Volume 33, Issue 6, June 2006
Index of content:
- Therapy Scientific Session: 224C
- Clinical Application of IMRT
33(2006); http://dx.doi.org/10.1118/1.2241809View Description Hide Description
Purpose: To determine if the dose calculation accuracy is suitable for the daily dose verification of patient treatment; and develop quality assurance tests for dose reconstruction with a commercial dose reconstruction system. Methods and Materials: Dose reconstruction accuracy was evaluated using MVCT and kVCT images of three test phantoms: 1.) A Electron Density Phantom phantom (Gammex); 2.) A water‐equivalent cylindrical phantom (TomoPhantom), and 3.) an anthropomorphic RANDO phantom. A parotid‐sparing head & neck plan was created for the phantoms. Using the kVCT images, inverse treatment plans simulating clinical prostate and head & neck helical tomotherapy treatments were created. The phantom MVCT images were fused with the kVCT images using a co‐registration algorithm. Planned‐Adaptive software was then used to re‐calculate the doses on the MVCT images. The kVCT‐based and MVCT‐based doses were then exported to the RIT113 Dosimetry System. Accurate dose reconstruction is dependent on the quality of the MVCT images used.As such, a genetic algorithm was developed to ensure proper image fusion and a principle component analysis was used to determine the most influential factors for image quality. Results: Based on the phantom results, the calculation accuracy of MVCT and kVCT images are typically within ±3% of each other. The greatest discrepancy occurs in the high‐dose gradient regions, which is most likely due to slight alignment errors between the two dose files. PCA indicated a correlation with the person performing the machine warm‐up and image quality. This was confirmed as MVCT dose reconstruction suffered due to an individual's error. Conclusions: Based on comparisons of calculated doses in phantom plans, the accuracy of using MVCT image data in dose reconstruction is typically within ±3%, subject to image quality. A process was developed for assessing the image contrast and resolution on a daily basis.
WE‐E‐224C‐02: Investigation of Simple IMRT Delivery to Stage I Lung Cancer Patients with Significant Respiratory Motion Using Respiratory Gated CT Scans33(2006); http://dx.doi.org/10.1118/1.2241810View Description Hide Description
Purpose: To investigate the use of IMRT for the treatment of stage I lungcancer associated with significant respiratory motion using 4DCT data. Method and Materials: A 4DCT scan ‐ covering a full respiratory cycle in 10 phases ‐ of a patient with a 1.8 cm diameter lungcancer was used to design several static step‐and‐shoot IMRT plans. Three plans were designed: two using snapshots of the tumor (mid‐inhale, full‐exhale), one using the superposition of all phases. Because of the significant tumor motion (maximum excursion of 2.5 cm) the effect of different margins around the CTV were studied. To reduce interplay between the MLC movement and respiratory motion in an ungated delivery, the number of intensity‐levels was minimized while maintaining coverage to the PTV and minimizing dose to OARs. Results: In this case‐study five‐field IMRT plans were generated using 18 MV photons delivering a total dose of 66 Gy in 33 fractions to the PTV. Plans based on snapshot scans of the lung only resulted in full coverage, if large margins (3 cm) were incorporated. All plans based on superimposed scans achieved full coverage, while allowing tight margins and minimizing the dosage to OARs. A small number of intensity‐levels (3–5 per beam) were sufficient for PTV coverage, thereby reducing the risk of unwanted interplay effects between MLC movement and respiratory motion. Conclusion: Using snapshot free‐breathing CT scans for treatment planning can lead to geometrical misses and underdosage of the target volume unless large PTV margins are applied at the expense of increased dose to OARs. Taking the superimposed CT scans of all respiratory phases for treatment planning ensures the full coverage of the tumor volume, without increasing dose to OARs. For this case‐study only a small number of segments were needed, allowing the application of IMRT despite significant tumor motion.
33(2006); http://dx.doi.org/10.1118/1.2241812View Description Hide Description
Purpose: This work investigates advanced mixed beam radiation therapy (MBRT) treatment of breast and head and neck cancer using energy‐ and intensity‐modulated electron (MERT) and photon(IMRT) beams. Methods and Materials: The new MBRT system consists of MLCs for both photon and electron beam modulation and associated software for dose calculation, treatment optimization, and beam delivery to ensure superior target coverage and normal tissue sparing. Accurate and efficient dose calculation tools for Monte Carlo based treatment planning, and effective treatment optimization and leaf sequencing algorithms for efficient and accurate beam delivery for advanced MBRT have been developed with the use of existing MLCs. This technique is being implemented clinically for breast and head and neck treatment through pilot studies and clinical trials that are specially designed for dose escalation and hypofractionation. Partial breast treatment is also investigated using advanced MBRT as it is being developed. Results: MBRT uses IMRT to achieve lateral dose conformity and MERT for conformity in the depth direction, which provides excellent target coverage for treatments involving shallow target volumes such as breast and head and neck. Our preliminary results based on 76 patients showed that grade II skin complications were significantly reduced in a hypofractionated breast trial. The whole breast received 20 fractions of 2.25Gy and the tumor bed received an additional 0.55Gy/day concurrent electron boost. The elimination of 10% hot spots in the whole breast volume ensures the whole breast dose to be under 2.5Gy beyond which significant skin complications have been reported in the literature. Conclusions: A set of software and hardware tools have been developed for conformal radiation therapy of shallow targets with much improved target dose conformity and uniformity, adequate skin coverage/avoidance and significant reduction in the dose to the adjacent normal organs and critical structures.
WE‐E‐224C‐04: A New CT Reconstruction Technique for Removal of Streak Artifacts Due to Metallic Dental Fillings and Implants for the Treatment of Head and Neck Cancer with Intensity Modulated Radiation Therapy33(2006); http://dx.doi.org/10.1118/1.2241813View Description Hide Description
Purpose: To remove or reduce the streak artifacts induced by metallic dental fillings and implants in CTimages for head and neck patients treated with intensity modulated radiation therapy(IMRT).Materials and Method: A cylindrical phantom was constructed using tissue‐equivalent plastic to simulate a human head. The phantom was filled with distilled water. A piece of 1 cm‐wide half elliptic tissue‐equivalent bolus was attached to a thin circular plastic plate inside the phantom to mimic the gingiva. Two human second molars with metallic fillings were implanted into the bolus on each side. The phantom was scanned on a GE LightSpeed CT scanner with a slice thickness of 2.5 mm. The acquired CTimages were first decomposed into spatio‐frequency components using a wavelet transform. The decomposition tree allowed us to examine and characterize different localized frequency information related to undesired artifacts. Once the spatio‐frequency signatures of metal objects were modeled, the corresponding coefficients in the wavelet domain were thresholded using a dynamic thresholding scheme. The thresholded images gave metal objets, which were then superimposed on a neighbor slice. New data were generated by re‐projecting the original slice and superimposed slice. The final image was reconstructed with no or reduced image artifacts due to dental fillings and implants. Results: Comparison of original images to reconstructed images indicates that the streak artifacts were either completely removed or significantly reduced. The structural details near the metal objects were clearly identifiable. The image quality was sufficient for target delineation in IMRTtreatment planning.Conclusions: The proposed technique can greatly reduce the streak artifacts induced by metallic dental fillings and implants without sacrifice of spatial resolution. The technique has the potential to significantly improve the accuracy of target and critical organ delineation and dose calculation in the head and neck IMRTtreatment planning.
WE‐E‐224C‐05: Uncertainty Analysis of Risk of Secondary Fatal Malignancies From Radiotherapy Treatments Including IMRT33(2006); http://dx.doi.org/10.1118/1.2241814View Description Hide Description
Purpose: Radiation away from the treatment field may induce secondary cancers in long‐term survivors. Risk estimates have received increased attention with the increase in out‐of‐field dose associated with IMRT. However, uncertainty in these risk estimates were not previously established, which has left unresolved the significance of these risk estimates. This work examines the uncertainty in the absolute risk estimates, as well as uncertainties in the ratio of risks between treatment modalities. Method and Materials: Effective dose equivalents and estimated risks were taken from the literature for several treatment modalities including IMRT at 6, 10, 15, and 18 MV and conventional therapy. The most recent risk estimates (5.75%/Sv) along with uncertainties in this risk estimate and uncertainties in the dose‐response model were considered in generating 90% confidence intervals for the absolute risk estimates and ratio of the risk estimates. Results: The absolute risks of fatal secondary malignancy were associated with very large uncertainties, which precluded distinguishing between the risks for the different treatment modalities considered. However, a much smaller confidence interval existed for the ratio of the risk. Because of the confidence intervals generated, an effective dose equivalent difference of 50% resulted in a statistically different ratio of the risks. Such differences were observed between some of the treatment modalities considered including 6MV as compared to 18MV IMRT.Conclusion: While no statistically significant difference existed in the absolute risk estimates of the treatment options examined in this study, the ratio of the risks was found to yield statistically significant differences between some treatment modalities considered.
WE‐E‐224C‐06: Differential Smoothing IMRT Planning for Head and Neck Cancer Patients with Mediastinal Involvement33(2006); http://dx.doi.org/10.1118/1.2241815View Description Hide Description
Purpose: Head and neck cancer patients with mediastinal involvement present a planning challenge. The superior fields are ideal for IMRT but possible interplay effects between leaf and mediastinal motion makes IMRT less desirable for the inferior fields. A new, differential smoothing IMRT technique is compared to matched and extended field IMRT plans. Materials and Methods: The differential smoothing IMRT technique treated the superior portion of the target with 7 fixed, low smoothing rate (LSR) beams while the inferior portion was treated by 3–4 highly smoothed, fixed beams with an overlap region. All beams existed in a single IMRT plan and were optimized simultaneously. This technique was used to plan treatments for three head and neck cancer patients with mediastinal involvement. These patients were also planned using two alternative techniques: (1) A matched field technique, with conformal radiation therapy used for the inferior portion, and an LSR IMRT plan used for the superior portion; (2) An extended field LSR IMRT plan treating the full extent of the disease. The plans were compared dosimetrically.Results: The differential smoothing technique provided homogenous, continuous coverage throughout the target volume while the matched field plan demonstrated discontinuous coverage in the match region. The extended field IMRT plan had coverage comparable to the differential smoothing plan. However, patient and leaf motion could compromise coverage in practice. The average MLC leaf opening in the inferior differentially smoothed beams was 1.5–2 times greater than the average opening in any of the LSR beams (1.55cm for LSR beams verses 3.04cm for highly smoothed beams) thereby potentially reducing the impact of patient motion. Lung DVH's were similar for all three techniques. Conclusion: The differential smoothing technique offers continuous, more homogenous coverage than the matched field method and is probably less susceptible to patient motion than the extended field technique.
WE‐E‐224C‐07: Image‐Guided Helical Tomotherapy for Localized Prostate Cancer: Technique and Initial Clinical Observations33(2006); http://dx.doi.org/10.1118/1.2241816View Description Hide Description
Purpose: The purpose of this study was to develop a technique for daily CT based IGRT, and to report clinical observations on treatment planning,imaging and delivery based on the first two years of experience. Method and Materials: Patients with previously untreated stage T1 through T3 biopsy‐proven adenocarcinoma of the prostate were considered eligible for treatment with daily CT guided helical tomotherapy. The prostate was targeted daily using megavoltage CT (MVCT) images that were fused with treatment planningCTimages based on anatomical alignments. All patients were treated at 2 Gy per fraction to 76 to 78 Gy (mean 76.7 Gy). Results: Thirty‐three prostate patients were planned, imaged, and treated as part of this study for a total of 1266 CT guided fractions. The prostate, rectum, bladder, femoral heads, and pubis symphysis were visible in one or more slices for all 1266 MVCT image sets. The typical range of measured prostate displacement in this study was 2–10 mm (3.4 mm standard deviation) in the anterior‐posterior direction, 2–8 mm (3.7 mm standard deviation) in the lateral direction, and 1–6 mm (2.4 mm standard deviation) in the superior‐inferior direction. The obese patients in this study had a substantially larger lateral variation (8.2 mm standard deviation) due to mobility of the external skin marks. Conclusions: A technique has been developed, and clinical implemented for daily MVCT based image‐guidedradiation therapy. The level of conformal avoidance increased with treatment planning experience, and the level of conformal avoidance can be greater than fix‐gantry based intensity modulation.