Midline Dose Research Articles

Correlation functions for Elekta aSi EPIDs used as transit dosimeter for open fields

In‐vivo dosimetry techniques are currently being applied only by a few Centers because they require time‐consuming implementation measurements, and workload for detector positioning and data analysis. The transit in‐vivo dosimetry performed by the electronic portal imaging device (EPID) avoids the problem of solid‐state detector positioning on the patient. Moreover, the dosimetric characterization of the recent Elekta aSi EPIDs in terms of signal stability and linearity make these detectors useful for the transit in‐vivo dosimetry with 6, 10 and 15 MV photon beams. However, the implementation of the EPID transit dosimetry requires several measurements. Recently, the present authors have developed an in‐vivo dosimetry method for 3D CRT based on correlation functions defined by the ratios between the transit signal, st(w,L), by the EPID and the phantom midplane dose, Dm(w,L), at the source to axis distance (SAD) as a function of the phantom thickness, w, and the square field dimensions, L. When the phantom midplane was positioned at distance, d, from the SAD, the ratios st(w,L)/st'(d,w,L) were used to take into account the variation of the scattered photon contributions on the EPID as a function of d and L.The aim of this paper is the implementation of a procedure that uses generalized correlation functions obtained by nine Elekta Precise linac beams. The procedure can be used by other Elekta Precise linacs equipped with the same aSi EPIDs, assuming the stabilities of the beam output factors and the EPID signals. The procedure here reported avoids measurements in solid water equivalent phantoms needed to implement the in‐vivo dosimetry method in the radiotherapy department. A tolerance level ranging between ±5% and ±6% (depending on the type of tumor) was estimated for the comparison between the reconstructed isocenter dose, Diso, and the computed dose, Diso,TPS, by the treatment planning system (TPS).PACS number: 87.55.Qr; 87.56.Fc

Midline Dose Verification with Diode In Vivo Dosimetry for External Photon Therapy of Head and Neck and Pelvis Cancers During Initial Large-Field Treatments

During radiotherapy treatments, quality assurance/control is essential, particularly dose delivery to patients. This study was designed to verify midline doses with diode in vivo dosimetry. Dosimetry was studied for 6-MV bilateral fields in head and neck cancer treatments and 10-MV bilateral and anteroposterior/posteroanterior (AP/PA) fields in pelvic cancer treatments. Calibrations with corrections of diodes were performed using plastic water phantoms; 190 and 100 portals were studied for head and neck and pelvis treatments, respectively. Calculations of midline doses were made using the midline transmission, arithmetic mean, and geometric mean algorithms. These midline doses were compared with the treatment planning system target doses for lateral or AP (PA) portals and paired opposed portals. For head and neck treatments, all 3 algorithms were satisfactory, although the geometric mean algorithm was less accurate and more uncertain. For pelvis treatments, the arithmetic mean algorithm seemed unacceptable, whereas the other algorithms were satisfactory. The random error was reduced by using averaged midline doses of paired opposed portals because the asymmetric effect was averaged out. Considering the simplicity of in vivo dosimetry, the arithmetic mean and geometric mean algorithm should be adopted for head/neck and pelvis treatments, respectively.

Calibration of portal imaging devices for radiotherapy in-vivo dosimetry

The complexity of radiotherapy techniques requires an accurate verification of the dose delivered to the patient during treatment. Recently, the present authors have developed an in patient dose reconstruction method with X-ray beams for 3D conformal radiotherapy. The procedure is based on correlation functions defined by the ratios of the transit signal measured by an electronic portal imaging device (EPID) to the mid-plane dose measured by calibrated ion chambers in a solid water phantom. The dosimetric characterization of aSi EPIDs in terms of signal stability, linearity and dependence on field dimension pointed out that these detectors are useful for the transit dosimetry of photon beams. However, the aSi EPIDs manufactured by Varian, Elekta and Siemens for their linacs are at present used for the visual inspection of the patient's set-up, and their use as transit dosimeters needs a special calibration that requires an effort for every beam. The aim of this paper has been the determination of a generalized EPID calibration that can be used by linacs of different manufacturers equipped with aSi EPIDs. The transit dosimetry method here proposed could supply for every linac the reconstruction in real time of the isocenter dose in patients with a tolerance level ranging between ±4% and ±6% depending on the treatment type and body district.

Dose-guided radiotherapy for lung tumors

The transit in vivo dosimetry performed by an electronic portal imaging device (EPID) is a very practical method to check error sources in radiotherapy. Recently, the present authors have developed an in vivo dosimetry method based on correlation functions, F (w, L), defined as the ratio between the transit signal, S(t) (w, L), by the EPID and the mid-plane dose, D(m) (w, L), in a solid water phantom as a function of the phantom thickness, w, and of the field dimensions, L. In particular, generalized correlation functions F (w, L) for 6, 10 and 15 MV X-ray beams supplied by a pilot Varian linac, are here used by other three linacs operating in two centers. This way the workload, due to measurements in solid water phantom, needed to implement the in vivo dosimetry method was avoided. This article reports a feasibility study on the potentiality of this procedure for the adaptive radiotherapy of lung tumors treated by 3D conformal radiotherapy techniques. In particular, the dose reconstruction at the isocenter point D(iso) in the lung tumor has been used as dose-guided radiotherapy (DGRT), to detect the inter-fraction tumor anatomy variations that can require new CT scans and an adaptive plan. When a difference greater than 6% between the predicted dose by the treatment planning system (TPS), D (iso,TPS) and the D(iso) was observed, the clinical action started to detect possible anatomical lung tumor changes. Twelve over twenty patients examined presented in vivo dose discrepancies due to the tumor morphological changes during treatments, and these results were successively confirmed by new CT scans. In this work, for a patient that showed for all beams, D (iso) values over the tolerance level, the new CT scan was used for an adaptive plan. The lung dose volume histogram for D (iso,TPS) = 2 Gy per fraction suggested the adaptive plan. In particular, the lung volume included in 2 Gy increased from 350 cm(3) of the original plan to 550 cm(3) of the hybrid plan, while for the adaptive plan the lung volume included in 2 Gy decreased to 15 cm(3). Moreover, the mean doses to the organs at risk were reduced to 70%. The results of this research show that the DGRT procedure by the D(iso) reconstruction, integrated with radiological imaging, was feasible for periodic investigation on morphological lung tumor changes. This feasibility study takes into account the accuracy of two algorithms based on the pencil beam and collapsed cone convolution models for dose calculations where large density inhomogeneities are present.

Verification of the Accuracy of the Delivered Dose in Pelvic and Breast Cancer Radiotherapy by in-vivo Semi-Conductor Dosimetry

Introduction: Delivering maximum dose to tumor and minimum dose to normal tissues is the most important goal in radiotherapy. According to ICRU, the maximum acceptable uncertainty in the delivered dose compared to the prescribed dose should be lower than 5%, and this is because of the relationship between absorbed dose, tumor control and normal tissue damage. Absorbed dose accuracy is investigated by an in vivo dosimetry method. In this paper, we compared absorbed dose in the tumors of the breast and pelvic region against the calculated dose. The amount of deviations and the factors that cause this deviation in dose delivery to patients and some methods for decreasing them were evaluated. Materials and methods: The entrance and exit doses of 36 pelvic-region cancer patients and 38 breast cancer patients who were treated by cobalt-60 teletherapy were measured using p-type diodes. It should be noted that the transmission method was used to assess the dose at isocenter. Two ionization chambers (0.6 cc and 0.3 cc) were used for calibration and determination of the correction coefficients in water and slab phantoms. Deviations between calculated and measured doses of entrance, exit and midline points were calculated and the results were shown using histograms. Results: The average and standard deviation for entrance, exit and midline points for pelvis cancer were assessed to be about 0.10%, -1.86% and -1.35% for mean deviation and 5.03%, 7.32% and 5.86% for standard deviation, respectively. The corresponding data for breast cancer were 0.78%, 5.29% and 3.59% for mean deviation and 5.97%, 10.23% and 9.86%, respectively. There was no significant difference between the calculated and measured doses (p > 0.1), except exit dose in breast cancer (p < 0.05). The temperature and angle of incidence correction factors were neglected due to their less than 1% deviations. Discussion and Conclusions: Some error sources are patient setup error, patient motion and dose calculation algorithm error (due to ignoring inhomogeneity and patient curvature). As no significant deviations were found in midline dose, the method used has an acceptable accuracy. In vivo dosimetry can perform a basic role in the quality control of radiotherapy departments.

Poster - Thurs Eve-39: Full 3D dose calculation for total body irradiation: A comparison study between treatment planning systems in homogeneous and heterogeneous conditions.

To perform full 3D heterogeneous dose calculations for total body irradiation (TBI) cases and compare different treatment planning softwares (TPS). A retrospective study was performed on 7 patients. Dose distributions obtained with Pinnacle3 v.7.9u (Philips Medical Systems) were compared with the ones calculated using our actual TBI planning system Theraplan Plus (TPP) by MDS Nordion/Nucletron. Two different Pinnacle3 models were studied: standard beam commissioning (std_Pinnacle3 ) and TBI commissioning (TBI_Pinnacle3 ). For the later case, commissioning was adapted for the special TBI conditions (extended SSD of 190cm, large field, acrylic beam spoiler, and out of field dose (OFD)). Significant differences are found between the TPP, std_Pinnacle3 and TBI_Pinnacle3 dose distributions. For the relative mid-line doses, differences up to 12% were observed. Systematic overestimations of 5% were found in patients extremities between TPP and TBI_Pinnacle3 . Average dose underestimation of 3% was observed between std_Pinnacle3 and TBI_Pinnacle3 . Differences in patient extremities are attributed to the OFD contribution which is not correctly computed in TPP and std_Pinnacle3 . Dose comparison outside the patient's mid-line showed greater differences (up to 20%) between models. Accurate 3D heterogeneous dose calculations with TBI_Pinnacle3 model show major differences (homogeneous versus heterogeneous) in high and low density regions. Dose overestimation of 5% was observed in bony regions and dose underestimation of 5% to 10% was observed in lung regions. Those results are of major interest since they show a strong dependence of the dose calculation outcome on both TPS and commissioning used, potentially leading to significant dose misevaluation.

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 patient's anatomy and on its tissue inhomogeneities along the beam central axis in the a direction. The C(alpha) factors are obtained by processing the patient's 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.

Testicular shield for para-aortic radiotherapy and estimation of gonad doses

For radiotherapy of para-aortic and abdominal regions in male patients, gonads are to be protected to receive less than 2% of the prescribed dose. A testicular shield was fabricated for abdominal radiotherapy with 15 MV X-rays ((Clinac 2300 CD, Varian AG) with low melting point alloy (Cerroband). The dimensions of the testicular shield were 6.5 cm diameter and 3.5 cm depth with 1.5 cm wall thickness. During treatment, this shield was held in position by a rectangular sponge and Styrofoam support. Phantom measurement was carried out with a humanoid phantom and a 0.6 cc ion chamber. The mean energy of the scattered photon was calculated for single scattering at selected distances from the beam edge and with different field dimensions. One patient received radiotherapy with an inverted Y field and gonad doses were estimated using calibrated thermo-luminescent detector (TLD) chips. Measured doses with the ion chamber were 7.1 and 3.5% of the mid-plane doses without a shield at 3 and 7.5 cm off-field respectively. These values decreased to 4.6 and 1.7% with the bottom shield alone, and to 1.7 and 0.8% with both bottom and top shields covering the ion chamber. The measured doses at the gonads during the patient’s treatment were 0.5–0.92% for the AP field (0.74 ± 0.17%, n = 5) and 0.5–1.2% for the PA field (0.88 ± 0.24%, n = 5). The dose received by the testis for the full course of treatment was 32 cGy (0.8%) for a total mid-plane dose of 40 Gy. The first-scatter energy estimated at the gonads is around 1.14 MeV for a primary beam of 15 MV for a long axis dimension of 37 cm of primary field. During the patient’s treatment, the estimated absorbed doses at the gonads were comparable with reported values in similar treatments. The testicular shield reported in this study is of light weight and could be used conveniently in treatments of abdominal fields.

In vivo dose verification for photon treatments of head and neck carcinomas using MOSFET dosimeters

In vivo dosimetry was performed for the head and neck carcinoma patients during the treatment of a large photon field using MOSFETs. This study followed the protocols recommended by the European Society for Therapeutic Radiology and Oncology. A total of 32 portals belonging to 12 patients were under investigation. Results showed that the deviation between in vivo midline doses and planned target doses was partly due to the manual dose calculations in the treatment planning which used the patient geometric thickness rather than the radiological thickness. Other factors responsible for this deviation included the difficult positioning of MOSFETs on the face mask, the asymmetric positioning of MOSFETs on the left and right sides of the mask, and the asymmetric tissue inhomogeneities with respect to the body midline. To reduce the deviation contributed from these factors, in vivo midline doses were calculated by averaging the results for each bilaterally opposed portals and compared with corresponding planned target doses. This comparison showed that MOSFET dosimeters are suitable for in vivo dosimetry of the present study.

Impact of Postoperative Radiation Therapy on Postmastectomy Breast Reconstruction

Impact of Postoperative Radiation Therapy on Postmastectomy Breast Reconstruction

Total Body Irradiation, Toward Optimal Individual Delivery: Dose Evaluation With Metal Oxide Field Effect Transistors, Thermoluminescence Detectors, and a Treatment Planning System

To predict the three-dimensional dose distribution of our total body irradiation technique, using a commercial treatment planning system (TPS). In vivo dosimetry, using metal oxide field effect transistors (MOSFETs) and thermoluminescence detectors (TLDs), was used to verify the calculated dose distributions. A total body computed tomography scan was performed and loaded into our TPS, and a three-dimensional-dose distribution was generated. In vivo dosimetry was performed at five locations on the patient. Entrance and exit dose values were converted to midline doses using conversion factors, previously determined with phantom measurements. The TPS-predicted dose values were compared with the MOSFET and TLD in vivo dose values. The MOSFET and TLD dose values agreed within 3.0% and the MOSFET and TPS data within 0.5%. The convolution algorithm of the TPS, which is routinely applied in the clinic, overestimated the dose in the lung region. Using a superposition algorithm reduced the calculated lung dose by approximately 3%. The dose inhomogeneity, as predicted by the TPS, can be reduced using a simple intensity-modulated radiotherapy technique. The use of a TPS to calculate the dose distributions in individual patients during total body irradiation is strongly recommended. Using a TPS gives good insight of the over- and underdosage in a patient and the influence of patient positioning on dose homogeneity. MOSFETs are suitable for in vivo dosimetry purposes during total body irradiation, when using appropriate conversion factors. The MOSFET, TLD, and TPS results agreed within acceptable margins.

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, Diso, of brain thorax and pelvic treatments using a transit signal St. The use of a stable detector for the measurement of the signal St (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 St and the mid-plane dose value, Dm, in standard water-equivalent phantoms, both determined along the beam central axis.The in vivo measurement of Diso 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 Diso with the stated doses, Diso,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 60Co γ-rays. In particular, in four centers small ion-chambers were positioned below the patient and used for the St measurement. In only one center, the St 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 St 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.

Evaluating the Figure of Merit in mammography utilizing Monte Carlo simulation

Mammography represents the most powerful screening tool available for the early detection of breast cancer. As a screening technique, it addresses to asymptomatic women and therefore the implementation of the “As Low As Reasonably Achievable” (ALARA) principle is of increased importance. The overall optimization of any radiographic procedure requires the definition of a suitable Figure of Merit (FOM), based on a representative combination of image quality and dose characteristics. For the present study, Monte Carlo simulation of the mammographic process was utilized to derive the energy deposition inside the breast phantom and the signal beneath it. Certain signal characteristics (Subject Contrast (SC), Contrast to Noise Ratio (CNR), squared CNR) were studied (normalized to the output signal) and combined with appropriate dose indices (Entrance Surface Dose (ESD), Average Glandular Dose (AGD), Mid Plane Dose (MPD)) to extract an appropriate FOM index, suitable for spectrum based optimization. Results, for different breast glandularities and lesion sizes and compositions, demonstrate that the choice of the optimum spectrum strongly depends not only on the breast and lesion characteristics, but also on the selection of the utilized FOM index. For screen-film mammography, where the primary image quality index is considered the SC, the traditionally used Mo and Rh-anode spectra demonstrate improved overall performance, despite their inferior dosimetric characteristics compared to W-anode spectra. If digital mammography is considered, where the CNR is of primary importance, W-anode spectra demonstrate a noticeable improved performance.

Dosimetry and verification of 60Co total body irradiation with human phantom and semiconductor diodes

Total Body Irradiation (TBI) is a form of radiotherapy used for patients prior to bone marrow or stem cell transplant to destroy any undetectable cancer cells. The dosimetry characteristics of a 60Co unit for TBI were studied and a simple method for the calculation of the prescribed dose for TBI is presented. Dose homogeneity was verified in a human phantom. Dose measurements were made in water phantom (30 × 30 × 30 cm3), using farmer ionization chamber (0.6 cc, TM30010, PTW) and a parallel plate ionization chamber (TM23343, PTW). Point dose measurements for AP/PA irradiation were measured in a human phantom using silicon diodes (T60010L, PTW). The lung dose was measured with an ionization chamber (0.3 cc, TM31013). The validity of the proposed algorithm was checked at TBI distance using the human phantom. The accuracy of the proposed algorithm was within 3.5%. The dose delivered to the mid-lobe of the lung was 14.14 Gy and it has been reduced to 8.16 Gy by applying the proper shield. Dose homogeneity was within ±7% for all measured points. The results indicate that a good agreement between the total prescribed and calculated midplane doses can be achieved using this method. Therefore, it could be possible to use calculated data for TBI treatments.

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.

Correction to BrainSCAN central axis dose calculations for 6-MV photon beams to lung with lateral electron disequilibrium

Purpose: To develop and evaluate a correction method for lateral electron disequilibrium and tissue inhomogeneities in lung tissues applicable to the BrainSCAN treatment planning system. Methods and Materials: Four noncoplanar 6-MV photon beams with different beam diameters were applied to the right lung of a thorax phantom. The measured/calculated dose value ratio was evaluated as a function of a parameter that describes the degree of the lateral electron disequilibrium based on the primary dose. Results: The dose ratio showed a clearcut linear dependency on the disequilibrium parameter. Applying the proposed correction method, only minor differences between the measured and calculated doses were found for lesions >1 cm. However, for lesions Conclusion: The data have indicated a relevant magnitude of the correction factor only for lung lesions

Clinical experience with EPID dosimetry for prostate IMRT pre‐treatment dose verification

The aim of this study was to demonstrate how dosimetry with an amorphous silicon electronic portal imaging device (a-Si EPID) replaced film and ionization chamber measurements for routine pre-treatment dosimetry in our clinic. Furthermore, we described how EPID dosimetry was used to solve a clinical problem. IMRT prostate plans were delivered to a homogeneous slab phantom. EPID transit images were acquired for each segment. A previously developed in-house back-projection algorithm was used to reconstruct the dose distribution in the phantom mid-plane (intersecting the isocenter). Segment dose images were summed to obtain an EPID mid-plane dose image for each field. Fields were compared using profiles and in two dimensions with the y evaluation (criteria: 3%/3 mm). To quantify results, the average gamma (gamma avg), maximum gamma (gamma max), and the percentage of points with gamma < 1(P gamma < 1) were calculated within the 20% isodose line of each field. For 10 patient plans, all fields were measured with EPID and film at gantry set to 0 degrees. The film was located in the phantom coronal mid-plane (10 cm depth), and compared with the back-projected EPID mid-plane absolute dose. EPID and film measurements agreed well for all 50 fields, with (gamma avg) =0.16, (gamma max)=1.00, and (P gamma < 1)= 100%. Based on these results, film measurements were discontinued for verification of prostate IMRT plans. For 20 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original gantry angles. The planned isocenter dose (plan(iso)) was verified with the EPID (EPID(iso)) and an ionization chamber (IC(iso)). The average ratio, (EPID(iso)/IC(iso)), was 1.00 (0.01 SD). Both measurements were systematically lower than planned, with (EPID(iso)/plan(iso)) and (IC(iso)/plan(iso))=0.99 (0.01 SD). EPID mid-plane dose images for each field were also compared with the corresponding plane derived from the three dimensional (3D) dose grid calculated with the phantom CT scan. Comparisons of 100 fields yielded (gamma avg)=0.39, gamma max=2.52, and (P gamma < 1)=98.7%. Seven plans revealed under-dosage in individual fields ranging from 5% to 16%, occurring at small regions of overlapping segments or along the junction of abutting segments (tongue-and-groove side). Test fields were designed to simulate errors and gave similar results. The agreement was improved after adjusting an incorrectly set tongue-and-groove width parameter in the treatment planning system (TPS), reducing (gamma max) from 2.19 to 0.80 for the test field. Mid-plane dose distributions determined with the EPID were consistent with film measurements in a slab phantom for all IMRT fields. Isocenter doses of the total plan measured with an EPID and an ionization chamber also agreed. The EPID can therefore replace these dosimetry devices for field-by-field and isocenter IMRT pre-treatment verification. Systematic errors were detected using EPID dosimetry, resulting in the adjustment of a TPS parameter and alteration of two clinical patient plans. One set of EPID measurements (i.e., one open and transit image acquired for each segment of the plan) is sufficient to check each IMRT plan field-by-field and at the isocenter, making it a useful, efficient, and accurate dosimetric tool.

Adjuvant axillary radiotherapy for breast cancer: Is CT planning with nodal contouring better than traditional planning?

606 Background: Historically, adjuvant radiotherapy planning for breast cancer has been based on clinical mark-up then subsequently bony landmarks. The objective of this study is to investigate whether individualized CT-based nodal contour guided planning of axillary fields in breast cancer improves nodal coverage and minimizes dose to normal tissues. There have been no previous studies addressing this issue. Methods: Thirty 4-field radiotherapy plans were selected as ‘traditional’ plans: 15 without nodal contours (traditional field placement) and 15 with radiation oncologists’ nodal contours. The following structures were contoured on each patient CT, regardless of previously contoured structures: level I, level II/III, supraclavicular(SCV)/infraclavicular(ICV) lymph nodes, ipsilateral brachial plexus and lung. Dose volume histograms (DVHs) of the listed contoured structures were obtained for the 30 original plans. All 30 patients were then re-planned with the same anterior dose prescription as the original plan (4000cGy/16 fractions (#) or 4500cGy/25#) but adjusted depth of midplane dose prescription based on nodal depth; MLC blocking was adjusted to the ‘study’ nodal contours. DVHs of the contoured structures for the new nodal-based plans were compared with the DVHs of the original plans, using two-tailed paired t-tests. Results: Volume receiving 90% dose (V90) was significantly improved for SCV nodes: original plan 84.67% vs nodal plan 95.76%(p=0.0005). V90 were similar for level I and level II/III nodes, but hot spots in these nodal groups were significantly hotter in the original vs nodal plan: mean hot spot for level I 120.8% vs 116.3%(p=0.0008), mean hot spot for level II/III 118.1% vs 113.2% (p=0.000003). Dose to 90% of the brachial plexus (D90) was significantly higher in the original vs nodal plan: 79.92% vs 40.92%(p=0.0028). V20 lung were not significantly different. Mean total body dose was significantly higher in the original vs nodal plan 831.8cGy vs 677.7cGy (p=0.0015). Conclusions: CT-based nodal contour guided planning significantly improves coverage of the nodes, particularly supraclavicular nodes, while markedly reducing the dose to critical normal structures, such as brachial plexus. No significant financial relationships to disclose.

Changes in lateral dimensions of irradiated volume and their impact on the accuracy of dose delivery during radiotherapy for head and neck cancer

Background and purpose To assess changes in lateral dimensions of irradiated volume during head and neck cancer radiotherapy and to determine their impact on the accuracy of dose delivery. Patients and methods Lateral dimensions of irradiated volumes were measured in five predefined points prior to treatment and then bi-weekly. For each measurement, midline dose was calculated and verified using in vivo dosimetry. Early radiation reactions, patient weight changes and the need to modify radiotherapy accessories were also recorded. The study included 33 head and neck cancer patients irradiated using parallel opposed megavoltage fields. Results Body mass changes during radiotherapy ranged from −18 to +4 kg (median −5). Lateral dimension changes >5 mm (range −37 to +16) occurred in 32 patients (97%). For axis measurements, the degree of lateral dimension changes were correlated with treatment field size ( P=0.022) and degree of mucositis ( P=0.017). Axis doses calculated for changed dimensions varied from those prescribed by −2.5 to +6% (median +2%). Differences larger than 5% were present in 4.8% of calculations. In 17 patients (52%), radiotherapy accessories had to be modified during treatment. The need to modify radiotherapy accessories correlated with larger treatment portals ( P=0.004), more weight loss during treatment ( P=0.01) and higher initial N stage ( P=0.04). Conclusions Changes of irradiated volume lateral dimensions during head and neck cancer radiotherapy may lead to considerable dose delivery inaccuracies. Watchful monitoring, corrections to calculated dose when changes observed are significant and radiotherapy accessories modification during the course of treatment are strongly recommended.

Chemo-radiation in advanced nasopharyngeal carcinoma, disease free after 6 years--a case report.

This is a case report of a patient with advanced nasopharyngeal Carcinoma, (T4 N2 MO) who had chemo-radiation with Cisplatin based chemotherapy and total midplane dose of 60 Gray external beam radiation. Six years after treatment patient has remained disease free and the primary site histologically confirmed disease free with no clinical evidence of regional or distance metastases