Dosimetric Implications Research Articles

PD-0478: Reproducibility of deep inspiration breath-hold and dosimetric implications in treatments of left-breast cancer

PD-0478: Reproducibility of deep inspiration breath-hold and dosimetric implications in treatments of left-breast cancer

EP-1674: Dosimetric implications of manual correction for automated deformable image registration in RT applications

EP-1674: Dosimetric implications of manual correction for automated deformable image registration in RT applications

A Clinical Dosimetric Perspective Uncovers New Evidence and Offers New Insight in Favor of 99mTc-Macroaggregated Albumin for Predictive Dosimetry in 90Y Resin Microsphere Radioembolization

TO THE EDITOR: At first glance, the results of a recent study by Wondergem et al. ([1][1]) may appear discouraging for the evolving science of personalized predictive dosimetry for 90Y radioembolization, especially to less experienced readers. However, the dosimetric implications of their data may

Respiratory-Correlated Duodenum Motion and Its Implications in Pancreatic SBRT

Radiation therapy of pancreatic cancer is challenging due to the close proximity of the target and surrounding organs at risk (OARs). Both target and OARs exhibit respiratory-correlated motion. Efforts have been devoted to address the target motion, but the dosimetric effect of OAR motion is largely unknown. In the current study, we investigated the motion of duodenum, the most dose-limiting OAR, and its dosimetric implications in hypofractionated stereotactic body radiation therapy (HFSBRT) of pancreatic head tumors.

TU‐E‐108‐06: Investigation of Prostate Deformation and Associated Dosimetric Implications in IMRT of the Prostate

Purpose: Prostate deformation is generally considered a secondary correction and is consequently often ignored in planning margin design. In this study we estimated prostate deformation and investigate the associated dosimetric impact on target coverage. Methods: A cohort of ten prostate cancer patients were retrospectively selected for the study, each with three fiducial markers implanted in the prostate. All CBCT images were registered to respective planning CT images using the Velocity AI deformable image registration (B‐spline‐based) software. For cases with accurate prostate registrations (defined by mean marker misalignments of <1.5 mm), deformation‐vector‐fields (DVFs) of the prostate and seminal vesicles (SVs) were quantified using deformation‐volume histograms. For the case with the largest prostate deformation, target coverage degradation was analyzed in each of three treatment plans with PTV margins of 10, 5, and 3 mm. Results: Deformation of the prostate and SVs was most significant in the anterior‐posterior (AP) direction. Maximum prostate deformation of > 10mm, 5mm, and 3mm occurred in 0.5%, 10.0%, and 68.5% of cases, respectively. Based on the deformation‐volume histograms, DVF magnitudes greater than 5mm and 3mm occurred in 2% and 27% of cases, respectively. In general, SV deformation was larger than that of the prostate. For the case with the largest prostate deformation, prostate coverage (D95%) was reduced by 0.5%, 9.0%, and 17.0% for 10mm, 5mm, and 3mm margin plans respectively. Conclusion: These preliminary results are suggestive that prostate deformation over a course of fractionated prostate radiotherapy may not be insignificant, and thereby that proper margin design may be necessary to account for deformation. A consequence of these results is that use of tight planning margins of (e.g. less than 5 mm) must be viewed with severe caution.

Does the presence of an implant including expander with internal port alter radiation dose? An ex vivo model

There is a lack of literature examining the dosimetric implications of irradiating breast implants and expanders with internal ports inserted at the time of mastectomy. To determine whether the presence of breast expanders with port in saline or silicone implants affect the dose uniformity across the breast when irradiated with various photon and electron energies. One tissue-equivalent torso phantom with overlying tissue expanders in saline or silicone implants were irradiated using tangential fields with 6 MV and 18 MV photons and 9 MeV and 12 MeV electrons. All dose measurements were performed using thermoluminescent dosimeters (TLDs). The TLDs were arranged around the port and the perimeters of either the expander, or saline or silicone implant. Comparisons of measured radiation doses, and between the expected and measured doses of radiation from the TLDs on each prosthesis, were performed. Data were analyzed using two-tailed t tests. There were no differences in TLD measurements between the expander and the saline implant for all energy modalities, and for the expected versus actual measurements for the saline implant. Higher than anticipated measurements were recorded for a significant number of TLD positions around the silicone implants. Radiation doses around saline implants or expanders with internal port were unaltered, whereas dose recordings for silicone implants were higher than predicted in the present laboratory/ex vivo study.

Rectal dose-volume differences for scanning proton beam therapy with or without an endorectal balloon for the treatment of prostate cancer.

243 Background: Endorectal balloons (ERB) are routinely used in proton radiation for prostate cancer (PCa). The dosimetric implications, however, in conjunction with scanning proton beam therapy (SPT) have not been studied. We sought to describe dose-volume values with the use of an ERB vs no ERB for SPT for PCa. Methods: We analyzed 10 patients with localized PCa. Each patient underwent two CT simulations, one with a water-filled ERB (50-60 ml) and one with no ERB. For 8 patients, the prostate and proximal 1 cm of the seminal vesicles was the clinical target volume (CTV), with the prostate only as CTV for 2 patients. We defined 3 structures: rectum (R), anterior rectal wall (ARW) as a 3 mm rim of the anterior half of the rectum, and AR-CTV as the ARW only on slices with a contoured CTV. SPT plans were created in both cases for each patient, with the creation of a scanning target volume (STV), which expanded the CTV by 6 mm in all dimensions except 4 mm posteriorly and 12 mm laterally. Two opposed lateral beams were used to plan to the STV. Total dose was 78 Co-60 Gy equivalent in 39 fractions, to cover 95% of the STV. RBE was 1.1. We compared the dose-volume values for the R, ARW, and AR-CTV between the two arms. Results: The ERB significantly increased the R volume for all cases (61 ± 11 ml vs 105 ± 10 ml (p<0.01)). Rectal volumes radiated for all cases: V10, 55%; V50, 23%; and V70, 11%. The ERB significantly decreased the R volume radiated for all dose levels V5-V82 (p<0.05). The absolute difference was larger at low dose levels (8% at V10) and intermediate dose levels (5% at V50) compared to high dose levels (3% at V70). For the ARW, the ERB did not significantly change the volume radiated at any dose level. For the AR-CTV, the ERB significantly decreased the volume radiated for dose levels V20-V60 (p<0.05). Conclusions: The ERB provided a significant decrease in R volume radiated at all dose levels, and significantly decreased the AR-CTV volume, which represents a high risk volume, radiated at intermediate dose levels. Rectal doses with SPT, however, were low whether using an ERB or not. In the setting of the low rectal doses delivered, further study regarding the clinical benefit is warranted.

Biokinetic models for radiocaesium and its progeny

The International Commission on Radiological Protection (ICRP) is preparing a series of reports that will provide updated biokinetic and dosimetric models and dose coefficients for occupational intake of radionuclides. The biokinetic modelling scheme continues a trend in ICRP reports towards physiologically realistic descriptions of the time-dependent behaviour of absorbed radionuclides and their radioactive progeny. This paper proposes systemic biokinetic models for caesium isotopes and their ingrowing chain members and examines the dosimetric implications of the proposed models. Comparisons of D68 = tissue dose per unit input to blood based on current ICRP models for workers (ICRP Publication 68, 1994) with DP = corresponding values based on the proposed biokinetic models (but using the dosimetry models of Publication 68) yields the following ranges of the ratios DP:D68 for the tissues addressed in Publication 68: 0.5–25 for 130Cs (T1/2 = 29.2 min), 0.6–9.5 for 134mCs (2.9 h), 0.7–1.7 for 131Cs (9.69 d), 0.7–1.1 for 134Cs (2.06 y), 0.5–1.9 for 137Cs (30.2 y) and 0.2–3.7 for 135Cs (2.3 × 106 y). The large differences in the derived dose coefficients for some tissues and caesium isotopes, particularly short-lived isotopes, result mainly from differences in predictions of the time-dependent distributions of caesium in the body. For example, the proposed model and the current ICRP model for occupational intake of caesium predict peak kidney contents of ∼22% and ∼0.4%, respectively, following intravenous injection of stable caesium. Based on the proposed models for caesium and its progeny, the only dosimetrically significant chain members of caesium isotopes with half-life ≥10 min are 137mBa, which represents 32–85% of the estimated tissue doses from injected 137Cs, and 134Cs, which represents 4–53% of the estimated tissue doses from injected 134mCs.

Clinical and Dosimetric Implications of Air Gaps between Bolus and Skin Surface during Radiation Therapy

Purpose: The main objective of the study was to evaluate the effect of air gaps of 0 - 5.0 cm between bolus and skin for 1.0 cm Superflab bolus on surface dose (DSurf) and depth of maximum dose (dmax) in solid water and Rando ® phantoms. Methods: In this work, the effects of bolus to surface distance on DSurf and variation in dmax were analyzed in a solid water phantom and in an anthropomorphic Rando ® phantom for different field sizes, using Gafchromic ® EBT films and

Effect of the embolization material in the dose calculation for stereotactic radiosurgery of arteriovenous malformations

It is reported in the literature that the material used in an embolization of an arteriovenous malformation (AVM) can attenuate the radiation beams used in stereotactic radiosurgery (SRS) up to 10% to 15%. The purpose of this work is to assess the dosimetric impact of this attenuating material in the SRS treatment of embolized AVMs, using Monte Carlo simulations assuming clinical conditions. A commercial Monte Carlo dose calculation engine was used to recalculate the dose distribution of 20 AVMs previously planned with a pencil beam dose calculation algorithm. Dose distributions were compared using the following metrics: average, minimal and maximum dose of AVM, and 2D gamma index. The effect in the obliteration rate was investigated using radiobiological models. It was found that the dosimetric impact of the embolization material is less than 1.0Gy in the prescription dose to the AVM for the 20 cases studied. The impact in the obliteration rate is less than 4.0%. There is reported evidence in the literature that embolized AVMs treated with SRS have low obliteration rates. This work shows that there are dosimetric implications that should be considered in the final treatment decisions for embolized AVMs.

Dosimetric Implications of Abdominal Compression for Lung SBRT Treatment

One of the methods to limit lung tumor motion stemming from respiration is through the use of abdominal compression devices. The purpose of this study is to evaluate how the use of such a device influences the dosimetry of the final plan in terms of normal lung volumes covered by the 5Gy and 20Gy isodose lines (i.e. V5 and V20, respectively).

Dosimetric Implications of CT Metallic Artifacts Correction in Current Radiation Therapy Treatment Planning Systems

Metal implants are recognized as a problem that limits the application of CT imaging in radiotherapy planning. Besides the loss of diagnostic quality of the artifacted image, incorrect HUs cause incorrect electron density information that may introduce errors in the calculations of the radiation dose distribution. This study investigates two methods for correction of CT metallic artifacts to reduce the uncertainty of dose calculation in popular external beam treatment planning systems.

Dosimetric Impact of Using the Acuros XB Algorithm for Intensity Modulated Radiation Therapy and RapidArc Planning in Nasopharyngeal Carcinomas

To assess the dosimetric implications for the intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy with RapidArc (RA) of nasopharyngeal carcinomas (NPC) due to the use of the Acuros XB (AXB) algorithm versus the anisotropic analytical algorithm (AAA). Nine-field sliding window IMRT and triple-arc RA plans produced for 12 patients with NPC using AAA were recalculated using AXB. The dose distributions to multiple planning target volumes (PTVs) with different prescribed doses and critical organs were compared. The PTVs were separated into components in bone, air, and tissue. The change of doses by AXB due to air and bone, and the variation of the amount of dose changes with number of fields was also studied using simple geometric phantoms. Using AXB instead of AAA, the averaged mean dose to PTV70 (70 Gy was prescribed to PTV70) was found to be 0.9% and 1.2% lower for IMRT and RA, respectively. It was approximately 1% lower in tissue, 2% lower in bone, and 1% higher in air. The averaged minimum dose to PTV70 in bone was approximately 4% lower for both IMRT and RA, whereas it was approximately 1.5% lower for PTV70 in tissue. The decrease in target doses estimated by AXB was mostly contributed from the presence of bone, less from tissue, and none from air. A similar trend was observed for PTV60 (60 Gy was prescribed to PTV60). The doses to most serial organs were found to be 1% to 3% lower and to other organs 4% to 10% lower for both techniques. The use of the AXB algorithm is highly recommended for IMRT and RapidArc planning for NPC cases.

Evaluation of the deformation and corresponding dosimetric implications in prostate cancer treatment

The cone-beam computed tomography (CBCT) imaging modality is an integral component of image-guided adaptive radiation therapy (IGART), which uses patient-specific dynamic/temporal information for potential treatment plan modification. In this study, an offline process for the integral component IGART framework has been implemented that consists of deformable image registration (DIR) and its validation, dose reconstruction, dose accumulation and dose verification. This study compares the differences between planned and estimated delivered doses under an IGART framework of five patients undergoing prostate cancer radiation therapy. The dose calculation accuracy on CBCT was verified by measurements made in a Rando pelvic phantom. The accuracy of DIR on patient image sets was evaluated in three ways: landmark matching with fiducial markers, visual image evaluation and unbalanced energy (UE); UE has been previously demonstrated to be a feasible method for the validation of DIR accuracy at a voxel level. The dose calculated on each CBCT image set was reconstructed and accumulated over all fractions to reflect the ‘actual dose’ delivered to the patient. The deformably accumulated (delivered) plans were then compared to the original (static) plans to evaluate tumor and normal tissue dose discrepancies. The results support the utility of adaptive planning, which can be used to fully elucidate the dosimetric impact based on the simulated delivered dose to achieve the desired tumor control and normal tissue sparing, which may be of particular importance in the context of hypofractionated radiotherapy regimens.

SU-E-T-138: Quantification of Dwell Position Inaccuracy in Varian GammaMed HDR Titanium Ring Applicators.

To quantify the dwell position inaccuracy in Titanium ring applicators and develop a test to be performed quarterly, after source exchange. All three rings from our Titanium kit (30, 45, 60 deg.) were used for this study. EDR2 film was placed on the Simulator table and a ring was taped to the film, with a solid water slab as buildup. A 1-cm spacing dummy wire was inserted into the ring. The film was exposed using 135 kV, 80 mA, 400 mAs. An HDR treatment was then delivered using the even source dwell positions from 2 to 16, with a 5 mm step size, nominal dwell time 0.4 sec/position. The procedure was repeated three times for each ring. The films were scanned and analyzed with the RIT software. The distance between the center of each source position to the adjacent dummy dots was measured for each ring on all three films. An average shift (AS) was obtained for each ring.New films were exposed with a treatment offset equal and in the opposite direction relative to the AS for the ring used. The films were visually inspected to assess if the source positions are centered in between two adjacent dummy dots, and also scanned and analyzed with the RIT software. This test will be performed quarterly to verify if the shifts remain stable. The average shift was 2.5, 2.4, and 2.4 mm distally for the 30, 45, and 60 deg. rings, respectively. The offset for the quarterly test was set to 2 mm proximally, to take into account the 1 mm tolerance for the source position. The dwell position inaccuracy in Titanium ring applicators was quantified and the quarterly test was successfully performed for two quarters. Work is started to assess the dosimetric implications of this shift.

SU-E-T-648: Comparison of VMAT Vs Arc Treatment Plans for Patients Undergoing SBRT of Early-Stage Non-Small Cell Lung Cancer (NSCLC).

To evaluate the dosimetric implications of using VMAT (Volume Modulated Arc Therapy) treatment planning techniques compared to traditional Arc therapy methods for patients undergoing SBRT (Stereotactic Body Radiation Therapy) for early-stage Non-Small Cell Lung Cancer (NSCLC). Ten NSCLC cancer patients are planned with both VMAT and Arc techniques. The SBRT treatment plans comparison was quantified by several Dose-Volume Histogram (DVH) indicators including mean, maximum and minimum doses for GTV, ITV, PTV, OAR (Organs At Risk), and V95 (volume receiving at least 95% of the prescribed dose) for PTV. On average VMAT plans require for treatment delivery 16.6 ± 20.2 % more monitor units (MU) than the traditional Arc plans. The average PTV minimum, maximum and mean doses as a percentage of prescribed dose are 94.5 ± 3.9 %, 114.1 ± 3.3 % and 106.6 ± 1.6 % for VMAT vs 91.6 ± 4.4 %, 119.5 ± 5.3 % and 109.5 ± 2.5 % for the Arc technique. The V95 PTV coverage for VMAT plans range from 99.4 % to 100 % with a mean of 99.7 %, compared with a range of 96.8 % to 100 % with a mean of 99 % for the Arc plans. The maximum dose received by the lungs, spinal cord and chest wall show on average significant increases for Arc plans as opposed to VMAT plans (5.7 ± 6 % increase for lungs, 4.4 ± 9.2 % for cord and 2.4 ± 6.3 % for chest wall). The average mean doses and minimum doses for the OAR are similar for both techniques. The comparison of VMAT vs Arc plans for SBRT of NSCLC patients is subject to many variables, including GTV and PTV volume sizes, shape and their proximity relative to the OAR.

Optimisation of exposure conditions for in vitro radiobiology experiments

Despite the long history of using cell cultures in vitro for radiobiological studies, there is to date no study specifically addressing the dosimetric implications of flask selection and exposure environment in clonogenic assays. The consequent variability in dosimetry between laboratories impedes the comparison of results. In this study we compare the dose to cells adherent to the base of three types of commonly used culture flasks or plates. The cells are exposed to a 6MV clinical photon beam using either an open or a half blocked field. The depth of medium in each flask is varied with the medium surrounding the flask either water or air. The results show that the dose to the cells is more affected by the scattering conditions surrounding the flasks than by the level of filling within the flask. It is recommended that water or a water equivalent phantom material is used to surround the flasks or plates to approximate full scatter conditions at the cell layer. However for modulated fields, surrounding the 24 well plates with water-equivalent material is inadequate because of the large volume of air surrounding individual wells. Our results stress the importance of measuring the dose for new experimental configurations.

Dosimetric implications of residual seminal vesicle motion in fiducial-guided intensity-modulated radiotherapy for prostate cancer

To determine whether residual interfraction seminal vesicle (SV) displacement necessitates specific planning target volume (PTV) margins during fiducial-guided intensity modulated radiation therapy (IMRT) of the prostate. A planning computed tomography (CT) scan and 2 subsequent CT scans were prospectively obtained for 20 prostate cancer patients with intraprostatic fiducial markers. After CT registration, SV displacement relative to the prostate was quantified as a function of margin size for both the proximal (1 cm) SV (PSV) and the full SV (FSV). Two IMRT plans were simulated for each patient (prostate + PSV and prostate + FSV) both with a uniform 5-mm PTV margin. Minimum clinical target volume (CTV) dose (Dmin) and the volume of SV receiving 95% of the prescription dose (V95%) were assessed during treatment and compared with the initial plan. In all cases, SV displacement with respect to the prostate was greater for the FSV compared with the PSV. To ensure at least 95% geometrical coverage of the CTV for 90% of patients, margins of 5 and 8 mm were required for the PSV and FSV, respectively. Dosimetrically, residual SV displacement had minimal impact on PSV coverage compared with FSV coverage. For the PSV Dmin was ≥95% of the prescribed dose in 90% of patients with an overall mean V95% of 99.6 ± 0.8%; for the FSV Dmin was ≥95% of the prescribed dose in only 45% of patients with a mean V95% of 97.9 ± 2.4%. The SVs move differentially from the prostate and exhibit greater variation with increasing distance from the prostate. For plans targeting just the prostate and PSVs, 5-mm PTV expansions are adequate. However, despite daily localization of the prostate, larger PTV margins are required for cases where the intent is to completely cover the FSV.

Correlação entre volume tireoidiano determinado pelo método de ultrassonografia versus cintilografia e sua implicação em cálculos dosimétricos na terapia com radioiodo na doença de Graves

Graves disease (GD) is the most common cause of hiperthyroidism, and the most common treatment options are surgery, antithyroid drugs and radioiodine therapy. In radiodosimetric calculations to determine radioiodine dosage it is possible to use thyroid volume estimatives based on ultrasound or scintigraphy. The present study aimed to correlate these methodologies emphasizing volume estimatives and dosimetric implications. Were included 103 patients with GD diagnosis and indication of radioiodine treatment. They were submitted to thyroid ultrasound and thyroid scintigraphy. Good correlation between both methods was observed, although scintigraphy systematically obtained greater volumes than ultrasound implying in lower estimatives of absorbed dose when scintigraphy is used.

Image-guided radiotherapy using surgical clips as fiducial markers after prostatectomy: A report of total setup error, required PTV expansion, and dosimetric implications

PurposeTo determine the total setup error and the required planning target volume (PTV) margin for prostate bed without image guided radiotherapy (IGRT), and to demonstrate the feasibility and dosimetric benefit of IGRT post prostatectomy using surgical clips. Materials and methodsSeventeen patients were treated with intensity modulated radiotherapy (IMRT) to the prostate bed with a 1cm PTV margin. Three-dimensional shifts of the surgical clips inside the prostate bed were measured with respect to the isocenter from 364 orthogonal kV image pairs, and the total setup error was calculated to determine the required PTV margin. Alternative IMRT plans using 5mm or 1cm PTV expansion were generated and compared for rectal and bladder sparing. ResultsSurgical clips were reproducibly and reliably identified. The mean (standard deviation) shifts in the left–right (LR), superior–inferior (SI), and anterior–posterior (AP), axes were: −0.1mm (1.7mm), 0.6mm (2.4mm), and −2.1mm (2.6mm), respectively. The required PTV margins were calculated to be 6, 8, and 9mm in the LR, AP, and SI axis, respectively. A PTV expansion of 5mm, compared to 1cm, significantly reduced V65Gy to the rectum by 10%. ConclusionsIn the absence of IGRT, a non-uniform PTV margin of 6mm LR, 8mm AP, and 9mm SI should be considered. Use of clips as fiducial markers can decrease the total setup error, enable a smaller PTV margin, and improve rectal sparing.