Skin Dose Reduction Research Articles

Decreasing Skin Toxicity in Whole Breast Irradiation with Helical Tomotherapy IMRT

To decrease skin toxicity resulting from whole breast irradiation with Helical Tomotherapy IMRT. Patients treated with the Helical Tomotherapy IMRT planning yielded an average mean skin dose and maximum skin dose of about 85% and 102% of the prescribed dose respectively resulting in brisk erythema. Target volumes and dose parameters were in accordance to the NSABP B51 protocol guidelines using a breast PTV cropped 5mm from the skin surface. A unique treatment planning technique was designed to decrease the skin toxicity while delivering the desired dose to the breast PTV. We started by determining a tolerable skin dose while not compromising breast PTV coverage. This was determined by evaluating the skin dose of patients treated via conventional tangent fields using similar dose prescription. The conventional tangent fields yielded an average mean skin dose and maximum skin dose of about 70% and 100% of the prescribed dose respectively. These patients had G1 skin toxicity. The next step was to design a helical tomotherapy IMRT treatment planning technique that would deliver a comparable tolerable mean skin dose to the conventional tangent fields. This technique was implemented by utilizing a 3mm skin rind limiting the dose to a maximum point ≤ 95% and a mean of 70%-75% of the prescribed dose. Treatment plans were generated using the 3mm skin rind as an OAR and a Breast PTV at 6mm, 7mm and 8mm from the skin surface to mimic the coverage seen in the conventional tangential fields. The findings for the 3 groups were as followed: 6mm: The Breast PTV coverage was good with above 90% and 97.7% of the volume receiving 100% and 95% of the prescribed dose respectively. Only 1.5% or 17.32cc of the volume received 105% of the prescribed dose. The skin rind received a mean dose of 69.4% and a maximum dose of 90.1%. 7mm: The Breast PTV coverage was at 88.4% and 96.2% of the volume receiving 100% and 95% of the prescribed dose respectively. Only 1.6% or 19.88cc of the volume received 105% of the prescribed dose. The skin rind received a mean dose of 67.5% and a maximum dose of 86.6%. 8mm: The Breast PTV coverage was good with above 79.9% and 90% of the volume receiving 100% and 95% of the prescribed dose respectively. Only 2.2% or 26.16cc of the volume received 105% of the prescribed dose. The skin rind received a mean dose of 64.3% and a maximum dose of 79.6%. 5/7patients treated prior to this technique experienced G2 toxicity while none of the 13 patients treated with technique developed greater than G1 toxicity. With a Breast PTV cropped 6mm from skin surface a mean dose of 75% and a maximum dose of 95% was achieved in these 13 patients CONCLUSION: Adding the skin rind as an OAR allowed a reduction in skin dose without compromising coverage of the breast PTV with a breast PTV cropped 6mm from the skin surface.

PO-1852 Novel Multi Jet Fusion 3D Printed Immobilizers Allowing Significant Reduction of Skin Dose

PO-1852 Novel Multi Jet Fusion 3D Printed Immobilizers Allowing Significant Reduction of Skin Dose

Reduction of the skin‑effect dose of IMRT plan for patients with cancer in pelvic region.

In the present study, it was aimed to investigate the optimized plan of radiotherapy with dose modulation in the pelvis to reduce the dose on the skin in patients having pelvic region radiotherapy. The series of images of 45 pelvic cancer patients were selected, intensity-modulated radiation therapy (IMRT) plan was made, the skin dose reduction was optimized, and evaluated verifying the plan verification. As a result, skin volume receiving dose ≥10, ≥20, ≥30, ≥40 and ≥50 Gy of the IMRT Skin plan were all less than those of the IMRT plan. Particularly, skin volumes receiving doses ≥20, ≥30, ≥40 and ≥50 Gy of the Skin IMRT plan were markedly lower than those of the IMRT plan, the reduction values were 8.76, 18.83, 46.84 and 100%, respectively. Furthermore, the Skin IMRT plan was no longer affected by the 50 Gy dose. In conclusion, the present study revealed that the skin's dose can be decreased with optimal plan processing; thus, this decrease of the skin's dose ensures the continuation of radiotherapy and improved life quality of the patient.

IMPT of head and neck cancer: unsupervised machine learning treatment planning strategy for reducing radiation dermatitis

Radiation dermatitis is a major concern in intensity modulated proton therapy (IMPT) for head and neck cancer (HNC) despite its demonstrated superiority over contemporary photon radiotherapy. In this study, dose surface histogram data extracted from forty-four patients of HNC treated with IMPT was used to predict the normal tissue complication probability (NTCP) of skin. Grades of NTCP-skin were clustered using the K-means clustering unsupervised machine learning (ML) algorithm. A new skin-sparing IMPT (IMPT-SS) planning strategy was developed with three major changes and prospectively implemented in twenty HNC patients. Across skin surfaces exposed from 10 (S10) to 70 (S70) GyRBE, the skin's NTCP demonstrated the strongest associations with S50 and S40 GyRBE (0.95 and 0.94). The increase in the NTCP of skin per unit GyRBE is 0.568 for skin exposed to 50 GyRBE as compared to 0.418 for 40 GyRBE. Three distinct clusters were formed, with 41% of patients in G1, 32% in G2, and 27% in G3. The average (± SD) generalised equivalent uniform dose for G1, G2, and G3 clusters was 26.54 ± 6.75, 38.73 ± 1.80, and 45.67 ± 2.20 GyRBE. The corresponding NTCP (%) were 4.97 ± 5.12, 48.12 ± 12.72 and 87.28 ± 7.73 respectively. In comparison to IMPT, new IMPT-SS plans significantly (P < 0.01) reduced SX GyRBE, gEUD, and associated NTCP-skin while maintaining identical dose volume indices for target and other organs at risk. The mean NTCP-skin value for IMPT-SS was 34% lower than that of IMPT. The dose to skin in patients treated prospectively for HNC was reduced by including gEUD for an acceptable radiation dermatitis determined from the local patient population using an unsupervised MLA in the spot map optimization of a new IMPT planning technique. However, the clinical finding of acute skin toxicity must also be related to the observed reduction in skin dose.

Proton Beam Therapy for Pancreas Cancer: PTCOG Consensus Recommendations for Simulation, Treatment Planning and Treatment Delivery

<h3>Purpose/Objective(s)</h3> The proximity of gastrointestinal (GI) luminal organs-at-risk (OAR) poses a challenge to radiotherapy (RT) for pancreas cancer (PC). Proton beam therapy (PBT) may reduce toxicities compared to photon therapy by avoiding radiation exposure to large volumes of the abdomen while enabling safe dose escalation. The purpose of this study is to create recommendations for the safe treatment of PC with PBT. <h3>Materials/Methods</h3> The Particle Therapy Co-Operative Group (PTCOG) Gastrointestinal Subcommittee consisting of radiation oncologists and medical physicists with extensive clinical experience using PBT for GI cancers developed expert consensus recommendations for PC describing simulation, treatment planning, and treatment delivery through their collective experience and systematic review of the literature. <h3>Results</h3> The following general recommendations were established: <i>Simulation</i>: Indexed immobilization is recommended to reduce patient setup variability. Patients should be supine with arms above the head. Additional imaging such as PET/CT and/or MRI should be considered for target delineation. Contrast CT is recommended but should be performed subsequent to the planning CT at simulation to avoid the need to override Hounsfield Units. Dual-energy CT should be used, if available, to reduce CT-related proton range uncertainties. Motion management strategies are imperative, including 4DCT for motion accounting and breath-hold, or other techniques for motion mitigation. Patients should also have an empty stomach. <i>Treatment planning</i>: Beam angle selection is critical. Posterior and lateral beam angles are preferred to minimize the dosimetric impact of variable anatomy such as bowel. Robust optimization should be used for pencil beam scanning (PBS) techniques. When treating with PBS, single-field optimization (SFO) is often preferrable to increase robustness. If OAR constraints cannot be met with SFO, then multi-field optimization may be used. Higher linear energy transfer (LET) at the end of range should be considered in the context of sparing distal OARs. Increased beam angle separation should be used to reduce skin dose and mitigate the impact of higher LET. <i>Treatment delivery</i>: Volumetric image guidance, where available, should be used daily. Verification CT scans should be acquired <i>ad-hoc</i> as deemed necessary, or at least every other week, to assess the dosimetric impact of changes in anatomy, such as weight changes and variation in bowel gas and gastric filling. Adaptive re-planning should be performed as required. <h3>Conclusion</h3> The PTCOG Gastrointestinal Subcommittee has developed PBT expert consensus recommendations for PC. These recommendations enable PBT facilities to develop protocols for safe dose escalation studies.

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Skin Sparing in Intensity-Modulated Radiation Therapy of Nasopharyngeal Carcinoma.

Radiation therapy of nasopharyngeal carcinomas (NPCs) involves high doses to the target structures which are superficial to the skin surfaces. As a result, the skin toxicities involved are higher and sometimes worsens to such an extent that radiotherapy needs to be interrupted unplanned. This leads to a break in radiation therapy which overall affects the local control and cure rates. The aim of this study is to decrease the skin dose by contouring skin as an organ at risk (OAR) to include in inverse planning calculation. Seventy-three cases of nasopharyngeal cancers were planned for 60 Gy to intermediate-risk planning target volume (PTVIntermediate) and 70 Gy to high risk (PTVHigh), by three different modes of Intensity-modulated radiation therapy (IMRT)- namely conventional sequential intensity-modulated radiation therapy (S-IMRT PH-I and PH-II), Skin Spared sequential intensity-modulated radiation therapy (SS-IMRT PH-I and PH-II), and Skin Spared simultaneously instantaneous boost intensity-modulated radiation therapy (SS-SIB IMRT). The plans were compared by dose volume histograms and dose statistics to the PTV as well as to the OAR's. For PTV, mean dose (Dmean), maximum dose (Dmax), and minimum dose (Dmin) were compared to check the homogeneity index (HI) while sparing the skin. For other OAR's Dmean, Dmax and dose to to 1 cubic cm was used for comparison. The skin doses to various volumes from volume to receive 5 Gy (V5) to volume to receive 70 Gy (V70) were evaluated and compared between the three techniques. Statistical analysis was done using one away ANOVA on the data editor SPSS Version 26.0 (SPSS Inc., Chicago, Illinois, USA) to evaluate the results. Continuous variables were expressed as mean ± standard deviation, and categorical variables were summarized as frequencies and percentages. Survival analysis was done by Kaplan-Meier Estimator. When the skin was considered as an OAR, the skin volume to receive 5, 10, 15, 20, 30, 40, 50, 60, 70 Gy was reduced by 6.5%, 6.5%, 6%, 11.5%, 7%, 6%, 6%, 5%, 2%, respectively, by SS-IMRT PH-I and II and 2%, 4.05%, 4%, 7%, 5%, 3%, 6%, 5%, 1%, respectively, by SS-SIB IMRT when both the SS techniques were compared with S-IMRT PH-I and II. Volume of skin to receive 20 Gy showed maximum reduction in SS-IMRT PH-I and II. A one-way ANOVA was carried out to find the differences in the skin doses between the three techniques. The skin dose in the two SS techniques, i.e., SS-IMRT PH-I and PH-II and SS-SIB IMRT was found significantly lower than that of IMRT plans without skin as an OAR, i.e., S-IMRT PH-I and PH-II (P = 0.000). The PTV doses were well within the 95%-107% of the prescribed dose (HI) and there were no significant differences in the means of the prescribed dose between the simple and skin spared IMRT techniques. The other OARs doses were also evaluated and there were no significant differences between the means of the doses among the techniques. SS IMRT for NPC has demonstrated reduction in skin dose while using skin as an OAR in the optimization. Moreover, decreased skin dose can decrease the skin related toxicities provided there is no compromise on Target dose coverage and OAR dose. We recommend that skin should be contoured as an OAR for NPC, provided PTV is minimally 3-5 mm beneath skin surface, in order to have a better disease control with lesser toxicities and less unplanned treatment interruptions.

Gold-nanoparticle-enriched breast tissue in breast cancer treatment using the INTRABEAM® system: a Monte Carlo study.

Using a 50-kV INTRABEAM® system after breast-conserving surgery, breast skin injury and long treatment time remain the challenging problems when large-size spherical applicators are used. This study has aimed to address these problems using gold (Au) nanoparticles (NPs). For this, surface and isotropic doses were measured using a Gafchromic EBT3 film and a water phantom. The particle propagation code EGSnrc/Epp was used to score the corresponding doses using a geometry similar to that used in the measurements. The simulation was validated using a gamma index of 2%/2mm acceptance criterion in the gamma analysis. After validation Au-NP-enriched breast tissue was simulated to quantify any breast skin dose reduction and shortening of treatment time. It turned out that the gamma value deduced for validation of the simulation was in an acceptable range (i.e., less than one). For 20mg-Au/g-breast tissue, the calculated Dose Enhancement Ratio (DER) of the breast skin was 0.412 and 0.414 using applicators with diameters of 1.5cm and 5cm, respectively. The corresponding treatment times were shortened by 72.22% and 72.30% at 20mg-Au/g-breast tissue concentration, respectively. It is concluded that Au-NP-enriched breast tissue shows significant advantages, such as reducing the radiation dose received by the breast skin as well as shortening the treatment time. Additionally, the DERs were not significantly dependent on the size of the applicators.

Impact of a contrast-to-noise ratio driven and material specific exposure control on image quality and radiation exposure in angiography

Conventional detector-dose driven exposure controls (DEC) do not consider the contrasting material of interest in angiography. Considering the latter when choosing the acquisition parameters should allow for optimization of x-ray quality and consecutively lead to a substantial reduction of radiation exposure. Therefore, the impact of a material-specific, contrast-to-noise ratio (CNR) driven exposure control (CEC) compared to DEC on radiation exposure was investigated. A 3D-printed phantom containing iron, tantalum, and platinum foils and cavities, filled with iodine, barium, and gas (carbon dioxide), was developed to measure the CNR. This phantom was placed within a stack of polymethylmethacrylate and aluminum plates simulating a patient equivalent thickness (PET) of 2.5–40 cm. Fluoroscopy and digital radiography (DR) were conducted applying either CEC or three, regular DEC protocols with parameter settings used in abdominal interventions. CEC protocols where chosen to achieve material-specific CNR values similar to those of DEC. Incident air kerma at the reference point(Ka,r), using either CEC or DEC, was assessed and possible Ka,r reduction for similar CNR was estimated. We show that CEC provided similar CNR as DEC at the same or lower Ka,r. When imaging barium, iron, and iodine Ka,r was substantially reduced below a PET of 20 cm and between 25 cm and 30 cm for fluoroscopy and Dr When imaging platinum and tantalum using fluoroscopy and DR and gas using DR, the Ka,r reduction was substantially higher. We estimate the Ka,r reduction for these materials between 15% and 84% for fluoroscopy and DR between 15% and 93% depending on the PET. The results of this study demonstrate a high potential for skin dose reduction in abdominal radiology when using a material-specific CEC compared to DEC. This effect is substantial in imaging materials with higher energy K-edges, which is beneficial, for example, in long-lasting embolization procedures with tantalum-based embolization material in young patients with arterio-venous malformations.

ANODE HEEL EFFECT ATTENUATION IN LUMBAR SPINE RADIOGRAPHY: CAN THE USE OF ALUMINIUM FILTERS IMPROVE CLINICAL PRACTICE OF RADIOGRAPHERS?

Purpose: The aim of this study was to design an aluminium-based filter to reduce the anode heel effect in lumbar spine radiographs. Methods: Initially, lumbar spine examinations were observed in a public imaging department to determine the standard exposure parameters. Then, the characterization of the anode heel effect was made using the Unfors Xi R/F detector and based on the data collected, aluminium filters were designed with a wedge shape, with thicknesses ranging from 0.1 to 4.0 mm. The assessment of the entrance skin dose (ESD) reduction was performed on the anthropomorphic phantom, with and without filters, using the universal dosimeter UNIDOS E equipped with an ionization chamber. Lastly, the image quality assessment was performed with the Pehamed Phantom Digrad A+K and image quality surveys were applied to radiographers and radiologists. Results and Discussion: Uniformity of the beam was achieved, especially with the filter number 2, which presents a significant variation of 9% between cathode and anode side. This filter contributes to ESD reduction of 35% and 36% for AP and lateral projection, respectively. Also, according to the radiographers and radiologists, it improves the image quality of lumbar spine radiography. Conclusion: The use of aluminium filters can be advantageous in the clinical practice of radiographers when carrying out lumbar spine radiographs, since it allows to standardize the anode heel effect, reducing the radiation dose to the patient and without compromising the image quality.

Skin Dose Reduction by Layer-Stacking Irradiation in Carbon Ion Radiotherapy for Parotid Tumors.

Background: Layer-stacking irradiation (LSI) results in the accumulation of multiple small spread-out Bragg peaks along the beam direction. Although the superiority of LSI to conventional passive irradiation (CPI) regarding normal tissue sparing is theoretically evident, the clinical benefit of LSI has not been demonstrated. Here, we compared LSI with CPI using the same treatment planning-computed tomography images used for carbon ion radiotherapy (CIRT).Methods: Twenty-one parotid tumors were analyzed. The clinical target volume (CTV) 1 and CTV2 encompassed the parotid grand and the tumor, respectively. CTV1 and CTV2 received 36 Gy (RBE: relative biological effectiveness) in nine fractions and 64 Gy (RBE) in 16 fractions, respectively, using either LSI or CPI. CTV coverage was assessed by DX%, which is the dose covering at least X% of the target volume. Skin dose was assessed by SX, which is the skin surface area receiving at least X Gy (RBE).Results: For CTV1 and CTV2, there were no significant differences in D2% between LSI and CPI. D50% and D98% were slightly higher for CPI; however, the absolute difference between the two methods was <3%. S10–S60 (in increments of 10) were significantly lower for LSI than for CPI (P < 0.001 for all parameters). LSI was associated with a significant trend toward dose reduction at the skin area irradiated with a higher dose by CPI (P < 0.001).Conclusions: LSI achieved better skin sparing than CPI without sacrificing target volume coverage in parotid tumor patients.

Oncological outcome, postoperative complications, and mammographic changes after intraoperative radiotherapy with electrons (IOERT) as a boost in a large single-institution cohort of breast cancer patients.

Advantages of using intraoperative radiotherapy with electrons (IOERT) as a boosting modality in breast-conserving therapy include the direct visualization of the tumor bed, a reduced skin dose, and patient convenience. We report oncological outcome, postoperative complication rate, and mammographic changes on follow-up imaging in women treated at our institution with IOERT as a boost modality in breast-conserving therapy for early-stage breast carcinoma. Between January 2007 and June 2018, 763 consecutive patients were enrolled. During breast-conserving surgery, an IOERT boost of 9Gy was applied, followed by whole breast irradiation (WBI). At a median follow-up of 62.2months (range: 0.5-135), 13 in-breast recurrences were observed, yielding a local tumor control rate of 98.4% at 5years. In multivariable analysis, high tumor grading was predictive for local recurrence (HR=5.6; 95%CI: 1.19-26.2). A total of 27 (3.5%) patients developed any kind of postoperative complication. None of the tumor characteristics nor any of the IOERT technical parameters were predictive for development of a postoperative complication. On follow-up imaging, 145 patients with mammographic changes BIRADS score ≥3 were found of which 50.3% required a biopsy. Only 17 patients had positive biopsies; none of the IOERT parameters were predictive for false-positive imaging. A 9Gy IOERT boost combined with postoperative WBI provided outstanding local control rates, was well-tolerated, with limited postoperative complications. However, radiologists must be aware of a presumable higher prevalence of mammographic changes after IORT as a boost.

Quantitative Effect of Bolus on Skin Dose in Postmastectomy Radiation Therapy

BackgroundIn postmastectomy radiation therapy (PMRT), some centres prescribe the use of a tissue-equivalent bolus to the skin to reduce the risk of chest wall recurrence. The addition of bolus causes an increase in the skin dose, which may lead to increased risk of radiodermatitis. Radiodermatitis can decrease patients’ overall quality of life, bringing into question the benefit of using the bolus. The purpose of this retrospective chart review was to quantify the increase in skin dose associated with the use of bolus in the PMRT setting. Materials and MethodsWe evaluated 70 patients who underwent PMRT at our institution during 2012–2018. Two similar treatment plans were generated for each patient: one with bolus and one without. Skin dose-volume histogram values were evaluated, and statistical analysis was performed using MATLAB R2015b. ResultsThere was no significant difference in the maximum skin dose within a depth of 5 mm for bolus versus nonbolus plans (P = 0.4). However, within a depth of 3 mm, bolus plans had a maximum skin dose 7% ± 2.5% higher than the nonbolus plans (P < .00001). Mean skin dose within depths of 3 and 5 mm were both significantly higher (P < .00001) for bolus plans. The photon beam energy and chest wall separation showed minimal or no effect on skin dose. ConclusionGiven the differing opinions in the literature regarding the role for bolus in PMRT, there is still uncertainty of the optimal treatment method. This retrospective study demonstrates a 20%–30% reduction in mean skin dose when bolus is not used.

A novel, yet simple MLC-based 3D-crossfire technique for spatially fractionated GRID therapy treatment of deep-seated bulky tumors.

PurposeTreating deep‐seated bulky tumors with traditional single‐field Cerrobend GRID‐blocks has many limitations such as suboptimal target coverage and excessive skin toxicity. Heavy traditional GRID‐blocks are a concern for patient safety at various gantry‐angles and dosimetric detail is not always available without a GRID template in user’s treatment planning system. Herein, we propose a simple, yet clinically useful multileaf collimator (MLC)‐based three‐dimensional (3D)‐crossfire technique to provide sufficient target coverage, reduce skin dose, and potentially escalate tumor dose to deep‐seated bulky tumors.Materials/methodsThirteen patients (multiple sites) who underwent conventional single‐field cerrobend GRID‐block therapy (maximum, 15 Gy in 1 fraction) were re‐planned using an MLC‐based 3D‐crossfire method. Gross tumor volume (GTV) was used to generate a lattice pattern of 10 mm diameter and 20 mm center‐to‐center mimicking conventional GRID‐block using an in‐house MATLAB program. For the same prescription, MLC‐based 3D‐crossfire grid plans were generated using 6‐gantry positions (clockwise) at 60° spacing (210°, 270°, 330°, 30°, 90°, 150°, therefore, each gantry angle associated with a complement angle at 180° apart) with differentially‐weighted 6 or 18 MV beams in Eclipse. For each gantry, standard Millenium120 (Varian) 5 mm MLC leaves were fit to the grid‐pattern with 90° collimator rotation, so that the tunneling dose distribution was achieved. Acuros‐based dose was calculated for heterogeneity corrections. Dosimetric parameters evaluated include: mean GTV dose, GTV dose heterogeneities (peak‐to‐valley dose ratio, PVDR), skin dose and dose to other adjacent critical structures. Additionally, planning time and delivery efficiency was recorded. With 3D‐MLC, dose escalation up to 23 Gy was simulated for all patient's plans.ResultsAll 3D‐MLC crossfire GRID plans exhibited excellent target coverage with mean GTV dose of 13.4 ± 0.5 Gy (range: 12.43–14.24 Gy) and mean PVDR of 2.0 ± 0.3 (range: 1.7–2.4). Maximal and dose to 5 cc of skin were 9.7 ± 2.7 Gy (range: 5.4–14.0 Gy) and 6.3 ± 1.8 Gy (range: 4.1–11.1 Gy), on average respectively. Three‐dimensional‐MLC treatment planning time was about an hour or less. Compared to traditional GRID‐block, average beam on time was 20% less, while providing similar overall treatment time. With 3D‐MLC plans, tumor dose can be escalated up to 23 Gy while respecting skin dose tolerances.ConclusionThe simple MLC‐based 3D‐crossfire GRID‐therapy technique resulted in enhanced target coverage for de‐bulking deep‐seated bulky tumors, reduced skin toxicity and spare adjacent critical structures. This simple MLC‐based approach can be easily adopted by any radiotherapy center. It provides detailed dosimetry and a safe and effective treatment by eliminating the heavy physical GRID‐block and could potentially provide same day treatment. Prospective clinical trial with higher tumor‐dose to bulky deep‐seated tumors is anticipated.

In vivo dosimetry using MOSkin detector during Cobalt-60 high-dose-rate (HDR) brachytherapy of skin cancer.

The MOSkin, a metal-oxide semiconductor field-effect transistor based detector, is suitable for evaluating skin dose due to its water equivalent depth (WED) of 0.07mm. This study evaluates doses received by target area and unavoidable normal skin during a the case of skin brachytherapy. The MOSkin was evaluated for its feasibility as detector of choice for in vivo dosimetry during skin brachytherapy. A high-dose rate Cobalt-60 brachytherapy source was administered to the tumour located at the medial aspect of the right arm, complicated with huge lymphedema thus limiting the arm motion. The source was positioned in the middle of patients' right arm with supine, hands down position. A 5mm lead and 5mm bolus were sandwiched between the medial aspect of the arm and lateral chest to reduce skin dose to the chest. Two calibrated MOSkin detectors were placed on the target and normal skin area for five treatment sessions for in vivo dose monitoring. The mean dose to the target area ranged between 19.9 and 21.1Gy and was higher in comparison with the calculated dose due to contribution of backscattered dose from lead. The mean measured dose at normal skin chest area was 1.6Gy (1.3-1.9Gy), less than 2Gy per fraction. Total dose in EQD2 received by chest skin was much lower than the recommended skin tolerance. The MOSkin detector presents a reliable real-time dose measurement. This study has confirmed the applicability of the MOSkin detector in monitoring skin dose during brachytherapy treatment due to its small sensitive volume and WED 0.07mm.

Understanding Parameters Affecting Surface Dose to Reduce Skin Toxicity in Patients Treated with SBRT

Skin toxicity has been reported with stereotactic body radiation therapy (SBRT) treatments. Skin dose is a complex function of many variables including beam energy, treatment distance (SSD), materials underlying beam path (t), and field size (FS). Various skin dose constraints have been reported and toxicity is noted despite meeting the published/proposed dose constraints predicted by the standard treatment planning systems. In vivo dosimetry assessments support that toxicity may be related to the bolus effect from the immobilization device. This study was undertaken to analyze the above parameters in the context of the BodyFix® (BF) system and their impact on skin dose. A custom BF device was created with three thicknesses (2.5, 5.0, 12.5 cm). Measurements were taken using solid water phantom with and without the BF using a PTW Markus parallel-plate chamber. The impact of BF on skin dose was investigated as a function of beam energy (6MV, 10MV, 10FFF), source to detector distance (SDD) (75, 85, 95cm), thickness (2.5, 7.5, 12.5cm), and depth (0, 1, 2, 3, 5mm & dmax) at clinically relevant field FS for SBRT (2x2 to 5x5cm2). Changes in percentage depth dose (PDD) with the same SDD were analyzed in the presence of BF, as well as relative skin dose (RSD), defined as dose with BF to dose without BF. The impact of BF on skin dose was significant for all beam energies as shown in Table. Comparing a 6MV beam without and with BF (6MV-BF), the PDD at 0mm depth (PDD0mm) ranged from 18.6% to 20.6% (61.0% to 83.5%) for all tested FS at 75cm SDD. The 10MV (10MV-BF) beam ranged from 10.8% to 12.6% (46.0% to 69.3%), and 10FFF (10FFF-BF) ranged from 16.6% to 17.7% (55.3% to 77.8%). RSD increased due to the presence of BF for all beams, increasing by 4-6x, 2-3x, and 1-2x at 0mm, 1mm, and 2mm depth, respectively. The effect of relative increase in RSD with BF decreased with depth. PDD decreased with larger BF thickness from 2.5-12.5 cm for all tested energies and FS. Compared to 2.5cm BF thickness, the PDD0mm decreased by ∼13% for 12.5cm thickness at 3x3cm2 FS for 10MV, indicating dose deposition is inside the BF for thicker BF. For 2.5cm of BF, more dose deposition is close to skin, resulting in increased skin dose. The RSD increased for all photon energies with increasing treatment distance. RSD at 0mm depth was 1.10 for 95cm SDD compared to 75cm SDD for t-2.5 cm & 2x2cm2. This increase is primarily due to the inverse square factor. BF parameters combined with dosimetric variables can create up to 6-fold increase in skin dose, which may result in acute skin toxicity. Using higher beam energy (up to 10x), larger BF thickness, and smaller SSD will help reduce skin dose in SBRT treatments. Accounting for immobilization devices and modifying planning parameters to reduce the bolus effect can potentially reduce skin dose and lead to lower toxicity.Abstract 3806; Table 13x3cm2PDD0mmPDD2mmPDD5mm6MV+BF72.9987.5897.256MV18.7260.0683.7110MV+BF55.3470.7184.4610MV10.6239.4364.0310FFF +BF63.5877.5188.9110FFF16.2550.4672.37 Open table in a new tab

104. Use of a RDIM system to analyse and optimize an interventional radiology procedure

Purpose To use NEXO[DOSE], a Radiation Dose Index Monitoring system, to collect and analyse dosimetric data to drive the optimization of the interventional radiology procedure for prostatic artery embolization (PAE). Methods NEXO[DOSE] extracts geometric (primary and secondary angles, table height) and dosimetric (irradiation type, KAP, voltage, filter) parameters from the Radiation Dose Structured Report, and calculates a skin dose map. NEXO[DOSE] separately lists the dose contribution from the acquisition mode and the fluoroscopy. In our institution, the PAE interventional procedure involved high Kerma Area Product (KAP) compared to other angiographic protocols and was associated with high risk of skin lesions. A low dose protocol was introduced and phantom characterization, i.e. entrance dose and image quality (LCD), was assessed on 24 to 30 cm PMMA phantom to better represent the patient geometry, considering that the highest dose contributions came from events with oblique angles. After optimization, patient dose was monitored using KAP and peak skin dose. Results We retrospectively analysed the exposure values of 84 patients, using NEXO[DOSE]. The acquisition dose in PAE is 70% of the total procedure: the first step was the introduction of an acquisition protocol with low entrance detector dose. The frame rate was reduced from 3 to 2 fps, and additional X-ray filtration of 1 mmAl + 0.1 mmCu was added. The fluoroscopy frame rate was also reduced from 15 to 7.5 fps. Entrance dose at the phantom surface was reduced by 49%, while the LCD reduction was 30% for details dimensions from 0.5 to 6 mm. The physicians assessed the loss of image quality to be acceptable. After optimization PAE procedures showed a 60% KAP reduction and a 49% peak skin dose reduction. Conclusions A RDIM software is a useful tool to highlight high dose procedures and to understand what parameters to modify, after a multidisciplinary team discussion.

Afatinib and Cetuximab in Four Patients With EGFR Exon 20 Insertion-Positive Advanced NSCLC.

EGFR exon 20 insertions comprise 4% to 9% of EGFR mutated NSCLC. Despite being an oncogenic driver, they are associated with primary resistance to EGFR tyrosine kinase inhibitors (TKIs). We hypothesized that dual EGFR blockade with afatinib, an irreversible EGFR TKI, and cetuximab, a monoclonal antibody against EGFR, could induce tumor responses. Four patients with EGFR exon 20 insertion-positive NSCLC were treated with afatinib 40 mg once daily and cetuximab 250 mg/m2 to 500 mg/m2 every 2 weeks. All patients had stage IV adenocarcinoma of the lung harboring an EGFR exon 20 insertion mutation. Previous lines of treatment consisted of platinum doublet chemotherapy (n= 4) and EGFR TKI (n= 2). Three of four patients showed a partial response according to Response Evaluation Criteria in Solid Tumors (RECIST 1.1). Median progression-free survival was 5.4 months (95% confidence interval: 0.0 - 14.2 months; range 2.7 months - 17.6 months). Toxicity was manageable with appropriate skin management and dose reduction being required in two patients. Dual EGFR blockade with afatinib and cetuximab may induce tumor responses in patients with EGFR exon 20 insertion-positive NSCLC.

Impact of low dose settings on radiation exposure during pediatric fluoroscopic guided interventions

PurposeTo evaluate the effects of lowering the detector entrance exposure in children undergoing interventional radiology procedures. Materials and MethodsThe study retrospectively investigated radiation dose levels in pediatric patients aged 0–18 years before (n = 39) and after (n = 26) lowering detector entrance dose, undergoing embolization of peripheral Arteriovenous malformations, Portal Vein Interventions or Percutaneous Transhepatic Cholangio Drainage (PTCD) between 2014 and 2017. Patient characteristics, fluoroscopy time, protocols used as well as resulting Skin Dose and Dose Area Product (DAP) were compared in each cohort. Image quality was assessed by two independent readers. ResultsThe two patient cohorts did not differ in terms of patient demographics. Similarly, fluoroscopy time did not differ before and after implementation of the low dose settings. An overall reduction of skin dose of 75.1% for AVM embolizations, 80.5% for Portal Vein Interventions and 85.3% for PTCD placement was observed. The DAP decrease was 82.5% for AVM embolizations, 72.2% for Portal Vein Interventions and 79.8% for PTCD placement. Image quality was generally considered to be good with an insignificant difference between pre and post implementation of the low dose approach and good agreement between the two readers. Manual inroom-switching to higher dose levels was possible, however this was not performed more frequently after implementation of the low dose settings. ConclusionLowering the detector entrance dose in pediatric interventional radiology procedures results in a significant decrease of the radiation dose burden.

Measurement of Electron Return Effect and Skin Dose Reduction by a Bolus in an Anthropomorphic Physical Phantom under a Magnetic Resonance Guided Linear Accelerator (MR-LINAC) System

Background: Magnetic resonance image-guided radiation therapy (MR-IGRT) promises more precise and effective radiation treatments compared to conventional IGRT by using real-time on-board MR imaging. Under the influence of a magnetic field, however, secondary electrons exiting a surface can be forced in a circular path and re-enter the medium, resulting in dose increase at a beam-exit surface, called the electron return effect (ERE). The purpose of the study is to compare the exit skin dose computed by Monte Carlo dose calculation with measurements using an adult anthropomorphic phantom and to measure the effect of skin dose reduction by adding 1 cm-thick bolus. Method: The plan was compared with measurements using an adult anthropomorphic phantom combined with radiochromic films and thermoluminescent dosimeters. We also measured the skin dose reduction by adding 1 cm-thick bolus on the frontal surface of the phantom. Results: We found that 1 cm-thick bolus reduced the skin dose by up to 20% both in measurements and calculations. The plan was found to overestimate the measured skin dose by about 10% and there was no significant difference in the bolus effect between the breast skin and the skin (without breast attachment) doses. Conclusion: In conclusion, we confirmed the ERE effect on the anthropomorphic phantom under the magnetic field and the exit skin dose reduction by adding a bolus. Skin dose measurements using anthropomorphic phantom may be helpful to evaluate more realistic skin dose and the bolus effect in the magnetic field.