Gerardina Stimato

In vivo dosimetry by an aSi‐based EPID

A method for the in vivo determination of the isocenter dose, Diso, and mid-plane dose, Dm, using the transmitted signal St measured by 25 central pixels of an aSi-based EPID is here reported. The method has been applied to check the conformal radiotherapy of pelvic tumors and supplies accurate in vivo dosimetry avoiding many of the disadvantages associated with the use of two diode detectors (at the entrance and exit of the patient) as their periodic recalibration and their positioning. Irradiating water-equivalent phantoms of different thicknesses, a set of correlation functions F(w, l) were obtained by the ratio between St and Dm as a function of the phantom thickness, w, for a different field width, l. For the in vivo determination of Diso and Dm values, the water-equivalent thickness of the patients (along the beam central axis) was evaluated by means of the treatment planning system that uses CT scans calibrated in terms of the electron densities. The Diso and Dm values experimentally determined were compared with the stated doses D(iso,TPS) and D(m,TPS), determined by the treatment planning system for ten pelvic treatments. In particular, for each treatment four fields were checked in six fractions. In these conditions the agreement between the in vivo dosimetry and stated doses at the isocenter point were within 3%. Comparing the 480 dose values obtained in this work with those obtained for 30 patients tested with a similar method, which made use of a small ion-chamber positioned on the EPIDs to obtain the transmitted signal, a similar agreement was observed. The method here proposed is very practical and can be applied in every treatment fraction, supplying useful information about eventual patient dose variations due to the incorrect application of the quality assurance program based on the check of patient setup, machine setting, and calculations.

Adding Ipsilateral V20 and V30 to Conventional Dosimetric Constraints Predicts Radiation Pneumonitis in Stage IIIA–B NSCLC Treated With Combined-Modality Therapy

PURPOSE To determine lung dosimetric constraints that correlate with radiation pneumonitis in non-small-cell lung cancer patients treated with three-dimensional radiation therapy and concurrent chemotherapy. METHODS AND MATERIALS Between June 2002 and December 2006, 97 patients with locally advanced non-small-cell lung cancer were treated with concomitant radiochemotherapy. All patients underwent complete three-dimensional treatment planning (including dose-volume histograms), and patients were treated only if the percentage of total lung volume exceeding 20 Gy (V(20)) and 30 Gy (V(30)), and mean lung dose (MLD) had not exceeded the constraints of 31%, 18%, and 20 Gy, respectively. The total and ipsilateral lung dose-volume histogram parameters, planning target volume, and total dose delivered were analyzed and correlated with pneumonitis incidence. RESULTS If dose constraints to the total lung were respected, the most statistically significant factors predicting pneumonitis were the percentage of ipsilateral lung volume exceeding 20 Gy (V(20)ipsi), percentage of ipsilateral lung volume exceeding 30 Gy (V(30)ipsi), and planning target volume. These parameters divided the patients into low- and high-risk groups: if V(20)ipsi was 52% or lower, the risk of pneumonitis was 9%, and if V(20)ipsi was greater than 52%, the risk of pneumonitis was 46%; if V(30)ipsi was 39% or lower, the risk of pneumonitis was 8%, and if V(30)ipsi was greater than 39%, the risk of pneumonitis was 38%. Actuarial curves of the development of pneumonitis of Grade 2 or higher stratified by V(20)ipsi and V(30)ipsi were created. CONCLUSIONS The correlation between pneumonitis and dosimetric constraints has been validated. Adding V(20)ipsi and V(30)ipsi to the classical total lung constraints could reduce pulmonary toxicity in concurrent chemoradiation treatment. V(20)ipsi and V(30)ipsi are important if the V(20) to the total lung, V(30) to the total lung, and mean lung dose have not exceeded the constraints of 31%, 18%, and 20 Gy, respectively.

Application of a practical method for the isocenter point in vivo dosimetry by a transit signal

This work reports the results of the application of a practical method to determine the in vivo dose at the isocenter point, D(iso), of brain thorax and pelvic treatments using a transit signal S(t). The use of a stable detector for the measurement of the signal S(t) (obtained by the x-ray beam transmitted through the patient) reduces many of the disadvantages associated with the use of solid-state detectors positioned on the patient as their periodic recalibration, and their positioning is time consuming. The method makes use of a set of correlation functions, obtained by the ratio between S(t) and the mid-plane dose value, D(m), in standard water-equivalent phantoms, both determined along the beam central axis. The in vivo measurement of D(iso) required the determination of the water-equivalent thickness of the patient along the beam central axis by the treatment planning system that uses the electron densities supplied by calibrated Hounsfield numbers of the computed tomography scanner. This way it is, therefore, possible to compare D(iso) with the stated doses, D(iso,TPS), generally used by the treatment planning system for the determination of the monitor units. The method was applied in five Italian centers that used beams of 6 MV, 10 MV, 15 MV x-rays and (60)Co gamma-rays. In particular, in four centers small ion-chambers were positioned below the patient and used for the S(t) measurement. In only one center, the S(t) signals were obtained directly by the central pixels of an EPID (electronic portal imaging device) equipped with commercial software that enabled its use as a stable detector. In the four centers where an ion-chamber was positioned on the EPID, 60 pelvic treatments were followed for two fields, an anterior-posterior or a posterior-anterior irradiation and a lateral-lateral irradiation. Moreover, ten brain tumors were checked for a lateral-lateral irradiation, and five lung tumors carried out with three irradiations with different gantry angles were followed. One center used the EPID as a detector for the S(t) measurement and five pelvic treatments with six fields (many with oblique incidence) were followed. These last results are reported together with those obtained in the same center during a pilot study on ten pelvic treatments carried out by four orthogonal fields. The tolerance/action levels for every radiotherapy fraction were 4% and 5% for the brain (symmetric inhomogeneities) and thorax/pelvic (asymmetric inhomogeneities) irradiations, respectively. This way the variations between the total measured and prescribed doses at the isocenter point in five fractions were well within 2% for the brain treatment, and 4% for thorax/pelvic treatments. Only 4 out of 90 patients needed new replanning, 2 patients of which needed a new CT scan.

Integration between in vivo dosimetry and image guided radiotherapy for lung tumors

The article reports a feasibility study about the potentiality of an in vivo dosimetry method for the adaptive radiotherapy of the lung tumors treated by 3D conformal radiotherapy techniques (3D CRTs). At the moment image guided radiotherapy (IGRT) has been used for this aim, but it requires taking many periodic radiological images during the treatment that increase workload and patient dose. In vivo dosimetry reported here can reduce the above efforts, alerting the medical staff for the commissioning of new radiological images for an eventual adaptive plan. The in vivo dosimetry method applied on 20 patients makes use of the transit signal St on the beam central axis measured by a small ion chamber positioned on an electronic portal imaging device (EPID) or by the EPID itself. The reconstructed in vivo dosimetry at the isocenter point Diso requires a convolution between the transit signal St and a dose reconstruction factor C that essentially depends on (i) tissue inhomogeneities along the beam central axis and (ii) the in-patient isocenter depth. The C factors, one for every gantry angle, are obtained by processing the patients computed tomography scan. The method has been recently applied in some Italian centers to check the radiotherapy of pelvis, breast, head, and thorax treatments. In this work the dose reconstruction was carried out in five centers to check the Diso in the lung tumor during the 3D CRT, and the results have been used to detect the interfraction tumor anatomy variations that can require new CT imaging and an adaptive plan. In particular, in three centers a small ion chamber was positioned below the patient and used for the St measurement. In two centers, the St signal was obtained directly by 25 central pixels of an a-Si EPID, equipped with commercial software that enabled its use as a stable detector. A tolerance action level of +/- 6% for every checked beam was assumed. This means that when a difference greater than 6% between the predicted dose by the treatment planning system, Diso,TPS, and the Diso was observed, the clinical action started to detect possible errors. 60% of the patients examined presented morphological changes during the treatment that were checked by the in vivo dosimetry and successively confirmed by the new CT scans. In this work, a patient that showed for all beams Diso values outside the tolerance level, new CT scans were commissioned for an adaptive plan. The lung dose volume histograms (DVHs) for a Diso,TPs=2 Gy for fraction suggested the adaptive plan to reduce the dose in lung tissue. The results of this research show that the dose guided radiotherapy (DGRT) by the Diso reconstruction was feasible for daily or periodic investigation on morphological lung tumor changes. In other words, since during 3D CRT treatments the anatomical lung tumor changes occur frequently, the DGRT can be well integrated with the IGRT.

Molecularly targeted therapy and radiotherapy in the management of localized gastrointestinal stromal tumor (GIST) of the rectum: a case report.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract. The main treatment for localized gastrointestinal stromal tumors is surgical resection. These tumors respond poorly to conventional cytotoxic chemotherapy agents and to radiotherapy. Imatinib mesylate, a small-molecule kinase inhibitor, has proved useful in the treatment of recurrent or metastatic GISTs and is now being tested in the adjuvant and neoadjuvant setting. The role of radiotherapy in the management of patients with GIST is currently restricted to symptomatic palliation. We present the case of a 54-year-old man affected by rectal GIST extending to the anal canal, with constipation, hematochezia, and anal pain. He received imatinib, 400 mg orally per day, for a week before and during radiation therapy. Irradiation was delivered to the gross tumor volume by 3D conformal therapy. The planned total dose was 50.4 Gy in fractions of 1.8 Gy daily. We observed a partial clinical response 3 weeks after the end of combination treatment. The patient then underwent a sphincter-saving surgical procedure. There was no perioperative morbidity and a complete pathological response was obtained. At the present time, the role of radiotherapy in the management of patients with GIST is restricted to symptomatic palliation. The introduction of molecularly targeted therapy combined with radiation therapy could improve the outcomes for patients diagnosed with GIST.

Whole-breast irradiation: a subgroup analysis of criteria to stratify for prone position treatment.

To select among breast cancer patients and according to breast volume size those who may benefit from 3D conformal radiotherapy after conservative surgery applied with prone-position technique. Thirty-eight patients with early-stage breast cancer were grouped according to the target volume (TV) measured in the supine position: small (≤400 mL), medium (400-700 mL), and large (≥700 ml). An ad-hoc designed and built device was used for prone set-up to displace the contralateral breast away from the tangential field borders. All patients underwent treatment planning computed tomography in both the supine and prone positions. Dosimetric data to explore dose distribution and volume of normal tissue irradiated were calculated for each patient in both positions. Homogeneity index, hot spot areas, the maximum dose, and the lung constraints were significantly reduced in the prone position (p < 0.05). The maximum heart distance and the V(5Gy) did not vary consistently in the 2 positions (p = 0.06 and p = 0.7, respectively). The number of necessary monitor units was significantly higher in the supine position (312 vs. 232, p < 0.0001). The subgroups analysis pointed out the advantage in lung sparing in all TV groups (small, medium and large) for all the evaluated dosimetric constraints (central lung distance, maximum lung distance, and V(5Gy), p < 0.0001). In the small TV group, a dose reduction in nontarget areas of 22% in the prone position was detected (p = 0.056); in the medium and high TV groups, the difference was of about -10% (p = NS). The decrease in hot spot areas in nontarget tissues was 73%, 47%, and 80% for small, medium, and large TVs in the prone position, respectively. Although prone breast radiotherapy is normally proposed in patients with breasts of large dimensions, this study gives evidence of dosimetric benefit in all patient subgroups irrespective of breast volume size.

DYNAMIC CONFORMAL ARC THERAPY: TRANSMITTED SIGNAL IN-VIVO DOSIMETRY

A method for the determination of the in vivo isocenter dose, D(iso), has been applied to the dynamic conformal are therapy (DCAT) for thoracic tumors. The method makes use of the transmitted signal, S(t,alpha), measured at different gantry angles, a, by a small ion chamber positioned on the electronic portal imaging device. The in vivo method is implemented by a set of correlation functions obtained by the ratios between the transmitted signal and the midplane dose in a solid phantom, irradiated by static fields. The in vivo dosimetry at the isocenter for the DCAT requires the convolution between the signals, S(t,alpha), and the dose reconstruction factors, C(alpha), that depend on the patients anatomy and on its tissue inhomogeneities along the beam central axis in the a direction. The C(alpha) factors are obtained by processing the patients computed tomography scan. The method was tested by taking measurements in a cylindrical phantom and in a Rando Alderson phantom. The results show that the difference between the convolution calculations and the phantom measurements is within +/-2%. The in vivo dosimetry of the stereotactic DCAT for six lung tumors, irradiated with three or four arcs, is reported. The isocenter dose up to 17 Gy per therapy fraction was delivered on alternating days for three fractions. The agreement obtained in this pilot study between the total in vivo dose D(iso) and the planned dose D(iso,TPS) at the isocenter is +/-4%. The method has been applied on the DCAT obtaining a more extensive monitoring of possible systematic errors, the effect of which can invalidate the current therapy which uses a few high-dose fractions.

Breast in vivo dosimetry by a portal ionization chamber

This work reports a practical method for the determination of the in vivo breast middle dose value, D(m) on the beam central axis, using a signal S(t), obtained by a small thimble ion chamber positioned at the center of the electronic portal imaging device, and irradiated by the x-ray beam transmitted through the patient. The use of a stable ion chamber reduces many of the disadvantages associated with the use of diodes as their periodic recalibration and positioning is time consuming. The method makes use of a set of correlation functions obtained by the ratios S(t)/D(m), determined by irradiating cylindrical water phantoms with different diameters. The method proposed here is based on the determination of the water-equivalent thickness of the patient, along the beam central axis, by the treatment planning system that makes use of the electron densities obtained by a computed tomography scanner. The method has been applied for the breast in vivo dosimetry of ten patients treated with a manual intensity modulation with four asymmetric beams. In particular, two tangential rectangular fields were first delivered, thereafter a fraction of the dose (typically less than 10%) was delivered with two multi leaf-shaped beams which included only the mammarian tissue. Only the two rectangular fields were tested and for every checked field five measurements were carried out. Applying a continuous quality assurance program based on the tests of patient setup, machine settings and dose planning, the proposed method is able to verify agreements between the computed dose D(m,TPS) and the in vivo dose value D(m), within 4%.