PTCOG Gastrointestinal Subcommittee Lower Gastrointestinal Tract Malignancies Consensus Statement

Purpose

Radiotherapy delivery in the definitive management of lower gastrointestinal (LGI) tract malignancies is associated with substantial risk of acute and late gastrointestinal (GI), genitourinary, dermatologic, and hematologic toxicities. Advanced radiation therapy techniques such as proton beam therapy (PBT) offer optimal dosimetric sparing of critical organs at risk, achieving a more favorable therapeutic ratio compared with photon therapy.

Materials and Methods

The international Particle Therapy Cooperative Group GI Subcommittee conducted a systematic literature review, from which consensus recommendations were developed on the application of PBT for LGI malignancies.

Results

Eleven recommendations on clinical indications for which PBT should be considered are presented with supporting literature, and each recommendation was assessed for level of evidence and strength of recommendation. Detailed technical guidelines pertaining to simulation, treatment planning and delivery, and image guidance are also provided.

Conclusion

PBT may be of significant value in select patients with LGI malignancies. Additional clinical data are needed to further elucidate the potential benefits of PBT for patients with anal cancer and rectal cancer.

Immunotherapy and radiotherapy for older patients with locally advanced rectal cancer unfit for surgery or decline surgery: a practical proposal by the International Geriatric Radiotherapy Group

The standard of care for locally advanced rectal cancer is total neoadjuvant therapy followed by surgical resection. Current evidence suggests that selected patients may be able to delay or avoid surgery without affecting survival rates if they achieve a complete clinical response (CCR). However, for older cancer patients who are too frail for surgery or decline the surgical procedure, local recurrence may lead to a deterioration of patient quality of life. Thus, for clinicians, a treatment algorithm which is well tolerated and may improve CCR in older and frail patients with rectal cancer may improve the potential for prolonged remission and potential cure. Recently, immunotherapy with check point inhibitors (CPI) is a promising treatment in selected patients with high expression of program death ligands receptor 1 (PD- L1). Radiotherapy may enhance PD-L1 expression in rectal cancer and may improve response rate to immunotherapy. We propose an algorithm combining immunotherapy and radiotherapy for older patients with locally advanced rectal cancer who are too frail for surgery or who decline surgery.

Immunotherapy and Hypofractionated Radiotherapy in Older Patients with Locally Advanced Cutaneous Squamous-Cell Carcinoma of the Head and Neck: A Proposed Paradigm by the International Geriatric Radiotherapy Group

Cutaneous skin carcinoma is a disease of older patients. The prevalence of cutaneous squamous-cell carcinoma (cSCC) increases with age. The head and neck region is a frequent place of occurrence due to exposure to ultraviolet light. Surgical resection with adjuvant radiotherapy is frequently advocated for locally advanced disease to decrease the risk of loco-regional recurrence. However, older cancer patients may not be candidates for surgery due to frailty and/or increased risk of complications. Radiotherapy is usually advocated for unresectable patients. Compared to basal-cell carcinoma, locally advanced cSCC tends to recur locally and/or can metastasize, especially in patients with high-risk features such as poorly differentiated histology and perineural invasion. Thus, a new algorithm needs to be developed for older patients with locally advanced head and neck cutaneous squamous-cell carcinoma to improve their survival and conserve their quality of life. Recently, immunotherapy with checkpoint inhibitors (CPIs) has attracted much attention due to the high prevalence of program death ligand 1 (PD-L1) in cSCC. A high response rate was observed following CPI administration with acceptable toxicity. Those with residual disease may be treated with hypofractionated radiotherapy to minimize the risk of recurrence, as radiotherapy may enhance the effect of immunotherapy. We propose a protocol combining CPIs and hypofractionated radiotherapy for older patients with locally advanced cutaneous head and neck cancer who are not candidates for surgery. Prospective studies should be performed to verify this hypothesis.

Impact of COVID-19 pandemic on older cancer patients: Proposed solution by the International Geriatric Radiotherapy Group

Older cancer patients are disproportionally affected by the Coronavirus 19 (COVID-19) pandemic. A higher rate of death among the elderly and the potential for long-term disability have led to fear of contracting the virus in these patients. This fear can, paradoxically, cause delay in diagnosis and treatment that may lead to a poor outcome that could have been prevented. Thus, physicians should devise a policy that both supports the needs of older patients during cancer treatment, and serves to help them overcome their fear so they seek out to cancer diagnosis and treatment early. A combination of telemedicine and a holistic approach, involving prayers for older cancer patients with a high level of spirituality, may improve vaccination rates as well as quality of life during treatment. Collaboration between health care workers, social workers, faith-based leaders, and cancer survivors may be crucial to achieve this goal. Social media may be an important component, providing a means of sending the positive message to older cancer patients that chronological age is not an impediment to treatment.

Immunotherapy and Modern Radiotherapy Technique for Older Patients with Locally Advanced Head and Neck Cancer: A Proposed Paradigm by the International Geriatric Radiotherapy Group

The standard of care for locally advanced head and neck cancer is concurrent chemoradiation or postoperative irradiation with or without chemotherapy. Surgery may not be an option for older patients (70 years old or above) due to multiple co-morbidities and frailty. Additionally, the standard chemotherapy of cisplatin may not be ideal for those patients due to oto- and nephrotoxicity. Though carboplatin is a reasonable alternative for cisplatin in patients with a pre-existing hearing deficit or renal dysfunction, its efficacy may be inferior to cisplatin for head and neck cancer. In addition, concurrent chemoradiation is frequently associated with grade 3-4 mucositis and hematologic toxicity leading to poor tolerance among older cancer patients. Thus, a new algorithm needs to be developed to provide optimal local control while minimizing toxicity for this vulnerable group of patients. Recently, immunotherapy with check point inhibitors (CPI) has attracted much attention due to the high prevalence of program death-ligand 1 (PD-L1) in head and neck cancer. In patients with recurrent or metastatic head and neck cancer refractory to cisplatin-based chemotherapy, CPI has proven to be superior to conventional chemotherapy for salvage. Those with a high PD-L1 expression defined as 50% or above or a high tumor proportion score (TPS) may have an excellent response to CPI. This selected group of patients may be candidates for CPI combined with modern radiotherapy techniques, such as intensity-modulated image-guided radiotherapy (IM-IGRT), volumetric arc therapy (VMAT) or proton therapy if available, which allow for the sparing of critical structures, such as the salivary glands, oral cavity, cochlea, larynx and pharyngeal muscles, to improve the patients' quality of life. In addition, normal organs that are frequently sensitive to immunotherapy, such as the thyroid and lungs, are spared with modern radiotherapy techniques. In fit or carefully selected frail patients, a hypofractionated schedule may be considered to reduce the need for daily transportation. We propose a protocol combining CPI and modern radiotherapy techniques for older patients with locally advanced head and neck cancer who are not eligible for cisplatin-based chemotherapy and have a high TPS. Prospective studies should be performed to verify this hypothesis.

Is immunotherapy at reduced dose and radiotherapy for older patients with locally advanced non-small lung cancer feasible?—a narrative review by the international geriatric radiotherapy group

Background and Objective

The standard of care for locally advanced non-small cell lung cancer (NSCLC) is either surgery followed by adjuvant chemotherapy with or without radiotherapy or concurrent chemotherapy and radiotherapy. However, older patients (70 years old or above) with multiple co-morbidities may not be able to tolerate the combined treatment due to its toxicity. Since lung cancer prevalence increases significantly with age, a new algorithm needs to be investigated to allow curative treatment for those with locally advanced disease.

Methods

A literature search of the literature was conducted through PubMed and Google Scholar using search terms such as locally advanced NSCLC, older cancer patients, immunotherapy with check point inhibitors (CPI), and image-guided radiotherapy (IGRT). Abstracts were screened, full articles fitting the article topic were reviewed, and duplicated and non-English articles were excluded.

Key Content and Findings

Recently, CPI has been introduced and proven effective for selected patients with increased program death ligand 1 (PD-L1) expression (50% or above). A reduced dose for CPI (RDCPI) may be as effective as a full dose and may decrease treatment cost. New radiation technique such as IGRT may also minimize radiotherapy complication through normal lung and cardiac sparing.

Conclusions

IGRT and RDCPI may be an innovative option for older patients with locally advanced NSCLC and high PD-L1 expression and needs to be investigated in future prospective studies.

Executive Summary of Clinical and Technical Guidelines for Esophageal Cancer Proton Beam Therapy From the Particle Therapy Co-Operative Group Thoracic and Gastrointestinal Subcommittees

Radiation therapy (RT) is an integral component of potentially curative management of esophageal cancer (EC). However, RT can cause significant acute and late morbidity due to excess radiation exposure to nearby critical organs, especially the heart and lungs. Sparing these organs from both low and high radiation dose has been demonstrated to achieve clinically meaningful reductions in toxicity and may improve long-term survival. Accruing dosimetry and clinical evidence support the consideration of proton beam therapy (PBT) for the management of EC. There are critical treatment planning and delivery uncertainties that should be considered when treating EC with PBT, especially as there may be substantial motion-related interplay effects. The Particle Therapy Co-operative Group Thoracic and Gastrointestinal Subcommittees jointly developed guidelines regarding patient selection, treatment planning, clinical trials, and future directions of PBT for EC.

Consensus Statement on Proton Therapy for Prostate Cancer

Proton therapy is a promising but controversial treatment in the management of prostate cancer. Despite its dosimetric advantages when compared with photon radiation therapy, its increased cost to patients and insurers has raised questions regarding its value. Multiple prospective and retrospective studies have been published documenting the efficacy and safety of proton therapy for patients with localized prostate cancer and for patients requiring adjuvant or salvage pelvic radiation after surgery. The Particle Therapy Co-Operative Group (PTCOG) Genitourinary Subcommittee intends to address current proton therapy indications, advantages, disadvantages, and cost effectiveness. We will also discuss the current landscape of clinical trials. This consensus report can be used to guide clinical practice and research directions.

A Virtual Reality Intervention for the Treatment of Phantom Limb Pain: Development and Feasibility Results

OBJECTIVE

To describe the development of a virtual reality (VR) treatment for phantom limb pain (PLP) and phantom sensations and provide feasibility data from testing the treatment in a population of veterans.

DESIGN & SUBJECTS

Fourteen participants completed a baseline visit evaluating their amputation, PLP, and phantom sensations. Subsequently, participants completed a VR treatment modeled after mirror therapy for PLP, navigating in a VR environment with a bicycle pedaler and motion sensor to pair their cadence to a VR avatar. The VR avatar enabled visualization of the participant's intact phantom limb in motion, a hypothesized mechanism of mirror therapy.

SETTING

Laboratory.

METHODS

Participants completed pre- and post-treatment measures to evaluate changes in PLP, phantom sensations, and rate helpfulness, realism, immersion, adverse experiences, and treatment satisfaction.

RESULTS

Eight of 14 participants (57.1%) reported PLP pre-VR treatment, and 93% (13/14) reported one or more unpleasant phantom sensations. After treatment, 28.6% (4/14) continued to report PLP symptoms (t[13] = 2.7, P = 0.02, d = 0.53) and 28.6% (4/14) reported phantom sensations (t[13] = 4.4, P = 0.001, d = 1.7). Ratings of helpfulness, realism, immersion, and satisfaction were uniformly high to very high. There were no adverse experiences. Four participants completed multiple VR treatments, showing stable improvements in PLP intensity and phantom sensations and high user ratings.

CONCLUSIONS

This feasibility study of a novel VR intervention for PLP was practical and was associated with significant reductions in PLP intensity and phantom sensations. Our findings support continued research in VR-based treatments in PLP, with a need for direct comparisons between VR and more established PLP treatments.

The role of proton beam therapy in the management of elderly breast cancer patients

Breast cancer is the most common noncutaneous malignancy in women. The prevalence increases with age such that nearly 7% of women in the United States over age 70 will be diagnosed with breast cancer. Radiation therapy (RT) is a standard component of the treatment course for women of all ages with breast cancer. RT is commonly encountered in the adjuvant setting for women with nonmetastatic disease, but also works for disease palliation in women with metastatic or recurrent disease. Different techniques for delivering RT for breast cancer include whole breast irradiation (WBI), accelerated partial-breast irradiation (APBI), and chest wall irradiation. Although these techniques often employ external beam radiation therapy (EBRT) delivered with photons, proton beam radiation therapy (PBRT) may also be used for each of these methods. Dosimetric breast cancer studies demonstrate clinical benefits of PBRT compared to photon EBRT. PBRT reduces the radiation dose delivered to the heart, particularly in women with left-sided breast cancer. This may subsequently reduce cardiac toxicity and associated cardiovascular disease. PBRT minimizes radiation dose to the lung and secondary tissues resulting in reduced pulmonary toxicity and secondary malignancies, respectively. PBRT offers superior target homogeneity and lymphatic coverage possibly leading to a lower risk of disease recurrence. A phase 3 prospective randomized clinical trial is currently being conducted to evaluate the efficacy of PBRT compared to EBRT with photons in patients with stage II–III breast cancer. Patients over age 70 with favorable stage I breast cancer may omit adjuvant RT. Elderly patients who are candidates for WBI, APBI and chest wall irradiation can receive PBRT and enjoy the same aforementioned benefits with potentially less toxicities. PBRT also plays a role in disease palliation and definitive therapy in patients who are not surgical candidates. In the elderly population, screening tests, such as the Timed Up and Go and G-8, can help determine which patients are suitable candidates for PBRT.

Radioembolization with Yttrium-90 microspheres for patients with unresectable hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is aggressive primary malignancy of the liver that most commonly presents late in the disease course. As a result, the majority of patients are not candidates for curative therapies. Locoregional therapies including Yttrium-90 (Y-90) radioembolization play an important role in management of the vast majority of patients with HCC.

METHODS

Patients with unnresectable HCC (n=17) treated with Y-90 radioembolization from 2005 to 2014 were evaluated retrospectively. Data was abstracted from medical records including patient charts, laboratory data, and imaging. Toxicities were recorded using Common Terminology Criteria 3.0. Response was recorded according to modified RECIST (mRECIST) criteria.

RESULTS

Seventeen patients received 33 treatments with Y-90 radioembolization. A majority (65%) received TheraSphere with a minority (35%) receiving SIR-Spheres. The median treatment activity delivered was 1.725 gBq (range, 1.4-2.5 gBq). The median treatment dose delivered was 100 Gy (range, 90-120 Gy). The median lung shunt fraction was 2.02% (range, 1.5-4.1%). The most common clinical toxicity among all patients was nausea and vomiting (59%), primarily grade 1 and 2. Other post-treatment findings included abdominal pain (29%), fatigue (53%), and weight loss (18%). One patient developed a grade 5 gastric ulcer after the treatment. A clinical benefit, defined as patients achieving complete response (CR), partial response (PR) or stable disease (SD), was seen in 48% of patients. PR was seen in 24% of cases; progressive disease (PD) was noted in 35%. Patients survived for a median of 8.4 months (range, 1.3 to 21.1 months) after the first radioembolization treatment. Median survival after Y-90 treatment was 8.4 months among patients treated TheraSphere as compared with 7.8 months in patients treated with SIR-Spheres. The mean overall survival from the time of diagnosis was 11.7 months (range, 3.4 to 43.2 months).

CONCLUSIONS

For patients with unresectable HCC, Y-90 radioembolization is a safe and well-tolerated procedure. Our experience suggests that a significant percentage of patients achieve clinical benefit including many with PR. Survival after treatment from this single-center, transplant center is in line with prior reports. Prospective, randomized data is required to compare radioembolization with other therapies including chemoembolization and systemic therapy with sorafenib.

Hepatic intra-arterial infusion of yttrium-90 microspheres in the treatment of recurrent hepatocellular carcinoma after liver transplantation: A case report

Hepatocellular carcinoma (HCC) recurs with a reported frequency of 12%-18% after liver transplantation. Recurrence is associated with a mortality rate exceeding 75%. Approximately one-third of recurrences develop in the transplanted liver and are therefore amenable to local therapy. A variety of treatment modalities have been reported including resection, transarterial chemo-embolization (TACE), radiofrequency ablation (RFA), ethanol ablation, cryoablation, and external beam irradiation. Goals of treatment are tumor control and the minimization of toxic effect to functional parenchyma. Efficacy of treatment is mitigated by the need for ongoing immunosuppression. Yttrium-90 microspheres have been used as a treatment modality both for primary HCC and for pre-transplant management of HCC with promising results.

Twenty-two months after liver transplantation for hepatitis C cirrhosis complicated by HCC, a 42-year old man developed recurrence of HCC in his transplant allograft. Treatment of multiple right lobe lesions with anatomic resection and adjuvant chemotherapy was unsuccessful. Multifocal recurrence in the remaining liver allograft was treated with hepatic intra-arterial infusion of yttrium-90 microspheres (SIR-Spheres, Sirtex Medical Inc., Lake Forest, IL, USA). Efficacy was demonstrated by tumor necrosis on imaging and a decrease in alpha-fetoprotein (AFP) level. There were no adverse consequences of initial treatment.

A multicenter, randomized, dose-finding study of gamma intracoronary radiation therapy to inhibit recurrent restenosis after stenting

OBJECTIVES

The objective of this double-blind, randomized study was to determine the safety and efficacy of intracoronary radiation therapy (ICRT) with a dose of 17 Gray (Gy) compared to the currently recommended dose prescription of 14 Gy for the treatment of in-stent restenosis within bare metal stents.

BACKGROUND

While gamma ICRT for in-stent restenosis has been proven efficacious, the optimal dose is unknown, and radiation failure due to recurrent neointimal hyperplasia remains a significant clinical problem for some patients. A higher radiation dose may improve outcomes, but may potentially increase adverse events.

METHODS

Following coronary intervention, 336 patients with in-stent restenosis were randomly assigned to receive ICRT with either 14 Gy or 17 Gy at 2 mm from an 192-iridium source.

RESULTS

At 8-month follow up, fewer patients in the 17 Gy group underwent target lesion revascularization (TLR = 15.2% versus 27.2%; p = 0.01), target vessel revascularization (21.3% versus 33.1%; p = 0.02), or reached the composite endpoint of death, myocardial infarction, thrombosis, or TLR (17.1% versus 28.4%; p = 0.02). There were no differences in late thrombosis or mortality between treatment groups. There was a strong trend toward reduced in-lesion late loss (0.36 +/- 0.63 mm vs. 0.51 +/- 0.64 mm; p = 0.09) and a significantly lower rate of binary restenosis (23.9% versus 38.1%; p = 0.031) in the high dose group.

CONCLUSIONS

Gamma ICRT with 17 Gy is safe and, compared to 14 Gy, reduces recurrent stenosis and clinical events at 8-month follow up. An increase in the currently recommended gamma radiation dose prescription from 14 Gy to 17 Gy should be strongly considered.

Intracoronary radiation therapy for multi-drug resistant in-stent restenosis: Initial clinical experience

While randomized clinical trials have demonstrated the excellent efficacy of sirolimus-eluting stents (SES) for de novo lesions, the optimal treatment for SES-restenosis is not known. Management may include stand-alone balloon angioplasty, repeat SES implantation, or placement of a drug-eluting stent (DES) with an alternative antiproliferative agent (i.e., a paclitaxel-eluting stent, PES). The appropriate management strategy for recurrent restenosis after PES implantation for SES restenosis is even less clear. We report the initial clinical experience with intracoronary radiation therapy (ICRT) for multi-DES resistant restenosis. We performed ICRT in five patients with recurrent restenosis after treatment with both SES and PES. Over a median follow-up of 256 days (range 75-489 days), one patient had a target lesion revascularization at 182 days and subsequently died at 483 days following the procedure. Our findings support the further study of this management approach.

Required treatment margin for coronary endovascular brachytherapy with iridium-192 seed ribbon

PURPOSE

Preliminary clinical trials (SCRIPPS I, WRIST and Gamma 1) employing catheter-based endovascular brachytherapy (EVBT) with iridium-192 (Ir-192) seeds show promising results in reducing restenosis after coronary intervention. Failure analysis of these studies showed a significant number of restenosis at the treatment margin called "edge effect." The objective of this study is to investigate the factors that contribute to the adequacy of treatment margin.

METHODS AND MATERIALS

The factors contributing to the margins are penumbra effect at the end of the seed train, uncertainty in target localization, longitudinal seed movement during cardiac cycle and barotrauma due to stent deployment. The magnitudes of the penumbra effect, which refers to the tapering off the prescribed isodose line near the ends of the source train, were calculated for various source lengths of Ir-192 seed ribbon using AAPM TG-43 algorithm. Uncertainty in target localization refers to the fact that the visual estimation of proximal and distal extent of the injury is not accurate, and this can be obtained by comparing the "estimate" from the interventional cardiologist with careful review of the cine-angiogram. Longitudinal seed movements relative to the coronary vessel during the cardiac cycle were determined by frame-by-frame reviewing cine-angiograms of 30 patients. The proximal and distal source points were measured in reference to branching vessels during the contrast phase of the cine-angiogram. The maximum proximal and distal longitudinal movement was captured and source displacement was measured from the closest proximal and distal branching vessel. Barotrauma, additional injury to the vessel arising from the stent deployment balloon, was obtained by reviewing specifications from commercially available stent delivery systems.

RESULTS

The penumbra effect ranges from 3.9 to 4.5 mm for 6-22 Ir-192 seed ribbons. The uncertainty in target localization is within 3 mm for our interventional cardiologists. The results of seed movements were categorized by three major coronary vessels and by proximal versus distal ends. The mean and standard deviation of seed movement are 1.1 and 0.8 mm, respectively. The average length of barotrauma beyond the stent margins for reviewed stents was 1.7 mm, ranging from 0.5 to 2.5 mm.

CONCLUSION

A minimum of 8-mm treatment margin is recommended for coronary vascular brachytherapy with Ir-192 seed ribbon. This was derived by considering the above contributing factors. Excessive margins should be avoided due to possible increase risk of late effect. By providing adequate treatment margins, one can avoid geographic miss; hence, one can further improve the effect of EVBT in reducing restenosis.

How long is enough? Defining the treatment length in endovascular brachytherapy

The International Commission on Radiation Units and Measurement (IRCU) 50 has clearly defined treatment volumes in radiation therapy in the management of neoplasms. These concepts are applied to the field of endovascular brachytherapy (EVBT) for the prevention of postangioplasty restenosis. The following definitions are proposed: gross target length (GTL) is defined as the narrowed segment of the artery that requires intervention. Clinical target length (CTL) is defined as the intervened or injured length, which could be due to angioplasty, stent strut injury, stent deployment, or debulking procedures. Planning target length (PTL) is the CTL plus a margin to account for heart/catheter movement and uncertainty in target localization. The final treatment length (TL) is the PTL plus the effect of penumbra. The accurate specification of treatment length serves several important purposes. Based on an understanding of the different factors constituting the treatment length, adequate margins can be provided beyond the GTL; this will avoid geographic misses and minimize edge failures. These definitions of target length ensure treatment consistency and provide a standard terminology for communication among practitioners of EVBT, something of critical importance in the conduct of multi-institutional trials in this new and multidisciplinary therapy. Finally, since the efficacy of EVBT is critically dependent on the precision of radiation delivery, these guidelines ensure that the benefits of EVBT seen in prospective randomized trials can be translated into daily clinical practice at the community level.

Barotrauma due to stent deployment in endovascular brachytherapy for restenosis prevention

PURPOSE

In this study, the effect of barotrauma due to stent deployment is investigated for several commonly used commercial stents used in endovascular brachytherapy for restenosis prevention.

METHODS AND MATERIALS

Restenosis due to intimal hyperplasia can occur anywhere along the clinical target volume, which is defined as the length of vessel receiving intervention or injury. The injury may be due to angioplasty, atherectomy (tissue removing), stenting, and stent deployment. Manufacturer specifications for several commonly used stents were reviewed and the results were tabulated.

RESULTS

The barotrauma length of stents reviewed in this study ranges from 0.5 to 2.5 mm; the average was 1.7 mm.

CONCLUSIONS

By considering specific barotrauma into the treatment length, one can provide adequate treatment margins to minimize edge failure or to avoid "geographic miss," which may improve the efficacy of endovascular brachytherapy.

Theoretical assessment of late cardiac complication from endovascular brachytherapy for restenosis prevention

PURPOSE

In this study, a theoretical assessment of late cardiac complication from endovascular brachytherapy is performed using the integrated logistic model.

MATERIALS AND METHODS

Calculation were performed for various lengths of Ir-192 sources using alpha/beta = 3.2 for the endpoint of chronic ischemia, TD50/5 = 7,000 cGy, and TD5/5 = 5,000 cGy. The dose distribution over a standard heart was divided into volume elements with uniform dose (dose-volume histogram). Using linear-quadratic equation, the dose in each of the volume elements was converted into dose equivalent to standard fractionation external beam irradiation. The normal tissue complication probability (NTCP) for each volume element was calculated and combined together to arrive at the cumulative risk of late cardiac complication. The NTCP was plotted against the dose prescribed at 2-mm radial distance for four treatment lengths.

RESULTS

(1) The overall risk of late cardiac toxicity (chronic ischemia within 5 years) was estimated to be less than 1% for current clinical trials using Ir-192. (2) There is a volume effect with higher risk for larger irradiated volume, which can come from longer treatment time, the same dose prescribed at a greater radial distance, and a longer source train. (3) The NTCP vs. dose demonstrates a sigmoidal relationship. There is a threshold dose (about 500 cGy), below which the risk is minimal; the gradient of the curve is greater for longer treatment length.

CONCLUSION

If the prediction from this model is validated with clinical data, it will contribute to guidelines for dose prescription, dose escalation, evaluation of new source design, and multivessel treatment.

Theoretical assessment of dose-rate effect in endovascular brachytherapy

PURPOSE

Several prospective randomized clinical trials utilizing endovascular brachytherapy after coronary angioplasty have shown promising preliminary results. Numerous clinical trials have been initiated to evaluate different delivery systems and source types. In this study, the dose-rate effect is investigated using a biophysical model derived from linear-quadratic formalism.

MATERIALS AND METHODS

The dose-rate effect is quantified using the Dale's formulation, which is based on a linear-quadratic model. This model converts the total absorbed dose into the biological equivalent dose (BED) based on the dose rate, total dose, treatment duration, biological endpoint (alpha/beta ratio), and sublethal damage repair constant. The calculations are performed for two common source configurations used in current clinical trials (192Ir and 90Sr/Y).

RESULTS

At smaller radial distance, the dose rate is higher, hence BED increases due to the increase in the relative effectiveness per unit dose (RE) and absorbed dose for a given treatment duration. For 90Sr/Y source, a similar trend is observed; however, it is at a much greater magnitude. The RE for 192Ir is close to unity, which is equivalent to that of external beam irradiation.

CONCLUSION

Although current clinical trials in endovascular brachytherapy report similar absorbed dose, the biological effects may be different due to the extremely high gradient of dose rate near the sources, a variety of isotopes and delivery systems, and different dose prescriptions. If the theoretical predictions in this study are validated in clinical trials, the proposed model can be useful to compare different protocols, design new delivery systems and isotopes, and optimize how radiation is delivered.

Endovascular brachytherapy for peripheral vascular disease

The most common cause of morbidity and mortality in the United States is vascular disease, which afflicts a wide spectrum of organs such as the heart (cardiovascular system), brain (cerebrovascular system), kidney (renal system), liver (hepatic system), and extremities (peripheral vascular system). The most common pathology of vascular diseases is occlusion. Percutaneous Transluminal Angioplasty (PTA) is currently the most common nonsurgical treatment for obstructive arteries. Unfortunately, the long-term effectiveness of PTA is limited by a high restenosis rate. Efforts to reduce post-PTA restenosis, including laser, mechanical atherectomy, intravascular stenting, and pharmacologic agents, have not been successful. With recent advances in the pathogenesis of restenosis, we have learned that the major problem is the intimal hyperplastic reaction in response to vessel injury. Encouraging animal data in the use of various radiotherapeutic approaches to prevent restenosis has led to a large number of multi-national, multicenter, randomized trials on coronary vascular systems. Because early results have been in favor of radiation therapy, and because the basic process of restenosis is similar for coronary and noncoronary vascular systems, many investigators extend the application of radiotherapy to the prevention of restenosis in peripheral vascular systems. However, the clinical scenarios are much different for peripheral vascular systems than for the coronary vascular system. This article discusses the current views of the pathophysiology of restenosis, major clinical trials, and perspectives on future studies. Experimental studies on animal models have documented the profound effects of endovascular brachytherapy in reducing restenosis caused by angioplasty and stenting. Early results of clinical trials are encouraging and confirm these positive results. Long-term follow-up data are needed to show that radiation does prevent, not merely delay, restenosis; Several areas of opportunity exist for both basic science research and clinical studies to enhance our knowledge of the pathophysiology. This would optimize the treatment strategy, maximizing the benefits and minimizing late complications.