Dosimetric Characterization of Small Radiotherapy Electron Beams Collimated by Circular Applicators with the New Microsilicon Detector

High-energy small electron beams, generated by linear accelerators, are used for radiotherapy of localized superficial tumours. The aim of the present study is to assess the dosimetric performance under small radiation therapy electron beams of the novel PTW microSilicon detector compared to other available dosimeters. Relative dose measurements of circular fields with 20, 30, 40, and 50 mm aperture diameters were performed for electron beams generated by an Elekta Synergy linac, with energy between 4 and 12 MeV. Percentage depth dose, transverse profiles, and output factors, normalized to the 10 × 10 cm2 reference field, were measured. All dosimetric data were collected in a PTW MP3 motorized water phantom, at SSD of 100 cm, by using the novel PTW microSilicon detector. The PTW diode E and the PTW microDiamond were also used in all beam apertures for benchmarking. Data for the biggest field size were also measured by the PTW Advanced Markus ionization chamber. Measurements performed by the microSilicon are in good agreement with the reference values for all the tubular applicators and beam energies within the stated uncertainties. This confirms the reliability of the microSilicon detector for relative dosimetry of small radiation therapy electron beams collimated by circular applicators.

Association between exposure to radioactive iodine after the Chernobyl accident and thyroid volume in Belarus 10-15 years later

Background While there is a robust literature on environmental exposure to iodine-131 (131I) in childhood and adolescence and the risk of thyroid cancer and benign nodules, little is known about its effects on thyroid volume. Methods To assess the effect of 131I dose to the thyroid on the volume of the thyroid gland, we examined the data from the baseline screening of the Belarusian-American Cohort Study of residents of Belarus who were exposed to the Chernobyl fallout at ages ≤18 years. Thyroid dose estimates were based on individual thyroid activity measurements made shortly after the accident and dosimetric data from questionnaires obtained 10-15 years later at baseline screening. During baseline screening, thyroid gland volume was assessed from thyroid ultrasound measurements. The association between radiation dose and thyroid volume was modeled using linear regression where radiation dose was expressed with power terms to address non-linearity. The model was adjusted for attained age, sex, and place of residence, and their modifying effects were examined. Results The analysis was based on 10,703 subjects. We found a statistically significant positive association between radiation dose and thyroid volume (P < 0.001). Heterogeneity of association was observed by attained age (P < 0.001) with statistically significant association remaining only in the subgroup of ≥18 years at screening (P < 0.001). For this group, increase in dose from 0.0005 to 0.15 Gy was associated with a 1.27 ml (95% CI: 0.46, 2.07) increase in thyroid volume. The estimated effect did not change with increasing doses above 0.15 Gy. Conclusions This is the first study to examine the association between 131I dose to the thyroid gland and thyroid volume in a population of individuals exposed during childhood and systematically screened 10-15 years later. It provides evidence for a moderate statistically significant increase in thyroid volume among those who were ≥ 18 years at screening. Given that this effect was observed at very low doses and was restricted to a narrow dose range, further studies are necessary to better understand the effect.

Lung Stereotactic Radiation Therapy: Intercomparison of Several Irradiation Devices in Terms of Outcome and Predictive Factors

Background: Stereotactic body radiotherapy (SBRT) is recommended for the treatment of inoperable early stage non-small-cell lung cancer and lung oligometastases. The radiation oncology department of the Lausanne University Hospital (CHUV) gathers three different radiotherapy devices able to treat pulmonary lesions in SBRT conditions: CyberKnife® (CK), Helical Tomotherapy® (HT), and volumetric modulated arc therapy (VMAT). The aim of this study is to define the patients’ outcome in terms of irradiation efficacy and toxicities after lung SBRT depending of the choice of the SBRT technique.Methods: We retrospectively analyzed the clinical, radiological, and dosimetric data of patients with primary lung tumor or pulmonary oligometastases treated with SBRT between January 2016 and February 2020. We analyzed descriptive data using the Chi-2 test for proportions and the T-test for means comparisons, survival data by the Kaplan-Meier method and comparisons between groups by the Log-rank test and Cox-regression.Results: We identified 111 patients mostly in good condition (82.9% PS 0-1) with a median age of 71.4 years. One hundred forty-two lesions were treated with a typical fractionation of 55 Gy in 5 fractions, delivered by CK (59.9%), VMAT (38.0%), or HT (2.1%). Compared to other techniques, CK technique allowed to treat comparable gross tumor volume (GTV; 2.1 vs 1.4cc, p = 0.84) with smaller planning treatment volume (PTV; 12.3 vs 21.9 cc, p = 0.013), and was associated with a lower mean lung dose (MLD; 2.6 vs 4.1 Gy, p < 0.001), a lower V5 (13.5 vs 19.9 cc, p = 0.002) and a lower V20 (2.3 vs 5.4 cc, p < 0.001). Local control rates at 2 years were not different depending on the irradiation device, respectively of 96.2% (range, 90.8-100) and 98.1% (range, 94.4-100), p = 0.68. Toxicity incidence was significantly increased with V5 value > 17.2% (56.0 vs 77.4%, p = 0.021). Conclusions: Compared to other SBRT techniques, CK treatments permitted to treat comparable GTV with reduced PTV, MLD, V20, and V5. The dosimetric benefit of CK SBRT was not associated with a clear clinical benefit, with comparable outcome in terms of control rates and toxicity. Toxicity incidence was less frequent when reducing the V5. The use of CK is particularly attractive in case of multiple courses of lung SBRT or in case of local relapse requiring lung re-irradiation.Trial registration: Registered on February 24th 2021, ID 2021-00267, with the authorization of the CER-VD ethics committee (Switzerland).

Dosimetric Characterization of Small Radiation Therapy Electron Beams Collimated by Tubular Applicators With the New Micro-Silicon Detector

High-energy small electron beams generated by linear accelerators are used for radiotherapy of localized superficial tumors. The aim of the present study is to assess the dosimetric performance under small radiation therapy electron beams of the novel PTW microSilicon detector by comparison with commercially available dosimeters. Relative dose measurements of circular fields with 20, 30, 40 and 50 mm aperture diameters were performed for 4 to 12 MeV energy range of electron beams generated by an Elekta Synergy linac. Percentage depth dose, transverse profiles and output factors normalized to the 10 &times; 10 cm2 reference field were measured. All dosimetric data were collected in a PTW MP3 motorized water phantom at SSD of 100cm by using the novel PTW microSilicon detector. The PTW diode E and the PTW microDiamond were also used in all beam aperture for benchmarking. Data for the biggest field size were also measured by the PTW Advanced Markus ionization chamber.Measurements performed by the microSilicon are in good agreement with the reference values for all the tubular applicators and beam energies, within the stated uncertainties. This confirms the reliability of the microSilicon detector for relative dosimetry of small radiation therapy electron beams collimated by tubular applicators.

Toxicity Outcome Using Bladder-Rectum Spacer Balloon Versus Vaginal Gauze Packing in Brachytherapy in Cervical Cancer.

BACKGROUNDTo compare the clinical impact in terms of toxicity outcomes with RayshieldTM bladder rectum spacer balloon (BRSB) versus vaginal gauze packing (VGP) in patients treated with high dose rate intracavitary brachytherapy for carcinoma cervix.RESULTSFollow-up and dosimetric data of patients in whom BRSB and VGP were used in a previously reported randomized study were retrieved, 8. Out of 80 patients analysed, late toxicities assessment (according to Common Terminology Criteria for Adverse Events version 4.0 (CTCAE v4). was possible in 65 patients as 14 patients were lost to follow-up and one patient died. Grade 2 bladder toxicity was observed in 2 patients in each arm. Only 1 patient in VGP arm experienced grade 3 proctitis while none of the patient in BRSB arm had ³Grade 2 rectal toxicity. Vaginal toxicity was comparable in both the arms.CONCLUSION No significant difference was observed in bladder and rectal toxicities using the BRSB versus VGP. BRSB can be considered as an alternative to VGP in intracavitary brachytherapy for cervical cancer using tandem- ovoid applicatorsThe study was approved by the institute ethics committee and registered under Clinical Trial Registry of India (CTRI/2009/091/000840).

Radioablation of Upper Abdominal Malignancies by CT-Guided, Interstitial HDR Brachytherapy: A Multivariate Analysis of Catheter Placement Assisted by Ultrasound Imaging

Purpose To evaluate the use of ultrasound (US) during catheter placement in interstitial brachytherapy (iBT) of abdominal malignancies as an alternative to computed tomography (CT) fluoroscopy. Materials and Methods Catheter placement for CT-guided iBT was, if US visibility was sufficient, assisted by sonography in 52 consecutive patients with 82 lesions (liver N = 62; kidney N = 16; peritoneum N = 4) of various malignancies. We collected data on lesion visibility, location, depth, size, and dosimetry. Comparison of CT fluoroscopy versus US-assisted catheter placement was performed by Fisher’s exact test for frequencies and U-test for lesion visibility and dosimetric data. Factors predicting the utility of sonography were determined in a lesion-based multivariate regression analysis. A p-value < 0.05 was regarded as statistically significant. Results 150 catheters (1 to 6 per lesion; mean diameter 3.6 ± 2.4 cm) were implanted. CT fluoroscopy was used for 44 catheters, and US was used for 106 catheters. Lesion visibility assessed by 5-point Likert scale was significantly better in US (median 2 vs. 3; p = 0.011) and effective dose was significantly reduced if US guidance was applicable (median 1.75 vs. 8.19 mSv; p = 0.014). In a multivariate regression analysis, we identified increased lesion size and caudal location within the target organ to independently predict the utility of ultrasound in catheter placement for iBT. Conclusion Sonography is a helpful technique to assist CT-guided interstitial brachytherapy of upper abdominal malignancies. Especially for larger lesions localized in the lower liver segments or lower half of the kidney, superior visibility can be expected. As the effective dose of the patient is also reduced, radiation exposure of the medical staff may be indirectly lowered. Key Points: Citation Format

Dosimetric Analysis of Patients Receiving Radiotherapy with VMAT Technique in Localized Prostate Cancer and Its Correlation with Side Effects

Aim: It is aimed whether there is a relationship between dosimetric data of localized prostate cancer patients who have been treated with curative radiotherapy and gastrointestinal (GIS), genitourinary (GUS), anal and sexual side effects and whether there is a difference between dosimetric data and clinical findings between risk groups. Method : Eighty-seven patients who received curative radiotherapy for localized prostate cancer between 2014 - 2019 were included in the study. Dosimetrically; whether there was a relationship between V30, V40, V50, V60, V65, V70, V75 for rectum and bladder; D90 for the penile bulb, V72, V74, V76 for the bulbomembranous urethra, V30, V45, V53, Dmax for the anus and V45 (cc) for the intestine data and the side effects was analyzed. It was evaluated whether there was a relationship between testosterone values and sexual side effects. The Kolmogorov-Smirnov test, One Way ANOVA (F-test), and paired-sample T-test were used as statistical methods. For statistical significance, p <0.05 was accepted. Results : The mean age of the patients was 69 (50-86), the mean PSA (ng/dl) before RT was 25.1 (0.9-339), the median RT dose was 76 Gy (74-78 Gy), and the mean follow-up period was 38.2 months. PTVmax, PTVmean, PTVmin, bladder V40, bladder V50, rectum V30, rectum V40, rectum V50 and intestinal V45 (cc) were determined as dosimetric data showing differences between risk groups. A statistically significant relationship was found between rectum V30 (p = 0.017), V60 (p = 0.019), V65 (p = 0.008), V70 (p = 0.007) and V75 (p = 0.034) and chronic GIS side effects. G2 GIS side effects were observed in 4 patients (4.6%) in the entire patient group in the acute period. A statistically significant relationship was found between the patients receiving hormonotherapy (p = 0.021) and testosterone values ​​at the last control (p = <0.001) and chronic sexual side effects. Conclusion: Attention should be paid to the rectum V30, V60, V65, V70, and V75 values to minimize the long-term GIS side effects of patients who have undergone RT. Testosterone level and HT status affect chronic sexual toxicity.

Mitigation on Bowel Loops Daily Variations by 1.5-T MR-guided Daily-adaptive SBRT for Abdomino-pelvic Lymph-nodal Oligometastases

PurposeWe report preliminary dosimetric data concerning the use of 1.5-T MR-guided daily-adaptive radiotherapy for abdomino-pelvic lymph-nodal oligometastases. We aimed to assess the impact of this technology on mitigating daily variations for both target coverage and organs-at-risk (OARs) sparing. MethodsA total of 150 sessions for 30 oligometastases in 23 patients were analyzed. All patients were treated with MR-guided stereotactic body radiotherapy (SBRT) for a total dose of 35Gy in 5 fractions. For each fraction, a quantitative analysis was performed for PTV volume, V35Gy and Dmean. Similarly, for OARs we assessed daily variations of volume, Dmean, Dmax. Any potential statistically significant change between baseline planning and daily-adaptive sessions was assessed using the Wilcoxon signed-rank test, assuming a p-value< 0.05 as significant.ResultsAverage baseline PTV, bowel, bladder and single intestinal loop volumes were respectively 8.9cc (range, 0.7-41.2cc), 1176cc (119-3654 cc), 95cc (39.7-202.9 cc), 18.3cc (9.1-37.7 cc). No significant volume variations were detected for PTV (p=0.21) bowel (p=0.36), bladder (p=0.47), except for single intestinal loops, which resulted smaller (p=0.026). Average baseline V35Gy and Dmean for PTV were respectively 85.6% (72-98.8%) and 35.6 Gy (34.6-36.1 Gy). We recorded a slightly positive trend in favor of daily-adaptive strategy vs baseline planning for improved target coverage, although not reaching statistical significance. (p=0.11 and p=0.18 for PTV-V35Gy and PTV-Dmean). Concerning OARs, a significant difference was observed in favor of daily-adapted treatments in terms of single intestinal loop Dmax [23.05 Gy (13.2-26.9 Gy) at baseline vs 20.5 Gy (12.1-24 Gy); p-value=0.0377] and Dmean [14.4 Gy (6.5-18 Gy) at baseline vs 13.0 Gy (6.7-17.6 Gy); p-value=0.0003].Specifically for bladder, the average Dmax was 18.6 Gy (0.4-34.3 Gy) at baseline vs 18.3 Gy (0.7-34.3 Gy) for a p-value=0.28; the average Dmean was 7.0 Gy (0.2-16.6 Gy) at baseline vs 6.98 Gy (0.2-16.4 Gy) for a p-value=0.66. Concerning the bowel, no differences in terms of Dmean [4.78 Gy (1.3-10.9 Gy) vs 5.6 Gy (1.4-10.5 Gy); p-value=0.23] were observed between after daily-adapted sessions. A statistically significant difference was observed for bowel Dmax [26.4 Gy (7.7-34 Gy) vs 25.8 Gy (7.8-33.1 Gy); p-value=0.0086].ConclusionsDaily-adaptive MR-guided SBRT reported a significantly improved single intestinal loop sparing for lymph-nodal oligometastases. Also bowel Dmax was significantly reduced with daily-adaptive strategy. A minor advantage was also reported in terms of PTV coverage, although not statistically significant.

455 Proton Beam Therapy in Rectal Cancer: A Systematic Review and Meta-Analysis

Introduction Radiotherapy for locally advanced rectal cancer is conventionally performed using photon-based radiotherapy (PBR), carrying significant risk of toxicity to organs at risk (OAR). Proton beam therapy (PBT) potentially delivers equivalent dosimetric radiation to the targeted tissue with improved sparing of OAR. We aimed to compare dosimetric irradiation of OAR for PBT versus PBR in patients with rectal cancer and assess any oncological outcomes. Method An extensive electronic literature search was performed from inception till April 2020 and subsequent meta-analysis performed. Results Six articles met the inclusion criteria. Dosimetric data of irradiation delivered to OAR for PBT and PBR were calculated for the same patients. PBT had significantly less irradiated small bowel compared to 3DCRT and IMRT, (MD -16.95, 95% CI [-24.03, -9.88], p &lt; 0.00001) and (MD -6.96, 95% CI [-12.99, -0.94], p = 0.02) respectively. Similar results were observed for bladder and pelvic bone marrow. Two studies reported clinical and oncological results for PBT in recurrent rectal cancer with overall survival reported as 43% and 68%. Conclusions Dosimetric treatment plans have less irradiation of OAR for rectal cancer with PBT compared to PBR. There is a need for further research in PBT and rectal cancer, as promising results have been shown in recurrent rectal cancer.