Dosimetric evaluation of radiation dose rate effect in respiratory gated intensity modulated radiation therapy.

PURPOSE: To investigate the dosimetric accuracy of the sliding window gated IMRT compared with the static treatment, using varying dose rates. MATERIALS AND METHODS: This study measured changes in output and diode array response with changing dose rate, verified the precision of the motion table, and measured changes in dose distribution accuracy with film and diodes at two depths with changing dose rate. During 4DCT (4 Dimensional Computed Tomography), the patient's respiratory signals and target motion were recorded and imported to the XY4D simulation table of SUN NUCLEAR Corporation to simulate the patient's respiration and tumour motion. A single field of each sliding window IMRT plan with 30º wedge and one for lung cancer were used in this study. Three irradiating conditions, static and moving target with and without gating, were applied to both plans. RESULTS: The standard deviations of output, with the dose rates changing from 300-600 MU/min, were 0.065 cGy and 0.169 cGy for the ionisation chamber and diode, respectively. The verification of the motion table shows very good precision with 9.98 ± 0.02 cm (true value = 10.0 cm). The measurements by MapCheck show the gamma index of the planned absolute dose distribution in static and moving targets with gating, resulting in more than 96% passing for all dose rates. The absolute dose distribution measured by film for the static target was agreeable with the value of moving target with gating. CONCLUSION: The sliding window gated IMRT technique is able to deliver an accurate dose to a moving target with the dose rate of 300-600 MU/min that is suitable for clinical treatment.

Osteosarcoma: a study of 130 cases.

Multidisciplinary treatment of osteosarcoma in the Faculty of Medicine Ramathibodi Hospital, Mahidol University, using preoperative intraarterial and postoperative chemotherapy, with or without local irradiation, combined with surgery and prophylactic lung irradiation provided an excellent 5 years' survival of 55 per cent, the same rate as the 9 years' survival. The survival was stable after 4.4 years. The patients with local irradiation had more tumor destruction apparent on the surgical specimen. The administration of prophylactic whole lung irradiation provided an outcome without any undesirable complication. Sixteen per cent of the cases with PLI developed lung metastasis compared to 48 per cent without PLI. The most important prognostic factor was low level of serum lactic acid dehydrogenase. The unanswered question is what is the optimal treatment for osteosarcoma?

Results of treatment in stage IIB squamous cell carcinoma of the uterine cervix: comparison between two and one intracavitary insertion.

To compare the results of treatment in stage IIB squamous cell carcinoma of the uterine cervix of two treatment regimens, two radium insertions vs one insertion, a prospective randomized study was carried out at Ramathibodi Hospital from 1 January 1983 to 31 December 1986, and the results were evaluated at the end of March 1991. The patients in treatment I (90 cases) received 40-41.4 Gy whole pelvic external irradiation and two intracavitary radium insertions, while patients in treatment II (53 cases) received 50.0-50.4 Gy and one intracavitary insertion. Both groups received the same total dose at point A, about 85-90 Gy. At 4 and 5 years, by the Kaplan-Meier survival curve, the disease-free survivals were 76 and 76% vs 79 and 79%, respectively, in treatment I and treatment II, which showed no significant difference by the log-rank test. Both groups had comparable serious complication rates, 0% vs 1.9%, respectively. However, grade I complications in treatment II, 35.8%, were higher than those in treatment I, 17.8% (P less than 0.01). Therefore, we concluded that treatment II provided the same disease-free survival and a very low rate of serious complications. To replace treatment I, the dose at the rectum and urinary bladder should be maintained with caution.

Neutron leakage measurements from a medical linear accelerator.

The McCall method has been used to measure neutron leakage from the Mevatron 77, 18- and 15-MV photon beams. Gold foil activation has been used employing a beta counting technique for the 18-MV beam and a gamma counting technique for both the 18- and 15-MV beam. The two counting techniques were used to evaluate their relative merit. The measurements were made at various locations in the patient-treatment plane for different field sizes. The results show that the thermal-neutron dose equivalent contributes only about 1%-2% of the total neutron dose equivalent. At 100 cm, the neutron dose equivalent for the 18-MV beam is approximately six times that of the 15-MV beam, slightly exceeding the 0.1% of the useful beam criteria used by some of the regulatory agencies. In light of the uncertainty in fluence to dose equivalent conversion factors, the increased dose equivalent above 0.1% is insignificant.