Work in progress: treatment planning system for conformation radiotherapy

High-dose conformal radiotherapy for supratentorial malignant glioma: a historical comparison

BACKGROUND

Although radiotherapy remains the main postoperative treatment for patients with malignant glioma, modifications to regimens have not improved the poor outlook of patients with this disease. We aimed to investigate whether high-dose conformal radiotherapy improves the survival of patients with supratentorial malignant glioma compared with conventional radiotherapy.

METHODS

29 patients with anaplastic astrocytoma and 61 patients with glioblastoma who received high-dose conformal radiotherapy during 1990-2002 were compared with 34 patients with anaplastic astrocytoma and 60 patients with glioblastoma who received conventional 60 Gy radiotherapy during 1979-89. 77 of the 90 patients receiving high-dose radiotherapy were given 80 Gy; the remaining 13 patients, all with glioblastoma, received 90 Gy. Radiotherapy was planned on the basis of images taken before surgery, and doses were delivered in 2 Gy per fraction per day for 5 days a week. Hazard ratios for death were calculated with a Cox model, and were adjusted for age, Karnofsky performance scale, tumour size, and extent of resection.

FINDINGS

Patients who received high-dose radiotherapy had significantly longer overall survival compared with those who received conventional radiotherapy (adjusted hazard ratio 0.30 [95% CI 0.12-0.76], p=0.011 for anaplastic astrocytoma and 0.49 [0.28-0.87], p=0.014 for glioblastoma). Patients with anaplastic astrocytoma in the high-dose group have not yet reached median survival; median survival in the conventional radiotherapy group was 22.3 months (95% CI 20.6-24.0). 5-year survival was 51.3% (29.2-73.4) for the high-dose group and 14.7% (0.0-30.0) for the conventional group. Median survival in patients with glioblastoma was 16.2 months (12.8-19.6) for the high-dose group and 12.4 months (10.0-14.8) for the conventional group. 2-year survival was 38.4% (23.5-53.3) for the high-dose group and 11.4% (0.0-25.3) for the conventional group. Survival did not differ between those that received 80 Gy radiotherapy and those that received 90 Gy (hazard ratio 0.94 [95% CI 0.42-2.12]). The higher frequency of radiation-induced white matter abnormality in the high-dose group compared with the conventional radiotherapy group did not lead to increased disability.

INTERPRETATION

High-dose, standard-fractionated radiotherapy shows potential as the main postoperative treatment for patients with supratentorial malignant glioma.

Dynamic conical conformal radiotherapy using a C-arm-mounted accelerator: dose distribution and clinical application

PURPOSE

The aim of this study was to solve anisotropy in the dose distributions from rotational conformal radiotherapy (RCRT) by using a C-arm-mounted accelerator.

MATERIALS AND METHODS

The linac head was designed to move along the C-arm with a maximum angle of 60 degrees (from a vertical position toward the gantry). Simultaneous rotation of the gantry creates a dynamic conical irradiation technique. Dynamic conical conformal radiation therapy (Dyconic CRT) was developed by combining the technique with continuous motion of a multileaf collimator. Dose distributions were measured in phantoms using film densitometry and compared with conventional RCRT. Dose distributions in actual radiation therapy patients are also presented.

RESULTS

Dyconic CRT enabled the precise delivery of noncoplanar beams without rotating the table. The measurements showed that three-dimensionally isotropic dose falloff was achieved with Dyconic CRT. Dose inhomogeneity in the sagittal direction with Dyconic CRT was compensated for by use of wedge filters.

CONCLUSIONS

The drawbacks of the dose distributions produced by RCRT were overcome with the use of Dyconic CRT.

Megavoltage CT-assisted stereotactic radiosurgery for thoracic tumors: original research in the treatment of thoracic neoplasms

PURPOSE

The aim of the study was to evaluate the efficacy of stereotactic radiosurgery (SRS) for thoracic tumors with megavoltage computed tomography (MVCT) from the point of view of symptom palliation as well as local control.

METHODS AND MATERIALS

MVCT-assisted positioning verification and real-time monitoring for a multileaf collimator (MLC) were used to enhance the accuracy of the thoracic SRS. Twenty-two thoracic tumors in 15 patients underwent the present treatment. All but 1 tumor were metastases from various primary malignancies. Eleven patients were symptomatic. The treatment site was the chest wall/pleura in 10 tumors, and the lung in 12 tumors. The median volume of the clinical target was 4.5 cc and the median peripheral dose was 20 Gy, for the lung tumors. For the chest wall/pleura tumors, the median volume of the clinical target was 40 cc and the median peripheral dose was 20 Gy. Conventional fractionated conformal radiation therapy (CRT) followed SRS in 10 tumors.

RESULTS

Of 21 tumors eligible for evaluation, there were 13 with complete responses, 6 with partial responses, and 2 without response. Duration of local control ranged from 0.6 to 82 months with a median of 8 months, with only one local recurrence seen. Immediate palliation was obtained in most symptomatic patients. Interstitial changes in the lung were limited. Autopsy performed for a patient revealed remarkable histologic effects with minimal injuries to the lung.

CONCLUSION

The geometric accuracy of MVCT-assisted SRS appeared to enhance the clinical efficacy and safety of treatment to thoracic malignancies.

Improvement of image quality in megavoltage computed tomography with second generation scanning mode

Megavoltage computed tomographic (CT) scanning is a topic of interest in precision radiation therapy. It is useful in verifying and improving the accuracy of the patient's positioning. For this purpose, we developed a third generation mode megavoltage CT scanner. However, insufficient spatial resolution limits its clinical usefulness. A second generation mode megavoltage scanner using a turntable has been newly developed to investigate whether improvements in spatial sampling could result in image quality high enough for clinical use. Scanning is composed of 11 rotations and 12 translations of the table. The scanning beam is a 3 MV X-ray, and the detector consists of 75 elements of cadmium tungstate crystals combined with photodiodes. A spatial resolution of 0.5 mm and contrast resolution of approximately 5% were obtained. The image quality is inferior to that of conventional diagnostic CT scanners, but is estimated to be adequate for some clinical applications of radiation therapy. Based on the satisfactory results, a new third generation megavoltage CT scanner is under investigation.

A computer-controlled conformal radiotherapy system. I: Overview

PURPOSE

Equipment developed for use with computer-controlled conformal radiotherapy (CCRT) treatment techniques, including multileaf collimators and/or computer-control systems for treatment machines, are now available. The purpose of this work is to develop a system that will allow the safe, efficient, and accurate delivery of CCRT treatments as routine clinical treatments, and permit modifications of the system so that the delivery process can be optimized.

METHODS AND MATERIALS

The needs and requirements for a system that can fully support modern computer-controlled treatment machines equipped with multileaf collimators and segmental or dynamic conformal therapy capabilities have been analyzed and evaluated. This analysis has been used to design and then implement a complete approach to the delivery of CCRT treatments.

RESULTS

The computer-controlled conformal radiotherapy system (CCRS) described here consists of a process for the delivery of CCRT treatments, and a complex software system that implements the treatment process. The CCRS system described here includes systems for plan transfer, treatment delivery planning, sequencing of the actual treatment delivery process, graphical simulation and verification tools, as well as an electronic chart that is an integral part of the system. The CCRS system has been implemented for use with a number of different treatment machines. The system has been used clinically for more than 2 years to perform CCRT treatments for more than 200 patients.

CONCLUSIONS

A comprehensive system for the implementation and delivery of computer-controlled conformal radiation therapy (CCRT) plans has been designed and implemented for routine clinical use with multisegment, computer-controlled, multileaf-collimated conformal therapy. The CCRS system has been successfully implemented to perform these complex treatments, and is considered quite important to the clinical use of modern computer-controlled treatment techniques.

Real-time beam monitoring in dynamic conformation therapy

PURPOSE

Although portal imaging is a promising method of verification during static multiport irradiation, it cannot be applied directly to dynamic irradiation such as rotational conformation with multileaf collimator movement. A real-time beam monitoring system based on megavoltage computed tomography scanning has been developed to establish a verification method for the rotational conformation technique.

METHODS AND MATERIALS

Exit beam through the patient is extracted by the same detector unit as used for megavoltage scanning during the actual treatment. Beam edge is defined as the 50% level of the maximum dose of the detector array. Megavoltage computed tomography is done after patient setup and just prior to the actual irradiation. Detected beam pathways are overlaid on this image approximately every 1 s. Therapists can monitor correlation between the target and actual beam pathways on a real-time computer display.

RESULTS

The accuracy of field edge detection has been proven to be less than 2 mm from various measurements. Real-time monitoring is more useful in rotational conformation than in static multiport irradiation due to dynamic movement of the collimator. Field errors were identified in two of 54 sessions using this method.

CONCLUSIONS

Although several limitations remain to be solved, the method presented is a useful tool for treatment verification of high accuracy radiation therapy, particularly rotational conformation irradiation.

Dynamic field shaping for stereotactic radiosurgery: a modeling study

PURPOSE

This work assesses the relative field shaping advantages of dynamic field shaping devices for stereotactic radiosurgery using a linear accelerator.

METHODS AND MATERIALS

We selected 43 intracranial tumors (2.0-4.2 cm maximum dimension, 1.5-25.5 cc tumor volume) out of the first 64 intracranial tumors treated with radiosurgery at the Joint Center for Radiation Therapy. We modeled five field shaping devices, each including a fixed auxiliary circular collimator: (a) fixed circular collimator alone; (b) two independent parallel jaws; (c) four independent rectangular jaws; (d) four independent rotatable jaws; and (e) "ideal" multileaf collimator. We adjusted the model parameters until the minimum target isodose was 80% of the dose delivered to isocenter. We defined the treatment volume ratio as the target volume divided by the treatment volume (volume receiving at least the minimum target dose). We used the treatment volume ratio to compare the five models and the actual patient treatments.

RESULTS

For 34 tumors originally treated with one isocenter, the median Treatment Volume Ratio was higher for all of the device models except the fixed circular collimator compared to the actual patient treatments. For the nine tumors originally treated with multiple isocenters, the median Treatment Volume Ratio for the actual multiple isocenter treatments was similar to that for two parallel jaws, four rectangular jaws and four rotatable jaws. Only the median "ideal" collimator treatment volume ratio was higher for these nine tumors.

CONCLUSION

Simple field shaping devices have approximately 50% of the conformal advantage of an "ideal" multileaf collimator. Approximately 50% of typical radiosurgical tumors between 2 and 4 cm have field shaping advantages which exceed the geometrical uncertainties inherent in linear accelerator radiosurgery treatments. The three models, two parallel, four rectangular, or four rotatable independent jaws would improve current linear accelerator technology by providing homogeneous doses with equivalent field shaping for most tumors originally treated with inhomogeneous multiple isocenter plans (6/9 tumors in the current series).

Conformal radiotherapy for unresectable retroperitoneal soft tissue sarcoma

Local tumor control remains a continuing challenge in the treatment of retroperitoneal soft tissue sarcoma. Though complete resection by means of wide excision or excisional biopsy can be performed in a minority of patients only, aggressive surgical approach remains the treatment of choice. Unresectable sarcoma can rarely be controlled by conventionally applied radiotherapy--only a few percent of patients survive. A superior dose distribution of external radiation is demanded in order to spare healthy tissue. The presumably greatest advantage will occur when radiotherapy is used preoperatively. The possible clinical gain of superior dose distribution is demonstrated by results of the dynamic, 3-D conformal pion radiotherapy at PSI. Between April 1983 and June 1988 a total of 21 patients were treated with high doses (greater than or equal to 30 Gy) for unresectable retroperitoneal soft tissue sarcoma. The follow-up time is 13-74 months, median 24. Fifteen patients were treated with 20 fx, and 19 patients were treated with fraction sizes of 150 or 165 cGy. Except for one patient with thrombocytopenia after chemotherapy, no treatment interruption was necessary. Five patients developed late reactions, caused also by surgery and chemotherapy: two intestinal obstructions, one liver abscess, one leg edema, and one superficial skin necrosis. Nine patients had laparotomy after pion irradiation, five for resection of the previous unresectable tumor; 3/5 sarcoma were completely resected. Morbidity rate after post-pion laparotomy did not increase. Three patients had local tumor progression, 1/3 inside the treatment volume. The actuarial five-year local tumor control rate of these unresectable retroperitoneal sarcoma is 60%, the actuarial five-year survival rate is 33%. Out of the 21 patients, 15 are alive, two have died from local progression, one from peritoneal progression, and three from metastases.

Physical aspects of external beam radiotherapy for the treatment of malignant pleural mesothelioma

The optimization of radiotherapy for the treatment of malignant mesothelioma highlights many of the currently outstanding problems in clinical radiation physics. The experimental investigation of an intuitively attractive irradiation technique with combined photon and electron beams using a specially constructed phantom has established that, due to the penetration in low density material of both primary electrons and those secondary to photon irradiation, the normal lung tissue is not spared to any significant degree by such a technique. Furthermore, great care needs to be exercised in the treatment planning calculations for this approach if absolute dosimetry errors as large as 50% are to be avoided.

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.