Ideal line-source configuration in permanent prostate implants: dosimetric impact

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I. J. Radiation Oncology

● Biology ● Physics

Volume 57, Number 2, Supplement, 2003

margins. In contrast, performing no corrections would result in serious underdosage to the CTV for the 2 mm and the 1 mm plan. Performing the stochastic corrections did not add significant advantage over the conventional correction strategy, but did marginally improve on overall TCP. Furthermore, a day-to-day analysis of the dose distributions showed that use of the stochastic setup correction strategy enables to deliver the dose to the CTV with more uniform dose-per-fraction and therefore with a more optimal BED. The reason for this is that the stochastic correction strategy is more robust against outliers in the measurements. Conclusions: The described prostate localization technique along with daily ultrasound guidance will allow to safely deliver the 7 field plan with a margin of 3 mm, and even arguably less for definitive prostate treatments. The authors acknowledge the financial support of the NIH under grant contract 1PO1 CA88960.

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Update of the AAPM Task Group No. 43 Report - A Revised AAPM Protocol for Brachytherapy Dose Calculations

M.J. Rivard,1 B.M. Coursey,2 L.A. DeWerd,3 W.F. Hanson,4 M.S. Huq,5 G.S. Ibbott,4 M.G. Mitch,2 R. Nath,6 J.F. Williamson7 1 Radiation Oncology, Tufts-New England Medical Center, Boston, MA, 2Ionizing Radiation Division, National Institute of Standards and Technology, Gaithersburg, MD, 3Accredited Dosimetry and Calibration Laboratory, University of Wisconsin, Madison, WI, 4Radiological Physics Center, M. D. Anderson Cancer Center, Houston, TX, 5Radiation Oncology, Jefferson Medical College and Thomas Jefferson University, Philadelphia, PA, 6Therapeutic Radiology, Yale University, New Haven, CT, 7Radiation Oncology, Virginia Commonwealth University, Richmond, VA Purpose/Objective: Since publication of the American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report in 1995 (TG-43), both the utilization of permanent source implantation and the number of low-energy interstitial brachytherapy sources commercially available have dramatically increased. In addition, the National Institute of Standards and Technology (NIST) introduced a new primary standard of air-kerma strength, and the brachytherapy dosimetry literature has grown substantially, documenting both improved dosimetry methodologies and dosimetric characterization of particular source models. In response to these advances, the AAPM Photon Emitting Brachytherapy Dosimetry subcommittee herein has revised the TG-43 protocol for calculation of dose-rate distributions around photon-emitting brachytherapy sources. Materials/Methods: The revised protocol (TG-43U1) includes: (a) a revised definition of air-kerma strength; (b) elimination of the apparent activity parameter; (c) elimination of the anisotropy constant in favor of the distance-dependent 1-D anisotropy function; (d) guidance on extrapolating tabulated TG-43 parameters to longer and shorter distances; and (e) elimination of minor inconsistencies and omissions in the original protocol and its implementation. Among the latter are consistent guidelines for use of point- and line-source geometry functions. A unified approach to comparing reference dose distributions derived from different investigators to develop a single critically evaluated consensus dataset as well as guidelines for performing and describing future theoretical and experimental single-seed dosimetry studies is recommended. Consensus datasets are included in the form of dose-rate constants, radial dose functions and 1-D and 2-D anisotropy functions, for all source models that met the AAPM dosimetric prerequisites as of July 15th, 2001. These include the following 125I sources: Amersham-Health models 6702 and 6711, Best Industries model 2301, North American Scientific Inc. (NASI) model MED3631-A/M, Bebig model I25.SO6, and the Imagyn isostar model IS-12501. The 103Pd sources included are the Theragenics model 200 and NASI model MED3633. Revised data for 137Cs and 192Ir are also provided. The AAPM recommends that the revised dose-calculation protocol and revised source-specific dose-rate distributions be adopted by all end users for the purpose of clinical treatment planning. Depending upon the dose-calculation protocol and parameters currently used by individual physicists, adoption of this protocol may result in changes to patient dose calculations. These changes should be carefully evaluated and reviewed with the radiation oncologist preceding implementation of the current protocol. Results: The definition of air-kerma strength, SK, is revised to include a low-energy cutoff, ⌬. SK ⫽ K⌬(d)d2. The distance, d, from the source center to the point of specification (usually but not necessarily associated with the point of measurement) is defined to be located on the transverse-plane of the source. Also, d should be large relative to the maximum linear dimension of the radioactivity distribution so that SK is independent of its value. Using the revised SK definition, 1-D and 2-D dosimetry formalisms are recommended. The 2-D formalism is largely unchanged, but a new 1-D dose rate formalism is preferred: D(r) ⫽ ` [GL(r,␪0)/GL(r0,␪0)] gL(r) ␸an(r). SKA Conclusions: The AAPM TG-43 protocol has been revised, and consensus datasets based on the reported literature have been prepared. These datasets permit consistent clinical implementation among end-users, and contribute towards accuracy for dose-finding studies.

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Ideal Line-Source Configuration in Permanent Prostate Implants: Dosimetric Impact 1,2

G. Leclerc, D. Tubic,1,3 L. Beaulieu1,2 Radio-Oncologie, CHUQ - Hotel-Dieu de Quebec, Quebec, QC, Canada, 2Physique, Genie Physique et Optique, Universite Laval, Quebec, QC, Canada, 3Genie Electronique, Universite Laval, Quebec, QC, Canada 1

Purpose/Objective: To compare the post-implant dosimetry, using TG-43 line source formalism, of clinical implants for which the real 3D seed orientations have been precisely extracted to that of ideally aligned seeds allowing the determination of whether or not this approximation is valid. This study also allowed for real angular distributions of implanted sources to be determined. Materials/Methods: The angular study of the individual seeds was conducted on 16 clinical cases. Each seed position and orientation (␪, ␸) were first extracted from 3 fluoroscopic images using an automated seed segmentation algorithm (D. Tubic et al, Med. Phys., 28:2265-2271, 2001). The seeds were then matched to the post-implant CT images for post-implant analysis. From those extracted positions, the positions of the aligned sources were constructed by simply keeping the center of the source in place and rotating the source to superpose it on the axis of implantation. A commercial algorithm was then utilized to conduct

Proceedings of the 45th Annual ASTRO Meeting

the dosimetric analysis on the prostate and organs at risk (OARs). A comparison is made between the real and ideal cases based on the DVHs and differences (⌬ ⫽ X(3D) – X(I)) in relevant dosimetric quantities (⌬D10, ⌬D50, ⌬D90, ⌬V100 and ⌬V200) obtained. The dose values are given in [Gy] and the volumes in [%]. Results: The ␸ angle (measured from the axis of implantation on the coronal plane) has a narrow distribution centered on 0° with 30% of the sources within ⫾2,5° (⬍␸⬎ ⫽ 1,11°, ␴ ⫽ 22,9°). The q angle (measured from the axis of implantation on the sagittal plane) as on the other hand a large distribution centered on 0° (⬍␪⬎ ⫽ 1,11°, ␴ ⫽ 22,9°) in opposition to the hypothesis of a uniform distribution of sources used by other publications in the field. Cumulative DVHs or dosimetric quantities show no significant differences for the dose to the prostate (⬍⌬D90⬎ ⫽ 0,87, ␴ ⫽ 3,21; ⬍⌬V100⬎ ⫽ -0,25, ␴ ⫽ 1,26; ⬍⌬V200⬎ ⫽ 0,23, ␴ ⫽ 1,03 ). Differential DVHs show a difference between the real and ideal case: the volume receiving less than 100% of the prescribed dose is on average higher in the ideal situation (⬍⌬D90⬎ ⫽ – 0,25, ␴ ⫽ 1,27). When the OARs were considered, the dosimetric quantities to investigate were different than those of the prostate. The results show that there was a significant variation in some instances: rectum (⬍⌬D10⬎ ⫽ – 0,85, ␴ ⫽ 4,75), bladder (⬍⌬D10⬎ ⫽1,76, ␴ ⫽ 2,44) and penile bulb (⬍⌬D10⬎ ⫽ 1,75, ␴ ⫽ 3,40;⬍⌬D50⬎ ⫽ 1,88, ␴ ⫽ 2,64). Conclusions: The real distribution of 1625 individual sources was obtained revealing that the f angle distribution is strongly peaked while the q angle distribution is wider. It also shows that the difference between an ideally aligned situation and the real one does not affect significantly the cumulative or differential DVHs of the prostate or rectum. For the bladder and the penile bulb, the ideal situation gives a lower dose. That is attributed to the anisotropy of the Amersham 6711 seed model (which yields a lower dose at its tips than on the side) and the position of those organs relative to the implantation axis. This lower dose should be taken into account when reporting dose effects to these OARs in post-implant evaluation (compared to ideal source configuration or point source configuration).

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Impact of MLC Leaf Width Reduction on Normal Tissue Dose in the Treatment of Infratentorial Ependymoma

M.R. Sontag, R. Siochi, Y. Zhu, S.S. Samant, B. Crawford, X. Ying, T.E. Merchant St. Jude Children’s Research Hospital, Memphis, TN Purpose/Objective: The benefit of MLC leaf width diminution usually is based on inductive reasoning in which it is assumed that better conformity to the target volume will reduce irradiation of surrounding tissues. Decrease in beam edge scalloping as measured by reduction of the effective penumbral width has been demonstrated. Neither approach ascertains whether this decrease is clinically important. A study has been undertaken to determine the dosimetric benefit of two approaches to mitigate the effect of MLC width. Materials/Methods: 20 pediatric patients were selected from a group of 67 patients with infratentoral ependymoma that were treated at our institution with conformal 3DRTP on either a Siemens Primart (6MV x-rays) or Siemens Primus (6MV and/or 15MV x-rays), each having identical doubly focused 1cm leaf width MLCs. All patients were planned using CT and MR for target and normal tissue delineation. Dose was calculated using a 0.2 cm grid. Measurements were made using Kodak verification film to ensure correct dosimetric description of penumbra in the dose model. Patients received 54 Gy (8.76 ⫾ 2.86 beams) to the PTV followed by a 5.4 Gy (4.95 ⫾ 3.01 beams) off-cord boost. All beams were shaped using MLC and approximately half of the beams employed virtual wedges. Two additional plans were developed for each patient employing either a 0.43 cm leaf width Radionics mini MLC (mMLC) or the Siemens HD270 technique, which uses the 1 cm wide MLC and a set of 3 slightly shifted beams which produces smearing of the beam edge. The new plans differed from the original MLC plan only in actual beam shape. Beam energy, orientation and wedging were identical. For each plan, comparable PTV dose was obtained as ascertained by DVH analysis. Dose-volume histograms and integral dose were calculated for target volumes and pertinent critical structures for each of the three (MLC, mMLC and HD270) techniques. Results: Measurement by film finds that the 80-20% penumbral width of the Siemens MLC and the Radionics mMLC leaf edge differs ⬍1 mm. Replacement of the MLC with mMLC resulted in integral dose reductions for total brain (5.1 ⫾ 3.7%; mean ⫾ SD), left temporal lobe (6.3 ⫾ 5.5%) and right temporal lobe (5.8 ⫾ 5.5%). Greater integral dose reductions were observed for left cochlea (13.7 ⫾ 8.9%), right cochlea (14.1 ⫾ 12.2%), chiasm (20.8 ⫾ 21.4%), hypothalamus (18.7 ⫾ 25.2%) and pituitary (18.2 ⫾ 21.4%). Replacement of MLC with the HD270 technique resulted in integral dose reductions in total brain (0.5 ⫾ 1.0%), left temporal lobe (1.2 ⫾ 0.9%), right temporal lobe (1.0 ⫾ 1.0%), chiasm (2.8 ⫾ 3.9%), hypothalamus (3.7 ⫾ 5.1%) and pituitary (0.8 ⫾ 3.4%). Integral dose increases were found for left cochlea (0.2 ⫾ 3.3%) and right cochlea (0.4 ⫾ 5.9%). Conclusions: The closer distance to the patient of the mMLC with its non divergent leaf edge offsets the advantage of the divergent leaf edge of the MLC, resulting in similar penumbras. The 5-6% reduction in brain dose obtained utilizing mMLC instead of MLC is attributable to reduction in beam scalloping obtained with the former’s smaller leaf width. More considerable dose reductions using the mMLC occurred for the chiasm, auditory and hypothalamic-pituitary unit because these structures are adjacent to the PTV and benefit from the more precise shaping afforded by the narrower mMLC leaf width. There is some modest additional reduction due to the diminution of beam scalloping. The large standard deviation in the reported dose reductions resulted from inter-patient variability of organ location relative to the PTV. In the HD270 technique, only the leaves of the original beam were placed based the location of critical structures as well as the PTV. Since shifted beam leaf positions were based only on the leaf positions of the original beam, critical structures lying adjacent to PTV were randomly further shielded or further exposed to the shifted beams. The small systematic dose reductions found were due to decrease in beam scalloping alone. While use of the mMLC resulted in meaningful dose reduction, this was not found for HD270.

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Clinical Experience of a Compensator-based Intensity Modulated Treatment Technique

S. Chang, T. Cullip, K. Deschesne, J. Rosenman Radiation Oncology, University of North Carolina at Chapel Hill, Chapel Hill, NC Purpose/Objective: Compensators offer an alternative means to deliver intensity-modulated radiotherapy (IMRT). Compared to the multi-leaf collimator (MLC)-based IMRT delivery techniques the drawback of compensator-based IMRT techniques is the lack of automation. The advantages of the compensator-based IMRT techniques, however, are less understood. We have implemented a compensator-based IMRT delivery technique in our clinic since 1996 and have treated more than seven hundred patients to date. The

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