J. Jezioranski

Magnetic Resonance Imaging-Guided Intracavitary Brachytherapy for Cancer of the Cervix

PURPOSE To determine the feasibility and benefits of optimized magnetic resonance imaging (MRI)-guided brachytherapy (BT) for cancer of the cervix. METHODS AND MATERIALS A total of 20 patients with International Federation of Gynecology and Obstetrics Stage IB-IV cervical cancer had an MRI-compatible intrauterine BT applicator inserted after external beam radiotherapy. MRI scans were acquired, and the gross tumor volume at diagnosis and at BT, the high-risk (HR) and intermediate-risk clinical target volume (CTV), and rectal, sigmoid, and bladder walls were delineated. Pulsed-dose-rate BT was planned and delivered in a conventional manner. Optimized MRI-based plans were developed and compared with the conventional plans. RESULTS The HR CTV and intermediate-risk CTV were adequately treated (the percentage of volume treated to >or=100% of the intended dose was >95%) in 70% and 85% of the patients with the conventional plans, respectively, and in 75% and 95% of the patients with the optimized plans, respectively. The minimal dose to the contiguous 2 cm(3) of the rectal, sigmoid, and bladder wall volume was 16 +/- 6.2, 25 +/- 8.7, and 31 +/- 9.2 Gy, respectively. With MRI-guided BT optimization, it was possible to maintain coverage of the HR-CTV and reduce the dose to the normal tissues, especially in patients with small tumors at BT. In these patients, the HR percentage of volume treated to >or=100% of the intended dose approached 100% in all cases, and the minimal dose to the contiguous 2-cm(3) of the rectum, sigmoid, and bladder was 12-32% less than with conventional BT planning. CONCLUSION MRI-based BT for cervical cancer has the potential to optimize primary tumor dosimetry and reduce the dose to critical normal tissues, particularly in patients with small tumors.

Tumor and normal tissue dosimetry changes during MR-guided pulsed-dose-rate (PDR) brachytherapy for cervical cancer

BACKGROUND AND PURPOSE To analyze systematic changes in tumor and normal tissue anatomy and dosimetry using serial MR imaging during pulsed dose rate brachytherapy (PDR BT) for cervical cancer. MATERIAL AND METHODS Forty-three patients with cervical cancer underwent MR-guided PDR BT using an intrauterine applicator alone after external beam radiotherapy. MR imaging was repeated on days 2 and 3 of treatment and the day 1 plan was applied to the re-contoured volumes. RESULTS The mean uterine volume and mean HR CTV increased during treatment. This resulted in a decrease in the mean HR CTV D90 relative to the day 1 planned dose. There was no change in the mean bladder volume during treatment but the mean rectal volume increased. This correlated with an increase in the mean rectal dose. There were four local recurrences. There was no apparent relationship between either the planned or the delivered HR CTV D90 and local recurrence. There was only one case of late bladder toxicity but nine patients developed late rectal toxicity. The cumulative rectal dose during treatment was a better predictor of late rectal toxicity than the planned dose. CONCLUSIONS Significant changes in tumor and normal tissue anatomy and dosimetry can occur during PDR BT and should be tracked and corrected using serial imaging and plan adaptation, especially when the day 1 tumor or normal tissue doses are close to the planning constraints.

THE EFFECT OF CHANGING TECHNIQUE, DOSE, AND PTV MARGIN ON THERAPEUTIC RATIO DURING PROSTATE RADIOTHERAPY

PURPOSE To quantify the dosimetric and radiobiological changes seen when using intensity-modulated radiation therapy (IMRT) or planning target volume (PTV) margin reduction with consistent planning parameters in a representative sample of localized prostate cancer patients. METHODS AND MATERIALS Twenty patients were randomly selected from a cohort that received 79.8 Gy using six-field conformal radiotherapy. Using the clinical contours, PTV margin, planning system, and dose constraints, five-field IMRT plans were generated for 79.8, 83.8, and 88.0 Gy. The 88.0-Gy IMRT plan was then reoptimized with a PTV margin reduced to 3 mm. These plans were then compared using various dosimetric and radiobiological endpoints calculated for various alpha/beta. RESULTS Intensity-modulated RT resulted in greater conformity to the PTV (p < 0.001). No improvement in mean normal tissue complication probabilities in the rectal wall (NTCPrw) was seen, and the modified therapeutic ratio (TR(mod)) was largely unchanged between six-field conformal and IMRT for the majority of the patients. When IMRT was used to escalate dose, NTCPrw increased by 9% at each 5% prescription increase (p < 0.001). Reducing the posterior PTV margin from 7 mm to 3 mm for an IMRT plan reduced the mean NTCPrw by 12% (p < 0.001) and resulted in a trend toward increased TR(mod)(p = 0.005). Changes in TR(mod) between conformal and IMRT planning or PTV reduction showed large interpatient variability. CONCLUSIONS Changing from conformal to IMRT, or from PTV(10-7) to PTV(3), did not produce a uniform interpatient increase in TR(mod)when the CTV contained the prostate alone. Radiobiological benefits of these two methods seem to be dependent on the particular anatomy of individual patients, supporting the use of patient-specific margin, planning, and dose prescription strategies.

The effect of voxel size on the accuracy of dose-volume histograms of prostate 125I seed implants.

Cumulative dose-volume histograms (DVH) are crucial in evaluating the quality of radioactive seed prostate implants. When calculating DVHs, the choice of voxel size is a compromise between computational speed (larger voxels) and accuracy (smaller voxels). We quantified the effect of voxel size on the accuracy of DVHs using an in-house computer program. The program was validated by comparison with a hand-calculated DVH for a single 0.4-U iodine-125 model 6711 seed. We used the program to find the voxel size required to obtain accurate DVHs of five iodine-125 prostate implant patients at our institution. One-millimeter cubes were sufficient to obtain DVHs that are accurate within 5% up to 200% of the prescription dose. For the five patient plans, we obtained good agreement with the VariSeed (version 6.7, Varian, USA) treatment planning softwares DVH algorithm by using voxels with a sup-inf dimension equal to the spacing between successive transverse seed implant planes (5 mm). The volume that receives at least 200% of the target dose, V200, calculated by VariSeed was 30% to 43% larger than that calculated by our program with small voxels. The single-seed DVH calculated by VariSeed fell below the hand calculation by up to 50% at low doses (30 Gy), and above it by over 50% at high doses (>250 Gy).

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%.

A practical approach to inverse planning for high-precision dose escalated conformal prostate radiotherapy

The problem of choosing the best gantry angles and beam weights for dose-escalated conformal prostate treatment planning is formulated using a mixed-integer linear programming approach, to account for tumour dose homogeneity and dose-volume constraints. The formulation allows the number of beams to be restricted and for some of the beams to be compulsory. The present planning algorithm interfaces with and utilizes the three-dimensional planning capabilities of a commercial treatment planning system. A case study is illustrated, which represents a particularly challenging planning problem due to a large planning target volume and an unusually small bladder. Treatment plans with different numbers of beams are generated to compare with each other and with the standard six-field plan. Significant improvement is shown in the reduction of hot regions within the femoral heads and rectal wall, while not unduly compromising homogeneity constraints for the tumour.