Comparison of ionization chambers of various volumes for IMRT absolute dose verification

IMRT plans are usually verified by phantom measurements: dose distributions are measured using film and the absolute dose using an ionization chamber. The measured and calculated doses are compared and planned MUs are modified if necessary. To achieve a conformal dose distribution, IMRT fields are composed of small subfields, or "beamlets." The size of beamlets is on the order of 1 x 1 cm2. Therefore, small chambers with sensitive volumes < or = 0.1 cm3 are generally used for absolute dose verification. A dosimetry system consisting of an electrometer, an ion chamber, and connecting cables may exhibit charge leakage. Since chamber sensitivity is proportional to volume, the effect of leakage on the measured charge is relatively greater for small chambers. Furthermore, the charge contribution from beamlets located at significant distances from the point of measurement may be below the small chambers threshold and hence not detected. On the other hand, large (0.6 cm3) chambers used for the dosimetry of conventional external fields are quite sensitive. Since these chambers are long, the electron fluence through them may not be uniform ("temporal" uniformity may not exist in the chamber volume). However, the cumulative, or "spatial" fluence distribution (as indicated by calculated IMRT dose distribution) may become uniform at the chamber location when the delivery of all IMRT fields is completed. Under the condition of "spatial" fluence uniformity, the charge collected by the large chamber may accurately represent the absolute dose delivered by IMRT to the point of measurement. In this work, 0.6, 0.125, and 0.009 cm3 chambers were used for the absolute dose verification for tomographic and step-and-shoot IMRT plans. With the largest, 0.6 cm3 chamber, the measured dose was equal to calculated within 0.5%, when no leakage corrections were made. Without leakage corrections, the error of measurement with a 0.125 cm3 chamber was 2.6% (tomographic IMRT) and 1.5% (step-and-shoot IMRT). When doses measured by a 0.125 cm3 chamber were corrected for leakage, the difference between the calculated and measured doses reduced to 0.5%. Leakage corrected doses obtained with the 0.009 cm3 chamber were within 1.5%-1.7% of calculated doses. Without leakage corrections, the measurement error was 16% (tomographic IMRT) and 7% (step-and-shoot IMRT)

Improvement of dose distributions in abutment regions of intensity modulated radiation therapy and electron fields

In recent years, intensity modulated radiation therapy (IMRT) is used to radiate tumors that are in close proximity to vital organs. Targets consisting of a deep-seated region followed by a superficial one may be treated with abutting photon and electron fields. However, no systematic study regarding matching of IMRT and electron beams was reported. In this work, a study of dose distributions in the abutment region between tomographic and step-and-shoot IMRT and electron fields was carried out. A method that significantly improves dose homogeneity between abutting tomographic IMRT and electron fields was developed and tested. In this method, a target region that is covered by IMRT was extended into the superficial target area by ∼2.0 cm. The length and shape of IMRT target extension was chosen such that high isodose lines bent away from the region treated by the electrons. This reduced the magnitude of hot spots caused by the “bulging effect” of electron field penumbra. To account for the uncertainties in positioning of the IMRT and electron fields, electron field penumbra was modified using conventional (photon) multileaf collimator (MLC). The electron beam was delivered in two steps: half of the dose delivered with MLCs in retracted position and another half with MLCs extended to the edge of electron field that abuts tomographic IMRT field. The experimental testing of this method using film dosimetry has demonstrated that the magnitude of the hot spots was reduced from ∼45% to ∼5% of the prescription dose. When an error of ±1.5 mm in field positioning was introduced, the dose inhomogeneity in the abutment region did not exceed ±15% of the prescription dose. With step-and-shoot IMRT, the most homogeneous dose distribution was achieved when there was a 3 mm gap between the IMRT and electron fields

A modified method of planning and delivery for dynamic multileaf collimator intensity-modulated radiation therapy

Purpose: To develop a modified planning and delivery technique that reduces dose nonuniformity for tomographic delivery of intensity-modulated radiation therapy (IMRT). Methods and Materials: The NOMOS-CORVUS system delivers IMRT in a tomographic paradigm. This type of delivery is prone to create multiple dose nonuniformity regions at the arc abutment regions. The modified technique was based on the cyclical behavior of arc positions as a function of a target length. With the modified technique, two plans are developed for the same patient, one with the original target and the second with a slightly increased target length and the abutment regions shifted by ∼5 mm compared to the first plan. Each plan is designed to deliver half of the target prescription dose delivered on alternate days, resulting in periodic shifts of abutment regions. This method was experimentally tested in phantoms with and without intentionally introduced errors in couch indexing. Results: With the modified technique, the degree of dose nonuniformity was reduced. For example, with 1 mm error in couch indexing, the degree of dose nonuniformity changed from ∼25% to ∼12%. Conclusion: Use of the modified technique reduces dose nonuniformity due to periodic shifts of abutment regions during treatment delivery

Improvement of tomographic intensity modulated radiotherapy dose distributions using periodic shifting of arc abutment regions

Based on the study of treatment arc positioning versus target length, a method that allowed periodic shift of arc abutment regions through the course of intensity modulated radiotherapy (IMRT) was developed. In this method, two treatment plans were developed for the same tumor. The first plan contained the original target (Planning Target Volume as defined by radiation oncologist) and the second one contained a modified target. The modification of the original target consisted of simply increasing its length, adding a small extension to it, or creating a distant pseudo target. These modifications cause arc abutment regions in the second plan to be shifted relative to their positions in the first plan. Different methods of target modification were investigated because in some cases (for instance, when a critical structure might overlap with the target extension) a simple extension of the target would cause an unacceptable irradiation of the sensitive structures. The dose prescribed to the modified portion of the target varied from 10% to 100% of the original target dose. It was found that a clinically significant shift (⩾5 mm) in abutment region locations occurred when the dose prescribed to the extended portion of the target was ⩾95% of the original target dose. On the other hand, the pseudo target required only ∼10% to 20% of the original target dose to produce the same shift in arc positions. Results of the film dosimetry showed that when a single plan was used for the treatment delivery, the dose nonuniformity was 17% and 25% of the prescribed dose with 0.5 and 1 mm errors in couch indexing, respectively. The dose nonuniformity was reduced by at least half when two plans were used for IMRT delivery

An immobilization and localization technique for SRT and IMRT of intracranial tumors

A noninvasive localization and immobilization technique that facilitates planning and accurate delivery of both intensity modulated radiotherapy (IMRT) and linac based stereotactic radiotherapy (SRT) of intracranial tumors has been developed and clinically tested. Immobilization of a patient was based on a commercially available Gill‐Thomas‐Cossman (GTC) relocatable frame. A stereotactic localization frame (LF) with the attached NOMOS localization device (CT pointer) was used for CT scanning of patients. Thus, CT slices contained fiducial marks for both IMRT and SRT. The patient anatomy and target(s) were contoured on a stand‐alone CT‐based imaging system. CT slices and contours were then transmitted to both IMRT and SRT treatment planning systems (TPSs) for concurrent development of IMRT and SRT plans. The treatment method that more closely approached the treatment goals could be selected. Since all TPSs used the same contour set, the accuracy of competing treatment plans comparison was improved. SRT delivery was done conventionally. For IMRT delivery patients used the SRT patient immobilization system. For the patient setup, the IMRT target box was attached to the SRT LF, replacing the IMRT CT Pointer. A modified and lighter IMRT target box compatible with SRT LF was fabricated. The proposed technique can also be used for planning and delivery of 3D CRT, thus improving its accuracy. Day‐to‐day reproducibility of the patient setup can be evaluated using a SRT Depth Helmet. PACS number(s): 87.53.Kn, 87.53Ly, 87.56.D

Assessment of different IMRT boost delivery methods on target coverage and normal-tissue sparing

Because of biologic, medical, and sometimes logistic reasons, patients may be treated with 3D conformal therapy or intensity-modulated radiation therapy (IMRT) for the initial treatment volume (PTV 1) followed by a sequential IMRT boost dose delivered to the boost volume (PTV 2). In some patients, both PTV 1 and PTV 2 may be simultaneously treated by IMRT (simultaneous integrated boost technique). The purpose of this work was to assess the sequential and simultaneous integrated boost IMRT delivery techniques on target coverage and normal-tissue sparing. Fifteen patients with head-and-neck (H&N), lung, and prostate cancer were selected for this comparative study. Each site included 5 patients. In all patients, the target consisted of PTV 1 and PTV 2. The prescription doses to PTV 1 and PTV 2 were 46 Gy and 66 Gy (H&N cases), 45 Gy and 66.6 Gy (lung cases), 50 Gy and 78 Gy (prostate cases), respectively. The critical structures included the following: spinal cord, parotid glands, and brainstem (H&N structures); spinal cord, esophagus, lungs, and heart (lung structures); and bladder, rectum, femurs (prostate structures). For all cases, three IMRT plans were created: ( 1) 3D conformal therapy to PTV 1 followed by sequential IMRT boost to PTV 2 (sequential-IMRT 1), ( 2) IMRT to PTV 1 followed by sequential IMRT boost to PTV 2 (sequential-IMRT 2), and ( 3) Simultaneous integrated IMRT boost to both PTV 1 and PTV 2 (SIB-IMRT). The treatment plans were compared in terms of their dose–volume histograms, target volume covered by 100% of the prescription dose ( D 100%), and maximum and mean structure doses ( D max and D mean ). H&N cases: SIB-IMRT produced better sparing of both parotids than sequential-IMRT 1, although sequential-IMRT 2 also provided adequate parotid sparing. On average, the mean cord dose for sequential-IMRT 1 was 29 Gy. The mean cord dose was reduced to ∼20 Gy with both sequential-IMRT 2 and SIB-IMRT. Prostate cases: The volume of rectum receiving 70 Gy or more ( V >70 Gy) was reduced to 18.6 Gy with SIB-IMRT from 22.2 Gy with sequential-IMRT 2. SIB-IMRT reduced the mean doses to both bladder and rectum by ∼10% and ∼7%, respectively, as compared to sequential-IMRT 2. The mean left and right femur doses with SIB-IMRT were ∼32% lower than obtained with sequential-IMRT 1. Lung cases: The mean heart dose was reduced by ∼33% with SIB-IMRT as compared to sequential-IMRT 1. The mean esophagus dose was also reduced by ∼10% using SIB-IMRT as compared to sequential-IMRT 1. The percentage of the lung volume receiving 20 Gy ( V 20 Gy) was reduced to 26% by SIB-IMRT from 30.6% with sequential-IMRT 1. For equal PTV coverage, both sequential-IMRT techniques demonstrated moderately improved sparing of the critical structures. SIB-IMRT, however, markedly reduced doses to the critical structures for most of the cases considered in this study. The conformality of the SIB-IMRT plans was also much superior to that obtained with both sequential-IMRT techniques. The improved conformality gained with SIB-IMRT may suggest that the dose to nontarget tissues will be lower