Stereotactic body radiotherapy with volumetric intensity-modulated arc therapy and flattening filter-free beams: dosimetric considerations.

PURPOSE: The present study comparatively evaluates the impact of energy-matched flattening filter-free (FFF) photon beams with different energy levels on the physical-dosimetric quality of lung and liver stereotactic body radiotherapy (SBRT) treatment plans. METHODS: For this purpose, 54 different lung and liver lesions from 44 patients who had already received SBRT combined with volumetric modulated arc therapy (VMAT) were included in this retrospective planning study. Planning computed tomography scans already available were used for the renewed planning with 6 MV, 6 MV-FFF, 10 MV, and 10 MV-FFF under constant planning objectives. The treatment delivery data, dosimetric distributions, and dose-volume histograms as well as parameters such as the conformity index and gradient indices were the basis for the evaluation and comparison of treatment plans. RESULTS: A significant reduction of beam-on time (BOT) was achieved due to the high dose rates of FFF beams. In addition, we showed that for FFF beams compared to flattened beams of the same energy level, smaller planning target volumes (PTV) require fewer monitor units (MU) than larger PTVs. An equal to slightly superior target volume coverage and sparing of healthy tissue as well as organs at risk in both lung and liver lesions were found. Significant differences were seen mainly in the medium to lower dose range. CONCLUSION: We found that FFF beams together with VMAT represent an excellent combination for SBRT of lung or liver lesions with shortest BOT for 10 MV-FFF but significant dose savings for 6 MV-FFF in lung lesions.

Effects of iodinated contrast agent on HU-based dose calculation and dose delivered in iridium-192 high-dose-rate brachytherapy.

PURPOSE: This study compares the effect of iodinated contrast agent on Hounsfield unit (HU)-based TG-186 dose calculation vs. delivered dose for high-dose-rate (HDR) iridium-192 brachytherapy using a phantom model. MATERIAL AND METHODS: A reservoir filled with a diluted contrast agent was placed inside a water phantom. A single steel needle applicator was centrally positioned inside the reservoir. Computed tomography (CT) datasets of five different contrast agent dilutions (25 to 300 mg/ml iodine concentration) were acquired, and dose calculations were performed with TG-186 ACE dose calculation formalism of Oncentra®Brachy (Elekta). The dose was measured with a PinPoint® ionization chamber (PTW) inside the contrast agent. ACE calculated and measured data were compared. RESULTS: For the different contrast agent dilutions, averaged Hounsfield units from 453 ±21 to 2623 ±221 were obtained. Electron densities derived from CT data were significantly higher than corresponding electron densities calculated from chemical compositions. Consequently, the measured dose was higher than corresponding HU-based calculated dose. Relative deviation ranged from 2.5% to 7% per 10 mm penetration depth, depending on contrast agent concentration. CONCLUSIONS: The application of HU-based TG-186 dose formalisms in the presence of high-Z contrast agent bulks overestimates electron densities. Consequently, HU-based dose calculations result in a higher delivered dose than expected from the treatment planning system.

Planning Benchmark Study for Stereotactic Body Radiation Therapy of Liver Metastases: Results of the DEGRO/DGMP Working Group on Stereotactic Radiation Therapy and Radiosurgery.

PURPOSE: Our purpose was to investigate whether liver stereotactic body radiation therapy treatment planning can be harmonized across different treatment planning systems, delivery techniques, and institutions by using a specific prescription method and to minimize the knowledge gap concerning intersystem and interuser differences. We provide best practice guidelines for all used techniques. METHODS AND MATERIALS: A multiparametric specification of target dose (gross target volume [GTV]D50%, GTVD0.1cc, GTVV90%, planning target volume [PTV]V70%) with a prescription dose of GTVD50% = 3 × 20 Gy and organ-at-risk (OAR) limits were distributed with computed tomography and structure sets from 3 patients with liver metastases. Thirty-five institutions provided 132 treatment plans using different irradiation techniques. These plans were first analyzed for target and OAR doses. Four different renormalization methods were performed (PTVDmin, PTVD98%, PTVD2%, PTVDmax). The resulting 660 treatments plans were evaluated regarding target doses to study the effect of dose renormalization to different prescription methods. A relative scoring system was used for comparisons. RESULTS: GTVD50% prescription can be performed in all systems. Treatment plan harmonization was overall successful, with standard deviations for Dmax, PTVD98%, GTVD98%, and PTVDmean of 1.6, 3.3, 1.9, and 1.5 Gy, respectively. Primary analysis showed 55 major deviations from clinical goals in 132 plans, whereas in only <20% of deviations GTV/PTV dose was traded for meeting OAR limits. GTVD50% prescription produced the smallest deviation from target planning objectives and between techniques, followed by the PTVDmax, PTVD98%, PTVD2%, and PTVDmin prescription. Deviations were significant for all combinations but for the PTVDmax prescription compared with GTVD50% and PTVD98%. Based on the various dose prescription methods, all systems significantly differed from each other, whereas GTVD50% and PTVD98% prescription showed the least difference between the systems. CONCLUSIONS: This study showed the feasibility of harmonizing liver stereotactic body radiation therapy treatment plans across different treatment planning systems and delivery techniques when a sufficient set of clinical goals is given.

Individualized mould-based high-dose-rate brachytherapy for perinasal skin tumors: technique evaluation from a dosimetric point of view.

PURPOSE: Dosimetric treatment planning evaluations concerning patient-adapted moulds for iridium-192 high-dose-rate brachytherapy are presented in this report. MATERIAL AND METHODS: Six patients with perinasal skin tumors were treated with individual moulds made of biocompatible epithetic materials with embedded plastic applicators. Treatment plans were optimized with regard to clinical requirements, and dose was calculated using standard water-based TG-43 formalism. In addition, retrospective material-dependent collapsed cone calculations according to TG-186 protocol were evaluated to quantify the limitations of TG-43 protocol for this superficial brachytherapy technique. RESULTS: The dose-volume parameters D90, V100, and V150 of the planning target volumes (PTVs) for TG-43 dose calculations yielded 92.2% to 102.5%, 75.1% to 93.1%, and 7.4% to 41.7% of the prescribed dose, respectively. The max- imum overall dose to the ipsilateral eyeball as the most affected organ at risk (OAR) varied between 8.9 and 36.4 Gy. TG-186 calculations with Hounsfield unit-based density allocation resulted in down by -6.4%, -16.7%, and -30.0% lower average D90, V100, and V150 of the PTVs, with respect to the TG-43 data. The corresponding calculated OAR doses were also lower. The model-based TG-186 dose calculations have considered reduced backscattering due to environmental air as well as the dose-to-medium influenced by the mould materials and tissue composition. The median PTV dose was robust within 0.5% for simulated variations of mould material densities in the range of 1.0 g/cm3 to 1.26 g/cm3 up to 7 mm total mould thickness. CONCLUSIONS: HDR contact BT with individual moulds is a safe modality for routine treatment of perinasal skin tumors. The technique provides good target coverage and OARs' protection, while being robust against small variances in mould material density. Model-based dose calculations (TG-186) should complement TG-43 dose calculations for verification purpose and quality improvement.

HDR brachytherapy with individual epithetic molds for facial skin cancer: techniques and first clinical experience.

BACKGROUND: Facial skin cancer lesions in close proximity to critical organs require further development of radiotherapeutic techniques for highly conformal treatment, especially when treating elderly frail patients. We report on our treatment technique and first clinical experience for patients with perinasal/periorbital skin cancer treated with individualized epithetic mold high-dose-rate brachytherapy (BRT). METHODS: From January 2019, patients with complex shaped or unfavorably located skin cancer not eligible for surgery or external beam radiotherapy (RT) were screened for mold-based BRT. Six patients were identified. Toxicity and clinical response were documented during therapy and posttreatment follow-up. RESULTS: Median patient age was 80 years (74-92 years). Median prescription dose was 42 Gy (range, 33-44 Gy) delivered in once-daily fractions of 3 or 4 Gy. Two patients had treatment interruptions caused by acute conjunctivitis grade 2 and a nontreatment-related cardiac event, respectively. At a median follow-up of 335 days (96-628 days), no ≥ grade 2 late toxicity was documented with all patients showing complete clinical response. CONCLUSIONS: High-dose-rate BRT with individualized epithetic molds for perinasal/periorbital skin cancer is a well-tolerated and safe treatment option for patients not eligible for primary surgery or definitive external beam RT because of comorbidities or tumor location.

Interstitial multicatheter HDR-brachytherapy as accelerated partial breast irradiation after second breast-conserving surgery for locally recurrent breast cancer.

Our aim was to evaluate the efficacy and toxicity of interstitial multicatheter high dose rate brachytherapy (imHDR-BRT) as accelerated partial breast irradiation (APBI) after second breast-conserving surgery (BCS) in patients with ipsilateral breast tumor recurrence (IBTR). Between January 2010 and December 2019, 20 patients with IBTR who refused salvage mastectomy (sMT) were treated with second BCS and post-operative imHDR-BRT as APBI. All patients had undergone primary BCS followed by adjuvant external beam radiotherapy. Median imHDR-BRT dose was 32 Gy delivered in twice-daily fractions of 4 Gy. Five-year IBTR-free survival, distant metastasis-free survival (DMFS), overall survival (OS) as well as toxicity and cosmesis were evaluated in the present retrospective analysis. Median age at recurrence and median time from the first diagnosis to IBTR was 65.1 years and 12.2 years, respectively. After a median follow-up of 69.9 months, two patients developed a second local recurrence resulting in 5-year IBTR free-survival of 86.8%. Five-year DMFS and 5-year OS were 84.6% and 92.3%, respectively. Grade 1-2 fibrosis was noted in 60% of the patients with no grade 3 or higher toxicity. Two (10%) cases of asymptomatic fat necrosis were documented. Cosmetic outcome was classified as excellent in 6 (37.5%), good in 6 (37.5%), fair in 3 (18.75%) and poor in 1 (6.25%) patient, respectively. We conclude that imHDR-BRT as APBI re-irradiation is effective and safe for IBTR and should be considered in appropriately selected patients.

Real-world dosimetric comparison between CyberKnife SBRT and HDR brachytherapy for the treatment of prostate cancer.

PURPOSE: High-dose-rate (HDR) brachytherapy (BRT) and stereotactic body radiotherapy (SBRT) are currently the two treatment options for definitive radiotherapy of prostate cancer, employing extreme hypofractionation. There are only very few studies comparing their dosimetry, all using computed tomography for treatment planning. We present here a real-word dosimetric comparison between SBRT and ultrasound-based virtual HDR-BRT, with both imaging modalities coming from the same patient. METHODS AND MATERIALS: Patients with prostate cancer on a prospective trial evaluating the toxicity of robotic-based SBRT were treated to a total dose of 35 Gy in 5 fractions. Fifteen patients were included in this analysis. During ultrasound-based fiducial implantation, a three-dimensional data set as in real HDR-BRT procedure was acquired. Virtual HDR-BRT plans were generated and various organs at risk and prostate dosimetric parameters were evaluated. RESULTS: Concerning prostate, SBRT achieved significant higher D98, V35 Gy, and V37.5 Gy coverage, whereas virtual HDR-BRT achieved significant higher intratumoral doses reflected in the V42 Gy and V52.5 Gy. Rectal Dmax, V36 Gy, and V29 Gy were significantly lower for HDR-BRT with no difference as for V18 Gy. SBRT was significantly inferior regarding bladder dosimetry (Dmax, V36 Gy, V18 Gy), whereas urethra Dmax and V44 Gy where significantly higher at the expense of HDR-BRT. CONCLUSIONS: HDR-BRT is superior regarding rectum and bladder dosimetry, with SBRT being superior relative to urethra dosimetry. A randomized study is warranted to define the best extreme hypofractionated modality.

Matched-pair dosimetric comparison of cardiac radiation exposure between deep-inspiration breath-hold whole-breast radiation therapy with Active Breathing Coordinator and interstitial multicatheter high-dose-rate brachytherapy as accelerated partial breast irradiation in adjuvant treatment of left-sided breast cancer after breast-conserving surgery.

BACKGROUND AND PURPOSE: To compare dosimetrically the radiation exposure to heart, left ventricle (LV), and left anterior descending artery (LAD) between whole-breast radiotherapy (WBRT) with Active Breathing Coordinator (ABC; ABC-WBRT) and interstitial multicatheter high-dose-rate (HDR) brachytherapy as accelerated partial breast irradiation (ABPI; imHDR-APBI) for left-sided breast cancer (BCA) after breast-conserving surgery (BCS). MATERIALS AND METHODS: Between January 2016 and December 2019, 32 and 20 patients were treated with ABC-WBRT (63 Gy/2.25 Gy) and imHDR-APBI (32 Gy/4 Gy), respectively. Among them a matched-pair analysis was performed according to tumor location (clock position) before BCS as well as planning target volume of imHDR-APBI and boost volume of ABC-WBRT. This yielded 17 pairs of patients for whom dosimetric parameters for heart, LV, and LAD were evaluated. The Mann-Whitney test was used for comparison after adjusting for equivalent dose in 2­Gy fractions (EQD2). In addition, a second analysis of ABC-WBRT to 40.05 Gy in 15 fractions was performed in order to account for the EQD2 difference between the 63-Gy ABC-WBRT and the imHDR-APBI protocol. RESULTS: Tumor location for the 17 pairs of patients relative to breast quadrant was as follows: upper outer 8, lower outer 5, upper inner 3, and lower inner 1. There was no difference regarding mean heart dose (MHD) and V5, whereas D25%, D45%, V10, and V25 significantly favored imHDR-APBI. Likewise, mean dose- and V5-LV did not differ, while Dmax- and V23-LV were significantly higher for ABC-WBRT. For LAD, Dmax, D25%, and V30 significantly favored imHDR-APBI without differences for mean dose and V40. When comparing imHDR-APBI with the 40.05 Gy ABC-WBRT schedule, MHD and mean dose LV were significantly lower in favor of ABC-WBRT. CONCLUSION: ABC-WBRT and imHDR-APBI yield similar low heart and LV exposure for left-sided BCA after BCS, whereas LAD can be better spared with imHDR-APBI.

Second infield re-irradiation with a resulting cumulative equivalent dose (EQD2max ) of >180 Gy for patients with recurrent head and neck cancer.

BACKGROUND: For locally recurrent head and neck cancer, re-irradiation with modern techniques like stereotactic body radiotherapy is a promising, possibly curative alternative to palliative systemic therapy. METHODS: We report on 1 patient with nasopharyngeal carcinoma (NPC) and 1 patient with cutaneous squamous cell carcinoma (SCC). Both patients received full dose primary treatment (>66 Gy, EQD2), full dose re-irradiation (>50 Gy, EQD2), and a second course of re-irradiation via robotic radiosurgery (CyberKnife) for a second local recurrence. RESULTS: Both treatments resulted in adequate tumor response. No grade III-IV acute or late toxicities occurred. Follow-up at 6 months after third irradiation showed partial remission for the patient with NPC. In the second case (SCC), no toxicities occurred and the tumor was in remission 18 months after last treatment. CONCLUSION: These cases show that a second course of re-irradiation utilizing modern techniques like robotic radiosurgery might be feasible for carefully selected patients with head and neck cancer.

Treatment planning for spinal radiosurgery : A competitive multiplatform benchmark challenge.

PURPOSE: To investigate the quality of treatment plans of spinal radiosurgery derived from different planning and delivery systems. The comparisons include robotic delivery and intensity modulated arc therapy (IMAT) approaches. Multiple centers with equal systems were used to reduce a bias based on individual's planning abilities. The study used a series of three complex spine lesions to maximize the difference in plan quality among the various approaches. METHODS: Internationally recognized experts in the field of treatment planning and spinal radiosurgery from 12 centers with various treatment planning systems participated. For a complex spinal lesion, the results were compared against a previously published benchmark plan derived for CyberKnife radiosurgery (CKRS) using circular cones only. For two additional cases, one with multiple small lesions infiltrating three vertebrae and a single vertebra lesion treated with integrated boost, the results were compared against a benchmark plan generated using a best practice guideline for CKRS. All plans were rated based on a previously established ranking system. RESULTS: All 12 centers could reach equality (n = 4) or outperform (n = 8) the benchmark plan. For the multiple lesions and the single vertebra lesion plan only 5 and 3 of the 12 centers, respectively, reached equality or outperformed the best practice benchmark plan. However, the absolute differences in target and critical structure dosimetry were small and strongly planner-dependent rather than system-dependent. Overall, gantry-based IMAT with simple planning techniques (two coplanar arcs) produced faster treatments and significantly outperformed static gantry intensity modulated radiation therapy (IMRT) and multileaf collimator (MLC) or non-MLC CKRS treatment plan quality regardless of the system (mean rank out of 4 was 1.2 vs. 3.1, p = 0.002). CONCLUSIONS: High plan quality for complex spinal radiosurgery was achieved among all systems and all participating centers in this planning challenge. This study concludes that simple IMAT techniques can generate significantly better plan quality compared to previous established CKRS benchmarks.

A clinical example of extreme dose exposure for an implanted cardioverter-defibrillator : Beyond the DEGRO guidelines.

INTRODUCTION: Considering that the number of malignant diseases in patients over 65 years of age is increasing, it often occurs that patients who carry a cardiac implanted electronic device must undergo radiotherapy. Ionizing radiation can disturb the function of the implantable cardioverter-defibrillator (ICD). As a result of this, an update of the DEGRO/DKG guidelines for radiotherapy of this patient group has been published. METHODS: We report the case of a patient with an ICD and T­lymphoblastic lymphoma with cardiac involvement, who received i.a. a total body irradiation with 8 Gy followed by a consolidating radiotherapy of the pericardium with 14 Gy as well as additional radiotherapy courses after consecutive recurrences. For the purposes of the treatment, the antitachyarrhythmia (ATA) therapy was deactivated and temporarily replaced through a life vest. RESULTS: According to the current DEGRO guidelines for irradiation of patients with cardiac implanted electronic devices, a categorization of the patient in the "high-risk" group was made. Furthermore, regular telemetric checks of the ICD device were performed before and after treatment. Despite unavailable declaration of the manufacturer regarding the cumulative tolerable dose and DEGRO recommendation for a cumulative dose <2 Gy, the aftercare was unproblematic and normal values were assessed for all relevant ICD parameters, despite a cumulative dose >10 Gy in the device. CONCLUSION: This case shows that if the cardiac implanted electronic devices are not directly irradiated und the energy used is reduced to 6 MV, irradiation-induced damage is less likely and can possibly be prevented.

New possibilities for volumetric-modulated arc therapy using the Agility™ 160-leaf multileaf collimator.

PURPOSE: This study compares the quality of intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans optimized for an Elekta Agility(TM) (Elekta, Stockholm, Sweden) multileaf collimator (MLC; leaf width 5 mm) and an Elekta MLCi2 (leaf width 10 mm) for complex target volumes (anal, AC; head and neck, H&N and prostate cancer, PC). PATIENTS AND METHODS: For plan comparisons, 15 patients who had been treated with IMRT or VMAT using the MLCi2 were selected. For each patient, a retrospective treatment plan using the MLCi2 for the technique not applied was created, as were treatment plans for both techniques using the Agility(TM) MLC. Dose-volume histograms (DHVs) for planning target volumes (PTVs) and organs at risk (OARs) were compared. Further parameters relating to dose conformity, dose homogeneity and mean dose (Dmean) to the PTV, compliance with the intended OAR dose criteria and overall dose to normal tissue were analyzed. Verification measurements were performed and optimization and treatment times were compared. RESULTS: Compared to the MLCi2 plans, the Agility(TM) IMRT and VMAT plans show better or equivalent results in terms of PTV dose conformity and homogeneity. Compliance with the intended OAR dose criteria does not differ according to technique or MLC type. Slight differences are shown for dose distributions in OARs and normal tissue. Verification measurements show that all plans fulfill the acceptance criteria of a minimum of 95 % matched dose points for the 3 %/3 mm γ criterion. Optimization times for the VMAT plans increase compared to the IMRT plans, whereas treatment times decrease. CONCLUSION: With the MLCi2, treatment of complex target volumes with VMAT was only possible with compromises in terms of target coverage. Using the Agility(TM) MLC, even complex target volumes can be treated with VMAT without compromising target coverage or resulting in higher exposure of OARs or normal tissue.