Stereotactic Body Radiation Therapy Techniques Research Articles

Thoracic Reirradiation with Stereotactic Body Radiation Therapy (SBRT) for Recurrent Advanced Non-Small Cell Lung Cancer (NSCLC)

BackgroundNon-small cell lung cancer (NSCLC) local control remains suboptimal with rates around 75%. Stereotactic body radiation therapy (SBRT) is an option for isolated local recurrences of small volume recurrences. This study investigates the safety and efficacy of 60 Gy in eight fractions in large volume local recurrences. MethodsA retrospective chart review of patients treated with salvage SBRT for NSCLC lung parenchymal recurrence between July 2013 and February 2020. Reirradiation prescribed dose was 60 Gy in eight fractions utilizing SBRT technique. The primary endpoint was local control at most recent follow-up or death. Secondary endpoints included overall survival, disease-free interval, cancer-specific survival, and treatment related toxicities. ResultsSeven patients met criteria. Median follow up time was 38 months (18.1-72.4). Median age was 67 years (63-80). The median time to reirradiation was 18.2 months (7.3-28.6). Retreatment median ITV was 57.9cc (15.8-344.6) and PTV median was 113.6cc (38.3-506.9). Local control was maintained in four of seven patients (57.1%). Two of the seven patients (28.6%) remained alive. Median disease-free interval was 22.5 months (11-65). Three of seven patients (42.9%) had grade two toxicities. One patient (14.3%) had a grade 3 rib/chest wall toxicity with concurrent disease recurrence invading the chest wall. ConclusionThis study reports that SBRT of 60 Gy in eight fractions was delivered safely and effectively to large volume recurrent NSCLC previously treated with radiation therapy. The disease-free interval of nearly 2 years is meaningful for patients’ quality of life and duration of time off of systemic therapy.

Stereotactic Body Radiotherapy for Management of Pulmonary Oligometastases in Stage IV Colorectal Cancer: A Perspective

In pulmonary oligometastases from colorectal cancer (POM-CRC), metastasectomy is the primarily recommended treatment. Stereotactic body radiotherapy (SBRT) has been suggested as a viable alternative therapy. SBRT efficacy for POM-CRC is poorly delineated compared to selected non-CRC primaries. This perspective article aims to critically summarize the existing evidence regarding efficacy of SBRT in terms of overall survival (OS) and local control (LC), and factors modulating this, in the treatment of POM-CRC. Overall, reasonable LC and OS rates were observed. The wide range of expansions in planning target volume margins introduced variation in pretreatment protocols. Dose-fractionation schedules varied according to patient and tumor characteristics, though leverage of BED10 in select studies enabled standardization. An association between SBRT dose and improved OS and LC was observed across multiple studies. Prognostic factors that were associated with improved LC included: fewer oligometastases, absence of extra-pulmonary metastases, primary tumor histology, and smaller gross tumor volume. Differences in SBRT modality and techniques over time further confounded results. Many studies included patients receiving additional systemic therapies; preprotocol and adjuvant chemotherapies were identified as prognostic factors for LC. SBRT compared with metastasectomy showed no differences in short-term OS and LC outcomes. In conclusion, SBRT is an efficacious treatment for POM-CRC, in terms of OS and LC. Heterogeneity in study design, particularly pertaining to dose protocols, patient selection, and additional therapies should be controlled for future randomized studies to further validate SBRT efficacy.

State of the art and future challenges of urethra-sparing stereotactic body radiotherapy for prostate cancer: a systematic review of literature

PurposeDoses delivered to the urethra have been associated with an increased risk to develop long-term urinary toxicity in patients undergoing stereotactic body radiotherapy (SBRT) for prostate cancer (PCa). Aim of the present systematic review is to report on the role of urethra-sparing SBRT (US-SBRT) techniques for prostate cancer, with a focus on outcome and urinary toxicity.MethodA systematic review of the literature was performed on the PubMed database on May 2023. Based on the urethra-sparing technique, 13 studies were selected for the analysis and classified in the two following categories: “urethra-steering” SBRT (restriction of hotspots to the urethra) and “urethra dose-reduction” SBRT (dose reduction to urethra below the prescribed dose).ResultsBy limiting the urethra Dmax to 90GyEQD2 (α/β = 3 Gy) with urethra-steering SBRT techniques, late genitourinary (GU) grade 2 toxicity remains mild, ranging between 12.1% and 14%. With dose-reduction strategies decreasing the urethral dose below 70 GyEQD2, the risk of late GU toxicity was further reduced (< 8% at 5 years), while maintaining biochemical relapse-free survival rates up to 93% at 5 years.ConclusionUS-SBRT techniques limiting maximum doses to urethra below a 90GyEQD2 (α/β = 3 Gy) threshold result in a low rate of acute and late grade ≥ 2 GU toxicity. A better understanding of clinical factors and anatomical substructures involved in the development of GU toxicity, as well as the development and use of adapted dose constraints, is expected to further reduce the long-term GU toxicity of prostate cancer patients treated with SBRT.

Current uses and applications of stereotactic body radiation therapy.

Stereotactic body radiation therapy (SBRT) is a modality that delivers high doses of radiation to a well-defined tumor target in a single or a few fractions and with high precision, which significantly reduces the dose received by surrounding normal tissues. SBRT is indicated for inoperable, early stage (T1 and T2) primary non-small cell lung cancer, lung metastases with a controlled primary tumor, prostate tumors and oligometastatic disease. Despite the lack of long-term or phase III studies, efficacy results in local control are higher than 90%, with similar toxicity to that reported with conventional fractionated radiotherapy. This article describes SBRT technology and technique, along with clinical applications, indications and limitations of this therapeutic modality.

Error detection using EPID-based 3D in vivo dose verification for lung stereotactic body radiotherapy

PurposeTo investigate the error detectability limitations of an EPID-based 3D in vivo dosimetry verification system for lung stereotactic body radiation therapy (SBRT). MethodsThirty errors were intentionally introduced, consisting of dynamic and constant machine errors, to simulate the possible errors that may occur during delivery. The dynamic errors included errors in the output, gantry angle and MLC positions related to gantry inertial and gravitational effects, while the constant errors included errors in the collimator angle, jaw positions, central leaf positions, setup shift and thickness to simulate patient weight loss. These error plans were delivered to a CIRS phantom using the SBRT technique for lung cancer. Following irradiation of these error plans, the dose distribution was reconstructed using iViewDose™ and compared with the no error plan. ResultsAll errors caused by the central leaf positions, dynamic MLC errors, Jaw inwards movements, setup shifts and patient anatomical changes were successfully detected. However, dynamic gantry angle and collimator angle errors were not detected in the lung case due to the rotation-symmetric target shape. The results showed that the γmean and γpassrate indicators can detect 13 (81.3%) and 14 (87.5%) of the 16 errors respectively without including the gantry angle error, collimator angle error and output error. ConclusionsIn summary, iViewDose™ is an appropriate approach for detecting most types of clinical errors for lung SBRT. However, the phantom results also showed some detectability limitations of the system in terms of dynamic gantry angle and constant collimator angle errors.

Dosimetric Comparison of Supine vs. Prone Position Utilizing Traditional Linac-Based Stereotactic Body Radiation Therapy for Accelerated Partial Breast Irradiation

<h3>Purpose/Objective(s)</h3> Our institution recently completed a pilot trial using stereotactic body radiation therapy (SBRT) for accelerated partial breast irradiation (APBI) on a traditional Linac delivering 30 Gy in 5 fractions. In this study, we compared dosimetry of patients treated in supine versus prone positions. <h3>Materials/Methods</h3> Patients enrolled on our recent pilot trial and additional post-accrual patients using the same SBRT technique were included. All patients had early-stage breast cancer, met the APBI ASTRO suitability criteria, and were treated on a traditional Linac. Treatment position, breast laterality and volume, BMI, and organs at risk (OAR) dosimetry were recorded. Prone treatments were delivered on a prone breast board using sliding window IMRT, and supine plans used either IMRT or partial arcs. All prone and 15 of 18 supine plans used free breathing technique (3 utilized deep inspiration breath-hold). CTV was 1 cm expansion from GTV. PTV was a 3 mm expansion from CTV and excluded skin surface+3 mm. Laterality was controlled for heart dosimetry. A small subset of patients was simulated in both supine and prone positions with associated radiation plans available for comparison. For unpaired data, unadjusted comparisons of means were performed using two-group t-test, and analysis of covariance was used for adjusted comparisons. For paired data, unadjusted comparisons were performed using the paired t-test, and adjusted comparisons used mixed models repeated measures analysis. Statistical tests were two-sided. <h3>Results</h3> This study included 35 patients with 36 treated plans. One patient had bilateral breast cancer. Of the 36 treated plans, 18 were supine (11 right-sided breast cancer, 7 left-sided) and 18 prone (8 right-sided, 10 left-sided). Eight patients had comparison plans in both positions. For patients with left-sided breast cancer, heart mean dose supine vs prone was 0.49 and 1.14 Gy, respectively (p < 0.001), and continued to be significant when controlling for BMI and treated breast volume. Mean heart dose was not different for right-sided patients supine vs prone, 0.36 vs 0.47 Gy, respectively (p = 0.870). Ipsilateral lung V1.5 Gy was higher for supine vs prone, 39.44 vs 29.89% (p=0.017). Max rib dose was higher for supine compared to prone, 28.29 vs 16.12 Gy (p<0.001). For the subset of patients with plans in both positions, mean heart dose continued to be lower for left-sided patients treated supine vs prone (p = 0.001). Ipsilateral lung V1.5 Gy and max dose to ribs were lower for prone with p values of 0.003 and <0.001, respectively. <h3>Conclusion</h3> Supine treatment provided heart dosimetry benefit for left-sided breast cancer patients compared to prone. Benefit was independent of BMI and breast volume. Prone position offered lower dose to ipsilateral lung and ribs. The results from this study can help determine the best treatment position based on patient-specific OAR concerns.

Clinical Outcomes Following Proton and Photon Stereotactic Body Radiation Therapy for Early-Stage Lung Cancer.

Simple SummaryThe current study reports the clinical outcomes of proton and photon stereotactic body radiation therapy (SBRT) for early-stage lung cancer. Out of 202 patients who met the inclusion criteria, 34 received proton SBRT and 168 received photon SBRT. Patients at high risk of developing post-SBRT radiation pneumonitis tended to receive proton SBRT. Oncologic outcomes and toxicity profiles were comparable between treatment modalities. Proton SBRT could be considered for patients with high risk of radiation pneumonitis.We aimed to report the clinical outcomes following stereotactic body radiation therapy (SBRT) using photon or proton equipment in early-stage lung cancer. We retrospectively reviewed 202 cT1-2N0M0 lung cancer patients who underwent SBRT with 60 Gy in four consecutive fractions between 2010 and 2019 at our institution: 168 photon SBRT and 34 proton SBRT. Patients who underwent proton SBRT had relatively poor baseline lung condition compared to those who underwent photon SBRT. Clinical outcomes were comparable between treatment modalities: 5-year local control (90.8% vs. 83.6%, p = 0.602); progression-free survival (61.6% vs. 57.8%, p = 0.370); overall survival (51.7% vs. 51.9%, p = 0.475); and cause-specific survival (70.3% vs. 62.6%, p = 0.618). There was no statistically significant difference in grade ≥ 2 toxicities: radiation pneumonitis (19.6% vs. 26.4%, p = 0.371); musculoskeletal (13.7% vs. 5.9%, p = 0.264); and skin (3.6% vs. 0.0%, p = 0.604). In the binary logistic regression analysis of grade ≥3 radiation pneumonitis, poor performance status and poor baseline diffusion capacity of lung for carbon monoxide were significant. To summarize, though patients with high risk of developing lung toxicity underwent proton SBRT more frequently, the SBRT techniques resulted in comparable oncologic outcomes with similar toxicity profiles. Proton SBRT could be considered for patients at high risk of radiation pneumonitis.

SBRT for Localized Prostate Cancer: CyberKnife vs. VMAT-FFF, a Dosimetric Study.

In recent years, stereotactic body radiation therapy (SBRT) has gained popularity among clinical methods for the treatment of medium and low risk prostate cancer (PCa), mainly as an alternative to surgery. The hypo-fractionated regimen allows the administration of high doses of radiation in a small number of fractions; such a fractionation is possible by exploiting the different intrinsic prostate radiosensitivity compared with the surrounding healthy tissues. In addition, SBRT treatment guaranteed a better quality of life compared with surgery, avoiding risks, aftermaths, and possible complications. At present, most stereotactic prostate treatments are performed with the CyberKnife (CK) system, which is an accelerator exclusively dedicated for stereotaxis and it is not widely spread in every radiotherapy centre like a classic linear accelerator (LINAC). To be fair, a stereotactic treatment is achievable also by using a LINAC through Volumetric Modulated Arc Therapy (VMAT), but some precautions must be taken. The aim of this work is to carry out a dosimetric comparison between these two methodologies. In order to pursue such a goal, two groups of patients were selected at Instituto Nazionale Tumori—IRCCS Fondazione G. Pascale: the first group consisting of ten patients previously treated with a SBRT performed with CK; the second one was composed of ten patients who received a hypo-fractionated VMAT treatment and replanned in VMAT-SBRT flattening filter free mode (FFF). The two SBRT techniques were rescaled at the same target coverage and compared by normal tissue sparing, dose distribution parameters and delivery time. All organs at risk (OAR) constraints were achieved by both platforms. CK exhibits higher performances in terms of dose delivery; nevertheless, the general satisfying dosimetric results and the significantly shorter delivery time make VMAT-FFF an attractive and reasonable alternative SBRT technique for the treatment of localized prostate cancer.

Assessment of HDR brachytherapy-replicating prostate radiotherapy planning for tomotherapy, cyberknife and VMAT.

A dosimetric study was undertaken to assess the ability of Cyberknife (CK), Volumetric Modulated Arc Therapy (VMAT), and TomoTherapy (Tomo) to generate treatment plans that mimic the dosimetry of high dose-rate brachytherapy (HDR BT) for prostate cancer. The project aimed to assess the potential of using stereotactic body radiotherapy (SBRT) for boost treatment of high-risk prostate cancer patients where HDR BT in combination with conformal external beam radiotherapy (EBRT) is the standard of care. The datasets of 6 prostate patients previously treated with HDR BT were collated. VMAT, CK, and TomoTherapy treatment plans were generated for each dataset using the target and organ-at-risk structures as defined by the Radiation Oncologist during the HDR BT treatment process. The HDR BT plan isodoses were also converted into planning structures to assist the other modalities to achieve a HDR BT-like dose distribution. CK plans were created using both the iris collimator (IC) and a multileaf collimator (MLC). Comparison of the techniques was made based on dose-volume indices. Each plan was created at centres experienced using the respective treatment planning systems (TPS). Planning target volume (PTV V100%), i.e., the volume of the planning target volume (PTV) receiving 100% of the relative dose, in VMAT and TomoTherapy SBRT plans was higher than HDR BT plans. PTV V150% and V200%, i.e., volume of the PTV receiving 150% and 200% of the relative dose, were approached on all the CK MLC and TomoTherapy SBRT plans. However, it is not presently achievable for "virtual brachytherapy" SBRT to replicate the same high intraprostatic doses as HDR BT while meeting the constraints on the organs-at-risk (OARs). Half of the CK IC plans achieved PTV V150% but this was at the expense of high rectal dose. TomoTherapy and CK MLC plans achieved PTV V150% and V200% but the bladder dose was higher compared to CK IC plans. VMAT exhibited excellent PTV coverage based on V100 and OAR sparing, but without any ability to achieve the high intra-prostatic doses of HDR (V150% and V200%). SBRT techniques can be used to deliver hypofractionated radiotherapy to the PTV V100%. Based on the comparison of "physical" dose distributions, SBRT cannot presently achieve the same high intraprostatic doses as HDR BT while respecting the OAR constraints. SBRT still remains an attractive treatment option for delivering hypofractionated treatments for prostate cancer compared to HDR BT, in particular as it is less invasive and less resource intensive. Long-term outcomes of clinical trials comparing HDR BT and SBRT "prostate boosts" may show whether the high intraprostatic doses are clinically significant and correlate with outcomes.

Single-isocenter versus multiple-isocenters for multiple lung metastases: Evaluation of lung dose

Background and purposeA potential challenge in single-isocenter multi-lesion lung stereotactic body radiotherapy (SBRT) is that patient positioning is not based on each lesion individually, but on the average position of all lesions. This may lead to larger margins compared to treating with one isocenter per lesion, but increases workflow efficiency. The aim of this study was to investigate whether a single-isocenter technique leads to increased normal lung dose compared to a conventional multiple-isocenters technique. Materials and methodsA cohort of 15 NSCLC patients with two or three lesions previously treated with SBRT was subjected to treatment planning with a multiple-isocenter technique and a single-isocenter technique. For the latter, two margin approaches were evaluated: (1) identical margins for each internal target volume (ITV), assuming an average registration for all lesions in cone-beam CT (CBCT) positioning verification and (2) a smaller margin for the largest lesion, assuming an optimal registration for that lesion. For all 45 treatment plans, mean lung dose (MLD) and lungs-V20Gy were evaluated. The study was performed following RATING guidelines. ResultsThe MLD was 4.9 ± 1.9 Gy (mean ± SD) for multiple-isocenters and 5.4 ± 2.1 Gy and 5.3 ± 2.2 Gy for single-isocenter approach 1 and 2, respectively. V20Gy was 5.5 ± 3.7%, 5.5 ± 3.2% and 5.4 ± 3.3%. A median [range] increase in MLD of 11.6% [−14.9 – 26.8] was observed when comparing single-isocenter treatment plans to those with multiple isocenters. V20Gy increased by 0.2 [−3.4 – 1.3] percentage points. ConclusionA single-isocenter SBRT technique for lung patients with multiple targets results in clinically acceptable increases in normal lung dose.

Medical consequences of radiation exposure of the bronchi—what can we learn from high-dose precision radiation therapy?

The bronchial tolerance to high doses of radiation is not fully understood. However, in the event of a radiological accident with unintended exposure of the central airways to high doses of radiation it would be important to be able to anticipate the clinical consequences given the magnitude of the absorbed dose to different parts of the bronchial tree. Stereotactic body radiation therapy (SBRT) is a radiation treatment technique involving a few large fractions of photon external-beam radiation delivered to a well-defined target in the body. Despite generally favourable results, with high local tumour control and low-toxicity profile, its utility for tumours located close to central thoracic structures has been questioned, considering reports of severe toxic symptoms such as haemoptysis (bleedings from the airways), bronchial necrosis, bronchial stenosis, fistulas and pneumonitis. In conjunction with patient- and tumour-related risk factors, recent studies have analysed the absorbed radiation dose to different thoracic structures of normal tissue to better understand their tolerance to these high doses per fraction. Although the specific mechanisms behind the toxicity are still partly unknown, dose to the proximal bronchial tree has been shown to correlate with high-grade radiation side effects. Still, there is no clear consensus on the tolerance dose of the different bronchial structures. Recent data indicate that a too high dose to a main bronchus may result in more severe clinical side effects as compared to a smaller sized bronchus. This review analyses the current knowledge on the clinical consequences of bronchial exposure to high dose hypofractionated radiation delivered with the SBRT technique, and the tolerance doses of the bronchi. It presents the current literature regarding types of high-grade clinical side effects, data on dose response and comments on other risk factors for high-grade toxic effects.

Stereotactic Body Radiotherapy (SBRT) for the Palliation of Unirradiated Mucosal Head and Neck Tumors

We report the treatment outcomes of palliative SBRT to primary mucosal unirradiated head and neck cancer (HNC) patients that were ineligible to receive conventional radiation therapy (RT) MATERIALS/METHODS: We retrospectively reviewed patients with primary mucosal HNC treated with SBRT from 2011-2020. Patients with histologies other than squamous cell carcinoma, with primary skin disease or with recurrent tumors were excluded. SBRT was used in this cohort of patients because of associated comorbidities that precluded full course RT. All patients were followed regularly with clinical examination and imaging. The endpoints were local failure (LF) defined as recurrence of tumor at the irradiated site, overall survival (OS), acute and late grade 3 and 4 toxicity.Sixty patients with a median age of 81 years were treated with SBRT technique. The median follow-up was 7.8 months. 8(13%), 38(63%), 13(22%) and 1(2%) were ECOG 1, 2, 3 and 4, performance status respectively. Oral cavity was the most common cancer site in our cohort (60%, 36 patients), followed by oropharyngeal cancer (15%, 9 patients), larynx (7%, 4 patients), hypopharynx (5%, 3 patients), parotid cancer (5%, 3 patients), unknown primary (5%, 3 patients) and nasal cavity/sinonasal cancer (3%, 2 patients). Tx-2 and T3-4 lesions were found in 18(30%) and 42(70%), respectively, while 32(53%) and 28(47%) were staged N0-1, N2-3, respectively. GTV was prescribed to 45 Gy in 5 fractions, most commonly used (55%), followed by 40 Gy in 5 fractions (37%). A high dose CTV was not used around GTV although an elective nodal volume of 25 Gy in 5 fractions was also used. 52 patients were treated twice a week completed in 13-20 days, 2 patients completed their treatments in 9 and 10 days, while 5 patients over 21-28 days. The cumulative incidence of LF at 6 and 12 months was 5.3% (95% CI 1.4-13.3) and 12.4% (95% CI 4.9-23.7), respectively. The median (OS) was 9.2 months. The 6 and 12 month OS rates were 67.9% (95% CI 56.6-81.4) and 41.4% (95% CI 29.2-58.7), respectively. Cancer was the cause of death in 12 patients (38%), while 20 deaths (62%) were not related to malignant disease. Acute G3 toxicity was observed in 23 patients, including G3 mucositis in 19 patients, G3 dermatitis in 2 patients and G3 dysphagia in 2 patients. Five patients developed G3+ late toxicity, including osteoradionecrosis (n = 3), soft tissue supraglottic ulceration (n = 1) and trismus (n = 1). Four of these 5 patients did not have evidence of concurrent local recurrence.HN SBRT has been largely reported in the re-irradiation setting, however, SBRT may have greater utility in the palliative management of previously untreated HNC patients. Despite the advanced age and performance status noted in this group of patients with mucosal SCC of the head and neck, who were not eligible for conventional RT, SBRT was associated with low local failure rates and reasonable toxicity profile. A randomized controlled trial is warranted to confirm the efficacy of HN SBRT in this setting.

Post-Stereotactic Body Radiotherapy Circulating Immune Cells and Local Tumor Control for Patients With Recurrent Hepatocellular Carcinoma

Host immune status plays an important role in treatment outcome prediction and prognosis in patients with various cancers. However, the relationship between circulating immune cell and tumor control has not been well studied in hepatocellular carcinoma (HCC) after stereotactic body radiation therapy (SBRT).Patients with recurrent HCC received liver SBRT were eligible. Patients must be deemed surgically unresectable, and had Trans Arterial Chemotherapy Embolization (TACE). The local tumor control was defined as no disease progression by treatment response assessment CT or MRI around 3 months, largely according to mRECIST. Complete metabolic responses were defined either with no enhancement on MRI or no FDG activity. The risk factors of our interest included demographics factors, liver disease history, Child Pugh score, pathology stage, and prior treatments. The counts of whole leukocytes, lymphocyte and neutrophil pre- and post-SBRT were recorded from clinical laboratory. Leukopenia was graded according to CTCAE v 5.0. The associations of variables with in-field treatment response were analyzed using Chi-square test and one-way ANOVA.A total of 22 patients enrolled between August 2015 and September 2020. All patients had disease progression after TACE treatment and received 5 fractions of SBRT. Twenty patients received a definitive dose of 40-50 Gy. The local tumor was controlled in 86.4% (19/22), and increased to 90% (18/20) after excluding 2 patients receiving lower doses of palliative SBRT. There were 2 (10%), 5 (25%) and 13 (65%) defined as progressive disease, stable disease, partial response, and complete metabolic response, respectively. Large tumor/multiple tumor (P = 0.003), vascular invasion (P = 0.042), surgery history (P = 0.023), and BCLC stage (P = 0.040) were significantly associated with the tumor control, while age, gender, family history, heavy drinking, smoking history, BMI, ECOG, hepatitis, cirrhosis, Child Pugh score, portal vein tumor emboli, TNM, tumor size, and radiation dose were not. In 20 patients with complete blood testing peri-SBRT and uninterrupted radiation treatment, 17 (85%), 18 (90%), 13 (65%) and 9 (45%) patients had reduction in leukocyte, lymphocyte, neutrophil and monocyte after SBRT, respectively. Post-SBRT leukocyte counts (P = 0.036), pre-monocyte (P = 0.029), pre-lymphocyte (P = 0.073) and post- lymphocyte (P = 0.092) were associated with tumor control. The pre-RT leukocyte, neutrophil, post-RT neutrophil and monocyte were not.This limited study suggests that SBRT may provide excellent local tumor control for patients with recurrent HCC after surgical resection and TACE with no local treatment options. The significance in post-SBRT circulating immune cell suggest a role of immune function on radiation tumor control and a potential value of improving SBRT technique to decrease damage of immune cells. Further validation study with large sample size and longer follow-up is needed.

PO-1986 Early toxicity with stereotactic body radiation therapy for prostate cancer

Purpose or Objective: Stereotactic body radiotherapy (SBRT) has proven to be a new option for localized prostate cancer. SBRT specific indications has been transformed due to SARS-2-COVID19 scenario. This communication shows our data about acute toxicity in patients treated with SBRT during the last sixteen months. Materials and Methods: This retrospective and observational study was performed on prostate cancer patients admitted into our radiotherapy department for SBRT between October 2019 until January 2021. SBRT treatment at our institution has been standardized as an IGRT technique using VMAT radiation therapy due to deliver total doses of 36.25 or 40 Gy in 5 fractions. The following variables were recorded: age, past medical history, previous treatment with anticoagulant or antiplatelet drugs, Performance Status (PS), ACE-27 score, IPSS score, Gleason score, Prostate Volume, Tumor Stage, PSA levels, Risk group following NCCN, Hormonotherapy, Radiation Area, RT Doses as well as different toxicity types depending on Genitourinary (GU) or Gastrointestinal (GI) location. Toxicity review was recorded (after RT, six month and 12 months) according to Common Terminology Criteria for Adverse Events (CTCAE). For the descriptive analysis, analytical variables were represented using absolute and relative frequencies. All statistical analysis were conducted using software from SPSS (version 25.0, Chicago, IL). The hypothesis of our study is SBRT technique will reduce radiotherapy sessions with a low rate of toxicity. Results: A total of 47 patients were included, with a median age of 71 years, performance status 0-1, including high risk prostate cancer (25 patients with Gleason score 6, 17 with score 7, 4 with score 8 and 1 with score 9), all of them without seminal vesicles affected. Related to NCCN risk group, 57.4 % of the patient had very low risk, low risk and favorable intermediate risk and 42.6% were unfavorable intermediate, high and very high risk. Androgen deprivation was associated in 40.4% of the cases with a median of treatment in six months (RI:6-24). Image Guided Radiotherapy was performed by matching of fiducial markers on kvCBCT images that were acquired prior to treatment, for positioning, and during treatment, for intrafraction movement control in 74.5% of the patients. The remaining 25.5% were treated by means of a gating system guided by continuous transperineal ultrasound scanning (Clarity ®system). Acute toxicity grade 1 and 2 after RT was greater at GU (51.1%/21,3%) than GI (12.8%/4.3%) location. During subsequent follow-up no patient developed acute GU or GI toxicity greater than or equal to grade 2. At 6 and 12 months evaluation GU toxicity was greater than GI, 31.7% Vs. 17.1% and 12%Vs.4%, respectively. Conclusion: Evaluation of toxicity during the early phase SBRT has proven to be a well tolerated treatment for prostate cancer in our series.

Second malignancy probabilities in prostate cancer patients treated with SBRT and other contemporary radiation techniques

BackgroundProstate radiotherapy has been associated with an increased risk of developing a second malignancy (SM). However, relative SM probabilities following treatment with contemporary radiation techniques such as stereotactic body radiotherapy (SBRT) or moderately hypofractionated intensity modulated radiotherapy (HF-IMRT) remain unknown. MethodsA cohort analysis was performed of men from a nationally representative database with localized prostate cancer with at least 60 months of follow-up comparing SM probability amongst men receiving either radical prostatectomy (RP), conventionally fractionated intensity-modulated radiotherapy (CF-IMRT), HF-IMRT, brachytherapy (BT), or SBRT, using multivariable logistic models, which were used to generate predicted probabilities. Additionally, propensity score-adjusted pairwise assessments of modalities were performed. ResultsFor 303,432 patients included in the study, median follow-up was 9.08 years (IQR 7.01–11.21). Predicted rates of SM by treatment modality and adjusted odds ratios (AOR) for development of SM (referent: RP) were: 6.0% for RP (AOR n/a), 7.1% for CF-IMRT (AOR 1.20, 95%CI 1.14–1.25, P < 0.001), 7.3% for HF-IMRT (AOR 1.25, 95%CI 1.01–1.55, P = 0.045), 6.6% for BT (AOR 1.11, 95%CI 1.07–1.16, P < 0.001), and 5.7% for SBRT (AOR 0.95, 95%CI 0.81–1.12, P = 0.567). On propensity score-adjusted analysis, SBRT was associated with lower odds of SM compared to CF-IMRT (AOR 0.78, 95%CI 0.66–0.93, P = 0.005); no significant difference was found when SBRT was compared to RP (AOR 0.86, 95%CI 0.73–1.03, P = 0.102). ConclusionsConventionally fractionated intensity-modulated radiotherapy, moderately hypofractionated intensity-modulated radiotherapy, and brachytherapy but not stereotactic body radiotherapy were associated with increased probability of a second malignancy compared to radical prostatectomy. Patients treated with SBRT may be at lower risk of second malignancy due to improved conformality, radiobiological differences or patient selection. The possibility that SBRT in select patients may minimize the probability of SM underscores the need for assessment of second malignancy risk in prospective studies of SBRT.

Pilot study evaluating the feasibility of stereotactic body radiation therapy for canine anal sac adenocarcinomas.

The use of stereotactic body radiation therapy (SBRT) to treat many canine tumors is rapidly expanding. However, published studies are lacking regarding use of SBRT for management of canine anal sac adenocarcinoma (ASAC), primarily due to concerns regarding intolerable late effects. The objective of this retrospective, pilot study was to describe the efficacy and safety profile of coarse fractions administered with an SBRT regime to manage a group of dogs with ASAC. A total of 12 dogs with ASAC that received SBRT as a component of their treatment were sampled. Three patients had macroscopic primary tumors irradiated, while nine patients received SBRT following incomplete surgical resection. Seven patients also received metastatic regional lymph node irradiation. Primary tumor and nodal irradiation sites received three fractions totaling 22-24Gy and 22.5-24Gy, respectively, over three consecutive days. All patients developed acute effects including mild colitis, alopecia, and erythema. Late effects included alopecia, variable dermal pigmentation and leuko- or melanotrichia within radiation fields, and rectal stricture in one patient. A median progression free survival time of 549 days and median survival time of 991 days were achieved in this study. These results should be considered preliminary data suggesting that coarse fractionation administered with an SBRT technique is a safe and effective treatment regime for the management of canine ASAC, with the aim to conduct prospective studies in the future.

SBRT for HCC: Overview of technique and treatment response assessment.

Stereotactic body radiation therapy (SBRT) is an emerging locoregional treatment (LRT) modality used in the management of patients with hepatocellular carcinoma (HCC). The decision to treat HCC with LRT is evaluated in a multidisciplinary setting, and the specific LRT chosen depends on the treatment intent, such as bridge-to-transplant, down-staging to transplant, definitive/curative treatment, and/or palliation, as well as underlying patient clinical factors. Accurate assessment of treatment response is necessary in order to guide clinical management in these patients. Patients who undergo LRT need continuous imaging evaluation to assess treatment response and to evaluate for recurrence. Thus, an accurate understanding of expected post-SBRT imaging findings is critical to avoid misinterpreting normal post-treatment changes as local progression or viable tumor. SBRT-treated HCC demonstrates unique imaging findings that differ from HCC treated with other forms of LRT. In particular, SBRT-treated HCC can demonstrate persistent APHE and washout on short-term follow-up imaging. This brief review summarizes current evidence for the use of SBRT for HCC, including patient population, SBRT technique and procedure, tumor response assessment on contrast-enhanced cross-sectional imaging with expected findings, and pitfalls in treatment response evaluation.

Rotational positional error-corrected linear set-up margin calculation technique for lung stereotactic body radiotherapy in a dual imaging environment of 4-D cone beam CT and ExacTrac stereoscopic imaging.

Accurate calculation of set-up margin is a prerequisite to arrive at the most optimal clinical to planning target volume margin. The aim of this study was to evaluate the compatibility of different on-board and in-room stereoscopic imaging modalities by calculating the set-up margins (SM) in stereotactic body radiotherapy technique accounting and unaccounting for rotational positional errors (PE). Further, we calculated separate SMs one based on residual positional errors and another based on residual + intrafraction positional errors from the imaging data obtained in a dual imaging environment. A total of 22 lung cancer patients were included in this study. For primary image guidance, four-dimensional cone beam computed tomography (4-D CBCT) was used and stereoscopic ExacTrac was used as the auxiliary imaging. Following table position correction (TPC) based on the initial 4-D CBCT, another 4-D CBCT (post-TPC) and a pair of stereoscopic ExacTrac images were obtained. Further, during the treatment delivery, a series of ExacTrac images were acquired to identify the intrafraction PE. If a, b and c were the observed translational shifts in lateral (x-axis), longitudinal (y-axis) and vertical direction (z-axis) and α, β and γ were the rotational shifts in radians about the same axes, respectively, then the resultant translational vectors (A, B and C) were calculated on the basis of translational and rotational values. Set-up margins were calculated using residual errors post-TPC only and also using intrafraction positional errors in addition to the residual errors. Residual and residual + intrafraction SM were calculated from a dataset of 82 CBCTs and 189 ExacTrac imaging sessions. CBCT-based mean ± SD shifts in translational and rotational directions were 0.3 ± 1.8mm, 0.1 ± 1.8mm, - 0.4 ± 1.6mm, 0.1 ± 0.4°, 0.0 ± 1.0° and 0.3 ± 0.7°, respectively, and for ExacTrac - 0.1 ± 1.8mm, 0.2 ± 2.4mm, - 0.6 ± 1.8mm, 0.1 ± 1.2°, - 0.2 ± 1.3° and - 0.1 ± 0.6°, respectively. Residual SM without considering the rotational correction in x, y and z directions were 5.0mm, 4.5mm and 4.4mm; rotation-corrected SM were 4.4mm, 4.0mm and 5.5mm, respectively. Residual plus intrafraction SM were 5.5mm, 6.6mm and 6.2mm without considering the rotational corrections, whereas they were 5.0mm, 6.3mm and 6.2mm with rotational errors accounted for. Accurate calculation of set-up margin is required to find the clinical to planning target volume margin. Primary and auxiliary imaging margins fall in the range of 4.0 to 5.5mm and 5.0 to 7.0mm, respectively, indicating a higher SM for X-ray-based planar imaging techniques over three-dimensional cone beam images. This study established the degree of mutual compatibility between two different kinds of widely used set-up imaging modalities, on-board CBCT and in-room stereoscopic imaging ExacTrac. It also describes the technique to calculate the residual and residual plus intrafraction SM and its variation in a dual imaging environment accounting for rotational PE in stereotactic body radiotherapy of lung.

Pilot study evaluating stereotactic body radiation therapy for feline facial squamous cell carcinomas.

The use of stereotactic body radiation therapy (SBRT) has not been assessed in advanced-stage feline solar-induced facial squamous cell carcinomas (SCCs). The objective of this study was to provide preliminary data on the use and safety profile of coarse fractions administered with an SBRT regime to manage advanced-stage feline solar-induced facial SCCs. This retrospective study assessed five cats diagnosed with advanced-stage solar-induced facial SCCs that received SBRT as their primary treatment or, in one cat, following failed surgical intervention. Tumour sites received three fractions totalling 26.25-27 Gy over a 3-5-day period. All patients developed acute effects following SBRT including alopecia, epilation and erythema. Late effects were mild and included alopecia, variable pigmentation and leukotrichia within radiation fields. All patients were alive at the time of article submission, with overall survival times ranging from 118 to 991 days. The results suggest that coarse fractions administered with an SBRT technique is a safe and effective treatment tool for the management of advanced-stage feline solar-induced facial SCCs. These data provide preliminary evidence to support larger, prospective studies evaluating the management of feline facial SCCs with SBRT.

Dosimetric evaluation of analytic anisotropic algorithm and Acuros XB algorithm using in-house developed heterogeneous thorax phantom and homogeneous slab phantom for stereotactic body radiation therapy technique

To perform patient-specific quality assurance (QA), the accuracy of the dose calculation algorithm is vital, especially in the lung cancer stereotactic body radiation therapy (SBRT). The present study is based on the evaluation of two widely used algorithms, analytical anisotropic algorithm (AAA) and Acuros XB (AXB) inside the in-house developed heterogeneous thorax phantom (HTP) and a homogeneous slab phantom (HSP) simultaneously. To evaluate dosimetric differences between the two algorithms, point dose measurement was performed for pretreatment QA plans of 35 lung cancer patients by keeping the same monitor units and beam angles as those for the actual patient treatment. The dose was calculated on the Eclipse treatment planning system inside both the medium by using both AAA and AXB algorithms. Plans were delivered on the Edge linear accelerator (LA) (Varian Medical Systems, Palo Alto, CA, USA), and measurements were taken by using a 0.01 cc ion chamber and DOSE1 electrometer. Statistical analysis was performed on the observed data set, and percentage (%) variations between the measured and planned doses were calculated and analyzed. The mean % variations between the measured and planned doses inside HTP for all QA plans were found to be 2.61 (standard deviation [SD]: 0.66) and 2.19 (SD: 0.64) for AAA and AXB algorithms, respectively. Whereas, inside HSP, it was found to be 1.79 (SD: 0.74) and 1.64 (SD: 0.70) for AAA and AXB algorithms, respectively. The mean % difference between the measured dose and the planned dose was derived to be statistically significant for HTP, however, it was found to be statistically insignificant inside the HSP at P