Use of 10 MV spoiled x ray beam for treatment of head and neck tumors

Calculation of midplane dose for total body irradiation from entrance and exit dose MOSFET measurements

This work is the development of a MOSFET based surface in vivo dosimetry system for total body irradiation patients treated with bilateral extended SSD beams using PMMA missing tissue compensators adjacent to the patient. An empirical formula to calculate midplane dose from MOSFET measured entrance and exit doses has been derived. The dependency of surface dose on the air-gap between the spoiler and the surface was investigated by suspending a spoiler above a water phantom, and taking percentage depth dose measurements (PDD). Exit and entrances doses were measured with MOSFETs in conjunction with midplane doses measured with an ion chamber. The entrance and exit doses were combined using an exponential attenuation formula to give an estimate of midplane dose and were compared to the midplane ion chamber measurement for a range of phantom thicknesses. Having a maximum PDD at the surface simplifies the prediction of midplane dose, which is achieved by ensuring that the air gap between the compensator and the surface is less than 10 cm. The comparison of estimated midplane dose and measured midplane dose showed no dependence on phantom thickness and an average correction factor of 0.88 was found. If the missing tissue compensators are kept within 10 cm of the patient then MOSFET measurements of entrance and exit dose can predict the midplane dose for the patient.

Radiation therapy of large intact breasts using a beam spoiler or photons with mixed energies

Radiation treatment of large intact breasts with separations of more than 24 cm is typically performed using x-rays with energies of 10 MV and higher, to eliminate high-dose regions in tissue. The disadvantage of the higher energy beams is the reduced dose to superficial tissue in the buildup region. We evaluated 2 methods of avoiding this underdosage: (1) a beam spoiler: 1.7-cm-thick Lucite plate positioned in the blocking tray 35 cm from the isocenter, with 15-MV x-rays; and (2) combining 6- and 15-MV x-rays through the same portal. For the beam with the spoiler, we measured the dose distribution for normal and oblique incidence using a film and ion chamber in polystyrene, as well as a scanning diode in a water tank. In the mixed-energy approach, we calculated the dose distributions in the buildup region for different proportions of 6- and 15-MV beams. The dose enhancement due to the beam spoiler exhibited significant dependence upon the source-to-skin distance (SSD), field size, and the angle of incidence. In the center of a 20 x 20-cm(2) field at 90-cm SSD, the beam spoiler raises the dose at 5-mm depth from 77% to 87% of the prescription, while maintaining the skin dose below 57%. Comparison of calculated dose with measurements suggested a practical way of treatment planning with the spoiler--usage of 2-mm "beam" bolus--a special option offered by in-house treatment planning system. A second method of increasing buildup doses is to mix 6- and 15-MV beams. For example, in the case of a parallel-opposed irradiation of a 27-cm-thick phantom, dose to D(max) for each energy, with respect to midplane, is 114% for pure 6-, 107% for 15-MV beam with the spoiler, and 108% for a 3:1 mixture of 15- and 6-MV beams. Both methods are practical for radiation therapy of large intact breasts.

Outcome of patients treated with cobalt and 6 MV in head and neck cancers

BACKGROUND

Since 1992, we have been using a 6-MV linear accelerator instead of a cobalt machine. The aim of the study is to evaluate the impact of this on neck control, particularly on postoperative patients in which subcutaneous tissues are at risk.

METHOD

A retrospective study including all of 1,452 consecutive patients treated by definitive or postoperative radiotherapy between 1989 and 1997. All stages and subsites of the head/neck were included. Local and neck control were evaluated by the Kaplan Meier method, and comparisons were made between the cobalt and the 6-MV subgroups with a subsequent Cox analysis. For neck control analysis, the postoperative patients were divided in low and high risk (extracapsular extension [ECE], >two nodes, or T4).

RESULTS

Radical radiotherapy: A better local control (LC) is observed with 6 MV than with cobalt, but neck control was similar. Postoperative radiotherapy: A better LC is observed with 6 MV. In high-risk patients, the neck control was higher for the cobalt group (79%) vs 60% for the 6-MV group (p = .09 and .03 in a Cox model).

CONCLUSION

In postoperative patients at high risk for neck relapse, cobalt seems to give a better neck control. We are currently doing a prospective study in which a bolus is added for half the treatment when patients at high risk for neck relapse are treated with 6 MV.

The influence of beam energy on the outcome of postoperative radiotherapy in head and neck cancer patients: secondary analysis of RTOG 85-03

PURPOSE

To determine whether any difference in toxicity or efficacy occurs when head and neck cancer patients are treated postoperatively with (60)C0, 4 MV, or 6 MV photon beam.

METHODS AND MATERIALS

This is a secondary analysis of the Intergroup Study 0034. Three hundred ninety-two patients were evaluable for comparison between treatment with (60)C0, 4 MV, or 6 MV photon beam. All patients had advanced but operable squamous cell carcinoma of the oral cavity, oropharynx, hypopharynx, or larynx. Patients were randomized following surgical resection to receive treatment with either postoperative irradiation alone, or postoperative irradiation plus three cycles of cisplatin and 5-fluorouracil. Patients were categorized as having either "low risk" or "high risk" treatment volumes based on whether the surgical margin was 5 mm or less, presence of extra capsular nodal extension, and/or carcinoma in situ at the surgical margins. Low-risk volumes received 50-54 Gy, and high-risk volumes were given 60 Gy. Patients were compared in regards to acute and late radiotherapy toxicities as well as overall survival and loco-regional control according to the beam energy used.

RESULTS

One-hundred fifty-seven, 140, and 95 patients were treated by (60)C0, 4 MV, and 6 MV, respectively. No differences were seen in acute or late toxicity among treatment groups. Locoregional control was achieved in 75%, 79%, and 80% of patients treated with (60)C0, 4 MV, or 6 MV (p = 0.61). Patients treated with 6 MV had a higher incidence of ipsilateral neck failure as first event (13%) than patients treated by (60)C0 and 4 MV (9%). This difference was not statistically significant.

CONCLUSION

No differences in outcome, acute, or late toxicity were discernible in patients with advanced head and neck cancer treated with (60)C0, 4 MV, or 6 MV. This result should be interpreted with caution as increased incidence, albeit nonsignificant, of ipsilateral neck recurrence was observed in patients treated with 6 MV and the power of the study to detect a statistically significant difference is small.

Effects of beam spoiler on radiation dose for head and neck irradiation with 10-MV photon beam

PURPOSE

To determine the effects of a lucite beam spoiler on the dose distribution to points inside and outside the primary beam for head and neck irradiation with a 10-MV photon beam.

METHODS AND MATERIALS

Build-up and depth-dose measurements were performed with a parallel-plate ionization chamber for 5 x 5, 10 x 10, and 15 x 15-cm field sizes using lucite spoilers with two different thicknesses at two different lucite-to-skin distances (LSD) for a 10-MV x-ray beam. Corrections were applied to account for finite chamber size. Beam profiles and isodose curves were obtained at several depths using film dosimetry. Beam uniformity was determined from uniformity indices. Peripheral doses (PD) were measured at the surface and at 1.5- and 2.5-cm depths using film dosimetry and a parallel-plate ionization chamber. Measurement points were positioned at the edge of a 10 x 10-cm field and at distances extending to 5.0 cm away. The treatment planning data for the 10-MV x-ray beam were modified to account for the effects of the beam spoiler when treating head and neck patients.

RESULTS

The spoiler increased the surface and build-up dose and shifted the depth of maximum dose toward the surface. With a 10-MV x-ray beam and a 1.2-cm-thick lucite at 15 cm LSD, a build-up dose similar to a 6-MV x-ray beam was achieved. The beam uniformity was altered at shallow depths. The peripheral dose was enhanced particularly at the surface and at the points close to the beam edge. The effects of the beam spoiler on beam profile and PD were reduced with increasing depths.

CONCLUSION

The lucite spoiler allowed use of a 10-MV x-ray beam for head and neck treatment by yielding a build-up dose similar to that of a 6-MV x-ray beam while maintaining skin sparing. The increase in PD was at superficial depths and was reduced at points away from the edge; therefore, it is clinically nonsignificant. Spoiling the 10-MV x-ray beam resulted in treatment plans that maintained dose homogeneity without the consequence of increased skin reaction or treatment volume underdose for regions near the skin surface.

Use of a lucite beam spoiler for high-energy breast irradiation

Treatment planning for breast cancer is a challenge for bridge separations greater than 24 cm. protocols require dose homogeneity to the breast parenchyma to 10% while maintaining good cosmesis (skin sparing). Low-energy beams (6 MV or lower) may not achieve protocol specifications because of lateral or medial hot spots. The buildup region for high-energy beams (i.e., 18 MV) may lead to underdosing in the superficial regions. Adding bolus will pull the isodoses to surface, surrendering skin sparing. A spoiler will increase the contribution of scattered medium energy electrons and photons, thereby increasing superficial dosing and maintaining skin sparing due to displacement of the spoiler from the skin. We chose Lucite as the spoiler material for our 18-MV beam. Buildup measurements were performed with a parallel-plate ionization chamber to find the optimal spoiler thickness of 18 mm at a spoiler-to-skin distance of 22 cm. A breast phantom was designed allowing thermoluminescent dosimeter measurements. The spoiler has increased the percent depth dose from the depths of 0.5 cm to 3.0 cm to mimic those delivered by a 6 MV beam, while the relative skin dose was held to 50%. Our treatment schema is typically 6-MV photons with customized compensation for half or more of the fractions and uncompensated 18-MV photons with the spoiler for the remaining fractions. The spoiler may be used in conjunction with thin bolus (1.0 cm) for therapeutic dosing to the skin (scars, inflammatory disease, etc.). The treatment plans have maintained dose homogeneity for large patients, without consequence of skin reaction.

Calculation of the dose in the build-up region for high energy photon beam. Treatment planning when beam spoilers are employed

Recoil electrons from a polystyrene sheet (beam spoiler) placed in a photon beam were used to modulate the dose in the build-up region. The effects of the beam spoiler on the dosimetry in simple phantoms were studied for both 6 MV and 15 MV photon beams at 100 cm source-to-surface distance (SSD) as a function of (1) the thickness of the spoiler, (2) the spoiler-to-surface distance (SD) and (3) the field size. A radiotherapy treatment planning (RTP) algorithm was used that employs pencil beam dose deposition kernels to evaluate the three-dimensional (3D) dose distribution due to photons and electrons in the build-up region. To determine the energy deposition kernels for the recoil electrons emanating from the spoiler, their energy spectrum needs to be known. Monte Carlo simulations were performed to derive the spectrum of electrons from the beam spoiler. The recoil electron spectrum was found to be nearly independent of field size, SD, and spoiler thickness. The pencil beam energy deposition kernel was calculated for the recoil electrons from the spoiler and inserted in the treatment planning algorithm. The dose calculation algorithm permits merging the photon and recoil electron dose components. In all cases the calculation agrees to within 3% of the measured results.

Beam-hardening effects of wedges on a spoiled 6 MV beam

The beam-hardening effects of the wedges on a 6-MV spoiled beam has been studied. The beam quality of all the wedged beams was found to be the same as the open beam. The dmax also stayed unchanged for all the wedges at all field sizes. The relative wedge factors were found to reflect the beam-hardening effect, which is a function of the wedge angles, depths, and field sizes. For the 15 degrees and 30 degrees wedges, the relative wedge factor at depths less than 15 cm were found to deviate less than +2% for all fields, while those for the 45 degrees and 60 degrees wedges for the same depth range from +3% to +4%. The surface doses were found to decrease with the wedged fields. For 15 degrees, 30 degrees, and 45 degrees wedges, the decreases were found to be from 0 to -2.0%. For 60 degrees wedges, the largest deviation was found to be -2.5% for a field size of 10 cm x 10 cm at a depth of 2 mm. The wedge factors at dmax were found to depend slightly on the field sizes. The use of an averaged wedge factor for each individual wedge was found to produce less than +/- 1.2% of error for all field sizes.

Dose to superficial node for patients with head and neck cancer treated with 6 MV and 60Co photons

The superficial neck nodes in only 1 out of 7 patients with head and neck cancer studied received more than 90% of the prescribed dose when treated with opposed 6 MV photons. Beam spoilers placed upstream from the patient enhanced the dose to the superficial node at the expense of higher dose to the skin.

Design principles and clinical possibilities with a new generation of radiation therapy equipment. A review

The main steps in the development of isocentric megavoltage external beam radiation therapy machines are briefly reviewed identifying three principal types or generations of equipment to date. The new fourth generation of equipment presented here is characterized by considerably increased flexibility in dose delivery through the use of scanned elementary electron and photon beams of very high quality. Furthermore the wide energy range and the possibility of using high resolution multileaf collimation with all beam modalities makes it possible to simplify irradiation techniques and increase the accuracy in dose delivery. The main design features are described including a dual dipole magnet scanning system, a photon beam purging magnet, a helium atmosphere in the treatment head, a beam's eye view video read-out system of the collimator setting and a radiotherapeutic computed tomography facility. Some of the clinical applications of this new type of radiation therapy machine are finally reviewed, such as the ease of performing beam flattening, beam filtering and compensation, and the simplification of many treatment techniques using the wide spectrum of high quality electron and photon beams. Finally the interesting possibility of doing conformation and more general isocentric treatments with non-uniform beams using the multileaf collimator and the scanning system are demonstrated.

Dosimetry of rotational photon fields with gravity-oriented eye blocks

Evolving radiotherapeutic technique is aimed at routine use of an eye protection accessory with uniformity of dose to facial tumors. Rotational 4 MV photon fields with gravity-oriented eye blocks are described in applications to patients and a phantom. A machine-oriented compensator with a slit and beveled edges at eye levels has been applied to increase uniformity of dose over the entire target, including partly blocked regions. Treatment planning parameters are derived from simple geometric relationships. Dose distributions and average tissue-air ratios (TAR) are calculated with a conventional radiotherapeutic treatment planning system, and measured during initial treatments via thermoluminescent dosemeters (TLD) in facial cavities. Machine monitor units of initial treatments (MU1) are calculated from prescribed dose D, TAR, percentage isodose lines, dose rates in air, and compensator transmission; for subsequent treatments, MU1 are modified using averages of unblocked target doses D measured during initial treatment: MU2 = (D/D)MU1. Doses measured on patients' eyelids have been 10 to 15% of prescribed dose. Three dimensional dose distributions were determined using TLD in an anthropomorphic phantom. With beam compensation, eye doses were 15% of unblocked target doses, doses to partly blocked target were 86 to 129% of unblocked target doses, and doses to unblocked target showed 5% mean standard deviations. In vivo and in phantom data were consistent with computer-calculated distributions. For uniformity of target dose, eye-sparing, and simplicity of irradiating tumors surrounding the eyes, the technique investigated compares favorably with conventional techniques.