Specific recommendations for accurate and direct use of PET-CT in PET guided radiotherapy for head and neck sites.

PURPOSE: To provide specific experience-based guidance and recommendations for centers wishing to develop, validate, and implement an accurate and efficient process for directly using positron emission tomography-computed tomography (PET-CT) for the radiotherapy planning of head and neck cancer patients. METHODS: A PET-CT system was modified with hard-top couch, external lasers and radiotherapy immobilization and indexing devices and was subject to a commissioning and quality assurance program. PET-CT imaging protocols were developed specifically for radiotherapy planning and the image quality and pathway tested using phantoms and five patients recruited into an in-house study. Security and accuracy of data transfer was tested throughout the whole data pathway. The patient pathway was fully established and tested ready for implementation in a PET-guided dose-escalation trial for head and neck cancer patients. RESULTS: Couch deflection was greater than for departmental CT simulator machines. An area of high attenuation in the couch generated image artifacts and adjustments were made accordingly. Using newly developed protocols CT image quality was suitable to maintain delineation and treatment accuracy. Upon transfer of data to the treatment planning system a half pixel offset between PET and CT was observed and corrected. By taking this into account, PET to CT alignment accuracy was maintained below 1 mm in all systems in the data pathway. Transfer of structures delineated in the PET fusion software to the radiotherapy treatment planning system was validated. CONCLUSIONS: A method to perform direct PET-guided radiotherapy planning was successfully validated and specific recommendations were developed to assist other centers. Of major concern is ensuring that the quality of PET and CT data is appropriate for radiotherapy treatment planning and on-treatment verification. Couch movements can be compromised, bore-size can be a limitation for certain immobilization techniques, laser positioning may affect setup accuracy and couch deflection may be greater than scanners dedicated to radiotherapy. The full set of departmental commissioning and routine quality assurance tests applied to radiotherapy CT simulators must be carried out on the PET-CT scanner. CT image quality must be optimized for radiotherapy planning whilst understanding that the appearance will differ between scanners and may affect delineation. PET-CT quality assurance schedules will need to be added to and modified to incorporate radiotherapy quality assurance. Methods of working for radiotherapy and PET staff will change to take into account considerations of both parties. PET to CT alignment must be subject to quality control on a loaded and unloaded couch preferably using a suitable emission phantom, and tested throughout the whole data pathway. Data integrity must be tested throughout the whole pathway and a system included to verify that delineated structures are transferred correctly. Excellent multidisciplinary team communication and working is vital, and key staff members on both sides should be specifically dedicated to the project. Patient pathway should be clearly devised to optimize patient care and the resources of all departments. Recruitment of a cohort of patients into a methodology study is valuable to test the quality assurance methods and pathway.

Potential role of intensity-modulated radiotherapy in the treatment of tumors of the maxillary sinus.

PURPOSE: To assess 3-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) techniques to see whether doses to critical structures could be reduced while maintaining planning target volume (PTV) coverage in patients receiving conventional radiotherapy (RT) for carcinoma of the maxillary sinus because of the risk of radiation-induced complications, particularly visual loss. METHODS AND MATERIALS: Six patients who had recently received conventional RT for carcinoma of the maxillary sinus were studied. Conventional RT, 3D-CRT, and step-and-shoot IMRT plans were prepared using the same 2-field arrangement. The effect of reducing the number of segments in the IMRT beams was investigated. RESULTS: 3D-CRT and IMRT reduced the brain and ipsilateral parotid gland doses compared with the conventional plans. IMRT reduced doses to both optic nerves; for the contralateral optic nerve, 15-segment IMRT plans delivered an average maximal dose of 56.4 Gy (range 53.9-59.3) compared with 65.7 Gy (range 65.3-65.9) and 64.2 Gy (range 61.4-65.6) for conventional RT and 3D-CRT, respectively. IMRT also gave improved PTV homogeneity and improved coverage, with an average of 8.5% (range 7.0-11.7%) of the volume receiving <95% of the prescription dose (64 Gy) compared with 14.7% (range 14.1-15.9%) and 15.1% (range 14.4-16.1%) with conventional RT and 3D-CRT, respectively. Little difference was found between the 15 and 7-segment plans, but 5 segments resulted in a reduced minimal PTV dose. CONCLUSIONS: IMRT offers significant advantages over conventional RT and 3D-CRT techniques for treatment of maxillary sinus tumors. Good results can be obtained from 7 segments per beam without compromising the PTV coverage. This number of segments is practical for implementation in a busy RT department.

Improvements in target coverage and reduced spinal cord irradiation using intensity-modulated radiotherapy (IMRT) in patients with carcinoma of the thyroid gland.

BACKGROUND AND PURPOSE: External beam radiotherapy for thyroid carcinoma poses a significant technical challenge as the target volume lies close to or surrounds the spinal cord. The potential of intensity-modulated radiotherapy (IMRT) to improve the dose distributions was investigated. MATERIALS AND METHODS: A planning study was performed on patients with thyroid carcinoma. Plans were generated to irradiate the thyroid bed alone or to treat the thyroid bed and the loco-regional lymph nodes in two phases. Conventional plans with minimal beam shaping were compared to three-dimensional conformal radiotherapy (3DCRT) and inverse-planned IMRT plans to assess target coverage and normal tissue sparing. IMRT techniques were optimized to find the minimum number of equispaced beams required to achieve the clinical benefit and a concomitant boost technique was explored. RESULTS: For the thyroid bed alone and the thyroid bed plus loco-regional lymph nodes, conventional and conformal techniques produced low minimum doses to the planning target volume (PTV) if spinal cord tolerance was respected. 3DCRT reduced the irradiated volume of normal tissue (P=0.01). IMRT plans achieved the goal dose to the PTV (P<0.01) and also reduced the spinal cord maximum dose (P<0.01). IMRT, using a concomitant boost technique, produced better target coverage than a two-phase technique. For both the two-phase and concomitant boost techniques, IMRT plans with seven and five equispaced fields produced similar dose distributions to nine fields, but three fields were significantly worse. CONCLUSIONS: 3DCRT reduced normal tissue irradiation compared to conventional techniques, but did not improve PTV or spinal cord doses. IMRT improved the PTV coverage and reduced the spinal cord dose. A simultaneous integrated boost technique with five equispaced fields produced the best dose distribution. IMRT should reduce the risk of myelopathy or may allow dose escalation in patients with thyroid cancer.

Reduction of small and large bowel irradiation using an optimized intensity-modulated pelvic radiotherapy technique in patients with prostate cancer.

PURPOSE: To investigate the role of intensity-modulated radiation therapy (IMRT) to irradiate the prostate gland and pelvic lymph nodes while sparing critical pelvic organs, and to optimize the number of beams required. METHODS AND MATERIALS: Target, small bowel, colon, rectum, and bladder were outlined on CT planning scans of 10 men with prostate cancer. Optimized conventional (RT) and 3-dimensional conformal radiotherapy (3D-CRT) plans were created and compared to inverse-planned IMRT dose distributions using dose-volume histograms. Optimization of beam number was undertaken for the IMRT plans. RESULTS: With RT the mean percentage volume of small bowel and colon receiving >45 Gy was 21.4 +/- 5.4%. For 3D-CRT it was 18.3 +/- 7.7% (p = 0.0043) and for 9-field IMRT it was 5.3 +/- 1.8% (p < 0.001 compared to 3D-CRT). For 7, 5, and 3 IMRT fields, it was 6.4 +/- 2.9%, 7.2 +/- 2.8%, and 8.4 +/- 3.8% (all p < 0.001 compared to 3D-CRT). The rectal volume irradiated >45 Gy was reduced from 50.5 +/- 16.3% (3D-CRT) to 5.8 +/- 2.1% by 9-field IMRT (p < 0. 001) and bladder from 52.2 +/- 12.8% to 7 +/- 2.8% (p < 0.001). Similar benefits were maintained for 7, 5, and 3 IMRT fields. CONCLUSIONS: The reduction in critical pelvic organ irradiation seen with IMRT may reduce side effects in patients, and allow modest dose escalation within acceptable complication rates. These reductions were maintained with 3-5 IMRT field plans which potentially allow less complex delivery techniques and shorter delivery times.

The delivery of intensity modulated radiotherapy to the breast using multiple static fields.

BACKGROUND AND PURPOSE: To develop a method of using a multileaf collimator (MLC) to deliver intensity modulated radiotherapy (IMRT) for tangential breast fields, using an MLC to deliver a set of multiple static fields (MSFs). MATERIALS AND METHODS: An electronic portal imaging device (EPID) is used to obtain thickness maps of medial and lateral tangential breast fields. From these IMRT deliveries are designed to minimize the volume of breast above 105% of prescribed dose. The deliveries are universally-wedged beams augmented with a set of low dose shaped irradiations. Dosimetric and planning QA of this method has been compared with the standard, wedged treatment and the corresponding treatment using physical compensators. Several options for delivering the MSF treatment are presented. RESULTS: The MSF technique was found to be superior to the standard technique (P value=0.002) and comparable with the compensated technique. Both IMRT methods reduced the volume of breast above 105% dose from a mean value of 12.0% of the total breast volume to approximately 2.8% of the total breast volume. CONCLUSIONS: This MSF method may be used to reduce the high dose volume in tangential breast irradiation significantly. This may have consequences for long-term side effects, particularly cosmesis.

Inverse planning with constraints to generate smoothed intensity-modulated beams.

Highly conformal dose distributions can be generated by intensity-modulated radiotherapy. Intensity-modulated beams (IMBs) are generally determined by inverse-planning techniques designed to maximize conformality. Usually such techniques apply no constraints on the form of the IMBs which may then develop fine-scale modulation. In this paper we present a technique for generating smoother IMBs, which yields a dose distribution almost identical to that without the constraint on the form of the IMBs. The method applies various filters successively at intervals throughout the iterative inverse planning. It is shown that the IMBs so determined using a simple median window filter have desirable properties in terms of increasing the efficiency of delivery by the dynamic multileaf collimator method and may be 'more like conventional beams' than unconstrained, highly modulated IMBs.

Generation of discrete beam-intensity modulation by dynamic multileaf collimation under minimum leaf separation constraints.

An algorithm to generate discrete beam-intensity modulation by dynamic multileaf collimation is presented which incorporates constraints on minimum allowed leaf separations. MLC positioning information is derived simultaneously for all leaf pairs and back-up diaphragms as they progress across the field. A feedback mechanism allows corrections to be applied to eliminate potential violations of minimum separation conditions and any underexposure in the interleaf tongue-and-groove region as they are encountered. The resulting motion correctly delivers the intended modulation and is physically realizable. Implementation of the algorithm is described. Results of the algorithm can also alternatively be interpreted as defining a series of static fields to deliver the same modulation.

Matching wedge field characteristics on different treatment machines.

A method is presented for matching wedge field dose distributions on different treatment units, simplifying the transfer of patients between machines during machine failure or scheduled downtime and avoiding the need for a full re-plan in most cases. Differences in wedge field characteristics between machines are accounted for and differences in energy are easily accommodated.

Treatment delivery accuracy in intensity-modulated conformal radiotherapy.

The importance of geometric and dosimetric inaccuracies in the delivery of conformal radiotherapy using intensity-modulated x-ray fields is assessed. It is shown that although the incident intensity or fluence profiles may have large gradients, the resulting absorbed dose profile gradients are shallower because of electron transport and photon scatter. This makes the interleaving of dose contributions from the many intensity-modulated fields used less critically dependent on their relative positions and on the accuracy of profile generation. It is found that the expected geometric and dosimetric inaccuracies do not make this technique impracticable provided that good patient fixation and beam delivery can be assured.