The half field technique of radiation therapy for the cancers of head and neck

Optimizing laryngeal sparing with intensity modulated radiotherapy or volumetric modulated arc therapy for unilateral tonsil cancer

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Early stage tonsillar cancers may be treated with unilateral neck radiotherapy (RT).

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We performed a dosimetric study comparing three common radiotherapy modalities.

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No significant differences in target coverage existed between all plans.

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Laryngeal doses were reduced using whole-field intensity modulated RT and VMAT.

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Shorter treatment times and required monitor units were also associated with VMAT.

Background and purpose

Minimizing radiation dose exposure to nearby organs is key to limiting clinical toxicities associated with radiotherapy. Several treatment modalities such as split- or whole-field intensity-modulated radiotherapy (SF-IMRT, WF-IMRT) and volumetric modulated arc therapy (VMAT) are being used to treat tonsillar cancer patients with unilateral neck radiotherapy. Herein, we provide a modern dosimetric comparison of all three techniques.

Materials and methods

Forty patients with tonsillar cancer treated with definitive, ipsilateral neck SF-IMRT were evaluated. Each patient was re-planned with WF-IMRT and VMAT techniques, and doses to selected organs-at-risk (OARs) including the larynx, esophagus, and brainstem were compared.

Results

No significant differences in target coverage existed between plans; however, the heterogeneity index improved using WF-IMRT and VMAT relative to SF-IMRT. Compared to SF-IMRT, WF-IMRT and VMAT plans had significantly lower mean doses to the supraglottic larynx (31 Gy, 18.5 Gy, 17 Gy; p < 0.01), the MDACC-defined larynx (13.4 Gy, 10.5 Gy, 9.8 Gy; p < 0.01), and RTOG-defined larynx (15.8 Gy, 12.1 Gy, 11.1 Gy; p < 0.01), respectively. Mean esophageal dose was lowest with SF-IMRT over WF-IMRT and VMAT (5.9 Gy, 12.2 Gy, 11.1 Gy; p < 0.01) but only in the absence of lower neck disease. On average, VMAT plans had shorter treatment times and required less monitor units than both SF-IMRT and WF-IMRT.

Conclusion

In the setting of unilateral neck radiotherapy, WF-IMRT and VMAT plans can be optimized to significantly improve dose sparing of critical structures compared to SF-IMRT. VMAT offers additional advantages of shorter treatment times and fewer required monitor units.

Microtensile Bond Strength of Polyacid-modified Composite Resin to Irradiated Primary Molars

AIM

This study evaluated the influence of various doses of radiotherapy on the microtensile bond strength (pTBS) of compomer resin to dentin and enamel in primary molars.

MATERIALS AND METHODS

Thirty-five intact primary molars were collected and divided into seven groups. Teeth were irradiated with doses from 10 to 60 Gy, except for the control group. Compomer restorations were performed, and enamel-compomer resin beams and dentin-compomer resin beams were tested at a crosshead speed of 1 mm/min.

RESULTS

No statistically significant difference was found between the irradiated tooth enamel and the control group (F = 1.1468; p = 0.194). However, statistically significant differences were evident among the dentin groups (F = 11.050; p < 0.001).

CONCLUSION

Radiation may not cause a significant difference in the pTBS of compomer resin to primary tooth enamel, but appears to dose dependently decrease its bond strength to primary tooth dentin.

CLINICAL SIGNIFICANCE

Radiotherapy may affect the success rate of compomer fillings in primary teeth, especially in deeper cavities with exposed dentin.

A method for accurate zero calibration of asymmetric jaws in single-isocenter half-beam techniques

PURPOSE

To present a practical method for calibrating the zero position of asymmetric jaws that provides higher accuracy at the central axis and improves dose homogeneity in the abutting region of half-beams.

METHODS

Junction doses were measured for each asymmetric jaw using the double-exposure technique and electronic portal imaging devices. The junction dose was determined as a function of jaw position. The shift in the zero jaw position (or in its corresponding potentiometer readout) required to correct for the measured junction dose could thus be obtained. The jaw calibration was then modified to introduce the calculated shift and therefore achieve an accurate zero position in order to provide a relative junction dose that was as close to zero as possible.

RESULTS

All the asymmetric jaws from four medical linear accelerators were calibrated with the new calibration procedure. Measured relative junction doses at gantry 0° were reduced from a maximum of ±40% to a maximum of ±8% for all the jaws in the four considered accelerators. These results were valid for 6 MV and 18 MV photon beams and for any combination of asymmetric jaws set to zero. The calibration was stable over a long period of time; therefore, the need for recalibrating is seldom necessary.

CONCLUSIONS

Accurate calibration of the zero position of the jaws is feasible in current medical linear accelerators. The proposed procedure is fast and it improves dose homogeneity at the junction of half-beams, thus, allowing a more accurate and safer use of these techniques.

The influence of the field setup on the dosimetry of abutted fields in single-isocenter half-beam techniques

PURPOSE

To study the influence of the field setup on the dosimetry at the junction in single-isocenter half-beam techniques.

METHODS

The dosimetry at the junction for a two-field setup with the gantry at zero was first evaluated with radiochromic films. A three-field setup, with an anterior field and two opposed lateral fields, was also analyzed for two different relative positions of the fields involved. In all cases, the dose increase at the central axis, called the junction dose, was measured.

RESULTS

Junction doses varied greatly with the setup. For the three-field setup, the junction dose differed from that obtained with the two-field setup, and it greatly depended on the relative position of the fields. When the anterior field was closer to the gantry than the lateral fields, a field gap occurred and the junction dose was negative. When the anterior field was farther from the gantry than the lateral fields, a field overlap was obtained and the junction dose was positive. The difference in the junction dose between the three-field setups was around 18% for the three accelerators evaluated.

CONCLUSIONS

Having a uniform dose distribution for two fields at gantry 0 degrees does not guarantee a uniform distribution at other gantry angles. Junction doses are largely affected by the relative position of the radiation fields, which may have an impact in clinical practice. Therefore, any method aiming to assess or to optimize the dose homogeneity at the junction should take this effect into account.

Review of monoisocentric split-field technique for conventional and IMRT treatment in head and neck cancers: technical limitations and approaches for optimization

The importance of treatment planning of head and neck malignancies arises from the necessity to achieve homogenous doses to localized target volume surrounded by normal structures, which can produce acute and long-term morbidity. In many radiotherapy departments, a commonly employed strategy is a 3-field technique. Bilateral parallel-opposed fields are matched to anterior lower neck field. In recent years, Intensity Modulated Radiotherapy (IMRT) is used to radiate head and neck tumors. When the target extends to the lower neck regions, abutment of upper IMRT and lower neck fields is required. Field matching represents a technical challenge for the medical physicist, medical engineer, and radiation oncologist to treat multiple fields while avoiding their overlap on the spinal cord. The monoisocentric split field technique has recently become a common technique to achieve matchline homogenous dose while respecting normal tissue tolerance. The aim of this work is to review merits, limitations, and recent approaches to optimize matchline dose in monoisocentric technique in conventional and intensity modulated radiotherapy for head and neck cancers. Although the technique has many advantages, it is subjected to some systematic and random errors due to equipment and patient setup inaccuracies. To decrease the magnitude of matchline inhomogeneities, customized penumbra generator or multileaf collimator have been used. Both methods are viable and represent alternative approaches to the problem of field matching using the asymmetric jaws.

A dynamic supraclavicular field-matching technique for head-and-neck cancer patients treated with IMRT

PURPOSE

The conventional single-isocenter and half-beam (SIHB) technique for matching supraclavicular fields with head-and-neck (HN) intensity-modulated radiotherapy (IMRT) fields is subject to substantial dose inhomogeneities from imperfect accelerator jaw/MLC calibration. It also limits the isocenter location and restricts the useful field size for IMRT. We propose a dynamic field-matching technique to overcome these limitations.

METHODS AND MATERIALS

The proposed dynamic field-matching technique makes use of wedge junctions for the abutment of supraclavicular and HN IMRT fields. The supraclavicular field was shaped with a multileaf collimator (MLC), which was orientated such that the leaves traveled along the superoinferior direction. The leaves that defined the superior field border moved continuously during treatment from 1.5 cm below to 1.5 cm above the conventional match line to generate a 3-cm-wide wedge-shaped junction. The HN IMRT fields were optimized by taking into account the dose contribution from the supraclavicular field to the junction area, which generates a complementary wedge to produce a smooth junction in the abutment region. This technique was evaluated on a polystyrene phantom and 10 HN cancer patients. Treatment plans were generated for the phantom and the 10 patients. Dose profiles across the abutment region were measured in the phantom on films. For patient plans, dose profiles that passed through the center of the neck lymph nodes were calculated using the proposed technique and the SIHB technique, and dose uniformity in the abutment region was compared. Field mismatches of +/- 1 mm and +/- 2 mm because of imperfect jaw/MLC calibration were simulated, and the resulting dose inhomogeneities were studied for the two techniques with film measurements and patient plans. Three-dimensional volumetric doses were analyzed, and equivalent uniform doses (EUD) were computed. The effect of field mismatches on EUD was compared for the two match techniques.

RESULTS

For a perfect jaw/MLC calibration, dose profiles for the 10 patients in the 3-cm match zone had an average inhomogeneity range of -1.6% to +1.6% using the dynamic-matching technique and -3.7% to +3.8% according to the SIHB technique. Measurements showed that dose inhomogeneities that resulted from 1-mm and 2-mm jaw/MLC calibration errors were reduced from as large as 27% and 45% with the SIHB technique to less than 2% and 5.7% with the dynamic technique, respectively. For -1-mm, -2-mm, +1-mm, and +2-mm jaw/MLC calibration errors, respectively, treatment plans for the 10 patients yielded average dose inhomogeneities of -5.9%, -3.0%, +2.7%, and +5.8% with the dynamic technique as compared to -22.8%, -11.1%, +9.8%, and +22.1% with the SIHB technique. Calculation based on a dose-volume histogram (DVH) showed that the SIHB technique resulted in larger changes in EUD of the PTV in the junction area than did the dynamic technique.

CONCLUSION

Compared with the conventional SIHB technique, the dynamic field-matching technique provides superior dose homogeneity in the abutment region between the supraclavicular and HN IMRT fields. The dynamic feathering mechanism substantially reduces dose inhomogeneities that result from imperfect jaw/MLC calibration. In addition, isocenter location in the dynamic field-matching technique can be chosen for reproducible patient setup and for adequate IMRT field size rather than being dictated by the match position. It also allows angling of the supraclavicular field to reduce the volume of healthy lung irradiated, which is impractical with the SIHB technique. In principle, this technique should be applicable to any treatment site that requires the abutment of static and intensity-modulated fields.

Three-dimensional radiotherapy of head and neck and esophageal carcinomas: a monoisocentric treatment technique to achieve improved dose distributions

The specific aim of three-dimensional conformal radiotherapy is to deliver adequate therapeutic radiation dose to the target volume while concomitantly keeping the dose to surrounding and intervening normal tissues to a minimum. The objective of this study is to examine dose distributions produced by various radiotherapy techniques used in managing head and neck tumors when the upper part of the esophagus is also involved. Treatment planning was performed with a three-dimensional (3-D) treatment planning system. Computerized tomographic (CT) scans used by this system to generate isodose distributions and dose-volume histograms were obtained directly from the CT scanner, which is connected via ethernet cabling to the 3-D planning system. These are useful clinical tools for evaluating the dose distribution to the treatment volume, clinical target volume, gross tumor volume, and certain critical organs. Using 6 and 18 MV photon beams, different configurations of standard treatment techniques for head and neck and esophageal carcinoma were studied and the resulting dose distributions were analyzed. Film validation dosimetry in solid-water phantom was performed to assess the magnitude of dose inhomogeneity at the field junction. Real-time dose measurements on patients using diode dosimetry were made and compared with computed dose values. With regard to minimizing radiation dose to surrounding structures (i.e., lung, spinal cord, etc.), the monoisocentric technique gave the best isodose distributions in terms of dose uniformity. The mini-mantle anterior-posterior/posterior-anterior (AP/PA) technique produced grossly non-uniform dose distribution with excessive hot spots. The dose measured on the patient during the treatment agrees to within +/- 5 % with the computed dose. The protocols presented in this work for simulation, immobilization and treatment planning of patients with head and neck and esophageal tumors provide the optimum dose distributions in the target volume with reduced irradiation of surrounding non-target tissues, and can be routinely implemented in a radiation oncology department. The presence of a real-time dose-measuring system plays an important role in verifying the actual delivery of radiation dose.

Decreasing the dosimetric effects of misalignment when using a mono-isocentric technique for irradiation of head and neck cancer

PURPOSE

The purpose of this study was to quantify and develop methods to decrease inhomogeneities created with field edge mismatch when using a mono-isocentric beam-split technique.

METHODS AND MATERIALS

We validated techniques to determine dose across a half-blocked field edge and quantified potential sources of systematic matchline error. Then, two methods were used to evaluate matchline doses. The first used film dosimetry data from a half-beam field and a spreadsheet. Duplication and reversal provided two columns, each representing a beam-split field edge. Summation simulated perfect abutment and shifting created various gaps and overlaps. The second method involved obtaining dose profiles at midfield along the ray perpendicular to abutted, overlapped, and gapped beam-split fields on six linear accelerators. To enlarge the penumbra, we designed several field edge modifiers, then re-evaluated matchline doses. The field edge modifiers applicability to a 3-field head and neck treatment technique was also examined.

RESULTS

Film-determined dose profiles provide similar information across a beam-split field edge as an ionization chamber. With the mono-isocentric beam-split technique, a 4-mm overlap or gap produces inhomogeneities nearly 60% above or below the intended dose. A 2-mm overlap or gap produces inhomogeneities nearly 30% above or below the intended dose. A customized penumbra generator decreased the magnitude of these inhomogeneities to 20% and 10%, respectively.

CONCLUSION

The two methods of evaluating matchline dose described above gave similar results. When using the mono-isocentric half-field technique, small misalignments produce worrisome regions of inhomogeneity. Our penumbra generator substantially decreases the magnitude of the dose inhomogeneities, although the volume receiving an inhomogeneous dose increases.

The single-isocentre treatment of head and neck cancer: time gain using MLC and automatic set-up

PURPOSE

In this manuscript, we studied the difference in the treatment time required to execute a single-isocentre three-field irradiation of the head and neck, using either tray-mounted cerrobend blocks or a multileaf collimator (MLC) for field shaping and automatic set-up.

MATERIALS AND METHODS

A total of twenty consecutive, unselected patients (16 males, four females), were eligible for this study because the dose they were to received was 44 Gy (2 Gy/fraction) to the head, neck and supraclavicular regions. Patients were randomly allocated to one of two treatment groups. The first group (n = 11) was treated on a Philips SL-75 linear accelerator (SL-75), using 5 MV photons and tray-mounted cerrobend blocks. The second group (n = 9) was treated on a Philips SL-25 linear accelerator (SL-25-MLC), using 6 MV photons and a MLC. Patients of the second group were treated using the automatic set-up facility of the SL-25-MLC, without entering the treatment room between consecutive fields.

RESULTS

Overall treatment time was significantly shorter on the SL-25-MLC than on the SL-75 (P < 0.0001). The difference in total treatment-execution time was in the range of 157 s per treatment session. The largest difference was observed in the set-up time. There was an average of a 125 s time gain per treatment day (P < 0.0001) in favour of the SL-25-MLC.

CONCLUSIONS

Compared to tray-mounted cerrobend blocks, a MLC and automatic set-up results in a significant time advantage when a single isocentre technique is used to treat head and neck cancer.

A method of gap and field size determination for abutting a treatment field to a previously treated area

A simple method of determining the new field size and the skin gap needed when abutting to a previously treated (or simulated) field is presented. The more complex case of abutting a treatment field between two previously treated fields and determining the new field length and the skin gap at each end is also addressed. Both methods utilize a single measurement on the patient's skin and the commonly used gap formula based on similar triangles. Both methods easily lend themselves to implementation through a simple BASIC program which eases any difficulties in calculation encountered during simulation.

A method for delivering accurate and uniform radiation dosages to the head and neck with asymmetric collimators and a single isocenter

PURPOSE

To investigate the use of asymmetric collimators and a single isocenter for delivering a uniform, accurate dose of radiation to the head, neck, and supraclavicular lymph nodes.

METHODS AND MATERIALS

A linear accelerator with a pair of asymmetric collimators is required for this technique. An isocenter was placed at the junction of the lateral head and neck fields and the anterior supraclavicular field. The asymmetric collimators were set longitudinally, by collimator rotation if necessary. The collimators split the radiation beam to all portals. Dose uniformity was measured at the junction with films in solid-water phantoms.

RESULTS

Film dosimetry showed a uniform dose at the junction without hot or cold regions. A digital display tolerance of +/- 1.0 mm for a field size maintained an acceptable uniform dose (+/- 5% dose variation) at the junction. The single isocenter and asymmetric collimators reduced field setup time by half. No table rotation was required to match fields.

CONCLUSION

The asymmetric collimators lead to easy and accurate patient setup. The absence of the trapezoid effect resulted in the complete coverage of the submandibular and cervical nodes without any hot spots.

Clinical use of a match-line wedge for adjacent megavoltage radiation field matching

The divergence and sharp penumbra of linear accelerator beams pose notorious problems when joining such beams side by side. One way of reducing the dose distribution nonuniformity in the matching region is to create a wide pseudo-penumbra with the use of a "match-line wedge." A single match-line wedge shape has been developed for 6 MV and 10 MV photon beams. The wide pseudo-penumbra created by the wedge drastically reduces the effect of random set-up errors. Special attention has been paid to ensure simple and reliable clinical use of the wedge. Details of the design, construction, dosimetry, and rules of practical application are presented. Comparisons of several matching methods are made.