Krishna Komanduri

Synchrony – Cyberknife Respiratory Compensation Technology

Studies of organs in the thorax and abdomen have shown that these organs can move as much as 40 mm due to respiratory motion. Without compensation for this motion during the course of external beam radiation therapy, the dose coverage to target may be compromised. On the other hand, if compensation of this motion is by expansion of the margin around the target, a significant volume of normal tissue may be unnecessarily irradiated. In hypofractionated regimens, the issue of respiratory compensation becomes an important factor and is critical in single-fraction extracranial radiosurgery applications. CyberKnife is an image-guided radiosurgery system that consists of a 6-MV LINAC mounted to a robotic arm coupled through a control loop to a digital diagnostic x-ray imaging system. The robotic arm can point the beam anywhere in space with 6 degrees of freedom, without being constrained to a conventional isocenter. The CyberKnife has been recently upgraded with a real-time respiratory tracking and compensation system called Synchrony. Using external markers in conjunction with diagnostic x-ray images, Synchrony helps guide the robotic arm to move the radiation beam in real time such that the beam always remains aligned with the target. With the aid of Synchrony, the tumor motion can be tracked in three-dimensional space, and the motion-induced dosimetric change to target can be minimized with a limited margin. The working principles, advantages, limitations, and our clinical experience with this new technology will be discussed.

Prescription dose in permanent (131)Cs seed prostate implants.

Recently, Cs131 seeds have been introduced for prostate permanent seed implants. This type of seed has a relatively short half-life of 9.7 days and has its most prominent emitted photon energy peaks in the 29-34 keV region. Traditionally, 145 and 125 Gy have been prescribed for I125 and Pd103 seed prostate implants, respectively. Since both the half-life and dosimetry characteristics of Cs131 seed are quite different from those of I125 and Pd103, the appropriate prescription dose for Cs131 seed prostate implant may well be different. This study was designed to use a linear quadratic radiobiological model to determine an appropriate dose prescription scheme for permanent Cs131 seed prostate implants. In this model, prostate edema was taken into consideration. Calculations were also performed for tumors of different doubling times and for other related radiobiological parameters of different values. As expected, the derived prescription dose values were dependent on type of tumors and types of edema. However, for prostate cancers in which tumor cells are relatively slow growing and are reported to have a mean potential doubling time of around 40 days, the appropriate prescription dose for permanent Cs131 seed prostate implants was determined to be: 127-12+5Gy if the experiences of I125 seed implants were followed and 121-3+0Gy if the experiences of Pd103 seed implants were followed.

Planning based on postneedle volume with early dosimetric assessment is beneficial for Cesium-131 permanent prostate seed implantation

PURPOSE This study reports on prostate edema after prostate brachytherapy using Cesium-131 ((131)Cs) and describes our method to compensate. METHODS AND MATERIALS Thirty-one patients underwent brachytherapy using an afterloading technique. Volume measurements of the prostate were taken at various time intervals relative to the date of implant. Real-time operating room dosimetry was used for seed placement on the postneedle prostate volume. The prostate volumes at the various time points were used to determine the effect of prostate edema on dosimetry. RESULTS Increase in prostate volume occurred immediately after needle placement, as measured by both ultrasound (mean increase of 17.7% (0-75.0%) from 36.8 to 46.9 cc) and Day 0 CT (mean increase of 15.3% (0-54.8%) to 45.9 cc). Day 0 assessment of dosimetry revealed a median D(90) of 102.7% (86.7-133.4%), median V(100) of 91.8% (75.9-98.4%), median V(150) of 44.4% (23.8-81.3%), and median V(200) of 16.3% (7.8-36.9%). This edema dissipated over the next 4 weeks, with resultant changes in dosimetric parameters. By 4 weeks, prostate volume had returned to the preimplant volume (37.7 cc) with increased D(90) (118.2%), V(100) (95.6%), V(150) (63.9%), and V(200) (28.4%). CONCLUSIONS There is significant immediate edema with prostate brachytherapy. This affects the dosimetry of the implant substantially. Because of this edema, our planning for brachytherapy is done on the postneedle implant volume. Quality assurance studies should be done on the same day as the implant to avoid substantial overestimation of dosimetric parameters.

Dosimetric evaluation of radiation exposure during I-125 vicryl mesh implants: implications for ACOSOG z4032.

BackgroundSegmentectomy or wedge resection along with brachytherapy delivered via a vicryl mesh implant imbedded with 125I is a novel therapeutic modality to treat early stage lung cancer. This modality is being evaluated in a large national prospective randomized trial (ACOSOG Z4032). There has been concern that this method exposes physicians and staff to unacceptable amounts of radiation. In this prospective study, we measured the exposure to health care professionals during such a procedure.MethodsDosimetric readings using Special Microdosimeter thermoluminescent detectors (TLDs) (Landauer, Inc) were performed during 22 125I vicryl mesh implantations. Diodes were placed on the back of the each hand of the primary radiation oncologist and primary surgeon during the creation and implantation of the mesh. In addition, diodes were placed on the posterior shoulder of the patient to obtain a control reading.ResultsPatients had 40–60 125I seeds placed. Median activity per seed was 0.511 milli Curie (mCi), with a median total activity implanted of 23.0 mCi. Median radiation dose to the radiation oncologist was 1 milli rem (mrem), and that to the surgeon was 2 mrem. Median dose to the control diode on the patient was a median radiation dose to the outside of the patient of 5.4 mrem/h.ConclusionsThere is very little radiation exposure to physicians and staff during a segmentectomy and 125I vicryl mesh implantation. This is a safe method of lung cancer treatment with respect to health care professionals, although the ALARA (As Low As Reasonably Achievable) principle should still be followed.

Comparison between real-time intra-operative ultrasound-based dosimetry and CT-based dosimetry for prostate brachytherapy using cesium-131.

The purpose of this study was to evaluate the correlation between real-time intra-operative ultrasound-based dosimetry (USD) and day 0 post-implant CT dosimetry (CTD) 131 Cs permanent prostate brachytherapy. Fifty-two consecutive patients who underwent prostate brachytherapy with 131 Cs were evaluated. Real time operating room planning was performed using VariSeed 7.1 software. Post-needle placement prostate volume was used for real-time planning. Targets for dosimetry were D90 >110%, V100 >90%, V150 <50%, and V200 <20%. The CT scan for post-operative dosimetry was obtained on day 0. The mean values for USD, CTD, and the linear correlation, respectively, were, for D90: 114.0%, 105.61%, and 0.15; for V100: 95.1%, 91.6%, and 0.22; for V150: 51.5%, 46.4%, and 0.40; and for V200: 15.8%, 17.9%, and 0.42. The differences between the mean values for USD and CTD for D90 (p<0.01), V100 (p<0.01), and V150 (p<0.05) were statistically significant. For D90, 30.8% of patients had a >15% difference between USD and CTD and 51.9% of patients had a >10% difference between these values. In contrast, the USD and CTD for V100 were within 5% in 55.8% of patients and within 10% in 86.5% of patients. This study demonstrates a correlation between the mean intra-operative USD and post-implant day 0 CTD values only for V200. Significant variation in D90, V150, and V200 values existed for individual patients between USD and CTD. These results suggest that real-time intra-operative USD does not serve as a surrogate for post-operative CTD, and that post-operative CTD is still necessary.

Prescription dose in permanent {sup 131}Cs seed prostate implants

Recently, Cs131 seeds have been introduced for prostate permanent seed implants. This type of seed has a relatively short half-life of 9.7 days and has its most prominent emitted photon energy peaks in the 29-34 keV region. Traditionally, 145 and 125 Gy have been prescribed for I125 and Pd103 seed prostate implants, respectively. Since both the half-life and dosimetry characteristics of Cs131 seed are quite different from those of I125 and Pd103, the appropriate prescription dose for Cs131 seed prostate implant may well be different. This study was designed to use a linear quadratic radiobiological model to determine an appropriate dose prescription scheme for permanent Cs131 seed prostate implants. In this model, prostate edema was taken into consideration. Calculations were also performed for tumors of different doubling times and for other related radiobiological parameters of different values. As expected, the derived prescription dose values were dependent on type of tumors and types of edema. However, for prostate cancers in which tumor cells are relatively slow growing and are reported to have a mean potential doubling time of around 40 days, the appropriate prescription dose for permanent Cs131 seed prostate implants was determined to be: 127-12+5Gy if the experiences of I125 seed implants were followed and 121-3+0Gy if the experiences of Pd103 seed implants were followed.

SU‐FF‐J‐56: Patient Dose From Kilo‐Voltage Cone Beam Computed Tomography (kV‐CBCT) Imaging

Purpose: To investigate patient dose from on‐board imager‐based kV‐CBCT. Method and Materials: Radiation doses from kV‐CBCT were measured using TLDs at different locations in three anthropomorphic‐phantoms (H&N, chest and pelvis) and patients retrospectively. kV‐CBCT scans were performed in standard settings (125 kV, 80 mA and 25 ms) using a Varian Trilogy linear accelerator. Both full‐fan (FOV=24 cm) and half‐fan (FOV=40 cm) modes were evaluated for H&N case while only half‐fan (FOV=45 cm) technique was studied for chest and pelvic cases. The skin dose in both phantoms and patients were measured at 4 locations: anterior, posterior, Rt‐Lat, and Lt‐Lat. Doses measured in the phantoms included different critical organs. The dosimeters used were high sensitivity TLD‐100H and only those with standard‐deviations less than 3% and sensitivity within ±3% were selected for this study. Each TLD was individually calibrated using an ion chamber under the irradiation condition. Phantom data was averaged from 3 separate measurements and patient data was averaged from 5 measurements in each category. Results: The skin dose for H&N cases were 9–10cGy for half‐fan mode in both the phantom and patients. The dose for brain and brainstem were 7.1cGy and 7.6cGy, respectively. The doses in same locations were 2–3cGy lower if the full‐fan mode is used. The skin dose for chest cases was 8–10cGy and were same for the phantom and patient measurements. Measured mean lung dose was 8.5cGy and spinal cord dose was 6.2cGy. For pelvis, measured skin dose was 2.9–4.2cGy and the prostate and rectum dose were 2.9cGy. Conclusions: For pelvic cases, kV‐CBCT dose was comparable or less than that from portal imaging. For chest and H&N cases the dose can be two times higher than that for the pelvis cases. Daily CBCT may lead to extra 400cGy to skin and 250cGy to spinal cord in 40 fractions.

Prescription dose in permanent seed prostate implants

Recently, Cs131 seeds have been introduced for prostate permanent seed implants. This type of seed has a relatively short half-life of 9.7 days and has its most prominent emitted photon energy peaks in the 29-34 keV region. Traditionally, 145 and 125 Gy have been prescribed for I125 and Pd103 seed prostate implants, respectively. Since both the half-life and dosimetry characteristics of Cs131 seed are quite different from those of I125 and Pd103, the appropriate prescription dose for Cs131 seed prostate implant may well be different. This study was designed to use a linear quadratic radiobiological model to determine an appropriate dose prescription scheme for permanent Cs131 seed prostate implants. In this model, prostate edema was taken into consideration. Calculations were also performed for tumors of different doubling times and for other related radiobiological parameters of different values. As expected, the derived prescription dose values were dependent on type of tumors and types of edema. However, for prostate cancers in which tumor cells are relatively slow growing and are reported to have a mean potential doubling time of around 40 days, the appropriate prescription dose for permanent Cs131 seed prostate implants was determined to be: 127-12+5Gy if the experiences of I125 seed implants were followed and 121-3+0Gy if the experiences of Pd103 seed implants were followed.

Prescription dose in permanent Cs131 seed prostate implants: Prescription dose in permanent Cs131 seed prostate implants

Recently, Cs131 seeds have been introduced for prostate permanent seed implants. This type of seed has a relatively short half-life of 9.7 days and has its most prominent emitted photon energy peaks in the 29-34 keV region. Traditionally, 145 and 125 Gy have been prescribed for I125 and Pd103 seed prostate implants, respectively. Since both the half-life and dosimetry characteristics of Cs131 seed are quite different from those of I125 and Pd103, the appropriate prescription dose for Cs131 seed prostate implant may well be different. This study was designed to use a linear quadratic radiobiological model to determine an appropriate dose prescription scheme for permanent Cs131 seed prostate implants. In this model, prostate edema was taken into consideration. Calculations were also performed for tumors of different doubling times and for other related radiobiological parameters of different values. As expected, the derived prescription dose values were dependent on type of tumors and types of edema. However, for prostate cancers in which tumor cells are relatively slow growing and are reported to have a mean potential doubling time of around 40 days, the appropriate prescription dose for permanent Cs131 seed prostate implants was determined to be: 127-12+5Gy if the experiences of I125 seed implants were followed and 121-3+0Gy if the experiences of Pd103 seed implants were followed.

SU‐FF‐T‐193: Dosimetric Responses at Different Gantry and Collimator Angles in Dynamic MLC Beam Delivery

Purpose:IMRT uses Dynamic Multi‐Leaf Collimator (DMLC). This study is to investigate the DMLC performance at different gantry and collimator angles based on dosimetric measurements. Conformity index and symmetry index were introduced to quantify the performance. Method and Materials: We developed a technique to measure the dosimetric impact of DMLC delivery at different gantry and collimator angles. Various DMLC patterns were designed to observe the discrepancies of the DMLC delivery at different gantry collimator angles. The discrepancies were quantified by acquiring dosimetric information under the corresponding radiation delivery conditions. The designed dynamic fields were delivered using 6 MV photon beams to a Solid Water TM phantom with an ion chamber at the isocenter. Phantom was carefully set up so that both the phantom and the chamber remained the same, geometrically,with respect to the beams coordinates when the gantry angle was changed. The measurements were carried out for two Varians 23Ex Liancs. Conformity index was defined to measure the output ratios at different gantry angles for a same DMLC pattern. Symmetry index was defined to assess the dosimetric discrepancies of the same pattern with opposite collimator setting at a same gantry angle. Results: The same measurements were performed for each machine for five consecutive days. It was observed that conformity index varied within between 98% and 100%. And the results varied on daily basis, which may imply the slight instability of DMLC performance. A consistent value less than 100% for the conformity index may indicate a gravity effect on the DMLC performance. In the meantime at a fixed gantry angle and at different collimator angle with a same DMCL pattern, the symmetry index varied randomly from 97.5% to 102.2%. Conclusion: The gravity may affect the DMLC dosimetric performance. This impact on IMRT dynamic delivery warrants further investigation.