Bolus Electron Conformal Therapy Research Articles

SU‐E‐T‐400: Evaluation of a Pencil Beam Algorithm and Pencil Beam Redefinition Algorithm for Bolus Electron Conformal Therapy

Purpose: To evaluate the accuracy of the pencil beam algorithmˈs (PBA) and pencil beam redefinition algorithmˈs (PBRA) calculated electron conformal therapy (ECT) dose distributions by comparing with phantom measured dose distributions. Method: The planning target volumes (PTV) from a parotid and a chest wall patient were modified to fit the CT image set of a cylindrical polystyrene phantom. A bolus ECT plan for both PTV sets was developed using .decimal p.d software to design a custom bolus, with the distal bolus surface fitting to the phantom and the proximal bolus surface conforming the 90% isodose surface to the distal PTV edge. PBA and PBRA dose distributions were calculated using the Pinnacle treatment planning system and in‐house software, respectively. In‐phantom film measurements were taken for each plan in five transverse slices and one sagittal slice. Accuracy of both the PBA and PBRA calculations was tested by comparing the measured 2D film data to corresponding planar dose files from both the PBA and PBRA treatment plans, respectively. Passing criteria of 5% dose difference or 2mm distance to agreement were applied to each pixel (0.5mm pixel size) to quantify the accuracy of each dose calculation. Results: Comparison between measured and PBA calculated pixels for the post‐mastectomy chest wall and parotid cases for all 6 measured planes resulted in 93.3% and 95.3% pass rates, respectively. Using the same criteria, the PBRA had pass rates of 96.6% and 95.6%, respectively. In a phantom with no heterogeneities, the PBA and PBRA are both expected to have excellent agreement. Conclusions: The PBA and PBRA produced highly accurate calculations for bolus ECT dose distributions. As expected, the PBRA is slightly more accurate than the PBA, however this difference may not have any clinical significance unless significant heterogeneities are present, currently under study.Support: This work was supported in part by a research grant from .decimal, Inc.

SU-E-T-732: Accuracy of Electron Dose from Pencil-Beam Redefinition Algorithm in Patient-Like 2D Phantoms

Purpose: To evaluate the accuracy of electron dose calculated using the pencil‐beam redefinition algorithm (PBRA) in patient‐like 2D phantoms. With the recent availability of bolus electron conformal therapy (ECT), conforming of the 90% dose surface to the planning target volume (PTV) might require a more accurate dose calculation than the Hogstrom pencil beam algorithm (PBA). Methods: PBRA‐calculated dose distributions have been compared to those previously measured in three cylindrical phantoms (retromolar trigone, maxillary sinus, and nose), whose axial cross sections were based on the mid‐PTV CTanatomy for each site. The phantoms consisted of SR4 muscle substitute, SR4 bone substitute, and air. TLD measured doses were reported for a matrix of locations in the SR4 muscle only. Current CT scan data was used for planning with CT to stopping and scattering power tables created based on the chemical compositions and current densities of the materials. Differences between PBRA‐calculated and measured dose were expressed as distance‐to‐dose agreement in regions of high dose gradient (90%–10%) and as a percent of given dose elsewhere. Results: Results from the PBRA for the retromolar trigone phantom showed 100% (27/27) TLD dose points within 5% and 95% (19/20) within 4mm. Similar results for the maxillary sinus phantom showed 79% (26/33) and 100% (19/19), respectively, and results forthe nose showed 96%(23/24) and 100% (38/38), respectively. For each phantom, results were better than previously reported PBA results, e.g. results for the nose showed 81% (26/32) and 60% (18/30), respectively. Dose calculation times for the three sites (single 2.93GHz) were 1min, 45sec, and 1min. Conclusions: Results of this study were consistent with those previously reported, namely that the PBRA is more accurate than the PBA and that it should be suitable for clinical situations where dose accuracy is more important, e.g. bolus ECT. This work was supported in part by a research agreement with .decimal Inc.

SU‐E‐T‐328: Evaluation of Radiation Dosimetry of Bolus Electron Conformal Therapy (ECT)

Purpose: To evaluate the radiation dosimetry of bolus electron conformal therapy (ECT) using new commercially available bolus fabrication software.Methods: A customized electron bolus was designed using 3D planning system (TPS) Eclipse Ver.8.6.15 (Varian) and p.d ECT (.decimal) software. Computed tomography (CT) images of tough water was obtained and the planning target volume (PTV) was assumed. The proximal surface of the bolus was designed to conform to the 90% isodose line to the distal surface of the PTV using p.d ECT. Dose was calculated with a pencil beam algorithm for review with TPS. Based on structure set from TPS, bolus was fabricated using a computer‐controlled milling machine. To evaluate the dose distribution, CT images with the bolus on the tough water were acquired and final dose distribution was computed. Gafchromic EBT 2 film (Lot A09171002, ISP) were sandwiched between tough water at 1, 2 and 3 cm depth from the bottom of bolus and irradiated at same geometry with planning. Film was scanned by EPSON GTX970 scanner and that 2D dose distribution is compared with calculation. Results: 2D dose distribution showed qualitative agreement between calculation and measurement, absolute dose value is corresponded within 3% in the region of PTV. The pass rates of the gamma test were greater than 90% at 3 cm depth Conclusions: In this study the dose distribution for bolus ECT was measured by EBT 2 film. The 3D electron bolus which is fabricated by p.d ECT software reproduce the accurate dose distribution that is calculated with TPS.

SU‐E‐T‐807: Measured Data Set for Validation of Bolus Electron Conformal Therapy

Purpose: To measure sample planes of two 3D dose distributions that can be used to evaluate the accuracy of planning and delivery of bolus electron conformal therapy (ECT). Method: The planning target volumes (PTV) from a parotid and a chest wall patient were modified to a CT image set of a cylindrical polystyrene phantom. A bolus ECT plan for each PTV was developed using .decimal p.d software to design a custom bolus and dose was calculated using Pinnacle. Once acceptable, .decimal fabricated the bolus from machinable wax. Approximately 45 cGy given dose was delivered for each bolus ECT plan, and planar dose measurements were taken in the mid‐sagittal and five transverse phantom planes (±5cm, ±2.5cm, 0cm) using Kodak XV radiographic film sealed within the in‐phantom cassettes. Films were developed and digitized to (0.5mm pixels) using a Vidar DosimetryPRO Advantage, and converted to dose using a film calibration curve created at the time of measurement. Four film measurements were taken in each plane, with the mean dose and standard error being determined at each pixel. The precision of the data was quantified in the low gradient regions (Dose > 90% and Dose <10%) by the mean percent standard error of all pixels and in the high gradient region (90% > Dose > 10%) by the mean distance to dose agreement. Results: The percent average standard errors in the low gradient, high dose region were less than 1.5% for parotid and chest wall measurements and less than 3% in the low gradient, low dose regions. Average distances to agreement in the high gradient region were less than 0.6 mm. Conclusions: For two PTVs bolus, ECT dose distributions in 6 planes were measured and their precision quantified for three dose regions. These data are well‐suited for comparison with calculated electron dose distributions.This work was supported in part by a research grant from .decimal, Inc.

Image‐guided bolus electron conformal therapy – a case study

We report on our initial experience with daily image guidance for the treatment of a patient with a basal cell carcinoma of the nasal dorsum using bolus electron conformal therapy. We describe our approach to daily alignment using treatment machine‐integrated megavoltage (MV) planar imaging in conjunction with cone beam CT (CBCT) volumetric imaging to ensure the best possible setup reproducibility. Based on MV imaging, beam aperture misalignment with the intended treatment region was as large as 0.5 cm in the coronal plane. Four of the five fractions analyzed show induced shifts when compared to digitally reconstructed radiographs (DRR), in the range of 0.2−0.5 cm. Daily inspection of CBCT images show that the bolus device can have significant tilt in any given direction by as much as 13° with respect to beam axis. In addition, we show that CBCT images reveal air gaps between bolus and skin that vary from day to day, and can potentially degrade surface dose coverage. Retrospective dose calculation on CBCT image sets shows that when daily shifts based on MV imaging are not corrected, geometrical miss of the planning target volume (PTV) can cause an underdosing as large as 14% based on DVH analysis of the dose to the 90% of the PTV volume.PACS number: 87.55.kh

Dosimetric Impact of Image Guidance on Bolus Electron Conformal Therapy

We report on our initial experience with the use of daily image guidance for the treatment of H&N patients using bolus electron conformal therapy. We demonstrate the dosimetric effect of misalignment on PTV dose coverage by retrospective dose calculation on daily volumetric CBCT image sets while accounting for alignment shifts based on MV portal imaging. Daily MV planar images were acquired for two H&N patients with the electron apertures in place. The range of electron energies used was 9-15 MeV with cone sizes in the range of 6 x 6 to 15 x 15 cm2 cones. By comparing positions of bony structure landmarks from MV images to digitally reconstructed radiographs (DRRs), the magnitude of aperture shifts in the coronal plane is determined. Volumetric imaging using the treatment machine on-board imager (CBCT) was also acquired to determine the quality of fit of the bolus device on patient skin and the magnitude of tilt of the device in the sagittal and axial planes with respect to beam axis. To determine the effect of bolus device tilt and misalignment on PTV coverage, daily CBCT images were fused with the reference CT image sets and contours are transferred from the reference CT to the CBCTs. Density overrides of both image sets were performed to avoid CT number dose calculation dependencies. Analysis of daily shifts based on MV imaging, beam aperture misalignment with the intended treatment region was as large as 1.0 cm in the coronal plane. Volumetric CBCT images revealed tilts of the bolus device in the sagittal and axial planes as large as 130 with respect to beam axis. In addition, CBCT images show potentially significant air gaps between bolus and skin surfaces that can degrade surface dose coverage. Retrospective dose calculation on CBCT image sets show that when daily shifts based on MV imaging are not performed, geometrical miss of the planning target volume (PTV) can cause under dosing as large as 14% based on DVH analysis of the dose to the 90% of the PTV volume. Based on two H&N case studies, we show that daily image guidance using two treatment machine integrated imaging modalities, CBCT and planar MV, is essential for optimal dosimetric coverage of PTV volumes in bolus electron conformal therapy and to ensure maximum dose sparing of sensitive structures.

Evaluation of Bolus Electron Conformal Therapy in Post Mastectomy Chestwall Irradiation

Evaluation of Bolus Electron Conformal Therapy in Post Mastectomy Chestwall Irradiation

Custom electron bolus treatment planning with skin collimation using the pencil-beam redefinition algorithm

Purpose/Objective: The aims are to demonstrate (1) the improvement in the patient dose distribution from using skin collimation when treating with bolus electron conformal therapy (ECT) and (2) the utility of the pencil beam redefinition algorithm (PBRA) for calculating dose in the presence of bolus and skin collimation.

Utilization of custom electron bolus in head and neck radiotherapy.

Conventional methods of treating superficial head and neck tumors, such as the wedge pair technique or the use of multiple electron fields of varying energies, can result in excellent tumor control. However, in some cases, these techniques irradiate healthy tissue unnecessarily and/or create hot and cold spots injunction regions, particularly in patients with complex surface contour modification or varying planning target volume (PTV) thickness. The objective of this work is to demonstrate how bolus electron conformal therapy can be used for these patients. Two patients treated using this technique are presented. The first patient was diagnosed with malignant fibrous histiocytoma involving the right ear concha and was treated with 12‐MeV electrons. The second patient was diagnosed with acinic cell carcinoma of the left parotid gland and was treated with 20‐MeV electrons after having undergone a complete parotidectomy. Each patient's bolus was designed using bolus design tools implemented in an in‐house treatment‐planning system (TPS). The bolus was fabricated using a computer‐controlled milling machine. As part of the quality assurance process to ensure proper fabrication and placement of the bolus, the patients underwent a second computed tomography (CT) scan with the bolus in place. Using that data, the final dose distribution was computed using the Philips Pinnacle 3 TPS (Philips Medical Systems, Andover, MA). Results showed that the 90% isodose surface conformed well to the PTV and that the dose to critical structures such as cord, brain, and lung was well below tolerance limits. Both patients showed no evidence of disease six months post‐radiotherapy. In conclusion, electron bolus conformal therapy is a viable option for treating head and neck tumors, particularly patients having a variable thickness PTV or surface anatomy with surgical defects.PACS number(s): 87.53.Kn