Akbar Beiki-Ardakani

EDR2 film dosimetry for IMRT verification using low-energy photon filters

Recently the EDR2 (extended dose range) film has been introduced commercially for applications in radiation therapy dosimetry. In addition to characterizing the wide dynamic range, several authors have reported a reduced energy dependence of this film compared to that of X-Omatic Verification (XV) films for megavoltage photon beams. However, those investigations were performed under limited geometrical conditions. We have investigated the dosimetric performance of EDR2 film for the verification of IMRT fields at more clinically relevant conditions by comparing the film doses with the doses measured with an ion chamber and XV films. The effects of using a low energy scattered photon filter on EDR2 film dosimetry was also studied. In contrast to previous reports our results show that EDR2 film still exhibits considerable energy dependence (a maximum discrepancy of 9%, compared with an ion chamber) at clinically relevant conditions (10 cm depth for IMRT fields). However, by using the low-energy filters the discrepancy is reduced to within 3%. Therefore, EDR2 film, in combination with the filters, is found to be a promising two-dimensional dosimeter for verification of IMRT treatment fields.

A comparison between 125Iodine brachytherapy and stereotactic radiotherapy in the management of juxtapapillary choroidal melanoma

Aims To compare the treatment efficacy and radiation complications between 125Iodine brachytherapy and stereotactic radiotherapy in the management of juxtapapillary choroidal melanoma. Methods Consecutive juxtapapillary melanoma patients treated with radiotherapy were included. Patients were divided into two cohorts: patients treated with 125Iodine brachytherapy and patients with stereotactic radiotherapy. Comparison included the rates postradiotherapy local recurrence, secondary enucleation, metastasis and radiotherapy complications. Kaplan–Meier estimates were used to determine the actuarial rates, and logrank test to compare between the estimates. Results We included 94 patients with juxtapapillary melanoma treated with radiotherapy. The brachytherapy cohort included 30 patients and stereotactic radiotherapy was 64. The median follow-up was 46 months in both cohorts. No statistically significant differences existed between the two cohorts on comparing pretreatment clinical data and tumour characteristics. On comparing treatment efficacy, the actuarial rates at 50 months for tumour recurrence were 11% and 7% (p=0.61), secondary enucleation was 11% and 21% (p=0.30) and for metastasis were 4% and 16% (p=0.11), respectively. On comparing treatment complications, the actuarial rates at 50 months for cataracts were 62% and 75% (p=0.1), for neovascular glaucoma 8% and 47% (p=0.002), for radiation retinopathy 59% and 89% (p=0.0001), and for radiation papillopathy 39% and 74% (p=0.003), respectively. Conclusions Both 125Iodine brachytherapy and stereotactic radiotherapy demonstrate comparable efficacy in the management of juxtapapillary choroidal melanoma. However, stereotactic radiotherapy shows statistically significant higher radiation-induced ocular morbidities at 4 years postradiotherapy.

Improving quality assurance for assembled COMS eye plaques using a pinhole gamma camera

A quality assurance system has been designed to verify the location and strength of seeds loaded in a brachytherapy eye plaque. This system consists of (1) a pinhole camera in conjunction with a Lumisys ACR-2000i computed radiography (CR) unit to image the location and measure the relative strength of the seeds with autoradiography, and (2) a source strength jig with a survey meter to estimate the total activity of the seeds in the plaque. Five holders of different sizes were made for fixation of the COMS (Collaborative Ocular Melanoma Study) plaques (12, 14, 16, 18, and 20mm) in the camera. The plaque-to-pinhole distance (dpp) has been optimized to be 30mm to give approximately uniform intensity on the CR image for uniformly loaded COMS plaques. The pinhole-to-detector distance (dpd) can be kept at either 30mm for 1:1 scale, or at larger distances for higher magnification. For a 1:1 scaling and pinhole diameter of 0.345mm, useful images are obtained with time-activity product (mCi sec) ranging from 5to250mCisec. Within this range, the pinhole system is able to differentiate seed activities of >10%. The resulting pinhole autoradiograph is able to (1) confirm the correct number of seeds loaded in the plaque, (2) verify the proper sitting of the seeds in the silastic carrier and the plaque, (3) verify the relative activity distribution of the seeds loaded in the plaque, and (4) potentially evaluate the integrity of the seed. The source strength measurement system is able to measure the total strength of seeds in the plaque ranging from 10to80mCi with an uncertainty of 5%.

Comparison of dosimetric parameters derived from whole organ and wall contours for bladder and rectum in cervical cancer patients treated with intracavitary and interstitial brachytherapy

For volumes up to 2 cm3 of the bladder and possibly up to 5 cm3 of the rectum, doses computed from the whole organ were good estimates of the doses in the wall in cervix brachytherapy, and there were no significant differences between patients treated with or without interstitial needles.

Magnetic Resonance Imaging-guided Brachytherapy Re-irradiation for Isolated Local Recurrence of Soft Tissue Sarcoma

Management options for locally recurrent sarcoma of the pelvis in patients previously treated with external beam radiation and surgery are limited. Pelvic exenteration is often the only viable but unattractive option. We present a patient with recurrent myxoid round cell liposarcoma of the ischiorectal fossa treated in 2013 with preoperative radiation (50 Gy in 25 fractions) and subsequent wide local resection. Four years later, a follow-up magnetic resonance imaging (MRI) scan demonstrated a 1 x 1 cm T2 hypointense soft tissue pre-sacral nodule consistent with local recurrence (LR). The patient declined posterior pelvic exenteration and was treated with 12 Gy using high dose rate brachytherapy (BT) under MRI-guidance followed by a further external beam radiation to a dose of 30 Gy in 15 fractions.

SU-GG-T-86: Retrospective Evaluation of Symmetrically Loaded COMS Plaques for Juxtapapillary Choroidal Melanoma Patients Using Plaque Simulator

Purpose/Objective(s): The purpose of this study is to retrospectively evaluate the dosimetric coverage of I‐125 plaque brachytherapy (IBT) in treating Juxtapapillary Choroidal Melanoma (JCM) using the Plaque Simulator treatment planning system. Materials and Methods: Thirty cases of JCM patients with a medium‐sized tumor were treated by COMS IBT loaded non‐uniformly but with circular symmetry from 1997–2005. Median follow‐up was 48 months. The tumor was 0.75–2.5 mm from the optic disc. The median tumor thickness was 4.3 mm and the median longest tumor diameter was 9.9 mm. An in‐house treatment planning system had been used for actual treatments. The proximity of the tumor to the optic nerve did not allow the plaque to be centered over the tumor, so the tumor edge closest to the optic nerve did not receive the dose specified in the COMS study. We used the Plaque Simulator treatment planning system (BEBIG, Germany) for replanning and assessing dose coverage of the tumor for these patients. Results When all corrections for dose calculation in the Plaque Simulator were taken into account, mean dose to tumor edge nearest the optic nerve were significantly lower than COMS recommendations (62Gy vs. 69Gy, p<0.02). Meanwhile, 4‐year tumorcontrol and eye preservation rates were both 90%, respectively. Radiotherapy complications included cataract in 53%, neovascular glaucoma in 10%, retinopathy in 53%, and papillopathy in 33% of treated patients. These rates are comparable to COMS outcomes study for non‐ Juxtapapillary tumors.Conclusion Similar tumorcontrol rates for juxtapapillary tumors and tumors where the plaque can be centred over the tumor might imply that tumor edges do not need as high a dose as the tumor apex.

Sci—Fri AM(2): Brachy—08: Dosimetric Comparison of Different Radiation Techniques in the Treatment of Juxtapapillary Choroidal Melanoma

Purpose/Objective(s): To compare dosimetric parameters of linac‐based stereotactic radiotherapy (LB‐SRT), to Gamma Knife Perfexion (GK‐SRT) and 125 Iodine Brachytherapy (IBT) in treating Juxtapapillary Choroidal Melanoma (JCM). Materials/Methods: Three JCM cases of small, medium and large size tumours, based on the COMS definition, which previously received LB‐SRT using our current protocol of 70 Gy in 5 fractions over 10 days, were selected and re‐planned on GK‐SRT and IBT. Dosimetric parameters chosen for comparison were tumourdose coverage, including maximum, minimum and mean doses, as well as dose to tumour base and apex. Dose to organs at risk, such as optic disc, lens and D70 of the involved eye were also compared. All doses were normalized such that 99% of the tutors received at least a dose of 70 Gy. Tumourcontrol and treatment complications, such as radiation retinopathy, optic neuropathy and neovascular glaucoma, have been reported that can be directly related to the above dosimetric parameters. Results: Overall tumour mean doses by LB‐SRT, GK‐SRT and IBT were 73.11, 98.86 and 117.72 Gy, and maximum doses 74.33, 137.12 and 229.05 Gy, respectively. Corresponding overall doses to the lens center were 2.24, 8.75 and 12.49 Gy, and D70 of the eye were 17.91, 10.01 and 13.55 Gy, respectively. Conclusion: In this study we have shown that LB‐SRT, GK‐SRT and IBT provide comparable treatment for Choroidal Melanoma. Linac‐based SRT delivers most uniform dose to the tumour and smaller dose to the lens and anterior chamber, however has a higher D70, compared to the other two.

SU‐FF‐T‐196: Effect of Stranded and Loose Seeds in Relation to Prostate Edema in Permanent I‐125 Prostate Brachytherapy

Objectives: The dosimetric evaluation of permanent I‐125 seed prostate implant is often based on the assumption that prostate is static throughout the treatment time which lasts for months. However, the prostate in general becomes edematous due to the surgical procedure, followed by resolution of edema post implantation. Such changes in prostate volume would have an impact on the dosimetry. The aim of this study is to investigate the effect of use of stranded and loose seeds on prostate swelling. Methods: Thirty‐six prostate patients were treated with I‐125 seed brachytherapy. Nineteen of them were treated with stranded seeds while 17 were with loose seeds. Pretreatment prostate volume was measured with transrectal ultrasound and MRI was used to quantify the post‐implant volume at day 1, 8, and 30. Results: The average percentage swelling on day 1, 8, and 30 was the same for both cohorts. Edema half‐life was found to be 15 days for the loose seed and 10 days for the stranded seed patients respectively. When the data were stratified based on the pre‐implant volume = 35 cc percentage swelling was found to be significantly higher (44.8 vs. 26.2, p=0.013) only on day one for the loose seed group compared to the stranded seed for pre‐implant volume =35 cc). Conclusions: This study shows that for small pre‐implant prostate volume (V<35cc), stranded seeds show significant reduction in amount of swelling relative to the loose seeds on day one of implant.