Methodology for biologically-based treatment planning for combined low-dose-rate (permanent implant) and high-dose-rate (fractionated) treatment of prostate cancer

AAPM Task Group Report 267: A joint AAPM GEC-ESTRO report on biophysical models and tools for the planning and evaluation of brachytherapy

Brachytherapy utilizes a multitude of radioactive sources and treatment techniques that often exhibit widely different spatial and temporal dose delivery patterns. Biophysical models, capable of modeling the key interacting effects of dose delivery patterns with the underlying cellular processes of the irradiated tissues, can be a potentially useful tool for elucidating the radiobiological effects of complex brachytherapy dose delivery patterns and for comparing their relative clinical effectiveness. While the biophysical models have been used largely in research settings by experts, it has also been used increasingly by clinical medical physicists over the last two decades. A good understanding of the potentials and limitations of the biophysical models and their intended use is critically important in the widespread use of these models. To facilitate meaningful and consistent use of biophysical models in brachytherapy, Task Group 267 (TG-267) was formed jointly with the American Association of Physics in Medicine (AAPM) and The Groupe Européen de Curiethérapie and the European Society for Radiotherapy & Oncology (GEC-ESTRO) to review the existing biophysical models, model parameters, and their use in selected brachytherapy modalities and to develop practice guidelines for clinical medical physicists regarding the selection, use, and interpretation of biophysical models. The report provides an overview of the clinical background and the rationale for the development of biophysical models in radiation oncology and, particularly, in brachytherapy; a summary of the results of literature review of the existing biophysical models that have been used in brachytherapy; a focused discussion of the applications of relevant biophysical models for five selected brachytherapy modalities; and the task group recommendations on the use, reporting, and implementation of biophysical models for brachytherapy treatment planning and evaluation. The report concludes with discussions on the challenges and opportunities in using biophysical models for brachytherapy and with an outlook for future developments.

Tolerance limit of external beam radiotherapy combined with low-dose rate brachytherapy in normal rabbit tissue

BACKGROUND

Dosage-optimized multimodal radiotherapies that are safe for head and neck cancer patients are desirable. In this study, we investigated tissue tolerance to varying doses of external beam radiotherapy (EBRT) combined with low-dose rate brachytherapy in the neck of a rabbit model.

METHODS

Twenty rabbits were used in the four test groups (five each) with iodine-125 seeds implanted in the neck treated with EBRT in four doses at 50, 40, 30 and 20 Gy each. Twelve rabbits for three control groups (four each). Three months after implantation, all rabbits were euthanized, and target tissues were collected. Analyses included seed implantation assessment, histopathological evaluation, immunohistochemistry staining, terminal deoxynucleotidyl transferase dUTP nick end labeling assay, electron microscopy and statistics with the SPSS software.

RESULTS

Five rabbits died in the four test groups, and three rabbits died in the three control groups (one per group), which showed no significant difference by survival analysis. The calculated minimum peripheral dose was 17.6 Gy, the maximum dose near the seed was 1812.5 Gy, the D90 was 34.5 Gy and the mean dose was 124.5 Gy. In all groups that received radiation, apoptosis occurred primarily in the esophageal mucosa and corresponded to the dose of radiation; a higher dose caused a greater apoptosis, with significant difference between groups (P < 0.05). Electron microscopy of carotid arteries revealed that endothelial cells were swollen and some were shed from basement membrane, but no other noticeable tissue damages.

CONCLUSIONS

Limited EBRT at maximal dose (50 Gy) combined with the brachytherapy interstitially applied to the neck was tolerated well in the rabbit model.

GammaTile Brachytherapy Combined With External Beam Radiation Therapy for the Treatment of a Partially Resected Secondary Glioblastoma (WHO Grade 4 IDH-Mutant Astrocytoma): Matching External Beam Dose Gradient to Brachytherapy Dose Fall-Off

Reirradiation of recurrent glioblastomas is most commonly managed with hypofractionated external beam radiation with a modest overall effect. GammaTile, which is a Cesium-131 source embedded in collagen mesh, is an approach that allows the surgical bed of resectable intracranial tumors to receive a greater biological dose than is possible with any form of external beam radiation therapy (EBRT). In this case report, a 28-year-old male presents with a WHO grade 4 isocitrate dehydrogenase (IDH)-mutant astrocytoma (formerly secondary glioblastoma) of the left occipital/parietal lobe after receiving 45 Gy and two cycles of adjuvant temozolomide four years prior for a grade 3 IDH-mutant astrocytoma. The patient proceeded to undergo craniotomy with maximal safe resection and application of GammaTile to a dose of 60 Gy at 5mm depth. Shortly afterward, he developed symptomatic progression of disease in the bilateral splenium and left thalamus/basal ganglia. We irradiated the undertreated residual disease with EBRT to a dose of 35 Gy in 10 fractions without introducing excessive dose to the GammaTile irradiated volume. This was achieved by creating one portion of the planning target volume with a homogeneous dose and another part where the delivered dose decreased with the GammaTile dose buildup. Treatment planning utilized the Gradient Optimization feathering technique with non-coplanar volumetric modulated arc therapy. The resulting composite between the hypofractionated EBRT and GammaTile dose distribution created an approximate dose equivalent of 50 Gy in 2 Gy fractions to the residual disease with no hot spots or areas of under coverage. This is the first report showing the feasibility of combining GammaTile with dose-matched EBRT volumes in a reproducible manner to sub-totally resected, recurrent intracranial neoplasms.

A validated tumor control probability model based on a meta-analysis of low, intermediate, and high-risk prostate cancer patients treated by photon, proton, or carbon-ion radiotherapy

PURPOSE

A fully heterogeneous population averaged mechanistic tumor control probability (TCP) model is appropriate for the analysis of external beam radiotherapy (EBRT). This has been accomplished for EBRT photon treatment of intermediate-risk prostate cancer. Extending the TCP model for low and high-risk patients would be beneficial in terms of overall decision making. Furthermore, different radiation treatment modalities such as protons and carbon-ions are becoming increasingly available. Consequently, there is a need for a complete TCP model.

METHODS

A TCP model was fitted and validated to a primary endpoint of 5-year biological no evidence of disease clinical outcome data obtained from a review of the literature for low, intermediate, and high-risk prostate cancer patients (5218 patients fitted, 1088 patients validated), treated by photons, protons, or carbon-ions. The review followed the preferred reporting item for systematic reviews and meta-analyses statement. Treatment regimens include standard fractionation and hypofractionation treatments. Residual analysis and goodness of fit statistics were applied.

RESULTS

The TCP model achieves a good level of fit overall, linear regression results in a p-value of <0.000 01 with an adjusted-weighted-R(2) value of 0.77 and a weighted root mean squared error (wRMSE) of 1.2%, to the fitted clinical outcome data. Validation of the model utilizing three independent datasets obtained from the literature resulted in an adjusted-weighted-R(2) value of 0.78 and a wRMSE of less than 1.8%, to the validation clinical outcome data. The weighted mean absolute residual across the entire dataset is found to be 5.4%.

CONCLUSIONS

This TCP model fitted and validated to clinical outcome data, appears to be an appropriate model for the inclusion of all clinical prostate cancer risk categories, and allows evaluation of current EBRT modalities with regard to tumor control prediction.

Optimal schedules of fractionated radiation therapy by way of the greedy principle: biologically-based adaptive boosting

We revisit a long-standing problem of optimization of fractionated radiotherapy and solve it in considerable generality under the following three assumptions only: (1) repopulation of clonogenic cancer cells between radiation exposures follows linear birth-and-death Markov process; (2) clonogenic cancer cells do not interact with each other; and (3) the dose response function s(D) is decreasing and logarithmically concave. Optimal schedules of fractionated radiation identified in this work can be described by the following 'greedy' principle: give the maximum possible dose as soon as possible. This means that upper bounds on the total dose and the dose per fraction reflecting limitations on the damage to normal tissue, along with a lower bound on the time between successive fractions of radiation, determine the optimal radiation schedules completely. Results of this work lead to a new paradigm of dose delivery which we term optimal biologically-based adaptive boosting (OBBAB). It amounts to (a) subdividing the target into regions that are homogeneous with respect to the maximum total dose and maximum dose per fraction allowed by the anatomy and biological properties of the normal tissue within (or adjacent to) the region in question and (b) treating each region with an individual optimal schedule determined by these constraints. The fact that different regions may be treated to different total dose and dose per fraction mean that the number of fractions may also vary between regions. Numerical evidence suggests that OBBAB produces significantly larger tumor control probability than the corresponding conventional treatments.

On the probability of cure for heavy-ion radiotherapy

The probability of a cure in radiation therapy (RT)-viewed as the probability of eventual extinction of all cancer cells-is unobservable, and the only way to compute it is through modeling the dynamics of cancer cell population during and post-treatment. The conundrum at the heart of biophysical models aimed at such prospective calculations is the absence of information on the initial size of the subpopulation of clonogenic cancer cells (also called stem-like cancer cells), that largely determines the outcome of RT, both in an individual and population settings. Other relevant parameters (e.g. potential doubling time, cell loss factor and survival probability as a function of dose) are, at least in principle, amenable to empirical determination. In this article we demonstrate that, for heavy-ion RT, microdosimetric considerations (justifiably ignored in conventional RT) combined with an expression for the clone extinction probability obtained from a mechanistic model of radiation cell survival lead to useful upper bounds on the size of the pre-treatment population of clonogenic cancer cells as well as upper and lower bounds on the cure probability. The main practical impact of these limiting values is the ability to make predictions about the probability of a cure for a given population of patients treated to newer, still unexplored treatment modalities from the empirically determined probability of a cure for the same or similar population resulting from conventional low linear energy transfer (typically photon/electron) RT. We also propose that the current trend to deliver a lower total dose in a smaller number of fractions with larger-than-conventional doses per fraction has physical limits that must be understood before embarking on a particular treatment schedule.

An NTCP Analysis of Urethral Complications from Low Doserate Mono- and Bi-Radionuclide Brachytherapy

Urethral NTCP has been determined for three prostates implanted with seeds based on ¹²⁵I (145 Gy), ¹⁰³Pd (125 Gy), ¹³¹Cs (115 Gy), ¹⁰³Pd-¹²⁵I (145 Gy), or ¹⁰³Pd-¹³¹Cs (115 Gy or 130 Gy). First, DU₂₀, meaning that 20% of the urhral volume receive a dose of at least DU₂₀, is converted into an I-125 LDR equivalent DU₂₀ in order to use the urethral NTCP model. Second, the propagation of uncertainties through the steps in the NTCP calculation was assessed in order to identify the parameters responsible for large data uncertainties. Two sets of radiobiological parameters were studied. The NTCP results all fall in the 19%–23% range and are associated with large uncertainties, making the comparison difficult. Depending on the dataset chosen, the ranking of NTCP values among the six seed implants studied changes. Moreover, the large uncertainties on the fitting parameters of the urethral NTCP model result in large uncertainty on the NTCP value. In conclusion, the use of NTCP model for permanent brachytherapy is feasible but it is essential that the uncertainties on the parameters in the model be reduced.

Tumor control probability in radiation treatment

Patients undergoing radiation therapy (and their physicians alike) are concerned with the probability of cure (long-term recurrence-free survival, meaning the absence of a detectable or symptomatic tumor). This is not what current practice categorizes as "tumor control (TC);" instead, TC is taken to mean the extinction of clonogenic tumor cells at the end of treatment, a sufficient but not necessary condition for cure. In this review, we argue that TC thus defined has significant deficiencies. Most importantly, (1) it is an unobservable event and (2) elimination of all malignant clonogenic cells is, in some cases, unnecessary. In effect, within the existing biomedical paradigm, centered on the evolution of clonogenic malignant cells, full information about the long-term treatment outcome is contained in the distribution Pm(T) of the number of malignant cells m that remain clonogenic at the end of treatment and the birth and death rates of surviving tumor cells after treatment. Accordingly, plausible definitions of tumor control are invariably traceable to Pm(T). Many primary cancers, such as breast and prostate cancer, are not lethal per se; they kill through metastases. Therefore, an object of tumor control in such cases should be the prevention of metastatic spread of the disease. Our claim, accordingly, is that improvements in radiation therapy outcomes require a twofold approach: (a) Establish a link between survival time, where the events of interest are local recurrence or distant (metastatic) failure (cancer-free survival) or death (cancer-specific survival), and the distribution Pm(T) and (b) link Pm(T) to treatment planning (modality, total dose, and schedule of radiation) and tumor-specific parameters (initial number of clonogens, birth and spontaneous death rates during the treatment period, and parameters of the dose-response function). The biomedical, mathematical, and practical aspects of implementing this program are discussed.

Assessment of normal tissue complications following prostate cancer irradiation: comparison of radiation treatment modalities using NTCP models

PURPOSE

Normal tissue complication probability (NTCP) of the rectum, bladder, urethra, and femoral heads following several techniques for radiation treatment of prostate cancer were evaluated applying the relative seriality and Lyman models.

METHODS

Model parameters from literature were used in this evaluation. The treatment techniques included external (standard fractionated, hypofractionated, and dose-escalated) three-dimensional conformal radiotherapy (3D-CRT), low-dose-rate (LDR) brachytherapy (I-125 seeds), and high-dose-rate (HDR) brachytherapy (Ir-192 source). Dose-volume histograms (DVHs) of the rectum, bladder, and urethra retrieved from corresponding treatment planning systems were converted to biological effective dose-based and equivalent dose-based DVHs, respectively, in order to account for differences in radiation treatment modality and fractionation schedule.

RESULTS

Results indicated that with hypofractionated 3D-CRT (20 fractions of 2.75 Gy/fraction delivered five times/week to total dose of 55 Gy), NTCP of the rectum, bladder, and urethra were less than those for standard fractionated 3D-CRT using a four-field technique (32 fractions of 2 Gy/fraction delivered five times/week to total dose of 64 Gy) and dose-escalated 3D-CRT. Rectal and bladder NTCPs (5.2% and 6.6%, respectively) following the dose-escalated four-field 3D-CRT (2 Gy/fraction to total dose of 74 Gy) were the highest among analyzed treatment techniques. The average NTCP for the rectum and urethra were 0.6% and 24.7% for LDR-BT and 0.5% and 11.2% for HDR-BT.

CONCLUSIONS

Although brachytherapy techniques resulted in delivering larger equivalent doses to normal tissues, the corresponding NTCPs were lower than those of external beam techniques other than the urethra because of much smaller volumes irradiated to higher doses. Among analyzed normal tissues, the femoral heads were found to have the lowest probability of complications as most of their volume was irradiated to lower equivalent doses compared to other tissues.

The use of risk estimation models for the induction of secondary cancers following radiotherapy

Theoretical predictions of cancer risk from radiotherapy may be used as a complementary criterion for the selection of successful treatment plans together with the classical approach of estimating the possible deterministic effects. However, any such attempts must take into consideration the specific features of radiation treatment. This paper explores several possible methods for estimating the risk of cancer following radiotherapy in order to investigate the influences of the fractionation and the non-uniformity of the dose to the irradiated organ. The results indicate that dose inhomogeneity plays an important role in predicting the risk for secondary cancer and therefore for predictive purposes it must be taken into account through the use of the dose volume histograms. They also suggest that the competition between cell killing and the induction of carcinogenic mutations has to be taken into consideration for more realistic risk estimations. Furthermore, more realistic parameters could be obtained if this competition is also included in analyses of epidemiological data from radiotherapy applications.

Urethral stricture following high dose rate brachytherapy for prostate cancer

PURPOSE

To evaluate the incidence, timing, nature and outcome of urethral strictures following high dose rate brachytherapy (HDRB) for prostate carcinoma.

METHODS AND MATERIALS

Data from 474 patients with clinically localised prostate cancer treated with HDRB were analysed. Ninety percent received HDRB as a boost to external beam radiotherapy (HDRBB) and the remainder as monotherapy (HDRBM). Urethral strictures were graded according to the Common Terminology Criteria for Adverse Events v3.0.

RESULTS

At a median follow-up of 41 months, 38 patients (8%) were diagnosed with a urethral stricture (6-year actuarial risk 12%). Stricture location was bulbo-membranous (BM) urethra in 92.1%. The overall actuarial rate of grade 2 or more BM urethral stricture was estimated at 10.8% (95% CI 7.0-14.9%), with a median time to diagnosis of 22 months (range 10-68 months). All strictures were initially managed with either dilatation (n=15) or optical urethrotomy (n=20). Second line therapy was required in 17 cases (49%), third line in three cases (9%) and 1 patient open urethroplasty (grade 3 toxicity). Predictive factors on multivariate analysis were prior trans-urethral resection of prostate (hazard ratio (HR) 2.81, 95% CI 1.15-6.85, p=0.023); hypertension (HR 2.83, 95% CI 1.37-5.85, p=0.005); and dose per fraction used in HDR (HR for 1 Gy increase per fraction 1.33, 95% CI 1.08-1.64, p=0.008).

CONCLUSIONS

BM urethral strictures are the most common late grade 2 or more urinary toxicity following HDR brachytherapy for prostate cancer. Most are manageable with minimally invasive procedures. Both clinical and dosimetric factors appear to influence the risk of stricture formation.

Dosimetric guidance on using brachytherapy (low-dose-rate or high-dose-rate) to offset a flawed permanent prostate implant

PURPOSE

To provide practical dosimetric advice on mending suboptimal permanent implants using low-dose-rate (LDR) or high-dose-rate (HDR) brachytherapy. The problem is to make the combination of the two radiation treatments (the initial, flawed one and the compensatory boost) clinically isoeffective with the planned dose.

METHODS AND MATERIALS

The device of isoeffective dose is the appropriate tool for this purpose as it accounts for the physical (temporal distribution of dose) and biologic (radiosensitivity, repair kinetics, proliferation rate) treatment settings.

RESULTS

I give, as a function of separation time from the initial, flawed treatment, and stratified by risk group representative values for the additional dose (low-dose-rate or high-dose-rate) needed to make the combined treatment isoeffective with a prescription of 144Gy of permanently implanted (125)I seeds.

CONCLUSIONS

Although the isoeffective dose concept, within the constraints stated in the text, is rigorously valid, its practical implementation depends importantly on the relevant radiobiologic parameters, and in this respect the reader is urged to use his own critical judgment.

Technology Insight: Combined external-beam radiation therapy and brachytherapy in the management of prostate cancer

External-beam radiation therapy (EBRT) combined with brachytherapy is an attractive treatment option for selected patients with clinically localized prostate cancer. This therapeutic strategy offers dosimetric coverage if local-regional microscopic disease is present and provides a highly conformal boost of radiation to the prostate and immediate surrounding tissues. Either low-dose-rate (LDR) permanent brachytherapy or high-dose-rate (HDR) temporary brachytherapy can be combined with EBRT; such combined-modality therapy (CMT) is typically used to treat patients with intermediate-risk to high-risk, clinically localized disease. Controversy persists with regard to indications for CMT, choice of LDR or HDR boost, isotope selection for LDR, and integration of EBRT and brachytherapy. Initial findings from prospective, multicenter trials of CMT support the feasibility of this strategy. Updated results from these trials as well as those of ongoing and new phase III trials should help to define the role of CMT in the management of prostate cancer. In the meantime, long-term expectations for outcomes of CMT are based largely on the experience of single institutions, which demonstrate that CMT with EBRT and either LDR or HDR brachytherapy can provide freedom from disease recurrence with acceptable toxicity.

Quality assurance/quality control issues for intraoperative planning and adaptive repeat planning of image-guided prostate implants

The quality assurance/quality control purpose is this. We design a treatment plan, and we wish to be as certain as reasonably possible that the treatment is delivered as planned. In the case of conventionally planned prostate brachytherapy, implementing to the letter the implantation plan is rarely attainable and therefore can require adaptive replanning (a quality control issue). The reasons for this state of affairs include changes in the prostate shape and volume during implantation and treatment delivery (e.g., edema resolution) and unavoidable inaccuracy in the placement of the seeds in the prostate. As a result, quality-control activities (e.g., the need to monitor-ideally, on the fly-the target and urethral and rectal dosage) must be also addressed.

Combined brachytherapy with external beam radiotherapy for localized prostate cancer: reduced morbidity with an intraoperative brachytherapy planning technique and supplemental intensity-modulated radiation therapy

PURPOSE

To report the acute and late treatment-related toxicities of combined permanent interstitial (125)I implantation delivered via real-time intraoperative planning and supplemental intensity-modulated radiotherapy (IMRT) for patients with clinically localized prostate cancer.

METHODS AND MATERIALS

One hundred twenty-seven patients were treated with a combined modality (CM) regimen consisting of (125)I implantation (110Gy) using a transrectal ultrasound-guided approach followed 2 months later by 50.4Gy of IMRT directed to the prostate and seminal vesicles. Late toxicity was scored according to the NCI Common Terminology Criteria for Adverse Events toxicity scale. The acute and late toxicities were compared to a contemporaneously treated cohort of 216 patients treated with (125)I alone to a prescribed dose of 144Gy.

RESULTS

The incidence of Grade 2 acute rectal and urinary side effects was 1% and 10%, respectively, and 2 patients developed Grade 3 acute urinary toxicities. The 4-year incidence of late Grade 2 gastrointestinal toxicity was 9%, and no Grade 3 or 4 complications have been observed. The 4-year incidence of late Grade 2 gastrourinary toxicities was 15% and 1 patient developed a Grade 3 urethral stricture, which was corrected with urethral dilatation. The percentage of patients who experienced resolution of late rectal and urinary symptoms was 92% and 65%, respectively. Multivariate analysis revealed that in addition to higher baseline International Prostate Symptom Score, those patients treated with implant alone compared to CM were more likely to experience Grade 2 acute urinary symptoms. Increased Grade 2 late rectal toxicities were noted for CM patients (9% vs. 1%; p=0.001) as well as a significant increase for late Grade 2 urinary toxicities (15% vs. 9%; p=0.004).

CONCLUSIONS

Adherence to dose constraints with combination real-time brachytherapy using real-time intraoperative planning and IMRT is associated with a low incidence of acute and late toxicities. Acute urinary side effects were significantly less common for CM patients compared to those treated with implantation alone. Late Grade 2 rectal and urinary toxicities were more common for patients treated with CM compared to implant alone.

Prostate Postbrachytherapy Seed Distribution: Comparison of High-Resolution, Contrast-Enhanced, T1- and T2-Weighted Endorectal Magnetic Resonance Imaging Versus Computed Tomography: Initial Experience

PURPOSE

To compare contrast-enhanced, T1-weighted, three-dimensional magnetic resonance imaging (CEMR) and T2-weighted magnetic resonance imaging (T2MR) with computed tomography (CT) for prostate brachytherapy seed location for dosimetric calculations.

METHODS AND MATERIALS

Postbrachytherapy prostate MRI was performed on a 1.5 Tesla unit with combined surface and endorectal coils in 13 patients. Both CEMR and T2MR used a section thickness of 3 mm. Spiral CT used a section thickness of 5 mm with a pitch factor of 1.5. All images were obtained in the transverse plane. Two readers using CT and MR imaging assessed brachytherapy seed distribution independently. The dependency of data read by both readers for a specific subject was assessed with a linear mixed effects model.

RESULTS

The mean percentage (+/- standard deviation) values of the readers for seed detection and location are presented. Of 1205 implanted seeds, CEMR, T2MR, and CT detected 91.5% +/- 4.8%, 78.5% +/- 8.5%, and 96.1% +/- 2.3%, respectively, with 11.8% +/- 4.5%, 8.5% +/- 3.5%, 1.9% +/- 1.0% extracapsular, respectively. Assignment to periprostatic structures was not possible with CT. Periprostatic seed assignments for CEMR and T2MR, respectively, were as follows: neurovascular bundle, 3.5% +/- 1.6% and 2.1% +/- 0.9%; seminal vesicles, 0.9% +/- 1.8% and 0.3% +/- 0.7%; periurethral, 7.1% +/- 3.3% and 5.8% +/- 2.9%; penile bulb, 0.6% +/- 0.8% and 0.3% +/- 0.6%; Denonvillier's Fascia/rectal wall, 0.5% +/- 0.6% and 0%; and urinary bladder, 0.1% +/- 0.3% and 0%. Data dependency analysis showed statistical significance for the type of imaging but not for reader identification.

CONCLUSION

Both enumeration and localization of implanted seeds are readily accomplished with CEMR. Calculations with MRI dosimetry do not require CT data. Dose determinations to specific extracapsular sites can be obtained with MRI but not with CT.

The role of external beam in brachytherapy

In this paper we put forward the claim that the combination of low dose-rate brachytherapy (BRT) and fractionated external-beam radiotherapy (EBRT) may, when planned to take advantage of the relative advantages of each of these two modalities, result in enhanced tumor dose with no penalties to organs at risk. The concept of iso-effective dose (IED) serves the role of common currency for fusing BRT and EBRT and, for evaluation purposes, converting back the resulting IED distribution into a biologically equivalent plan delivered by any single modality. If we accept this view, there are further questions that must be answered regarding practical matters. We show how to deal with these questions by describing an actual patient plan.