast implant, median EQD2 (α/β = 4) was 42 Gy and 76 Gy (66-85) without and with dose gradient consideration, respectively. For prostate implant, median EQD2 (α/β = 1.5) was 39 Gy and 98 Gy (90-103) whether dose gradient was not or was taken into account, respectively.

CONCLUSIONS

This study pointed out that for brachytherapy, EQD2 calculation must take into account the dose gradient. Because this model is a mathematical one, it has to be cautiously applied. Nevertheless, it appears as a useful tool for EQD2 comparison between the same PPD delivered through EBRT or brachytherapy regarding trial result interpretation.

Consequences of dose heterogeneity on the biological efficiency of ¹⁰³Pd permanent breast seed implants

Brachytherapy is associated with highly heterogeneous spatial dose distributions. This heterogeneity is usually ignored when estimating the biological effective dose (BED). In addition, the heterogeneities of the medium including the tissue heterogeneity (TH) and the interseed attenuation (ISA) are also contributing to the heterogeneity of the dose distribution, but they are both ignored in Task Group 43 (TG43)-based protocols. This study investigates the effect of dose heterogeneity, TH and ISA on metrics that are commonly used to quantify biological efficiency in brachytherapy. The special case of 29 breast cancer patients treated with permanent (103)Pd seed implant is considered here. BED is compared to equivalent uniform BED (EUBED) capable of considering the spatial heterogeneity of the dose distribution. The effects of TH and ISA on biological efficiency of treatments are taken into account by comparing TG43 with Monte Carlo (MC) dose calculations for each patient. The effect of clonogenic repopulation is also considered. The analysis is performed for different sets of (α/β, α) ratios of (2, 0.3), (4, 0.27) and (10, 0.3) [Gy, Gy(-1)] covering the whole range of reported α/β values in the literature. BED is sometimes larger and sometimes smaller than EUBED(TG43) indicating that the effect of the dose heterogeneity is not similar among patients. The effect of the dose heterogeneity can be characterized by using the D(99) dose metric. For each set of the radiobiological parameters considered, a D(99) threshold is found over which dose heterogeneity will cause an overestimation of the biological efficiencies while the inverse happens for smaller D(99) values. EUBED(MC) is always larger than EUBED(TG43) indicating that by neglecting TH and ISA in TG43-based dosimetry algorithms, the biological efficiencies may be underestimated by about 10 Gy. Overall, by going from BED to the more accurate EUBED(MC) there is a gain of about 9.6 to 13 Gy on the biological efficiency. The efficiency gain is about 10.8 to 14 Gy when the repopulation is considered. Dose heterogeneity does not have a constant impact on the biological efficiencies and may under- or overestimate the efficacy in different patients. However, the combined effect of neglecting dose heterogeneity, TH and ISA results in underestimation of the biological efficiencies in permanent breast seed implants.

An NTCP Analysis of Urethral Complications from Low Doserate Mono- and Bi-Radionuclide Brachytherapy

Urethral NTCP has been determined for three prostates implanted with seeds based on ¹²⁵I (145 Gy), ¹⁰³Pd (125 Gy), ¹³¹Cs (115 Gy), ¹⁰³Pd-¹²⁵I (145 Gy), or ¹⁰³Pd-¹³¹Cs (115 Gy or 130 Gy). First, DU₂₀, meaning that 20% of the urhral volume receive a dose of at least DU₂₀, is converted into an I-125 LDR equivalent DU₂₀ in order to use the urethral NTCP model. Second, the propagation of uncertainties through the steps in the NTCP calculation was assessed in order to identify the parameters responsible for large data uncertainties. Two sets of radiobiological parameters were studied. The NTCP results all fall in the 19%–23% range and are associated with large uncertainties, making the comparison difficult. Depending on the dataset chosen, the ranking of NTCP values among the six seed implants studied changes. Moreover, the large uncertainties on the fitting parameters of the urethral NTCP model result in large uncertainty on the NTCP value. In conclusion, the use of NTCP model for permanent brachytherapy is feasible but it is essential that the uncertainties on the parameters in the model be reduced.