Single-Fraction Radiosurgery Using Conservative Doses for Brain Metastases: Durable Responses in Select Primaries With Limited Toxicity

BACKGROUND

Optimal doses for single-fraction stereotactic radiosurgery (SRS) in the treatment of brain metastases are not well established. Our institution utilized conservative dosing compared to maximum-tolerated doses from the Radiation Therapy Oncology Group 90-05 Phase I study.

OBJECTIVE

To report individual lesion control (LC) from conservative single-fraction doses and determine factors affecting LC.

METHODS

From 2003 to 2015, patients who underwent linear accelerator-based single-fraction SRS for cerebral/cerebellar metastases and receiving at least 1 follow-up magnetic resonance imaging (MRI) were identified. Lesion response was assessed by a size-based rating system and modified "Response Assessment in Neuro-Oncology Brain Metastases" (RANO-BM) criteria.

RESULTS

Among 188 patients with 519 lesions, median survival was 13.1 mo; median follow-up time with MRI was 9.6 mo per course. Median tumor-periphery dose was 15 Gy (range: 7.5-20.7). Median lesion volume was 0.5 cc and diameter was 9 mm (range: 2-45). Concordance between RANO-BM and size-based system was 93%. Crude 1-yr LC was 80%, 73%, 56%, and 38% for lesions 1 to 10, 11 to 20, 21 to 30, >31 mm, respectively. On multivariate analysis, increased size, melanoma and colorectal histology, and progression after whole brain radiation therapy predicted worse LC. When excluding lesions treated as a boost, dose was a significant predictor of LC in multivariate models (hazard ratio 0.89, P = .01). Symptomatic radiation necrosis occurred in 10 lesions in 10 patients.

CONCLUSION

Histology predicts LC after conservative SRS doses with evidence of a dose-response relationship. Conservative single-fraction SRS doses confer minimal toxicity and acceptable control in certain subgroups (breast cancer, <5 mm), with suboptimal control in larger lesions and in combination with whole brain radiation therapy.

NRG Oncology/RTOG 0438: A Phase 1 Trial of Highly Conformal Radiation Therapy for Liver Metastases

PURPOSE

This study aimed to determine the feasibility and maximally tolerated dose of hypofractionated, conformal radiation therapy (RT) in patients with liver metastases.

METHODS AND MATERIALS

Nonsurgical patients with ≤5 liver metastases (sum of maximal diameter of all lesions ≤8 cm) were included in the study. There were 4 dose levels: 35 Gy, 40 Gy (starting level), 45 Gy, and 50 Gy, in 10 fractions. The clinical target volume included metastases identified on contrast computed tomography or magnetic resonance imaging with a 5-mm margin within the liver. The planning target volume margin ranged from 4 to 30 mm, depending on breathing motion. Dose-limiting toxicities were defined as RT-related grade ≥4 hepatic or gastrointestinal toxicities or thrombocytopenia occurring within 90 days of the start of RT.

RESULTS

A total of 26 patients with metastases from colorectal (8 patients), breast (7 patients) and other malignancies (11 patients) were enrolled between November 2005 and December 2010. Twenty-three patients were evaluable (8, 7, and 8 on the 40, 45, and 50 Gy dose levels, respectively). Two patients assigned to 50 Gy received 35 Gy owing to normal tissue limits, so 2 additional patients were treated to 50 Gy. There were no dose-limiting toxicities on any of the dose levels. On the 45 Gy dose level, 1 patient developed reversible grade 3 enteritis (37 days from RT start) and diarrhea (22 days); another patient developed grade 3 lymphopenia (23 days). At the 50 Gy dose level, 1 patient had grade 3 hyperglycemia (74 days), and another patient developed grade 3 lymphopenia (13 days), colonic hemorrhage (325 days), and colonic gastrointestinal obstruction (325 days). With a potential median follow-up of 66.1 months (range, 34.6-89.0 months), no other late toxicities were observed.

CONCLUSIONS

Treatment of liver metastases with 50 Gy in 10 fractions was feasible and safe in a multi-institutional setting.

AAPM medical physics practice guideline 10.a.: Scope of practice for clinical medical physics

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline (MPPG) represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiation requires specific training, skills, and techniques as described in each document. As the review of the previous version of AAPM Professional Policy (PP)‐17 (Scope of Practice) progressed, the writing group focused on one of the main goals: to have this document accepted by regulatory and accrediting bodies. After much discussion, it was decided that this goal would be better served through a MPPG. To further advance this goal, the text was updated to reflect the rationale and processes by which the activities in the scope of practice were identified and categorized. Lastly, the AAPM Professional Council believes that this document has benefitted from public comment which is part of the MPPG process but not the AAPM Professional Policy approval process. The following terms are used in the AAPM's MPPGs:

Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline.

Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

Anatomical Study of Percutaneous Trigeminal Compressive Balloon Positioning on Merged 3-D Rotational X-Ray and Preprocedural Magnetic Resonance Imaging

BACKGROUND

Percutaneous trigeminal rhizotomy or balloon compression for trigeminal neuralgia carries a potential risk for the brainstem, the carotid artery, and the basilar artery.

OBJECTIVE

To detail the relation of critical neural and vascular structures to expanded balloons used for percutaneous compression of the trigeminal ganglion.

METHOD

A retrospective analysis of preprocedural magnetic resonance imaging (MRI) and procedural X-ray-based imaging for 9 patients detailed balloon proximity to the brainstem, carotid artery, and basilar artery.

RESULTS

Balloons extended 10.96 ± 5.54 mm (mean ± SD) posterior to the clival line. The average distance from the balloon to the brainstem was 6.89 mm, and that to the basilar artery was 12.12 mm (range: 0-18.2). The medial edge of the balloon was an average distance of 1.39 mm from the baseline position of the carotid lumen.

CONCLUSION

Preprocedural MRI, merged with 3-D rotational angiography suite imaging, detailed the proximity of the balloon to critical neural and vascular structures. Our study found that the standard technique for percutaneous trigeminal compression, with balloon placement at an average depth of 10.96 mm posterior to the clival line, on average, provided an additional 6.89 mm of space before the brainstem would have been encountered, demonstrating safe positioning.

Local control rates with five-fraction stereotactic body radiotherapy for oligometastatic cancer to the lung

OBJECTIVE

To report our institutional experience with five fractions of daily 8-12 Gy stereotactic body radiotherapy (SBRT) for the treatment of oligometastatic cancer to the lung.

METHODS

Thirty-four consecutive patients with oligometastatic cancers to the lung were treated with image-guided SBRT between 2008 and 2011. Patient age ranged from 38 to 81 years. There were 17 males and 17 females. Lung metastases were from the following primary cancer types: colon cancer (n=13 patients), head and neck cancer (n=6), breast cancer (n=4), melanoma (n=4), sarcoma (n=4) and renal cell carcinoma (n=3). The median prescription dose was 50 Gy in five fractions (range, 40-60 Gy) to the isocenter, with the 80% isodose line encompassing the planning target volume (PTV) [defined as gross tumor volume (GTV) + 7-11 mm volumetric expansion]. The follow-up interval ranged from 2.4-54 months, with a median of 16.7 months.

RESULTS

The 1-, 2-, and 3-year patient local control (LC) rates for all patients were 93%, 88%, and 80% respectively. The 1-, 2-, and 3-year overall survival (OS) rates were 62%, 44%, and 23% respectively. The 1- and 2-year patient LC rates were 95% and 88% for tumor size 1-2 cm (n=25), and 86% for tumor size 2-3 cm (n=7). The majority (n=4) of local failures occurred within 12 months. No patient experienced local failure after 12 months except for one patient with colon cancer whose tumors progressed locally at 26 months. All five patients with local recurrences had colorectal cancer. Statistical analyses showed that age, gender, previous chemotherapy, previous surgery or radiation had no significant effect on LC rates. No patient was reported to have any symptomatic pneumonitis at any time point.

CONCLUSIONS

SBRT for oligometastatic disease to the lung using 8-12 Gy daily fractions over five treatments resulted in excellent 1- and 2-year LC rates. Most local failures occurred within the first 12 months, with five local failures associated with colorectal cancer. The treatment is safe using this radiation fractionation schedule with no therapy-related pneumonitis.

A phase 3 trial of whole brain radiation therapy and stereotactic radiosurgery alone versus WBRT and SRS with temozolomide or erlotinib for non-small cell lung cancer and 1 to 3 brain metastases: Radiation Therapy Oncology Group 0320

BACKGROUND

A phase 3 Radiation Therapy Oncology Group (RTOG) study subset analysis demonstrated improved overall survival (OS) with the addition of stereotactic radiosurgery (SRS) to whole brain radiation therapy (WBRT) in non-small cell lung cancer (NSCLC) patients with 1 to 3 brain metastases. Because temozolomide (TMZ) and erlotinib (ETN) cross the blood-brain barrier and have documented activity in NSCLC, a phase 3 study was designed to test whether these drugs would improve the OS associated with WBRT + SRS.

METHODS AND MATERIALS

NSCLC patients with 1 to 3 brain metastases were randomized to receive WBRT (2.5 Gy × 15 to 37.5 Gy) and SRS alone, versus WBRT + SRS + TMZ (75 mg/m(2)/day × 21 days) or ETN (150 mg/day). ETN (150 mg/day) or TMZ (150-200 mg/m(2)/day × 5 days/month) could be continued for as long as 6 months after WBRT + SRS. The primary endpoint was OS.

RESULTS

After 126 patients were enrolled, the study closed because of accrual limitations. The median survival times (MST) for WBRT + SRS, WBRT + SRS + TMZ, and WBRT + SRS + ETN were qualitatively different (13.4, 6.3, and 6.1 months, respectively), although the differences were not statistically significant. Time to central nervous system progression and performance status at 6 months were better in the WBRT + SRS arm. Grade 3 to 5 toxicity was 11%, 41%, and 49% in arms 1, 2, and 3, respectively (P<.001).

CONCLUSION

The addition of TMZ or ETN to WBRT + SRS in NSCLC patients with 1 to 3 brain metastases did not improve survival and possibly had a deleterious effect. Because the analysis is underpowered, these data suggest but do not prove that increased toxicity was the cause of inferior survival in the drug arms.

Abstract 736: RTOG 0320:A phase III trial comparing whole brain radiation therapy (WBRT) and stereotactic radiosurgery (SRS) alone versus WBRT with temozolomide (TMZ) or erlotinib for non-small cell lung cancer (NSCLC) and 1-3 brain metastases

Background: A previous phase III RTOG study subset analysis demonstrated improvement in overall survival (OS) with the addition of SRS to WBRT in NSCLC patients with 1 to 3 brain metastases. As both TMZ and erlotinib are known to cross the blood brain barrier (potentially providing radiosensitization), and have documented activity in NSCLC, a phase III study was designed to test whether either of these drugs would improve outcome of WBRT/SRS. Methods: NSCLC patients (n=126) with 1-3 brain metastases were randomized (10/2005 to 8/2009; study closed prematurely due to slow accrual) to receive WBRT (2.5 Gy x 15 to 37.5Gy) + SRS alone, vs. WBRT/SRS with TMZ (75mg/m2/D x 21) or erlotinib (150mg/D). Erlotinib or TMZ (150-200 mg/m2/D x 5/mo) could be given in the drug arms post-WBRT/SRS at the discretion of the investigator. The primary endpoint was overall survival (OS). Results: Arms were stratified by RTOG recursive partitioning analysis (RPA) class and balanced for prognostic variables including the Graded Prognostic Assessment (GPA) score. Neither the addition of erlotinib nor TMZ to WBRT/SRS resulted in an improvement in OS, or time to CNS progression compared to WBRT/SRS alone. Patients in the WBRT/SRS arm had longer MST (Median Survival Time) (13.4 mo, 95% CI = 6.5-20.8 mo.) compared to the WBRT+SRS+ erlotinib (6.1 mo, 95% CI = 3.6-12.1 mo)[Hazard ratio (≥2 / α1) and 95% CI; 1.47 (0.92 to 2.36)], or TMZ (6.3 mo, 95% CI= 3.4-10.1 mo.) [Hazard ratio (β3 / α1) and 95% CI; 1.43 (0.89 to 2.31)]. This surprising result was not related to excess toxicity. In fact, patients experiencing grade 3+ Adverse Events (AE) appear to have longer OS than those patients without grade 3+ AE for both drug arms. The WBRT/SRS arm had significantly less deterioration in performance status at 6 mo. There were no significant differences between arms for steroid dependence at 6 mo, or causes of death. Conclusion: The addition of either TMZ or erlotinib to WBRT/SRS in this unselected population of NSCLC patients with 1-3 brain metastases provided no clinical advantage. Treatment with WBRT/SRS alone appeared to result in superior outcome data (compared to the addition of TMZ or erlotinib) relative to OS in this limited data set. Detailed analysis to date provides no obvious explanation for these unexpected results. Support: RTOG grant U10 CA21661, and CCOP grant U10 CA37422 from the National Cancer Institute (NCI)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 736. doi:1538-7445.AM2012-736

RTOG 0438: A phase I trial of highly conformal radiation therapy for patients with liver metastases

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Background: This multi-institutional phase I trial was conducted to determine the maximally tolerated dose (MTD) of hypofractionated, highly conformal radiation therapy (RT) in patients (pts) with liver metastases.

Methods: Eligibility criteria included non-surgical pts with ≤ 5 liver metastases; total measurement for all lesions ≤ 8 cm. There were four dose levels (DLs) escalating from 35 Gy to 50 Gy in 5 Gy increments given in 10 fractions with defined normal tissue dose limits. Accrual began at 40 Gy. The clinical target volume (CTV) included all metastases identified on contrast CT/MRI with a 5 mm margin. The planning target margin ranged from 4 to 30 mm around the CTV. For quality assurance the Image-Guided Therapy Center (ITC) remote review tool was used to evaluate treatment planning images and dosimetry information. Dose limiting toxicities (DLTs) were defined as treatment-related grade (Gr) ≥ 4 hepatic, gastrointestinal (GI), thrombocytopenia, or radiation induced liver disease (RILD) within 90 days (dys) of the start of RT.

Results: 26 pts were enrolled between 11/05 and 12/10, and 23 were evaluable; 8, 7, and 8 on the 40, 45, and 50 Gy DLs respectively. Two pts assigned to the 50 Gy DL received 35 Gy because of normal tissue constraints, therefore an additional 2 pts were accrued and treated at the 50 Gy DL. The study was temporarily closed for toxicity evaluation after 6 pts on each DL were followed for a minimum of 90 dys from start of treatment. There were no DLTs observed on any of the DLs. Four pts developed treatment-related Gr 3 toxicities; 2 each on the 45 and 50 Gy DLs. On the 45 Gy DL, 1 pt had two Gr 3 GI toxicities: enteritis (37 dys from RT start) and diarrhea (22 dys) while another pt had Gr 3 lymphopenia (23 dys). On the 50 Gy DL, 1 pt had Gr 3 hyperglycemia (74 dys) and another pt had three Gr 3 toxicities: lymphopenia (13 dys), colonic hemorrhage (325 dys), and GI obstruction (325 dys).

Conclusions: When normal tissue constraints could be met, treatment of liver metastases with 50 Gy in 5 Gy/fx is feasible and safe in a multi-institutional setting. Further studies looking at higher doses and alternate fractionation regimens are warranted. Supported by RTOG U10 CA21661, CCOP U10 CA37422 and ATC U24 CA 81647 NCI grants.

Stereotactic body radiation therapy: the report of AAPM Task Group 101

Task Group 101 of the AAPM has prepared this report for medical physicists, clinicians, and therapists in order to outline the best practice guidelines for the external-beam radiation therapy technique referred to as stereotactic body radiation therapy (SBRT). The task group report includes a review of the literature to identify reported clinical findings and expected outcomes for this treatment modality. Information is provided for establishing a SBRT program, including protocols, equipment, resources, and QA procedures. Additionally, suggestions for developing consistent documentation for prescribing, reporting, and recording SBRT treatment delivery is provided.

Stereotactic radiosurgery for glioblastoma: retrospective analysis

Purpose

This retrospective study was done to better understand the conditions for which stereotactic radiosurgery (SRS) for glioblastoma may be efficacious.

Methods

Between 2000 and 2007, 33 patients with a pathological diagnosis of glioblastoma received SRS with the Novalis^® Shaped Beam Radiosurgery system. Eighteen patients (54%) underwent salvage SRS for recurrence while 15 (45%) patients received upfront SRS following standard fractionated RT for newly diagnosed glioblastoma.

Results

There were no RTOG grade >2 acute side effects. The median survival after SRS was 6.7 months (range 1.4 – 74.7). There was no significant difference in overall survival (from the time of initial diagnosis) with respect to the timing of SRS (p = 0.2). There was significantly better progression free survival in patients treated with SRS as consolidation versus at the time of recurrence (p = 0.04). The majority of patients failed within or at the margin of the SRS treatment volume (21/26 evaluable for recurrence).

Conclusion

SRS is well tolerated in the treatment of glioblastoma. As there was no difference in survival whether SRS is delivered upfront or at recurrence, the treatment for each patient should be individualized. Future studies are needed to identify patients most likely to respond to SRS.

Descriptive analysis of oligometastatic lesions treated with curative-intent stereotactic body radiotherapy

PURPOSE

To characterize oligometastases in patients enrolled on two prospective pilot studies, treating oligometastases with hypofractionated stereotactic body radiotherapy and stereotactic radiosurgery to cranial lesions.

METHODS AND MATERIALS

We describe the characteristics and local control (LC) of 293 lesions in 121 patients with five or fewer metastases treated with stereotactic body radiation and/or cranial stereotactic radiosurgery. For each lesion, the primary cancer site, tumor histology, site of metastasis, gross tumor volume, and prescribed dose were ascertained. The prescribed dose is expressed by the biologically effective dose in 2-Gy fractions (BED2), calculated using the linear quadratic model, assuming an alpha/beta ratio of 10.

RESULTS

Lung lesions were significantly smaller than other lesions in our cohort, whereas liver lesions were significantly larger, possibly reflecting a detection and/or referral bias. The 2-year and 4-year tumor LC rates were 77% and 73% respectively. A larger gross tumor volume was significantly (p < 0.0001) correlated with worse lesion LC. Lesions originating from primary pancreatic, biliary or liver cancer exhibited significantly poorer LC, as did lesions from colorectal cancer. Lesions from breast cancer were better controlled. A higher BED2 did not correlate with improved tumor control.

CONCLUSIONS

Stereotactic body radiation to aggressively treat oligometastatic lesions results in good local tumor control. Bulkier lesions are more difficult to control and may benefit from dose escalation.

Stereotactic Body Radiation Therapy (SBRT) for lung metastases

The curative treatment of oligometastases with radiotherapy remains an area of active investigation. We hypothesise that treating oligometastases with SBRT can prolong life and potentially cure patients, while in patients with multiple lung metastases SBRT can improve quality of life. Fifty patients with lung metastases were treated on this study. Individuals with five or fewer total lesions were treated with curative intent. Individuals with > five metastases were treated palliatively. Most patients (62%) received 5 Gy/fraction for a total of 50 Gy. The number of targets treated per patient ranged from one to five (mean 2.6). Tumor sizes ranged from 0.3-7.7 cm in maximal diameter (median 2.1 cm). Mean follow-up was 18.7 months. Local control of treated lesions was obtained in 42 of 49 evaluable patients (83%). Of the 125 total lesions treated, eight progressed after treatment (94% crude local control). The median overall survival time from time of treatment completion of the curatively treated patients was 23.4 months. The progression-free survival of the same group of patients was 25% and 16% at 12 and 24 months, respectively. Grade 1 toxicity occurred in 35% of all the patients, 6.1% had grade 2 toxicity, and 2% had grade 3 toxicity. Excellent local tumor control rates with low toxicity are seen with SBRT. Median survival time and progression-free survival both appear better than that achieved with standard care alone. Long-term progression-free survival can be seen in a subset of patients when all tumors are targeted.

Hypofractionated stereotactic body radiation therapy (SBRT) for limited hepatic metastases

PURPOSE

To evaluate the feasibility and efficacy of hypofractionated stereotactic body radiation therapy (SBRT) for the treatment of liver metastases.

METHODS AND MATERIALS

The records of 69 patients with 174 metastatic liver lesions treated with SBRT between April 2001 and October 2004 were reviewed. The most common primary tumors were colorectal (n = 20), breast (n = 16), pancreas (n = 9), and lung (n = 5). The mean number of lesions treated per patient was 2.5 (range, 1-6). The longest diameter of the lesions ranged in size from 0.6 to 12.2 cm (median, 2.7 cm). Dose per fraction ranged from 2 Gy to 6 Gy, with a median total dose of 48 Gy (range, 30-55 Gy). Dose was prescribed to the 100% isodose line (IDL), with the 80% IDL covering the gross tumor volume with a minimum margin of 7 mm.

RESULTS

The median follow up was 14.5 months. Sixty patients were evaluable for response based on an abdominal computed tomography scan obtained at a minimum of 3 months after completion of SBRT. The actuarial overall infield local control rate of the irradiated lesions was 76% and 57% at 10 and 20 months, respectively. The median overall survival time was 14.5 months. The progression-free survival rate was 46% and 24% at 6 and 12 months, respectively. None of the patients developed Grade 3 or higher toxicity.

CONCLUSION

Hypofractionated SBRT provides excellent local control with minimal side effects in selected patients with limited hepatic metastases.

Bouquet brachytherapy: Feasibility and optimization of conically spaced implants

PURPOSE

To examine the dosimetric feasibility of a conical implantation approach to robotic-assisted prostate brachytherapy.

METHODS AND MATERIALS

An in-house inverse planning software based on the genetic algorithm (GA) was used to optimize the needle angulations and the seed positions along needles that form one or two bouquets. Volume data from 20 prostate seed implant patients (six 125I and 14 103Pd) previously treated using the conventional rectilinear template approach were used. The dosimetry outcomes of the optimized treatment plan in the conical approach were compared with those from the original treatment plans based on the conventional rectilinear template approach.

RESULTS

When seed spacing is restricted to nominal 1 cm center-to-center spacing, dosimetry results in the conical approach suffer from a higher urethra dose and higher dose heterogeneity compared with the original rectilinear template plans. When the seed loading patterns are optimized as part of inverse planning, the resulting dosimetry plans exhibit adequate dose coverage and uniformity through the target volume, as well as satisfactory sparing of the urethra and rectum.

CONCLUSIONS

Conically spaced implantation for prostate brachytherapy with 125I and 103Pd seeds is feasible in terms of dosimetry outcomes. Techniques for optimized inverse planning for this approach have been developed.

Long-term management of patients with multiple brain metastases after shaped beam radiosurgery. Case report and review of the literature

The role of radiosurgery in the treatment of patients with advanced-stage metastatic disease is currently under debate. Previous randomized studies have not consistently supported the use of radiosurgery to treat patients with numbers of brain metastases. In negative-results studies, however, intracranial tumor control was high but extracranial disease progressed; thus, patient survival was not greatly affected, although neurocognitive function was generally maintained until death. Because the future promises improved systemic (extracranial) therapy, the successful control of brain disease is that much more crucial. Thus, for selected patients with multiple metastases to the brain who remain in good neurological condition, aggressive lesion-targeting radiosurgery should be very useful. Although a major limitation to success of this therapy is the lack of control of extracranial disease in most patients, it is clear that well-designed, aggressive treatment substantially decreases the progression of brain metastases and also improves neurocognitive survival. The authors present the management and a methodology for rational treatment of a patient with breast cancer who has harbored 24 brain metastases during a 3-year period.

Long-term management of patients with multiple brain metastases after shaped beam radiosurgery

✓ The role of radiosurgery in the treatment of patients with advanced-stage metastatic disease is currently under debate. Previous randomized studies have not consistently supported the use of radiosurgery to treat patients with numbers of brain metastases. In negative-results studies, however, intracranial tumor control was high but extracranial disease progressed; thus, patient survival was not greatly affected, although neurocognitive function was generally maintained until death. Because the future promises improved systemic (extracranial) therapy, the successful control of brain disease is that much more crucial. Thus, for selected patients with multiple metastases to the brain who remain in good neurological condition, aggressive lesion-targeting radiosurgery should be very useful. Although a major limitation to success of this therapy is the lack of control of extracranial disease in most patients, it is clear that well-designed, aggressive treatment substantially decreases the progression of brain metastases and also improves neurocognitive survival. The authors present the management and a methodology for rational treatment of a patient with breast cancer who has harbored 24 brain metastases during a 3-year period.

Long-term management of patients with multiple brain metastases after shaped beam radiosurgery

✓ The role of radiosurgery in the treatment of patients with advanced-stage metastatic disease is currently under debate. Previous randomized studies have not consistently supported the use of radiosurgery to treat patients with numbers of brain metastases. In negative-results studies, however, intracranial tumor control was high but extracranial disease progressed; thus, patient survival was not greatly affected, although neurocognitive function was generally maintained until death. Because the future promises improved systemic (extracranial) therapy, the successful control of brain disease is that much more crucial. Thus, for selected patients with multiple metastases to the brain who remain in good neurological condition, aggressive lesion-targeting radiosurgery should be very useful. Although a major limitation to success of this therapy is the lack of control of extracranial disease in most patients, it is clear that well-designed, aggressive treatment substantially decreases the progression of brain metastases and also improves neurocognitive survival. The authors present the management and a methodology for rational treatment of a patient with breast cancer who has harbored 24 brain metastases during a 3-year period.

Whole brain radiation therapy with or without stereotactic radiosurgery boost for patients with one to three brain metastases: phase III results of the RTOG 9508 randomised trial

BACKGROUND

Brain metastases occur in up to 40% of all patients with systemic cancer. We aimed to assess whether stereotactic radiosurgery provided any therapeutic benefit in a randomised multi-institutional trial directed by the Radiation Therapy Oncology Group (RTOG).

METHODS

Patients with one to three newly diagnosed brain metastases were randomly allocated either whole brain radiation therapy (WBRT) or WBRT followed by stereotactic radiosurgery boost. Patients were stratified by number of metastases and status of extracranial disease. Primary outcome was survival; secondary outcomes were tumour response and local rates, overall intracranial recurrence rates, cause of death, and performance measurements.

FINDINGS

From January, 1996, to June, 2001, we enrolled 333 patients from 55 participating RTOG institutions--167 were assigned WBRT and stereotactic radiosurgery and 164 were allocated WBRT alone. Univariate analysis showed that there was a survival advantage in the WBRT and stereotactic radiosurgery group for patients with a single brain metastasis (median survival time 6.5 vs 4.9 months, p=0.0393). Patients in the stereotactic surgery group were more likely to have a stable or improved Karnofsky Performance Status (KPS) score at 6 months' follow-up than were patients allocated WBRT alone (43% vs 27%, respectively; p=0.03). By multivariate analysis, survival improved in patients with an RPA class 1 (p<0.0001) or a favourable histological status (p=0.0121).

INTERPRETATION

WBRT and stereotactic boost treatment improved functional autonomy (KPS) for all patients and survival for patients with a single unresectable brain metastasis. WBRT and stereotactic radiosurgery should, therefore, be standard treatment for patients with a single unresectable brain metastasis and considered for patients with two or three brain metastases.

Low-dose radiosurgery for benign intracranial lesions

This study assesses the efficacy and neurotoxicity of radiosurgical treatment of benign intracranial tumors using a linear accelerator, with relatively low dose and homogeneous dosimetry. Between June 1998 and July 2000, 27 patients were treated for benign lesions with radiosurgery using a 6-MV linear accelerator-based X-knife system and circular collimators. The lesions included schwannoma, meningioma, papillary cyst adenoma, and hemangioblastoma. Five patients had tissue diagnosis. The mean peripheral dose to the tumor margin was 12.8 Gy. The mean dose to the isocenter was 16.3 Gy. One to five isocenters were used to treat these lesions, with a mean of 10 arcs per isocenter and mean collimator size of 1.25 cm. Follow-up information was available on all patients, with a mean follow-up duration of 33 months. Six patients (22%) had improved symptoms and 21 (78%) had stable symptoms. Eight patients (30%) had regression of tumor and 19 had stable disease (70%). No patient had tumor progression, and Radiation Therapy Oncology Group (RTOG) grade III or IV toxicity did not occur in any patients. In 3 patients (11%), RTOG grade I or grade II neurotoxicity developed. Of these, one patient had worsening of a preexisting VIIth nerve deficit that required temporary oral methylprednisolone, and in two patients a mild trigeminal deficit developed that did not require any medical intervention. Low-dose homogeneous radiosurgery using a linear accelerator is an effective treatment for benign intracranial tumors. If lower, more homogeneous radiation doses produce responses as durable as higher doses, then toxicity might be further reduced.

Optimal marker placement in photogrammetry patient positioning system

A photogrammetry-based patient positioning system has been used instead of the conventional laser alignment technique for patient set-up in external beam radiotherapy. It tracks skin affixed reflective markers with multiple infrared cameras. The three-dimensional (3D) positions of the markers provide reference information to determine the treatment plan isocenter location and hence provide the ability to position the lesion at the isocenter of the treatment linear accelerator. However, in current clinical practice for lung or liver lesion treatments, fiducial markers are usually randomly affixed onto the patients' chest and abdomen, so that the actual target registration error (TRE) of the internal lesions inside the body may be large, depending on the fiducial registration error (FRE). There exists an optimal marker configuration that can minimize the TRE. In this paper, we developed methods to design the patient-specific optimal configurations of the surface makers to minimize the TRE, given the patient's surface contour, the lesion position and the FRE. Floating genetic algorithm (GA) optimization was used to optimize the positions of the skin markers. The surface curve of the patient body was determined by an automatic segmentation algorithm from the planning CT. The method was evaluated using a body phantom implanted with a metal ball (a simulated target). By registering two CT scans using the surface markers and measuring the displacement of the target, the TRE was measured. The TRE was also measured by taking two orthogonal portal films after positioning the phantom using the photogrammetry based patient positioning system. A 50% reduction in TRE has been achieved by using the optimal configuration compared to the random configuration. This result demonstrates that the optimization of a fiducial configuration can result in improved tumor targeting ability.

Dose broadening due to target position variability during fractionated breath-held radiation therapy

Recent advances in Stereotactic Radiosurgery/Conformal Radiotherapy have made it possible to deliver surgically precise radiation therapy to small lesions while preserving the surrounding tissue. However, because of physiologic motion, the application of conformal radiotherapy to extra-cranial tumors is, at present, geared toward slowing the progression of disease rather than obtaining a cure. At the University of Rochester, we are investigating the use of patient breath-holding to reduce respiratory-derived motion in fractional radiotherapy. The primary targeting problem then becomes the small variation in tumor location over repeated breath-holds. This paper describes the effects of residual target position uncertainty on the dose distribution observed by small extra-cranial tumors and their neighboring tissues during fractional radiation treatment using breath holding. We employ two computational methods to study these effects: numerical analysis via Monte Carlo simulation and analytical computation using three-dimensional convolution. These methods are demonstrated on a 2-arc, 10-fraction treatment plan used to treat a representative lung tumor in a human subject. In the same human subject, the variability in position of a representative lung tumor was measured over repeated end-expiration breath-holds using volumetric imaging. For the 7 x 7 x 10 mm margin used to treat this 12 mm diameter tumor and the measured target position variability, we demonstrated that the entire tumor volume was irradiated to at least 48 Gy-well above the tumoricidal threshold. The advantages, in terms of minimizing the volume of surrounding lung tissue that is radiated to high dose during treatment, of using end-expiration breath holding compared with end-inspiration breath-holding are demonstrated using representative tumor size and position variability parameters. It is hoped that these results will ultimately lead to improved, if not curative, treatment for small (5-20 mm diameter) lung, liver, and other extra-cranial lesions.

Multi-objective optimization in radiotherapy: applications to stereotactic radiosurgery and prostate brachytherapy

Treatment planning for radiation therapy is a multi-objective optimization process. Here we present a machine intelligent scheme for treatment planning based on multi-objective decision analysis (MODA) and genetic algorithm (GA) optimization. Multi-objective ranking strategies are represented in the L(p) metric under the displaced ideal model. Goal setting, protocol satisficing and fuzzy ranking of objective importance can be incorporated into the decision scheme to assimilate clinical decision making. For distance measures in the L(p) metric, a dynamic gauge function is defined based on the state energy of the decision system, which is assumed to undergo thermodynamic cooling with iteration time. The MODA scheme interacts with a robust GA engine, which adaptively evolves in the multi-modal landscape that defines the treatment plan quality. A conventionally challenging case of stereotactic radiosurgery of a brain lesion was selected for GA optimization. The resulting dose distributions are compared to human-developed plans, which are commonly regarded as clinically relevant and empirically optimal. The GA-optimized plans achieve substantially better sparing of critical normal neuroanatomy surrounding the brain lesion while respecting the preset constraints on tumor dose uniformity. In addition, machine optimization tends to produce novel treatment strategies which complements expert knowledge. The run time for producing an optimal plan is considerably shorter than the typical planning time for human experts, thus GA can also be used to aid the human treatment planning process. In prostate brachytherapy, MODA-GA was specifically applied to non-ideal conditions in which typical surgical uncertainties in seed implant positioning occur, where noisy objectives were introduced into the optimization scheme. The noisy system is found to be manageable by MODA-GA at uncertainty levels corresponding to reasonably proficient surgery teams. In contrast, noisy objectives would be very difficult to explore by human expert planners. Potential use of noisy optimization with time series analysis is being explored for error-corrective computer guidance in the operating room for prostate seed implantation. In conclusion, the combination of MODA and GA optimization offers both a solution to practical treatment planning tasks and the potential for real time applications in radiotherapy.

Toward a statistically relevant calibration end point for prostate seed implants

Interstitial brachytherapy for carcinoma of the prostate is achieved through the use of a configuration of radioactive seeds placed in a manner that delivers a customized, reasonably uniform dose to the target volume. Accurate dose delivery depends on both precise seed placement and reliable seed strength in the implanted configuration. This study assumes the independence of the two issues, and quantifies the reduction in the minimum dose to the surface of the gland due only to variability in individual seed strengths. Current AAPM guidelines pertaining to the acceptable limits on seed-to-seed variability are prudent for small configurations of seeds, yet are likely to be overly stringent for applications such as prostate seed implantation. In this study we determine the reduction in the minimum peripheral dose (mPD) caused by the introduction of source strength variability, and provide statistical insight into this effect. It is concluded that the current guidelines limit the reduction in mPD to < or =0.4% relative to the prescription value, for an average configuration, due to the inclusion of strength variability. The maximum observed reduction in mPD would be < or =1.5%. This value is an order of magnitude lower than the recommendations of the AAPM Task Group 40 for the overall accuracy of brachytherapy procedures, which suggests that seed strength variability is of limited concern and that constraints on this factor should perhaps be reevaluated.

Radiochromic film dosimetry of a high dose rate beta source for intravascular brachytherapy

Good clinical physics practice requires that dose rates of brachytherapy sources be checked by the institution using them, as recommended by American Association of Physicists in Medicine Task Group 56 and The American College of Radiology. For intravascular brachytherapy with catheter-based systems, AAPM Task Group 60 recommends that the dose rate be measured at a reference point located at a radial distance of 2 mm from the center of the catheter axis. AAPM Task Group 60 also recommends that the dose rate along the catheter axis at a radial distance of 2 mm should be uniform to within +/- 10% in the center two-thirds of the treated length, and the relative dose rate in the plane perpendicular to the catheter axis through the center of the source should be measured at distances from 0.5 mm to R90 (the distance from a point source within which 90% of the energy is deposited) at intervals of 0.5 mm. Radiochromic film dosimetry has been used to measure the dose distribution in a plane parallel to and at a radial distance of 2 mm from the axis of a novel, catheter-based, beta source for intravascular brachytherapy. The dose rate was averaged along a line parallel to the catheter axis at a radial distance of 2 mm, in the centered 24.5 mm of the treated length. This average dose rate agreed with the dose rate measured with a well ionization chamber by the replacement method using source trains calibrated with an extrapolation chamber at the National Institute of Standards and Technology. All of the dose rates in the centered 24.5 mm of a line parallel to the axis at a distance of 2 mm were within +/-10% of the average.

Permanent prostate seed implant brachytherapy: report of the American Association of Physicists in Medicine Task Group No. 64

There is now considerable evidence to suggest that technical innovations, 3D image-based planning, template guidance, computerized dosimetry analysis and improved quality assurance practice have converged in synergy in modern prostate brachytherapy, which promise to lead to increased tumor control and decreased toxicity. A substantial part of the medical physicist's contribution to this multi-disciplinary modality has a direct impact on the factors that may singly or jointly determine the treatment outcome. It is therefore of paramount importance for the medical physics community to establish a uniform standard of practice for prostate brachytherapy physics, so that the therapeutic potential of the modality can be maximally and consistently realized in the wider healthcare community. A recent survey in the U.S. for prostate brachytherapy revealed alarming variance in the pattern of practice in physics and dosimetry, particularly in regard to dose calculation, seed assay and time/method of postimplant imaging. Because of the large number of start-up programs at this time, it is essential that the roles and responsibilities of the medical physicist be clearly defined, consistent with the pivotal nature of the clinical physics component in assuring the ultimate success of prostate brachytherapy. It was against this background that the Radiation Therapy Committee of the American Association of Physicists in Medicine formed Task Group No. 64, which was charged (1) to review the current techniques in prostate seed implant brachytherapy, (2) to summarize the present knowledge in treatment planning, dose specification and reporting, (3) to recommend practical guidelines for the clinical medical physicist, and (4) to identify issues for future investigation.

Intraoperative optimized inverse planning for prostate brachytherapy: early experience

PURPOSE

To demonstrate the feasibility of an intraoperative inverse planning technique with advanced optimization for prostate seed implantation.

METHODS AND MATERIALS

We have implemented a method for optimized inverse planning of prostate seed implantation in the operating room (OR), based on the genetic algorithm (GA) driven Prostate Implant Planning Engine for Radiotherapy (PIPER). An integrated treatment planning system was deployed, which includes real-time ultrasound image acquisition, treatment volume segmentation, GA optimization, real-time decision making and sensitivity analysis, isodose and DVH evaluation, and virtual reality navigation and surgical guidance. Ten consecutive patients previously scheduled for implantation were included in the series.

RESULTS

The feasibility of the technique was established by careful monitoring of each step in the OR and comparison with conventional preplanned implants. The median elapsed time for complete image capture, segmentation, GA optimization, and plan evaluation was 4, 10, 2.2, and 2 min, respectively. The dosimetric quality of the OR-based plan was shown to be equivalent to the corresponding preplan.

CONCLUSION

An intraoperative optimized inverse planning technique was developed for prostate brachytherapy. The feasibility of the method was demonstrated through an early clinical experience.

Characterization of the dose perturbation by stents as a function of X-ray beam energy

PURPOSE

External beam irradiation of coronary arteries has been shown to be detrimental in an animal model for the prevention of neointimal hyperplasia in the presence of stents when orthovoltage x-ray beams are used. The present study investigated the effect of beam energy on the dose distribution in the wall of the artery in the presence of stents.

MATERIALS AND METHODS

We used 250-kVp x-rays and 6-MV x-rays to irradiate a stent placed in a homogeneous phantom. Radiochromic film densitometry and Monte Carlo calculations were used to measure and to simulate the dose distribution in the proximity of the stent.

RESULT

External beam irradiation not only failed to prevent neointimal hyperplasia, but actually accentuated the neointimal response to a prompt mechanical injury in the artery. The photoelectric effect, which dominates low-energy x-ray interactions, produces recoil electrons in the stent, which enhance the dose surrounding the intima. The photoelectrons generated in nickel and iron have an extremely short range in normal tissue, approximately 0.1 mm. Initial estimates of orthovoltage x-ray interactions with the stent indicate a dose enhancement in the orthovoltage range by a factor of 2-6 due to the rise in the photoelectric cross section in this energy range depending on the elemental composition of the stent. Film densitometry verifies this dose enhancement. The Monte Carlo calculation yields a dose enhancement and the dose fall-off with distance from the stent when irradiated with orthovoltage x-rays. Conversely when the tissue and stent are irradiated with megavoltage x-rays, the dose enhancement in this region is a factor of 1.15 in close proximity to the stent and 1.0 at distances greater than 0.1 mm. The 6-MV photon interactions in tissue and Ni/Ti are predominantly through Compton scattering. The Compton effect is dependent on the electron density in the medium, in contrast to the atomic number, which is more relevant for photoelectric absorption. The dose estimates for megavoltage x-rays adjacent to the stent are complicated by the lack of charged particle equilibrium.

CONCLUSIONS

There is a limited but definite increase in the dose delivery to the arterial wall when stents are irradiated with orthovoltage x-ray energies. This increase may explain the negative response in other studies. The presence of the stent does perturb the character and magnitude of the dose in the normal arterial wall as a function of beam quality.

Quality assurance in linac-based stereotactic radiosurgery and radiotherapy

Stereotactic Radiosurgery demands extraordinary attention to quality assurance issues. This is related to the high accuracy needed to perform a successful procedure, accuracy demanded by the proximity of the target lesion to neighboring fragile and eloquent structures in the head and large doses delivered. The nature of the linac-based radiosurgery procedure is that of a series of steps, each linked together and requiring quality control, for if one step is faulty the final result will be equally faulty. The salient points associated with the quality assurance of each step are laid out in this article. Implementation of a linac-based radiosurgery program in an institution must be well thought out and must be a team effort, involving expertise in medical physics, radiological imaging, radiation oncology, and specially trained radiation therapists in order to be successful and safe.

MR image-guided portal verification for brain treatment field

PURPOSE

To investigate a method for the generation of digitally reconstructed radiographs directly from MR images (DRR-MRI) to guide a computerized portal verification procedure.

METHODS AND MATERIALS

Several major steps were developed to perform an MR image-guided portal verification procedure. Initially, a wavelet-based multiresolution adaptive thresholding method was used to segment the skin slice-by-slice in MR brain axial images. Some selected anatomical structures, such as target volume and critical organs, were then manually identified and were reassigned to relatively higher intensities. Interslice information was interpolated with a directional method to achieve comparable display resolution in three dimensions. Next, a ray-tracing method was used to generate a DRR-MRI image at the planned treatment position, and the ray tracing was simply performed on summation of voxels along the ray. The skin and its relative positions were also projected to the DRR-MRI and were used to guide the search of similar features in the portal image. A Canny edge detector was used to enhance the brain contour in both portal and simulation images. The skin in the brain portal image was then extracted using a knowledge-based searching technique. Finally, a Chamfer matching technique was used to correlate features between DRR-MRI and portal image.

RESULTS

The MR image-guided portal verification method was evaluated using a brain phantom case and a clinical patient case. Both DRR-CT and DRR-MRI were generated using CT and MR phantom images with the same beam orientation and then compared. The matching result indicated that the maximum deviation of internal structures was less than 1 mm. The segmented results for brain MR slice images indicated that a wavelet-based image segmentation technique provided a reasonable estimation for the brain skin. For the clinical patient case with a given portal field, the MR image-guided verification method provided an excellent match between features in both DRR-MRI and portal image. Moreover, target volume could be accurately visualized in the DRR-MRI and mapped over to the corresponding portal image for treatment verification. The accuracy of DRR-MRI was also examined by comparing it to the corresponding simulation image. The matching results indicated that the maximum deviation of anatomical features was less than 2.5 mm.

CONCLUSION

A method for MR image-guided portal verification of brain treatment field was developed. Although the radiographic appearance in the DRR-MRI is different from that in the portal image, DRR-MRI provides essential anatomical features (landmarks and target volume) as well as their relative locations to be used as references for computerized portal verification.

Decision theoretic steering and genetic algorithm optimization: application to stereotactic radiosurgery treatment planning

Treatment planning for stereotactic radiosurgery and fractionated radiotherapy is currently a labor intensive, operator-dependent process. Many degrees of freedom exist to make rigorous optimization intractable except by computationally intelligent techniques. The quality of a given plan is determined by an aggregate of clinical objectives, most of which are subject to competing tradeoffs. In this work, we present an autonomous scheme that couples decision theoretic guidance with a genetic algorithm for optimization. Ordinal ranking among a population of viable treatment plans is based on a generalized distance metric, which promotes a decreasing hyperfrontier of the efficient solution set. The solution set is driven toward efficiency by the genetic algorithm, which uses the tournament selection mechanism based on the ordinal ranking. Goals and satisficing conditions can be defined to signal the ultimate and the minimum achievement levels in a given objective. A conventionally challenging case in radiosurgery was used to demonstrate the practical utility and the problem-solving power of the decision theoretic genetic algorithm. Treatment plans with one isocenter and four isocenters were derived under the autonomous scheme and compared to the actual treatment plan manually optimized by the expert planner. Quality assessment based on dose-volume histograms and normal tissue complication probabilities suggested that computational optimization could be driven to offer varying degrees of dosimetric improvement over a human-guided optimization effort. Furthermore, it was possible to achieve a high degree of isodose conformity to the target volume in computational optimization by increasing the degree of freedom in the treatment parameters. The time taken to derive an efficient planning solution was comparable and usually shorter than in the manual planning process, and can be scaled down almost linearly with the number of processors. Overall, the autonomous genetic algorithm scheme was found to be powerful and versatile as a computationally intelligent counterpart to human-guided strategies in treatment optimization for stereotactic radiosurgery and radiotherapy.

A genetic algorithm for the optimization of prostate implants

A genetic algorithm (GA) is presented for the optimization of template- and ultrasound-guided prostate implants. The end points for optimization are incorporated in an objective function of separable cardinal utility terms. As an application of the GA, the minimum 103Pd total source strength required to deliver a given dose was correlated with the average dimension for prostate implants carried out under the current template and seed spacing protocols. Significant improvements in quality were observed, in terms of both the minimum peripheral dose and tumor cell surviving fractions, when GA-optimized implants were compared to the corresponding unoptimized implants for given target volumes. In addition, numerical simulation of source displacements indicates that the dosimetric and radiobiologic advantages of GA optimization can tolerate a reasonable level of seed placement uncertainties observed clinically. In summary, the GA application provides an automated design strategy for prostate implant planning, and at the same time affords the potential for systematic optimization of a set of end points that can sustain practical variations.

An observer study for direct comparison of clinical efficacy of electronic to film portal images

PURPOSE

To directly compare clinical efficacy of electronic to film portal images.

METHODS AND MATERIALS

An observer study was designed to compare clinical efficacy of electronic to film portal images acquired using a liquid matrix ion-chamber electronic portal imaging device and a conventional metal screen/film system. Both images were acquired simultaneously for each treatment port and the electronic portal images were printed on gray-level thermal paper. Four radiation oncologists served as observers and evaluated a total of 44 sets of images for four different treatment sites: lung, pelvis, brain, and head/neck. Each set of images included a simulation image, a double-exposure portal film, and video paper prints of electronic portal images. Eight to nine anatomical landmarks were selected from each treatment site. Each observer was asked to rate each landmark in terms of its clinical visibility and to rate the ease of making the pertinent verification decision in the corresponding electronic and film portal images with the aid of the simulation image.

RESULTS

Ratings for the visibility of landmarks and for the verification decision of treatment ports were similar for electronic and film images for most landmarks. However, vertebral bodies and several landmarks in the pelvis such as the acetabulum and public symphysis were more visible in the portal film images than in the electronic portal images.

CONCLUSION

The visibility of landmarks in electronic portal images is comparable to that in film portal images. Verification of treatment ports based only on electronic portal images acquired using an electronic portal imaging device is generally achievable.

Dose-volume histogram analysis of techniques for irradiating pituitary adenomas

PURPOSE

Three-dimensional treatment planning was performed to evaluate three standard coplanar irradiation techniques (two-field parallel-opposed, three-field, and 110 degrees bilateral arcs), the 330 degrees single rotational arc, and a four noncoplanar arc technique for the treatment of pituitary adenomas. We sought to identify the optimal technique for minimizing the dose delivered to the normal tissues around the pituitary gland.

METHODS AND MATERIALS

Contours of the pituitary tumor and normal tissues were traced onto computed axial tomography (CT) scans and reconstructed in three dimensions using a three-dimensional planning system. A total dose of 45 Gy was delivered to the pituitary lesion with the five techniques using 6 MV and 18 MV photons, and dose-volume histograms were generated.

RESULTS

The 18 MV photons delivered a lower dose to the temporal lobe than did the 6 MV photons in the two-field technique, but this advantage was not evident for the other techniques. The three-field technique improved dose distribution throughout the temporal lobes with low doses being delivered to the frontal lobe. The bilateral arc and the 330 degrees arc techniques were superior to stationary two- and three-fields techniques for sparing the temporal lobes. The four noncoplanar arc technique delivered less doses to the temporal and frontal lobes than did the other techniques. However, the lens dose (3.6 Gy/25 fractions) was higher compared to the other techniques.

CONCLUSION

Analysis of the dose-volume histograms shows the various dosimetric advantages and disadvantages of the five techniques. Based upon individual considerations, including the patient's age and medical history, one can decide the optimal technique for treatment.

Three-dimensional conformal pancreas treatment: comparison of four- to six-field techniques

PURPOSE

We compare practical conformal treatment approaches to pancreatic cancer using 6 and 18 MV photons and contrast those approaches against standard techniques.

METHODS AND MATERIALS

A four-field conformal technique for treating pancreas cancer has been developed using nonopposed 18 MV photons. This approach has been extended to 6 MV photon application by the addition of one to two fields. These techniques have been optimized to increase sparing of normal liver and bowel, compared with opposed-field methods, to improve patient tolerance of high doses. In this study we compare these techniques in a simulated tumor model in a cylindrical phantom. Dose-volume analysis is used to quantify differences between the conformal, nonopposed techniques with conformal, opposed field methods. This model is also used to evaluate the effect of 1-2 cm setup errors on dose-volume coverage.

RESULTS

Dose-volume analysis demonstrates that five-to-six field conformal treatments using 6 MV photons provides similar or better dose coverage and normal tissue sparing characteristics as an optimized 18 MV, four-field approach when 1-2 cm margins are included for setup uncertainty. All approaches using nonopposed beam geometry provide significant reduction in the volume of tissue encompassed by the 30-50% isodose surfaces, as compared with four-field box techniques.

CONCLUSIONS

Three-dimensional (3D) conformal treatments can be designed that significantly improve dose-volume characteristics over conventional treatment designs without costing unacceptable amounts of machine time. Further, deep intraabdominal sites can be adequately accessed and treated on intermediate energy machines with a relatively moderate increase in machine time.

A technique of automating compensator design for lung inhomogeneity correction using an electron portal imaging device

A technique of automating compensator design for lung inhomogeneity correction using an electron portal imaging device (EPID) has been investigated. This technique utilizes exit-radiation information as detected by an EPID to determine the thickness of the compensator desired. In this particular study, the compensator thickness is determined to provide a uniform gray-level distribution (related to uniform exit-dose distribution) in the region of the portal image to be compensated. Initially, a compensation characteristic curve, which relates the compensator thickness to the pixel value of the electronic portal image, is measured for both the Lead and Lipowitz compensator materials and a 6-MV photon beam. Then, a chest-treatment field is simulated using an anthropomorphic phantom. Based on the analysis of the profile (gray-level distribution) across the lung and mediastinum regions in the electronic portal image, the average of pixel values within the mediastinum region is selected as the matching level and the regions to be compensated are determined. With the aid of the predetermined compensation characteristic curve and proper distance scaling, the compensator thickness at each pixel location is automatically calculated at the block tray level to correct lung inhomogeneity. In a simple test using a single anterioposterior (AP) chest field, the compensated profile in the electronic portal image presents a uniform gray-level distribution (related to uniform exit dose) compared to the uncompensated profile.(ABSTRACT TRUNCATED AT 250 WORDS)

Input/output characteristics of a matrix ion-chamber electronic portal imaging device

The input/output characteristics of a matrix liquid ion-chamber electronic portal imaging device (EPID) are investigated to elucidate the imaging properties of EPIDs. The radiation input to the detector, represented by dose rate, and the pixel value output from the device are related by a characteristic curve. Various incident radiation intensities are obtained by changing the source-to-detector distance (SDD). For each incident radiation intensity, an electronic portal image is obtained using a field size of 5 x 5 cm2. The output pixel value of the EPID is represented by the average pixel value of a region of interest of 9 x 9 pixels centered at a selected point. The effects of various accelerator settings, such as the repetition-rate setting and photon energy, gantry angle, field size, SDD, and acquisition mode of the EPID on characteristic curves are investigated at the central axis. The off-axis response of the detector is also examined. The derivative of the pixel value with respect to the input dose rate is used to analyze the detector contrast. Results indicate that the output pixel value is not a linear function of the incident radiation intensity. The detector contrast is comparable between photon energies of 10 and 6 MV and increases at low dose rates. The response of the imaging device varies substantially with acquisition mode, but is less sensitive to the SDD used for calibration. Characteristic curves are consistent for different gantry angles at the central axis and with the off-axis locations when the gantry angle is used for imaging and calibration, but vary with off-axis locations when the gantry angle is not at the calibration direction. Characteristic curves are also found to vary with different field sizes, but are similar in shape.

Uniform irradiation of the craniospinal axis with a penumbra modifier and an asymmetric collimator

PURPOSE

A technique is described that uses an independent, asymmetric collimator and a penumbra modifier to uniformly administer radiation over the craniospinal axis.

METHOD AND MATERIALS

An isocenter is used at the junction of the cranial and the upper spinal fields. These fields are defined by a single isocenter at spinal cord depth and an independent, asymmetric collimator. From the isocenter, the cranial and upper spinal fields are extended 1 cm inferiorly and superiorly, respectively. A modifier provides a 2-cm wide penumbra at the central axis of the beam (overlapping region) and is attached to a wedge tray in an accessory slot. This modifier allows the fields to be matched so a uniform dose is delivered over the isocenter and junction. Dose distribution was measured with an anthropomorphic head-and-neck wax phantom that included the seventh cervical vertebrae. A film was placed in a coronal cut at the spinal cord level.

RESULTS

The administered dose varied less than 10% through the craniospinal axis. Reproducibility with portal films has been very good. Advantages include dose homogeneity, easy reproducibility, and decreased setup time.

CONCLUSION

This single-field isocentric technique allows more uniform irradiation of the craniospinal axis than do previously described techniques.

Diode dosimetry of models 6711 and 6712 125I seeds in a water phantom

Two-dimensional relative dose distributions have been measured around 125I brachytherapy seeds. The two seed models studied, models 6711 and 6712, were manufactured by the 3M Company. Silicon detectors immersed in water phantoms were used to measure the dose. A computerized data acquisition system that controlled the radial position of the diode and the angular rotation of the seed, as well as a manually controlled system were used to collect and store the data. Our results show that the two seed models have relative dose distributions which are quite similar; however, the absolute dose distributions are sufficiently different to warrant separate look-up tables for the two seed models. Additionally, our results are compared with dose distribution data previously obtained for the model 6711 seed.

Clinical implications of I-125 dosimetry of bone and bone-soft tissue interfaces

The dose to bone from I-125 photon interactions is expected to be approximately five times greater than the dose to soft tissue for the same photon fluence because of the dominance of the photoelectric effect. However, adverse clinical effects are not observed for I-125 implants near bone. Both the strong absorption of I-125 photons in bone and the narrowness (about 10 mu) of the high dose transition zone at a bone-soft tissue interface act to limit the volume of radiation sensitive tissue in the high dose region. Examples of calculated implant dose distributions in bone and in soft tissue cavities in bone are presented. Radiobiological measurements are consistent with the theoretical interface calculations. Calculation of the macroscopic dose distribution uses a recently measured radial dose function, while at the bone-soft tissue interface an analytic theory of the transition zone that is applicable to regular shaped cavities is used. Radiobiological experiments comparing cell survival for cells irradiated with 70 kvP X rays at Al-water and polystyrene-water interfaces are consistent with the transition zone calculations.