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Using IMRT to repair unacceptable dose distributions of prostate implants
S434
I. J. Radiation Oncology
2155
● Biology ● Physics
Volume 57, Number 2, Supplement, 2003
Using IMRT to Repair Unacceptable Dose Distributions of Prostate Implants
X. Li, J.Z. Wang, P.P. Amin, M. Earl, D. Shepard Radiation Oncology, University of Maryland, Baltimore, MD Purpose/Objective: Unacceptable dose distributions (e.g., cold spots) in prostate permanent implants are frequently seen, due to many anatomical and/or technical reasons including pubic arch interference, prostate volume/shape change, seed migration, image artifacts, and seed placement errors. While the brachytherapy is typically not repeated, we propose to use image-guided intensity-modulated radiotherapy (IMRT) to repair these unacceptable dose distributions. To explore its feasibility, a dosimetric and biologic treatment planning study is carried out. Materials/Methods: The equivalent uniform dose (EUD) is applied to both brachytherapy and IMRT and is used to determine the required dose for IMRT to compensate an underdosing from an implant. For prostate tumor, the EUD is estimated based on the LQ model with the most recent parameters derived from clinical data (␣⫽0.15Gy-1, ␣/⫽3.1Gy). The EUD for normal tissue is computed based on the Lyman model. For an unsatisfactory implant, the EUD value in each sub-region (voxel), presently termed as voxel equivalent dose (VED), is calculated based on the dose distribution from post-implant CT. The obtained 3D VED distribution is used to form an objective function that aims to add a non-uniform dose distribution to compensate the underdosing regions within the prostate and, in the mean time, to spare the normal structures. This objective function is then used in the IMRT planning based on the same CT data. Either fixed-gantry or rotational IMRT is planned in order to deliver the required non-uniform dose distribution for different patient situations. The fixed gantry IMRT is planned using a commercial system (Corvus), and the rotational IMRT, termed as IMAT, is planned either by forward planning using a commercial system (Precise) or by inverse planning using an in-house developed algorithm (direct aperture optimization). A software tool is developed to (1) calculate the EUD value and the 3D VED distribution from the physical dose distribution for either brachytherapy or IMRT, (2) to display the combined VED distribution in 3D, and (3) to evaluate plans in terms of the EUD and the dose volume histogram (DVH) based on VED. To demonstrate the feasibility, the entire planning process is carried out on selected patient cases. Results: The required doses of IMRT for various unacceptable implant dose levels are tabulated in the table. The values in the table are physical doses (Gy) to be delivered in 2Gy daily fraction and they are calculated for two combined plans with EUD values of 70 and 80 Gy. According to the table, for example, an underdosing by 45Gy from an 125I implant with a intended prescription of 145Gy (EUD ⫽ 70Gy) can be compensated by an IMRT dose of 26Gy. Our results show that the required 3D IMRT dose distributions are highly non-uniform for patient cases studied. Dosimetric planning performed to achieve these dose distributions indicates that different unacceptable implant can be repaired by a preferable form of IMRT with satisfactory target coverage and normal structure sparing. For example, if underdosing is mainly in apex-anterior region (e.g., due to pubic arch interference), fixed-gantry IMRT is preferred because of its low integral dose and simplicity. If underdosing is mainly in anterior-lateral peripheral region (e.g., due to ultrasound image artifacts from large prostate), IMAT is advantageous because of its superior sparing of urethra. Because of its highly non-uniform dose distributions and/or small treatment volumes, IMRT needs to be guided by daily imaging (e.g., ultrasound). Conclusions: It is dosimetrically and biologically feasible for using image-guided IMRT to repair an unacceptable prostate implant. Necessary tools to address the biological and dosimetric issues for the clinical implementation of this approach are developed.
I. J. Radiation Oncology
2155
● Biology ● Physics
Volume 57, Number 2, Supplement, 2003
Using IMRT to Repair Unacceptable Dose Distributions of Prostate Implants
X. Li, J.Z. Wang, P.P. Amin, M. Earl, D. Shepard Radiation Oncology, University of Maryland, Baltimore, MD Purpose/Objective: Unacceptable dose distributions (e.g., cold spots) in prostate permanent implants are frequently seen, due to many anatomical and/or technical reasons including pubic arch interference, prostate volume/shape change, seed migration, image artifacts, and seed placement errors. While the brachytherapy is typically not repeated, we propose to use image-guided intensity-modulated radiotherapy (IMRT) to repair these unacceptable dose distributions. To explore its feasibility, a dosimetric and biologic treatment planning study is carried out. Materials/Methods: The equivalent uniform dose (EUD) is applied to both brachytherapy and IMRT and is used to determine the required dose for IMRT to compensate an underdosing from an implant. For prostate tumor, the EUD is estimated based on the LQ model with the most recent parameters derived from clinical data (␣⫽0.15Gy-1, ␣/⫽3.1Gy). The EUD for normal tissue is computed based on the Lyman model. For an unsatisfactory implant, the EUD value in each sub-region (voxel), presently termed as voxel equivalent dose (VED), is calculated based on the dose distribution from post-implant CT. The obtained 3D VED distribution is used to form an objective function that aims to add a non-uniform dose distribution to compensate the underdosing regions within the prostate and, in the mean time, to spare the normal structures. This objective function is then used in the IMRT planning based on the same CT data. Either fixed-gantry or rotational IMRT is planned in order to deliver the required non-uniform dose distribution for different patient situations. The fixed gantry IMRT is planned using a commercial system (Corvus), and the rotational IMRT, termed as IMAT, is planned either by forward planning using a commercial system (Precise) or by inverse planning using an in-house developed algorithm (direct aperture optimization). A software tool is developed to (1) calculate the EUD value and the 3D VED distribution from the physical dose distribution for either brachytherapy or IMRT, (2) to display the combined VED distribution in 3D, and (3) to evaluate plans in terms of the EUD and the dose volume histogram (DVH) based on VED. To demonstrate the feasibility, the entire planning process is carried out on selected patient cases. Results: The required doses of IMRT for various unacceptable implant dose levels are tabulated in the table. The values in the table are physical doses (Gy) to be delivered in 2Gy daily fraction and they are calculated for two combined plans with EUD values of 70 and 80 Gy. According to the table, for example, an underdosing by 45Gy from an 125I implant with a intended prescription of 145Gy (EUD ⫽ 70Gy) can be compensated by an IMRT dose of 26Gy. Our results show that the required 3D IMRT dose distributions are highly non-uniform for patient cases studied. Dosimetric planning performed to achieve these dose distributions indicates that different unacceptable implant can be repaired by a preferable form of IMRT with satisfactory target coverage and normal structure sparing. For example, if underdosing is mainly in apex-anterior region (e.g., due to pubic arch interference), fixed-gantry IMRT is preferred because of its low integral dose and simplicity. If underdosing is mainly in anterior-lateral peripheral region (e.g., due to ultrasound image artifacts from large prostate), IMAT is advantageous because of its superior sparing of urethra. Because of its highly non-uniform dose distributions and/or small treatment volumes, IMRT needs to be guided by daily imaging (e.g., ultrasound). Conclusions: It is dosimetrically and biologically feasible for using image-guided IMRT to repair an unacceptable prostate implant. Necessary tools to address the biological and dosimetric issues for the clinical implementation of this approach are developed.