Dosimetry limitations and pre-treatment dose profile correction for sliding window IMRT

This work investigated the dosimetry limitations of the random and systematic uncertainties of sliding window [SW] intensity modulated radiation therapy [IMRT]. A Varian 21EX linear accelerator, Pinnacle[3] treatment planning system and radiographic film dosimetry was used. The limitations of the SW were studied using beam modulation ranging from 2 to 100 MU/beam, DR from 100 to 600 MU min[-1], LV from 1 to 5 cm [s-1] and field size up to 12 x 12 cm[2]. The random and systematic errors were investigated using clinical and flat beams, as well as beams of high profile modulation including linear, exponential, and sinusoidal profiles. The leading edge and plateau of the SW profiles have a significant deformation for higher DR and for beams of < 10 MUs/beam. It was found that the error is directly proportional to the DR and LV, and inversely proportional to the number of MU/beam. The high DR and LV are limiting factors, producing random profile deformation when SW beams of small number of MU/beam are delivered. A very good agreement was found between the planned and delivered geometrical and clinical dose profiles when beams > 10 MUs irradiated by a DR from 100 to 600 MU min[-1] and LV from 1 to 5 cm s[-1]. After the proposed correction, an average difference < 0.5% for clinical profiles was measured for beams irradiated with DR = 600 MU min[-1] and LV= 5 cm s[-1]. It was concluded that this correction methodology may serve as a pre-treatment Quality Assurance tool for SW IMRT beams

Dosimetric study of inverse-planed intensity modulated, forward-planned intensity modulated and conventional tangential techniques in breast conserving radiotherapy.

OBJECTIVE: The authors present the result of a dosimetric comparison of inverse-planed intensity modulated, forward-planned intensity modulated, and conventional tangential technique in breast conserving radiotherapy. METHOD AND MATERIAL: The breasts (Right side: Left side = 1:1), heart, and lungs of 28 patients were contoured on all the computed tomography (CT)-slice. Three different treatment plans were created: (1) inverse IMRT (iIMRT), (2) forward IMRT (fIMRT), and (3) conventional tangential technique (CVT). The total prescribed dose for all plans was 50 Gy/25 fractions. All treatment plans were normalized at 95% of the prescribed dose covered the entire PTV and used inhomogeneity corrections. RESULTS: For the entire group, the mean breast volume was 517 cc. The V105% for iIMRT, fIMRT and conventional plans was 1.12%, 2.36% and 16.81%, which iIMRT better than fIMRT and CVT (p < 0.001) and fIMRT better than CVT (p < 0.05). The Dmax for the iIMRT plan received 105.03%, which was significantly less than those from the fIMRT(106.6%, p < 0.001) and the conventional (110.68%, p < 0.001) plan. The PTV coverage (V95-105%) for the iIMRT, fIMRTand conventional was 96%, 91% and 87%, which iIMRT better than fIMRT and CVT (p < 0.05) and fIMRT better than CVT (p < 0.05). The PTV CI for the iIMRT technique was 0.704, which was significantly more conformity than those from the fIMRT (0.639, p < 0.001) and the conventional (0.539, p < 0.001) techniques. The PTV CI of fIMRT is significantly better than CVT (p < 0.005). Mean ipsilateral lung dose was 642.7 cGy, 747.6 cGy and 882.25 cGy for iIMRT fIMRT and CVT respectively (p < 0.05) The V20Gy reduced from 14.87% for conventional plan to 12.82% for the fIMRT plan, while 0.88% was obtained for the iIMRT plan (P<0.05). The heart V30 Gy value was 3.124%, 4.65%, and 5.84% for iIMRT, fIMRT and conventional plans, respectively (p < 0.05). The mean dose of contralateral breast was 55.86 cGy, 60.33 cGy, 68.57 cGy for iIMRT, fIMRT and conventional plans, respectively (p < 0.05 both). The mean contralateral lung dose was 57.8 cGy, 43.87 cGy, and 32.28 cGy for iIMRT, fIMRT and conventional plans, respectively (p < 0.005 both). CONCLUSION: The iIMRT technique provides significantly improved PTV Dmax, PTV V105%, PTV V110%, target volume coverage, dose homogeneity and dose conformity throughout the target volume of breast and reduced doses to all critical structures, compared to the fIMRT and conventional techniques. In view of fIMRT technique, it significantly improved the dose distribution and reduced dose to OARs compared to conventional technique, although not better than iIMRT technique.

Multileaf collimator transmission from the first Hi-Art II helical tomotherapy machine in India.

The purpose of this study was to measure the multileaf collimator (MLC) transmission from the first Hi-Art II tomotherapy machine installed at the Advanced Center for Treatment, Research, and Education in Cancer (ACTREC). The MLC transmission was measured with an A1SL ion chamber and the radiographic extended dose range (EDR2) film in virtual water slabs at 1.5-cm depth with a source-to-surface distance of 85 cm. The MLC transmission was measured for 30 s with all leaves open and for 360 s with all leaves closed. The movable jaws were set to the calibration field size of 5 x 40 cm at isocenter. The MLC transmission was found to be 0.3% with the ion chamber and 0.32% with the film. Thus, the MLC transmission value was found well within the manufacturer tolerance of 0.5%. MLC can safely be used for the beam modulation during intensity-modulated radiotherapy (IMRT) to deliver accurate doses to the patients.

A two-year experience of implementing 3 dimensional radiation therapy and intensity-modulated radiation therapy for 925 patients in King Chulalongkorn Memorial Hospital.

BACKGROUND AND OBJECTIVE: Three dimensional conformal radiation therapy (3D CRT) and intensity-modulated radiation therapy (IMRT) have been implemented at Department of Therapeutic Radiation and Oncology, King Chulalongkorn Memorial Hospital (KCMH) since July 2005. This is the first study in Thailand to evaluate the pattern of care and utilization of 3D CRT and IMRT for treatment in each individual cancer. MATERIAL AND METHOD: Between July 2005 and July 2007, 925 newly diagnosed cancer patients underwent IMRT or 3D CRT at KCMH. The authors retrospectively reviewed the experience and utilization of 3D CRT and IMRT for each disease site and region. RESULTS: There were 471 males and 454 females. There were 332 patients (35.9%) treated with IMRT. Among the 332 IMRT patients, there were 100, 32 and 27 nasopharyngeal, lung and prostate cancers, respectively. On the contrary, 593 patients (64.1%) were treated with 3D CRT. Among these, breast, cervix and lung cancers were the most common diseases. Except for head and neck as well as genitourinary cancer 3D CRT was still the main technique used in more than 60% of the patients at KCMH. CONCLUSION: 3D CRT and IMRT have been successfully implemented at KCMH for 2 years. Three dimensional conformal radiation therapy was still the main technique used in more than 60% of the patients at KCMH. Prospective studies evaluating tumor control and treatment sequelae are expected.