Dosimetric Considerations Research Articles

RADT-03. CONTRAST-ENHANCING LESIONS INDUCED BY CNS-DIRECTED INTENSITY MODULATED PROTON THERAPY: DEMOGRAPHIC AND CARDIOVASCULAR RISK FACTORS ANALYSIS

Abstract INTRODUCTION Lesions exhibiting radiation-induced contrast enhancement (RICE) may develop following cranial intensity modulated proton therapy (IMPT). The mechanisms underlying their development may be shared or amplified by existing patient comorbidities. The objective of this study was to assess a potential relationship between individual or composite cardiovascular risk criteria and RICE development, independent of dosimetric considerations. METHODS Data were reviewed for patients treated at a single tertiary care institution between 2017-2023. Time to RICE development was assessed using Kaplan-Meier. Cox regression analysis was used to determine hazard ratios (HRs) with 95% confidence intervals (CIs). Host-related risk factors for RICE development were assessed in a univariable analysis (UVA). A binary logistic regression model was built to ascertain the effects of age and cardiovascular risk factors on the likelihood of developing RICE (expressed as QRISK3 relative risk). RESULTS The study cohort included 137 patients; the median age was 40 years (range: 1-85), 51% female, and 53% Hispanic. Cardiovascular risk factors included smoking (18% past, 6% current), diabetes (7%), hypertension (22%), dyslipidemia (15%) and high body mass index (BMI, 50% were obese with BMI>25). At a median follow-up of 3 years, RICE developed in 36 (26%) patients. On UVA, RICE development was only associated with age ≥18 compared to <18 (HR 3.3, 95% CI: 1.2-9.4, p=0.024). None of the other evaluated risk factors were significantly associated with RICE (p>0.05). Additionally, logistic regression analysis showed increasing age was associated with likelihood of developing RICE (χ2=6.95, p=0.031), while QRISK3 score was not. CONCLUSIONS In this contemporary series of patients treated with IMPT, age was the only host factor associated with RICE, while other cardiovascular risk factors, evaluated individually or using established composite risk scores (QRISK3) were not associated with RICE. Additional research is needed to identify potential patient characteristics associated with RICE.

Dosimetric evaluation and treatment planning considerations for GammaTile permanent brain implants - a pilot, institutional experience.

GammaTile® (GT) is a brachytherapy platform that received Federal Drug Administration (FDA) approval as brain tumor therapy in late 2018. Here, we reviewed our institutional experience with GT as treatment for recurrent glioblastomas and characterized dosimetric parameter and associated clinical outcome. A total of 20 consecutive patients with 21 (n = 21) diagnosis of recurrent glioblastoma underwent resection followed by intraoperative GT implant between 01/2019 and 12/2020. Data on gross tumor volume (GTV), number of GT units implanted, dose coverage for the high-risk clinical target volume (HR-CTV), measured by D90 or dose received by 90% of the HR-CTV, dose to organs at risk, and six months local control were collected. The median D90 to HR-CTV was 56.0Gy (31.7-98.7Gy). The brainstem, optic chiasm, ipsilateral optic nerve, and ipsilateral hippocampus median Dmax were 11.2, 5.4, 6.4, and 10.0Gy, respectively. None of the patients in this study cohort suffered from radiation necrosis or adverse events attributable to the GT. Correlation was found between pre-op GTV, the volume of the resection cavity, and the number of GT units implanted. Of the resection cavities, 7/21 (33%) of the cavity experienced shrinkage, 3/21 (14%) remained stable, and 11/21 (52%) of the cavities expanded on the 3-months post-resection/GT implant MRIs. D90 to HR-CTV was found to be associated with local recurrence at 6-month post GT implant, suggesting a dose response relationship (p = 0.026). The median local recurrence-free survival was 366.5days (64-1,098days), and a trend towards improved local recurrence-free survival was seen in patients with D90 to HR-CTV ≥ 56Gy (p = 0.048). Our pilot, institutional experience provides clinical outcome, dosimetric considerations, and offer technical guidance in the clinical implementation of GT brachytherapy.

Simulated pharmacokinetics of inhaled caffeine and melatonin from existing products indicate the lack of dosimetric considerations

Numerous commercially available inhalable products claim to improve sleep-wake cycle-related target indications by delivering a wide variety of chemicals like caffeine and melatonin. The resulting exposure-responses from inhaling different doses are unknown and obtaining early understanding of resulting pharmacokinetics is beneficial. This study applied a physiologically based pharmacokinetic modeling approach to predict the inhalation pharmacokinetics of caffeine and melatonin for different target indications related to the sleep-wake cycle. The model predicted rapid systemic delivery of caffeine and melatonin based on airway regional deposition of inhaled aerosol. A low inhaled dose of 1 mg of caffeine resulted in a 72.3-times lower plasma maximal concentration and was predicted to not improve cognitive performance task outcomes compared to oral consumption of coffee containing 80 mg of caffeine. Conversely, 2-mg oral and inhaled doses of melatonin under recommended directions of use result in more than 25.1- and 645-times higher plasma concentrations compared to endogenous melatonin, respectively. The recommended doses for inhalation products for potential improvement in the target indications vary widely. Additional research is needed to evaluate the human pharmacokinetics, efficacy, and safety of inhaled products. Given the lack of assessments, inhaled caffeine and melatonin must be consumed with caution as the toxicological concerns are not known and could outweigh the potential beneficial effects.

Radiation-Induced Sensorineural Hearing Loss and Potential Management

The cochlear apparatus is one of the major organs at risk when considering radiation therapy (RT) for brain, head, and neck tumors. Radiation oncologists currently consider mean dose constraints of <35 Gy for conventionally fractioned radiation therapy (RT), <4 Gy for single fraction stereotactic radiosurgery, and <17.1 or 25 Gy for 3- or 5-fraction stereotactic radiosurgery, respectively, as the standard of care. Indeed, dose adjustments are made in the setting of concurrent platinum-based chemotherapy or when prioritizing tumor coverage during treatment planning. Despite guidelines, in many patients, RT to the cochlea may still cause sensorineural hearing loss through progressive degeneration and ossification of the inner ear. There are several audiologic and otolaryngologic interventions for incident RT-induced hearing loss, including hearing aids, cochlear implants, or, in the context of vestibular schwannoma due to neurofibromatosis type 2, auditory brain stem implantation. Cochlear implants are the most effective at restoring hearing and improving quality of life for those with an intact cochlear nerve. An early multidisciplinary approach is essential to optimally manage RT-induced hearing loss, and this topic discussion serves as a guide for radiation oncologists on cochlear dosimetric considerations as well as how to address potential RT-induced adverse effects.

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

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. For this purpose, 54different lung and liver lesions from 44patients 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 6MV, 6MV-FFF, 10MV, and 10MV-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. Asignificant 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. We found that FFF beams together with VMAT represent an excellent combination for SBRT of lung or liver lesions with shortest BOT for 10MV-FFF but significant dose savings for 6MV-FFF in lung lesions.

Stereotactic Radiosurgery of Multiple Brain Metastases: A Review of Treatment Techniques.

The advancement of systemic targeted treatments has led to improvements in the management of metastatic disease, particularly in terms of survival outcomes. However, brain metastases remain less responsive to systemic therapies, underscoring the significance of local interventions for comprehensive disease control. Over the past years, the threshold for treating brain metastases through stereotactic radiosurgery has risen. Yet, as the number of treated metastases increases, treatment complexity and duration also escalate. This trend has made multi-isocenter radiosurgery treatments, such as those with the Gamma Knife, challenging to plan and lengthy for patients. In contrast, single-isocenter approaches employing linear accelerators offer an efficient and expeditious treatment option. This review delves into the literature, comparing different linear-accelerator-based techniques with each other and in relation to dedicated systems, focusing on dosimetric considerations and feasibility.

Commissioning of Aktina SRS cones and dosimetric validation of the RayStation photon Monte Carlo dose calculation algorithm

Clinical implementation of SRS cones demands particular experimental care and dosimetric considerations in order to deliver precise and safe radiotherapy to patients. The purpose of this work was to present the commissioning data of recent Aktina cones combined with a 6MV flattened beam produced by an Elekta VersaHD linear accelerator. Additionally, the modelling process, and an assessment of dosimetric accuracy of the RayStation Monte Carlo dose calculation algorithm for cone based SRS was performed. There are currently no studies presenting beam data for this equipment and none that outlines the modelling parameters and validation of dose calculation using RayStation’s photon Monte Carlo dose engine with cones. Beam data was measured using an SFD and a microDiamond and benchmarked against EBT3 film for cones of diameter 5–39 mm. Modelling was completed and validated within homogeneous and heterogeneous phantoms. End-to-end image-guided validation was performed using a StereoPHAN™ housing, an SRS MapCHECK and EBT3 film, and calculation time was investigated as a function of statistical uncertainty and field diameter. The TPS calculations agreed with measured data within their estimated uncertainties and clinical treatment plans could be calculated in under a minute. The data presented serves as a reference for others commissioning Aktina stereotactic cones and the modelling parameters serve similarly, while providing a starting point for those commissioning the same TPS algorithm for use with cones. It has been shown in this work that RayStation’s Monte Carlo photon dose algorithm performs satisfactorily in the presence of SRS cones.

Optimal Combination of Pitch, Modulation Factor and Dosimetric Considerations in Treatment Planning for Total Body Irradiation Using Helical Tomotherapy.

The aim of this study is to makethe standard total body irradiation (TBI) protocol for Helical tomotherapy© (HT) and to analyze the optimal pitch and modulation factor (MF) with respect to dose homogeneity index (HI), target dose coverage, target overdose, beam on time (BOT) and mean lung dose. Ten patients who underwent high-dose TBI were taken for this study. For each patient, 35 dose plans were created by different combination of pitch and MF. The optimal pitch and MF were deduced using scatter plot and regression methodology based on target coverage, HI, target volume receiving 103%(V103%), 105%(V105%) and 107% (V107%) of the prescription dose and BOT. Using these optimal pitch and MF, the final dose plan was made and the planning aim and achieved dose was compared using two tailed student's t-test. Radiochromic films and ionization chambers were used to measure the delivered dose using anthropomorphic phantom on various points for the head and pelvis regions to verify the skin flash margin and its effect on skin dose. The optimal pitch and MF value were 0.287 and 2.4 respectively. Based on optimal pitch and MF, the mean BOT was 1692 seconds with optimal inhomogeneity (7.4%). For target, D95 and D98 were 97.09% (range:94.7-99.6%, p=0.002) and 93.9% (range:91.5-94.4%,p=0.007) respectively, and mean D2 was within 107% with SD of ±1.22% (p=0.04). The mean of PTV receiving V103, V105 and V107 was 24.48% (range=7.7-36.6%, p=0.03), 5.76% (range=1.4-12.1%, SD=±3.3%), 1.93% (range=0.1-4.6%, p=0.008) respectively. Our measurements show that the flash margin did not increase the skin dose. In our study, the optimal combination of pitch value of 0.287 and MF value of 2.4 provided acceptable plans for all patients planned for TBI in HT. The flash margin can provide adequate coverage during patient position uncertainty without increasing the skin dose.

Interstitial Brachytherapy for Lip Cancer: Technical Aspects to Individualize Treatment Approach and Optimize Outcomes

Primary radiation therapy using interstitial brachytherapy (IBT) provides excellent local tumor control for early-stage squamous cell carcinoma of the lip. Technical aspects of treatment are important to optimize outcomes. In this report, we discuss patient selection criteria, procedural details, and dosimetric considerations for performing IBT for cancers of the lip. Catheters are inserted across the length of tumor entering and exiting approximately 5 mm beyond the palpable tumor extent. A custom mouthpiece is fabricated to facilitate normal tissue sparing. Patients undergo computed tomography imaging, the gross tumor volume is contoured based on physical examination and computed tomography findings, and an individualized brachytherapy plan is generated with the goals of achieving gross tumor volume D90% ≥ 90% and minimizing V150%. Ten patients with primary (n = 8) or recurrent (n = 2) cancers of the lip who received high-dose-rate lip IBT using 2.0- to 2.5-week treatment regimens are described (median prescription: 47.6 Gy in 14 fractions of 3.4 Gy). Local tumor control was 100%. There were no cases of acute grade ≥4 or late grade ≥2 toxicity, and cosmesis scores were graded as good to excellent in all patients. IBT represents an excellent treatment option for patients with lip squamous cell carcinoma. With careful attention to technical considerations furthered described in the present report, high rates of tumor control, low rates of toxicity, and favorable esthetic and functional outcomes can be achieved with IBT for lip cancer.

Accelerated partial breast irradiation in early stage breast cancer.

Accelerated partial breast irradiation (APBI) is increasingly used to treat select patients with early stage breast cancer. However, radiation technique, dose and fractionation as well as eligibility criteria differ between studies. This has led to controversy surrounding appropriate patients for APBI and an assessment of the toxicity and cosmetic outcomes of APBI as compared to whole breast irradiation (WBI). This paper reviews existing data for APBI, APBI delivery at our institution, and ongoing research to better define patient selection, treatment delivery, dosimetric considerations and toxicity outcomes.

PO44 Presentation Time: 4:45 PM: HDR Intracavitary Brachytherapy in the Management of Recurrent VaIN3: A Case Report and Review of the Literature

<h3>Purpose</h3> Vaginal intraepithelial neoplasia, or VaIN, is a pre-cancerous condition in the cell of the vagina. It has an estimated occurrence of 0.2-0.3 cases per 100,000 women in the U.S. It is classified into three groups based on histologic characteristics: low-grade (VaIN 1), intermediate-grade (VaIN 2), and high-grade (Vain 3). High-grade lesions includes vaginal carcinoma-in-situ and are known as a pre-malignant lesion most likely to progress to vaginal cancer. Risk factors include HPV, cigarette smoking and immunodeficiency. VaIN 1 are usually treated with close observation; VaIN 2 and 3 usually require prompt treatments including excision (wide local excision, partial or total vaginectomy), ablation (CO2 laser, ultrasonic surgical aspiration), topical therapy (imiquimod, fluorouracil), and radiation therapy. Yet no standard treatment exists. Given that it is a rare pre-malignant condition, there have been limited reports on the use of intracavitary brachytherapy. Proper dose prescription remains challenging, especially for patients with history of hysterectomy. <h3>Materials and methods</h3> We report a case of a 70 year-old female with history of hysterectomy 15 years ago for endometriosis and recurrent diffuse VaIN3 that was refractory to multiple lines of local therapy who received HDR cylinder brachytherapy. The report includes clinical and diagnostic studies, dosimetric considerations in treatment planning for improved target coverage, and a review of the limited existing literature of HDR brachytherapy for VaIN3. <h3>Results</h3> HDR, MDR and LDR had all been utilized to treat VaIN with comparable tumor control and toxicity. Reported acute side effects include vaginitis, urethritis, and proctitis; whereas late side effects include vaginal atrophy/fibrosis/telangiectasia/stenosis. What made this case especially challenging is her past surgical history of hysterectomy. The vaginal vault is difficult to assess after hysterectomy with potential abnormal cells sequestered above the suture line, and is thus prone to underdosing. Therefore some institutions prefer to use ovoids rather than cylinder to adequately cover the upper vagina; whereas others argue that cylinder is better for treating extensive lesions by providing a uniform coverage of the entire vagina despite higher toxicity. Upper vagina is also noted to tolerate a higher radiation dose compared to lower vagina. Doses are often prescribed to different depths based on location in the vagina, or on doses to the OARs. Our patient was treated with a dose of 30Gy in 5 fractions given twice weekly, prescribed to 3mm depth for the proximal 3cm vagina and to surface for the remaining 5cm vagina. She had complete clinical response and was encouraged to continue vaginal dilator use to prevent vaginal stenosis and regular follow-ups with pelvic exams. <h3>Conclusions</h3> HDR brachytherapy is a feasible and effective alternative approach to treat patients who are non-surgical candidates, have recurrent disease or disease refractory to previous treatments, or have extensive multifocal VaIN. Our case would be a valuable addition to the existing limited literature regarding the treatment efficacy and proper dose prescription for this rare condition.

PHSPP03 Presentation Time: 2:50 PM: Dosimetric Considerations of the Use of Synthetic MRI for Prostate HDR Brachytherapy

<h3>Purpose</h3> In prostate HDR brachytherapy, MRI, while providing exquisite soft-tissue contrast and anatomic delineation, its acquisition slows the treatment workflow and the image registration between CT and MRI adds uncertainty to the treatment planning process. Deep learning-based MRI synthesis from CT is possible but has yet to be clinically validated. To address this, we assessed the dosimetric and workflow impact of synthetic MRI (sMRI) use in prostate HDR brachytherapy. <h3>Materials and Methods</h3> 58 paired pelvis CT and T2-weighted MRI datasets from patients treated with HDR brachytherapy to either a prescription dose of 13.5 Gy or 15 Gy were collected. A deep learning-based framework was developed to synthesize MRI data from CT and was validated using 17 additional patient datasets. The prostate was contoured on the sMRI, which was used to create a new planning target volume (PTV) with a 3 mm isotropic expansion and removal of the urethra, bladder, and rectum. New brachytherapy treatment plans were generated to cover the sMRI PTV with the catheter positions visible in the CT scans by adjusting the source dwell times from the original treatments. This dose was overlaid onto a second PTV, created from the prostate contour by the same observer on real MRI (rMRI). This observer is different from the one who contoured the prostate for the clinically delivered treatment, so the inter-observer variability of the prostate contour on rMRI can be compared with the inter-modality variability between the prostate contour on sMRI and rMRI using the dice similarity coefficient (DSC). The impact of sMRI use was assessed with dosimetric volume histogram (DVH) parameters PTV V100%, V150%, V200%, bladder D1cc, urethra D1cc, and rectum D1cc (percent of prescription dose averaged over 17 test cases ± standard deviation, institutional constraints based on ABS prostate high-dose rate task group). The sMRI PTV DVH metrics were compared with the rMRI DVH metrics, such that we assessed how delivering dose to the sMRI PTV would cover the ground truth rMRI PTV. The bladder, rectum, and urethra DVH metrics were compared with those of the original treatment. The workflow impact was assessed by comparison of the time for sMRI generation with the time from the end of the CT acquisition to the end of the MRI acquisition during the clinically delivered plan. <h3>Results</h3> The inter-modality variability of the prostate contour between sMRI and rMRI was found to be statistically similar to the inter-observer variability of the prostate contour on rMRI assessed with DSC, indicating that the prostate contour is similarly sensitive to observer as it is to sMRI (table a). Target coverage of sMRI PTV was not statistically different to that of rMRI PTV (table b). While there were statistical increases (p<0.05) in bladder, rectum, and urethra doses in the sMRI-planned cases (table c), in no case did this lead to an increase above the constraint for D1cc. Lastly, a full sMRI volume can be generated from the corresponding CT in approximately 10 seconds, compared to the 73.6 minutes on average between the completion of the CT and rMRI scans during the clinically delivered treatment workflow, not including image registration which will add further time to the procedure. <h3>Conclusions</h3> To our knowledge, this is the first report which details the dosimetric similarity and workflow advantages from the use of synthetic MRI for prostate HDR brachytherapy.

AAPM Task Group Report 290: Respiratory motion management for particle therapy.

Dose uncertainty induced by respiratory motion remains a major concern for treating thoracic and abdominal lesions using particle beams. This Task Group report reviews the impact of tumor motion and dosimetric considerations in particle radiotherapy, current motion‐management techniques, and limitations for different particle‐beam delivery modes (i.e., passive scattering, uniform scanning, and pencil‐beam scanning). Furthermore, the report provides guidance and risk analysis for quality assurance of the motion‐management procedures to ensure consistency and accuracy, and discusses future development and emerging motion‐management strategies. This report supplements previously published AAPM report TG76, and considers aspects of motion management that are crucial to the accurate and safe delivery of particle‐beam therapy. To that end, this report produces general recommendations for commissioning and facility‐specific dosimetric characterization, motion assessment, treatment planning, active and passive motion‐management techniques, image guidance and related decision‐making, monitoring throughout therapy, and recommendations for vendors. Key among these recommendations are that: (1) facilities should perform thorough planning studies (using retrospective data) and develop standard operating procedures that address all aspects of therapy for any treatment site involving respiratory motion; (2) a risk‐based methodology should be adopted for quality management and ongoing process improvement.

The EANM guideline for radiosynoviorthesis

PurposeRadiosynoviorthesis (RSO) using the intraarticular application of beta-particle emitting radiocolloids has for decades been used for the local treatment of inflammatory joint diseases. The injected radiopharmaceuticals are phagocytized by the superficial macrophages of the synovial membrane, resulting in sclerosis and fibrosis of the formerly inflamed tissue, finally leading to reduced joint effusion and alleviation of joint pain.MethodsThe European Association of Nuclear Medicine (EANM) has written and approved these guidelines in tight collaboration with an international team of clinical experts, including rheumatologists. Besides clinical and procedural aspects, different national legislative issues, dosimetric considerations, possible complications, and side effects are addressed.ConclusionThese guidelines will assist nuclear medicine physicians in performing radiosynoviorthesis. Since there are differences regarding the radiopharmaceuticals approved for RSO and the official indications between several European countries, this guideline can only give a framework that must be adopted individually.

The Path to an Evidence-Based Treatment Protocol for Extraoral Photobiomodulation Therapy for the Prevention of Oral Mucositis.

Oral mucositis is a painful complication of hematopoietic stem cell transplantation for which photobiomodulation therapy (PBMT) is a safe and effective intervention. Extraoral delivery of PBMT has clinical advantages over intraoral delivery but requires additional dosimetric considerations due to the external tissue layers through which the light must propagate before reaching the oral mucosa. Additionally, to date there has been no dose modeling study, a task essential to developing a justified treatment protocol. We review here some of the complexities surrounding extraoral photobiomodulation therapy and offer that may help guide researchers toward an evidence-based treatment protocol for the prevention of oral mucositis.

Clinical and technical considerations for mediastinal Hodgkin lymphoma protontherapy based on a single-center early experience

PurposeProtontherapy for mediastinal Hodgkin lymphoma reduces cardiac, lung and breast exposure, which may limit radiation-induced adverse events. While this technique is already widely implemented in the United-States, clinical experience is still limited in France. This study analyses the practice of mediastinal Hodgkin lymphoma protontherapy at the Institut Curie to implement this technique at a larger scale. Materials and MethodsData from all mediastinal Hodgkin lymphoma patients from the hematology department of the Institut Curie who were subsequently evaluated at the Protontherapy Center of Orsay (CPO) of Institut Curie for adjuvant protontherapy were retrieved. We analyzed why these patients were ultimately treated with protontherapy or not. ResultsBetween January 2018 and January 2021, twenty mediastinal Hodgkin lymphoma patients from the hematology department of the Institut Curie have been screened for protontherapy at the CPO. Four of them (20%) were ultimately treated with proton beams. Treatment was well tolerated without grade 3–4 adverse events. With a median follow-up of two years, none of these patients relapsed. The others sixteen patients were not treated with protontherapy due to multiple reasons including: lack of treatment room disponibility, accessibility difficulties, psychiatric disorder, and anatomic or dosimetric considerations. ConclusionDespite notable dosimetric superiority over photon radiotherapy and excellent clinical tolerance, lack of availability of protontherapy facilities limit implementation of mediastinal Hodgkin lymphoma protontherapy. Additionally, strict selection criteria must be defined.

Selective internal radiation therapy in the management of primary and metastatic disease in the liver.

Selective internal radiation therapy is a type of brachytherapy used to provide targeted radiotherapy, most commonly to treat primary or metastatic disease within the liver. This review outlines current clinical practice, dosimetric considerations, the pre-treatment workup and safety considerations before treatment. It also examines the clinical evidence for its use in patients with both primary and metastatic disease within the liver.

Dosimetric considerations of 99mTc-MDP uptake within the epiphyseal plates of the long bones of pediatric patients

Skeletal scintigraphy is most performed in pediatric patients using the radiopharmaceutical 99mTc labelled methylene diphosphonate (99mTc-MDP). Reference biokinetic models for 99mTc-MDP indicate 50% of the administered activity is uniformly localized to the interior bone surfaces (trabecular and cortical regions), yet imaging data clearly show some preferential uptake to the epiphyseal growth plates of the long bones. To explore the dosimetric consequences of these regional activity concentrations, we have modified mesh-type computational phantoms of the International Commission on Radiological Protection (ICRP) reference pediatric series to explicitly include geometric models of the epiphyseal growth plates (2 mm in thickness) within the left/right, distal/proximal ends of the humeri, radii, ulnae, femora, tibia, and fibulae. Bone mineral activity from the ICRP Publication 128 biokinetic model for 99mTc-MDP (ICRP ) was then partitioned to the growth plates at values of 0.5%, 4.4%, 8.3%, 12.2%, 16.1%, and 20%. Radiation transport simulations were performed to compute 99mTc S-values and organ dose coefficients to the soft tissues and to bone site-specific regions of spongiosa. As the percentage of bone activity assigned to the growth plates was increased (from 0.5% to 20%), absorbed doses to the soft tissue organs, active bone marrow, bone endosteum (BE), as well as the detriment-weighted dose, were shown to decrease from their nominal values (no substantial growth plate activity), while epiphyseal plate self-doses increased. In the 15 year old male phantom, moving from 0.5% to 20% relative bone activity within the epiphyseal plates resulted in a 15% reduction in active marrow (AM) and BE dose, a 10% reduction in mean soft tissue and detriment-weighted dose, and a 6.3-fold increase in epiphyseal plate self-dose. In the newborn female phantom, we observed a 18% decrease in AM and BE dose, a 10% decrease in mean soft tissue dose, a 15% decrease in detriment-weighted dose, and 12.8-fold increase in epiphyseal plate self-dose. Increases (to 3 mm) and decreases (to 1 mm) in the assumed growth plate thickness of our models were shown to impact only the growth plate self-dose. Future work in differential quantification of 99mTc-MDP activity—growth plates versus other bone surfaces—is required to provide clinically realistic data on activity partitioning as a function of patient age, and perhaps skeletal site. The phantom series presented here may be used to develop more optimized age-related guidance on 99mTc-MDP administered activities to children.

Institut Curie guidelines on breast target volume delineation for patients treated in lateral position

PurposeTo develop guidelines for and describe the delineation of breast for patients treated in lateral position and to transform this three-dimensional technique based on the virtual simulation to volume-based modern intensity-modulated irradiation technique. Material and methodsIn our department, during the daily delineation, radiation oncologists specialized in breast cancer treatment sought consensus on the delineation of clinical treatment volume of the breast through dialogue based on cases. A radiation oncologist delineated clinical treatment volumes on CT scans of five to 20 patients, followed by a discussion and adaptation of the delineation between all radiation oncologists of the team. The consensus established between clinicians was discussed, corrected and improved. All patients were delineated in treatment position; skin markers were used to visualize the breast tissue after careful palpation. ResultsBreast clinical treatment volume was situated and delineated between pectoral muscle and 5mm below the skin (dosimetric considerations), within the space outlined by skin markers, that showed the limits of the palpable breast tissue. In lateral position some vessels were very useful to define the limits as rami mammarii (from thoracica interna) for the internal one and thoracica lateralis for the external. This is the first atlas proposed for the delineation of the breast clinical treatment volumes for breast cancer using alternative technique of breast irradiation (lateral). ConclusionThis atlas will be helpful for the volume definition in our daily practice of breast irradiation in lateral position and can open perspectives to develop also atlases for other alternative techniques as treatment in prone position.

On-line adaptive MR guided radiotherapy for locally advanced pancreatic cancer: Clinical and dosimetric considerations.

Magnetic Resonance-guided Radiation Therapy (MRgRT) allows online adaptations (OA) of the treatment plan to optimize daily dose distribution based on patient's anatomy, just before fraction delivery. The aim of this study is to evaluate feasibility and the dosimetric improvement of the OA workflow implemented in our institution for locally advanced pancreatic cancer (LAPC) patients, in terms of target coverage and organs at risk (OARs) sparing. We retrospectively analysed 8 LAPC patients treated with MRgRT in combination with the OA approach, using video-assisted inspiratory breath-hold for a total of 38 fractions with a dose ranging from 30 Gy to 40 Gy in 5 fractions.Dose distribution of the baseline plan was first calculated based on daily anatomy, obtaining a "predicted" plan to assess the dosimetric improvement. If the dose distribution did not meet the constraints set in the planning phase, PTV, GTV and OARs were re-contoured within a distance of 3 cm from the PTV external edge and a new online "adaptive" plan was generated. Other clinical and planning parameters were also evaluated to assess the feasibility and the dosimetic benefit of the online adaptive workflow. Out of 38 total fractions, 26 (68.4%) were adapted online and 12 (31.6%) were delivered using the baseline plan. The use of the adaptive workflow resulted to be feasible in our clinical practice and advantageous in all the patients: mean PTV V95% increased by 10.8% (5.7-20.8) while mean CTV V98% of 12.6% (7.3-17.7). Also OARs V33 and V25 showed a positive trend avoiding unnecessary irradiation. OA workflow improves the dosimetric benefit of MRgRT, preventing the occurrence of high-doses to OARs and increasing the safety of stereotactic treatment for LAPC, without any drawback for our daily clinical practice routine.