Current practice in total-body irradiation: results of a Canada-wide survey.

BACKGROUND: Total-body irradiation (tbi) is used to condition patients before bone marrow transplant. A variety of tbi treatment strategies have been described and implemented, but no consensus on best practice has been reached. We report on the results of a survey created to assess the current state of tbi delivery in Canada. RESULTS: A 19-question survey was distributed to 49 radiation oncology programs in Canada. Responses were received from 20 centres, including 12 centres that perform tbi. A variety of tbi dose prescriptions was reported, although 12 Gy in 6 fractions was used in 11 of the 12 centres performing tbi. Half of the centres also reported using a dose prescription unique to their facility. Most centres use an extended-distance parallel-opposed-pair technique, with the patient standing or lying on a stretcher against a wall. Others translate the patient under the beam, sweep the beam over the patient, or use a more complicated multi-field technique. All but 1 centre indicated that they attenuate the lung dose; only 3 centres indicated attenuating the dose for other organs at risk. The survey also highlighted the considerable resources used for tbi, including extra staff, prolonged planning and treatment times, and use of locally developed hardware or software. CONCLUSIONS: At transplant centres, tbi is commonly used, but there is no commonly accepted approach to planning and treatment delivery. The important discrepancies in practice between centres in Canada creates an opportunity to prompt more discussion and collaboration between centres, improving consistency and uniformity of practice.

Dose distributions and dose rate constants for new ytterbium-169 brachytherapy seeds.

Relative dose distributions in the vicinities of two new prototypes of ytterbium-169 brachytherapy sources have been measured using lithium fluoride thermoluminescent dosimeters (TLD) placed in a solid water phantom. The type 6 seed consists of four Yb2O3 spheres contained in a 0.075-mm thick titanium tube, with overall dimensions of 4.5 mm in length and 0.8 mm in diameter. The type 8 seed contains a cylindrical segment of Yb wire, 2.5 mm long, sealed in a 0.075-mm thick titanium tube 4 mm in length and 0.5 mm in diameter. The relative dose distributions, measured with TLD, were compared with those predicted by Monte Carlo simulations (MCNP code). Experimental and theoretical dose distributions were generally in agreement within 5% of the local dose value. Source strength was determined from activity measurements using a high purity germanium (HPGe) spectrometer. With source strengths established, thermoluminescence was then used to measure the dose rate constant, lambda 0, for the type 8 seed. The measured lambda 0 of 1.34 +/- 0.10 cGy U-1 for the type 8 seed agrees, within statistical uncertainty, with the value of 1.25 +/- 0.05 cGy U-1 predicted through Monte Carlo simulation. Comparisons are made with experimental data reported by other investigators.

Radioactivity measurements of ytterbium-169 brachytherapy sources.

Ytterbium-169 is being considered as a new radiation source for brachytherapy applications. This radioisotope emits photons with energies ranging from 50 to 308 keV (average energy 93 keV) and decays with a half life of 32 days. For these reasons, it is believed to offer radiological and radiobiological advantages over some other isotopes currently in use. One impediment to widespread clinical use of this isotope is the determination of source strength in units of air kerma rate [cGy h-1]. The source strength can be measured directly with an ion chamber or calculated indirectly from the source radioactivity [Bq] with corrections for encapsulation. Our attempts to reconcile these two approaches have led to the development of a spectrometric technique for determining the radioactivity of ytterbium-169 brachytherapy seeds. A High Purity Germanium (HPGe) spectrometer is used to count the 307.7 keV photon emitted within a defined solid angle. The intrinsic photopeak efficiency of the detector was determined by Monte Carlo simulation followed by experimental verification with an activity-calibrated europium-152 source. Finally, the HPGe system has been used to calibrate a re-entrant ionisation chamber, allowing routine of Ytterbium source activity for clinical applications.

Sci-Thur AM: Planning - 11: The impact of distributed calculation framework settings on plan calculation time.

Some treatment planning system can divide a treatment plan calculation into multiple threads and allow both local and network computing resources to perform the calculation concurrently, which significantly reduces the calculation time for a calculation-demanding planning such as Volumetric Modulated Arc Therapy (VMAT) or electron Monte Carlo (eMC). This study tested in Eclipse (Varian, V10.0.39) the impact of Distributed Calculation Framework (DCF, V10.0.0.757) settings on calculation time in a planning environment that consists of 20 workstations with 8 core processors and 16GB RAMs installed on most of them. It is found that for an arc plan increasing the control point field parallelization factor reduces the total calculation time at beginning but lengthens the total calculation time after a certain level as a result of data sending time increase. Further increasing the factor may cause a serious net work traffic or even failure of a calculation. For an eMC plan the calculation time decreases monotonously with the increase of Monte carlo field parallelization factor, and the data sending time is insignificant compared to the calculation time. Increasing the local servant numbers reduces the data sending time but raises the calculation time for arc and eMC plans. The calculation time increment is more and more significant with the increase of local servants. The optimal DCF setting for a facility depends on the total number of calculation workstations available, the hardware configuration of the workstations, and the data transfer rate of the network. No conflict of interest exists in the study.

Poster - Thur Eve - 74: A set of tests designed for electron dose calculation algorithm verification during a treatment planning system upgrade.

A set of tests were designed to verify an electron algorithm effectively and quickly during a treatment planning system upgrade. Based on TG-53 report's suggestion and the assumption that the algorithm is well commissioned before the upgrade, the tests spot-check the output factors, depth doses, off-axis doses and treatment field sizes. The field sizes of 4×4, 6×6, 10×10, 15×15, 20×20 and 25×25 are to be tested. Four test plans are created for each field size, i.e., for open field, for extended SSD, for shaped field, and for bolus field. Fixed MU setting is recommended to avoid a possible plan normalization issue. The parameters to be recorded and compared include doses at dmax , R50 and Rp along central axis, which contain output and depth dose information, doses at four off-axis points in dmax plane, which contain off-axis dose and beam symmetry information, and FWHMs at dmax . For the plans other than open field only doses at dmax are checked. The tests were performed successfully during a planning system upgrade. The whole test can be completed in approximately 12 hours if the workload is distributed into multiple task carriers. It was found that most of the data agree very well between the old and the new version of the algorithm while some of the Rp or R50 doses deviated more than other data, which prompted a depth dose check. PDD comparisons were performed for the involved fields and it was found there were less than 0.5 mm PDD shifts occurred.

Poster - Thurs Eve-30: Investigation of a patient immobilization system for breast tomotherapy.

Using tomotherapy to deliver adjuvant radiation therapy for breast cancer treatment requires more precise immobilization than can be achieved using gravity alone. We evaluated the use of a thermoplastic shell to immobilize the patient's torso during breast cancer treatment. To measure intrafraction breathing motion, 4DCT scans were performed for eight post-lumpectomy or post-mastectomy breast cancer patients with the thermoplastic shell in place. The 4DCT scans were then analyzed to determine the magnitude of motion of the breast surface, chest wall, and heart over the breathing cycle. Maximum surface motion was typically less than 2mm, with a maximum of 4mm. Maximum displacement of the chest wall was less than 3mm with a maximum of 5mm in a single patient. Comparison with the setup errors recorded prior to repositioning the patients suggests that, with the thermoplastic shell in place, patient setup error will be a more significant source of uncertainty in patient position than breathing motion.