Differential dosing of prostate and seminal vesicles using dynamic multileaf collimation

Single Course IMRT Plan to Deliver 45 Gy to Seminal Vesicles and 81 Gy to Prostate in 45 Fractions

We treat prostate and seminal vesicles (SV) to 45 Gy in 25 fractions (course 1) and boost prostate to 81 Gy in 20 more fractions (course 2) with Intensity Modulated Radiation Therapy (IMRT). This two-course IMRT with 45 fractions delivered a non-uniform dose to SV and required two plans and two QA procedures. We used Linear Quadratic (LQ) model to develop a single course IMRT plan to treat SV to a uniform dose, which has the same biological effective dose (BED) as that of 45 Gy in 25 fractions and prostate to 81 Gy, in 45 fractions. Single course IMRT plans were compared with two-course IMRT plans, retrospectively for 14 patients. With two-course IMRT, prescription to prostate and SV was 45 Gy in 25 fractions and to prostate only was 36 Gy in 20 fractions, at 1.8 Gy/fraction. With 45-fraction single course IMRT plan, prescription to prostate was 81 Gy and to SV was 52 or 56 Gy for a α/β of 1 and 3, respectively. 52 Gy delivered in 45 fractions has the same BED of 72 Gy3 as that of delivering 45 Gy in 25 fractions, and is called Matched Effective Dose (MED). LQ model was used to calculate the BED and MED to SV for α/β values of 1–10. Comparison between two-course and single course IMRT plans was in terms of MUs, dose-max, and dose volume constraints (DVC). DVC were: 95% PTV to be covered by at least 95% of prescription dose; and 70, 50, and 30% of bladder and rectum should not receive more than 40, 60, and 70% of 81 Gy. SV Volumes ranged from 2.9–30 cc. With two-course IMRT plans, mean dose to SV was non-uniform and varied between patients by 48% (54 to 80 Gy). With single-course IMRT plan, mean dose to SV was more uniform and varied between patients by only 9.6% (58.2 to 63.8 Gy), to deliver MED of 56 Gy for α/β − 1. Single course IMRT plan MUs were slightly larger than those for two-course IMRT plans, but within the range seen for two-course plans (549–959 MUs, n=51). Dose max for single-course plans were similar to two-course plans. Doses to PTV, rectum and bladder with single course plans were as per DVC and comparable to two-course plans. Single course IMRT plan reduces IMRT planning and QA time to half.

Variations in inter-observer contouring and its impact on dosimetric and radiobiological parameters for intensity-modulated radiotherapy planning in treatment of localised prostate cancer

Inter-observer variations in contouring and their impacts on dosimetric and radiobiological parameters in intensity-modulated radiotherapy (IMRT) treatment for localised prostate cancer patients were investigated. Four observers delineated the gross tumour volume (GTV) (prostate and seminal vesicles), bladder and rectum for nine patients. Contouring done by radiologist was considered as gold standard for comparison purposes and for IMRT plan optimisation. Maximum average variations in contoured prostate, bladder and rectum volumes were 3% (SD = 8.4), 2.5% (SD = 4.12) and 13.2% (SD = 6.77), respectively. The average conformity index for standard contouring set (observer A) was 0.85 (SD = 0.028) and statistically significant differences were observed for observers A–B (p = 0.008), A–C (p = 0.006) and A–D (p = 0.011). Average values of normal tissue complication probability for bladder and rectum for observer A were 0.361% (SD = 0.036) and 1.59% (SD = 0.14). Maximum average tumour control probability was 99.94% (SD = 0.035) and statistically significant difference was observed for observers A–B (p = 0.037) and observers A–C (p = 0.01). Inter-observer contouring variations have significant impact on dosimetric and radiobiological outcome in IMRT treatment planning. So accurate contouring of tumour and normal organs is a fundamental prerequisite to make good correlation between calculated and clinical observed results.

Intensity modulated neutron radiotherapy for the treatment of adenocarcinoma of the prostate

PURPOSE

This study investigates the enhanced conformality of neutron dose distributions obtainable through the application of intensity modulated neutron radiotherapy (IMNRT) to the treatment of prostate adenocarcinoma.

METHODS AND MATERIALS

An in-house algorithm was used to optimize individual segments for IMNRT generated using an organ-at-risk (OAR) avoidance approach. A number of beam orientation schemes were investigated in an attempt to approach an optimum solution. The IMNRT plans were created retrospectively for 5 patients previously treated for prostate adenocarcinoma using fast neutron therapy (FNT), and a comparison of these plans is presented. Dose distributions and dose-volume histograms (DVHs) were analyzed and plans were evaluated based on percentage volumes of rectum and bladder receiving 95%, 80%, and 50% (V(95), V(80), V(50)) of the prescription dose, and on V(60) for both the femoral heads and GM(muscle) group.

RESULTS

Plans were normalized such that the IMNRT DVHs for prostate and seminal vesicles were nearly identical to those for conventional FNT plans. Use of IMNRT provided reductions in rectum V(95) and V(80) of 10% (2-27%) and 13% (5-28%), respectively, and reductions in bladder V(95) and V(80) of 12% (3-26%) and 4% (7-10%), respectively. The average decrease in V(60) for the femoral heads was 4.5% (1-18%), with no significant change in V(60) for the GM(muscle) group.

CONCLUSIONS

This study provides the first analysis of the application of intensity modulation to neutron radiotherapy. The IMNRT technique provides a substantial reduction in normal tissue dose in the treatment of prostate cancer. This reduction should result in a significant clinical advantage for this and other treatment sites.

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.

A method for increased dose conformity and segment reduction for SMLC delivered IMRT treatment of the prostate

PURPOSE

The focus of this work is to develop a practical planning method that results in increased dose conformity and reduced treatment time for segmental multileaf collimation (sMLC) based intensity-modulated radiation therapy (IMRT) delivery.

METHODS AND MATERIALS

Additional regions for dose constraint are introduced within the normal tissue during the planning process by designing a series of concentric ellipsoids around the target. A dose gradient is then defined by assigning dose constraints to each concentric region. The technique was tested at two centers and data for 26 and 10 patients, respectively, are presented allowing for differences in treatment technique, beam energy, ellipsoid definition, and prescription dose. At both centers, a series of patients previously treated for prostate cancer with IMRT were selected, and comparisons were made between the original and new plans.

RESULTS

While meeting target dose specifications and normal tissue constraints, the average number of beam directions decreased by 1.6 with a standard error (SE) of 0.1. The average time for delivery at center 1 decreased by 29.0% with an SE of 2.0%, decreasing from 17.5 min to 12.3 min. The average time for delivery at center 2 decreased by 29.9% with an SE of 3.8%, decreasing from 11 min to 7.7 min. The amount of nontarget tissue receiving D(100) decreased by 15.7% with an SE of 2.4%. Nontarget tissue receiving D(95), D(90), and D(50) decreased by 16.3, 15.1, and 19.5%, respectively, with SE values of approximately 2% at center 1. Corresponding values for D(100), D(95), D(90), and D(50) decreased by 13.5, 16.7, 17.1, and 5.1%, respectively, with SE values of less than 3% at center 2.

CONCLUSION

By designating subsets of tissue as concentric regions around the target(s) and carefully defining each region's dose constraints, we have gained an increased measure of control over the region outside the target boundaries. This increased control manifests as two distinct endpoints that are beneficial to the IMRT process: increased dose conformity and decreased treatment time.

Analysis of various beamlet sizes for IMRT with 6 MV Photons

Application of intensity modulated radiation therapy (IMRT) using multileaf collimation often requires the use of small beamlets to optimize the delivered radiation distribution. Small-beam dose distribution measurements were compared to dose distributions calculated using a commercial treatment planning system that models its data acquired using measurements from relatively large fields. We wanted to evaluate only the penumbra, percent depth-dose (PDD) and output model, so we avoided dose distribution features caused by rounded leaf ends and interleaf leakage by making measurements using the secondary collimators. We used a validated radiochromic film dosimetry system to measure high-resolution dose distributions of 6 MV photon beams. A commercial treatment planning system using the finite size pencil beam (FSPB) dose calculation algorithm was commissioned using measured central axis outputs from 4.0x4.0 to 40.0x40.0 cm2 beams and radiographic-film profile measurements of a 4.0x4.0 cm2 beam at twice the depth of maximum dose (dmax). Calculated dose distributions for square fields of 0.5x0.5 cm2, and 1.0x1.0 cm2, to 6.0x6.0 cm2, in 1.0x1.0 cm2, increments were compared against radiochromic film measurements taken with the film oriented parallel to the beam central axis in a water equivalent phantom. The PDD of the smaller field sizes exhibited behavior typical of small fields, namely a decrease in dmax with decreasing field size. The FSPB accurately modeled the depth-dose and central axis output for depths deeper than the nominal dmax of 1.5 cm plus 0.5 cm. The dose distribution in the build-up and penumbra regions was not accurately modeled for depths less than 2 cm, especially for the fields of 2.0x2.0 cm2 and smaller. Using the gamma function with 2 mm and 2% criteria, the dose model was shown to accurately predict the penumbra. While for single small beams the compared dose distributions passed the gamma function criteria, the clinical appropriateness of these criteria is not clear for a composite IMRT plan. Further investigation of the cumulative impact of the observed dose discrepancies is warranted. We speculate that the observed differences in the penumbra regions arise from some energy dependent artifact in the radiographic-film profiles used for commissioning. In the future, radiochromic film based commissioning might provide a more accurate data set for dose modeling.

Benchmarking beam alignment for a clinical helical tomotherapy device

A clinical helical tomotherapy treatment machine has been installed at the University of Wisconsin Comprehensive Cancer Center. Beam alignment has been finalized and accepted by UW staff. Helical tomotherapy will soon be clinically available to other sites. Clinical physicists who expect to work with this machine will need to be familiar with its unique dosimetric characteristics, and those related to the geometrical beam configuration and its verification are described here. A series of alignment tests and the results are presented. Helical tomotherapy utilizes an array of post-patient xenon-filled megavoltage radiation detectors. These detectors have proved capable of performing some alignment verification tests. That is particularly advantageous because those tests can then be automated and easily performed on an ongoing basis.

Acceptance tests and quality control (QC) procedures for the clinical implementation of intensity modulated radiotherapy (IMRT) using inverse planning and the sliding window technique: experience from five radiotherapy departments

BACKGROUND AND PURPOSE

An increasing number of radiotherapy centres is now aiming for clinical implementation of intensity modulated radiotherapy (IMRT), but--in contrast to conventional treatment--no national or international guidelines for commissioning of the treatment planning system (TPS) and acceptance tests of treatment equipment have yet been developed. This paper bundles the experience of five radiotherapy departments that have introduced IMRT into their clinical routine.

METHODS AND MATERIALS

The five radiotherapy departments are using similar configurations since they adopted the commercially available Varian solution for IMRT, regarding treatment planning as well as treatment delivery. All are using the sliding window technique. Different approaches towards the derivation of the multileaf collimator (MLC) parameters required for the configuration of the TPS are described. A description of the quality control procedures for the dynamic MLC, including their respective frequencies, is given. For the acceptance of the TPS for IMRT multiple quality control plans were developed on a variety of phantoms, testing the flexibility of the inverse planning modules to produce the desired dose pattern as well as assessing the accuracy of the dose calculation. Regarding patient treatment verification, all five centres perform dosimetric pre-treatment verification of the treatment fields, be it on a single field or on a total plan procedure. During the actual treatment, the primary focus is on patient positioning rather than dosimetry. Intracavitary in vivo measurements were performed in special cases.

RESULT AND CONCLUSION

The configurational MLC parameters obtained through different methods are not identical for all centres, but the observed variations have shown to be of no significant clinical relevance. The quality control (QC) procedures for the dMLC have not detected any discrepancies since their initiation, demonstrating the reliability of the MLC controller. The development of geometrically simple QC plans to test the inverse planning, the dynamic MLC modules and the final dose calculation has proven to be useful in pointing out the need to remodel the single pencil beam scatter kernels in some centres. The final correspondence between calculated and measured dose was found to be satisfactory by all centres, for QC test plans as well as for pre-treatment verification of clinical IMRT fields. An intercomparison of the man hours needed per patient plan verification reveals a substantial variation depending on the type of measurements performed.

A review of intensity modulated radiation therapy: incorporating a report on the seventh education workshop of the ACPSEM--ACT/NSW branch. Australasian College of Physical Scientists and Engineers in Medicine

Intensity modulated radiation therapy (IMRT) is an evolving treatment technique that has become a clinical treatment option in several radiotherapy centres around the world. In August 2001 the ACT/NSW branch of the ACPSEM held its seventh education workshop, the subject was IMRT. This review considers the current use of IMRT and reports on the proceedings of the workshop. The workshop provided some of the theory behind IMRT, discussion of the practical issues associated with IMRT, and also involved presentations from Australian centres that had clinically implemented IMRT. The main topics of discussion were patient selection, plan assessment, multi-disciplinary approach, quality assurance and delivery of IMRT. Key points that were emphasised were the need for a balanced multi-disciplinary approach to IMRT, in both the establishment and maintenance of an IMRT program; the importance of the accuracy of the final dose distribution as compared to the minor in-field fluctuations of individual beams; and that IMRT is an emerging treatment technique, undergoing continuing development and refinement.

Intensity-modulated radiotherapy

Intensity-modulated radiotherapy represents a recent advancement in conformal radiotherapy. It employs specialized computer-driven technology to generate dose distributions that conform to tumor targets with extremely high precision. Treatment planning is based on inverse planning algorithms and iterative computer-driven optimization to generate treatment fields with varying intensities across the beam section. Combinations of intensity-modulated fields produce custom-tailored conformal dose distributions around the tumor, with steep dose gradients at the transition to adjacent normal tissues. Thus far, data have demonstrated improved precision of tumor targeting in carcinomas of the prostate, head and neck, thyroid, breast, and lung, as well as in gynecologic, brain, and paraspinal tumors and soft tissue sarcomas. In prostate cancer, intensity-modulated radiotherapy has resulted in reduced rectal toxicity and has permitted tumor dose escalation to previously unattainable levels. This experience indicates that intensity-modulated radiotherapy represents a significant advancement in the ability to deliver the high radiation doses that appear to be required to improve the local cure of several types of tumors. The integration of new methods of biologically based imaging into treatment planning is being explored to identify tumor foci with phenotypic expressions of radiation resistance, which would likely require high-dose treatments. Intensity-modulated radiotherapy provides an approach for differential dose painting to selectively increase the dose to specific tumor-bearing regions. The implementation of biologic evaluation of tumor sensitivity, in addition to methods that improve target delineation and dose delivery, represents a new dimension in intensity-modulated radiotherapy research.

Interleaf leakage for 5 and 10 mm dynamic multileaf collimation systems incorporating patient motion

Use of dynamic multileaf collimation (DMLC) for intensity modulated radiation therapy (IMRT) is accelerating. Delivery systems have the ailment of interleaf leakage (IL). This is compounded by the inefficiency of IMRT delivery, estimated to be a factor of 5 for DMLC. With IL on the order of 4%, it is possible to deliver as much as 20% of the prescribed dose to nonprescribed regions. However, IL is characterized by narrow Gaussian peaks of approximately 0.5-1.0 mm full-width-half-maximum (FWHM). We performed a leakage study for 5 and 10 mm leaf systems, accounting for intratreatment and intertreatment motions. In solid phantoms, film was placed perpendicular to beams. DMLC patterns delivered step-wedged distributions. The same field was duplicated using a collimating jaw in a segmented fashion to obtain baseline data of primary and scatter contributions. Longitudinal shifts up to 4 mm and angulations up to 4 degrees were introduced during beam delivery by running multiple patterns, to arrive at a composite delivery. The intent of these rigid body motion experiments was to replicate patient motion. Clinical IMRT fields using segmented MLC were also tested. Films were scanned and converted to dose. A microionization chamber confirmed film data at discrete points. In all cases shifts diminished IL peak values. In the step-wedge case, the net 18 MV IL peaks diminished from 3.6% to 3.2% for the 10 mm system. The 5 mm system IL values decreased from 4.0% to 3.2% with a 2 mm shift but increased to 4.0% with 4 mm shifts. The clinical field data followed the same pattern with a washing out of peak values, but the overall transmission to shielded regions slightly increased. Therefore nonprescribed regions are influenced by an effective transmission value rather than discrete peak IL values. The 5 mm leaf system does not introduce increased IL and is an appropriate system for IMRT.