The use of megavoltage cone-beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement

In Europe and the USA combined, over half a million people had a hip joint replaced in 2005, contributing to the increasing number of radiotherapy patients with metallic hip prostheses. The treatment plan for external beam radiation therapy is based on the delineation of the anatomy in the planning CT scan. When implanted objects of high atomic number (Z) material are present, however, severe image artefacts are generated in conventional CT, strongly hindering the ability to delineate some organs. This is particularly the case for the planning of prostate patients with hip prostheses. This short communication presents the use of a new imaging modality, megavoltage cone-beam CT, to complement the regular CT for target definition of prostate cancer treatment of patients with hip replacements.

Forward or inversely planned segmental multileaf collimator IMRT and sequential tomotherapy to treat multiple dominant intraprostatic lesions of prostate cancer to 90 Gy

PURPOSE

To investigate the technical feasibility of using forward or inversely planned segmental multileaf collimator (SMLC) intensity-modulated radiotherapy and sequential tomotherapy (ST) to escalate to a dose of 90 Gy to multiple dominant intraprostatic lesions within the prostate gland while delivering a dose of 75.6 Gy to the remaining prostate.

METHODS AND MATERIALS

A selected case with one dominant intraprostatic lesion located at the left base and a second dominant intraprostatic lesion at the right apex of the prostate was planned using three different intensity modulation techniques. Two plans were generated with inverse treatment planning, using either SMLC or ST with a special multivane collimator. The third plan also employed SMLC but was generated using forward planning. All three plans were compared based on dose-volume histograms, isodose distributions, and doses to sensitive normal structures.

RESULTS

All three plans meet and exceed the desired dose constraints, limiting doses to the rectum and bladder to an estimated RTOG Grade 2 complication rate of <10%. The ST plan achieved the best dose conformality, whereas the inverse SMLC plan gave the lowest dose to the rectal wall, and the forward SMLC plan obtained the best dose homogeneity inside the targets.

CONCLUSIONS

Using any of the three intensity-modulated techniques, it is technically feasible to concurrently treat multiple selected high-risk regions within the prostate to 90 Gy and the remaining prostate to 75.6 Gy, while keeping the doses to the rectum and the bladder significantly lower than those associated with a Grade 2 complication rate of 10%.

Dose of radiation received by the bulb of the penis correlates with risk of impotence after three-dimensional conformal radiotherapy for prostate cancer

OBJECTIVES

To evaluate the effect of the dose to the bulb of the penis on postradiation potency.

METHODS

Twenty-one patients reporting potency before three-dimensional conformal radiotherapy had the dose delivered to the bulb of the penis evaluated. This was then compared with the patient assessments of post-treatment sexual function to determine whether a dose-volume relationship exists.

RESULTS

Among the patients analyzed to date, a strong dose-volume relationship and the likelihood of remaining potent after treatment seems to exist. Patients receiving a dose of less than 40 Gy to 70% of the bulb of the penis appear to have a much greater likelihood of maintaining potency. Patients receiving 70 Gy or more to 70% of the bulb of the penis appear to be at very high risk of experiencing radiation-induced impotence (P = 0.03).

CONCLUSIONS

More studies are needed to confirm these observations. If confirmed, these data suggest that by using three-dimensional-based treatment planning and carefully designed treatment fields, the potency of men treated with radiotherapy might be substantially improved.

Toxicity following high-dose three-dimensional conformal and intensity-modulated radiation therapy for clinically localized prostate cancer

OBJECTIVES

To report the toxicity profile of patients treated with three-dimensional conformal radiation therapy (3D-CRT) or intensity-modulated radiation therapy (IMRT) receiving doses of 82 Gy or more to portions of their prostate.

METHODS

Forty-four patients treated with radiation therapy for prostate cancer between June 1992 and August 1998 at the University of California, San Francisco received a maximal dose within the target volume (Dmax) of 82 Gy or more. Eighteen patients were boosted selectively to a limited portion of their prostate using IMRT, whereas 26 patients were treated with 3D-CRT and had unselected "hot spots" within their prostate. The Radiation Therapy Oncology Group (RTOG) acute and late toxicity scales were used to score gastrointestinal (GI) and genitourinary (GU) morbidity.

RESULTS

Median follow-up and Dmax were 23.1 months (range 10.0 to 84.7) and 84.5 Gy (range 82.0 to 96.7), respectively. Of the patients, 59.1% and 34.1% developed some level of acute GU and GI toxicity, respectively. One patient experienced grade 3 acute GI toxicity. No other grade 3 or greater acute toxicity was observed. The 2-year actuarial rates for freedom from late GI and GU morbidity were 77.1% (95% confidence interval [CI] 60.4% to 87.5%) and 79.5% (95% CI 62.7% to 89.3%), respectively. Although no grade 3 or greater late GU morbidity has been observed to date, 3 patients experienced grade 3 late GI morbidity. However, these cases involved rectal bleeding and were effectively managed with laser coagulation/fulguration.

CONCLUSIONS

Doses of 82 Gy or more to a portion of the prostate gland can be tolerated with acceptable morbidity. This observation supports the continued investigation of IMRT as a means for improving disease control in prostate cancer.

Prostate volume change after radioactive seed implantation: possible benefit of improved dose volume histogram with perioperative steroid

PURPOSE

To evaluate the changes in prostate volume associated with radioactive seed implantation and identify factors that influence prostate swelling.

METHODS AND MATERIALS

Between June 1997 and August 1999, 161 patients implanted for prostate carcinoma at the University of California, San Francisco, had prostate volume measurements taken at 4 time points (preplan, preimplant, postimplant, postimplant dosimetry). Patient records were reviewed for treatment with perioperative steroids, hormone therapy (nHT), and external beam radiotherapy (EBRT). One and 2-way analysis of variance (ANOVA) methods were used to test differences in mean effects among patient subsets.

RESULTS

A mean 20% volume increase was noted immediately postimplant overall (p < 0.0001), and even with EBRT and/or HT. Steroids were associated with a mean volume decrease of 19.9%, by 3-4 weeks post-procedure (p < 0.0001). Without steroids, only a 3.8% mean change was seen (p = ns). Steroid use resulted in a significant increase in mean dose-volume histogram (DVH) (p = 0.001); however, this benefit was only observed among patients who did not receive steroid. A consistently high DVH occurred with steroid use.

CONCLUSION

A significant decrease in prostate volume and improved DVH are associated with steroid use. The diminished benefit of steroid use and higher mean DVH achieved in later years suggests the existence of a significant "learning curve" for brachytherapy procedures.

Minimal toxicity with 3-FAT radiotherapy of prostate cancer

Beam radiation with three-dimensional conformal planning appears to decrease morbidity of prostate cancer therapy. The 3-field, arc technique (3-FAT) technique was designed by computer modeling to improve radiation dose to the target and minimize dispersion to nearby organs. Toxicity was studied in patients with prostate cancer. We performed a retrospective study of 168 consecutive men with prostate cancer after 3-FAT radiotherapy with a median follow-up of 24 months. All patients, treated from 1996 through 1999 at the University of Colorado had a pathological diagnosis of cancer before irradiation. Therapy was designed with a urethrogram and planning computed tomography scan. The 3-FAT was employed using noncoplanar, rotational beams, and nonuniform blocking of portals. Patients were treated to a minimal tumor dose of 74 Gy in 37 fractions. Adverse effects were investigated. Definitive radiotherapy was given to 80% of the group, and 58% received total androgen blockade. 3-FAT produced favorable dose distributions for the rectum, bladder, femoral heads, and base of the penis. Patients routinely report minimal dysuria and frequency during treatment. There were minimal urinary complaints after irradiation and no proctitis, diarrhea, incontinence, or change in potency as a result of radiotherapy. The 3-FAT represents a technical improvement in the treatment of prostate cancer by minimizing radiation delivered to adjacent critical structures. There were minimal side effects to the rectum, bladder, and penis base despite high doses to the prostate and seminal vesicles. The large percentage of patients with preliminary prostate-specific antigen values below 1.0 portends efficacy.

Normal tissue dosimetric comparison between HDR prostate implant boost and conformal external beam radiotherapy boost: potential for dose escalation

PURPOSE

To compare the dose and volume of bladder and rectum treated using high-dose-rate (HDR) prostate implant boost versus conformal external beam radiotherapy boost, and to use the dose-volume information to perform a critical volume tolerance (CVT) analysis and then estimate the potential for further dose escalation using HDR brachytherapy boost.

METHODS AND MATERIALS

Using CT scan data collected before and after patients underwent HDR prostate implant, a 7-field conformal prostate-only external beam treatment plan and HDR brachytherapy treatment plan were constructed for each patient. Doses to the normal structures were calculated. Dose-volume histograms (DVH) were plotted for comparison of the two techniques. Wilcoxon signed rank test was performed at four dose levels to compare the dose to normal structures between the two treatment techniques. The acute and late effects of HDR brachytherapy were calculated based on the linear-quadratic (LQ) model. CVT analyses were performed to calculate the potential dose gain (PDG) using HDR brachytherapy boost.

RESULTS

The volume of bladder and rectum receiving high dose was significantly less from implant boost. On the average, 0.19 cc of the bladder received 100% of the brachytherapy prescription dose, compared with 5.1 cc of the bladder receiving 100% of the prescription dose in the 7-field conformal external beam radiotherapy boost. Similarly, 0.25 cc of the rectum received 100% of the dose with the implant boost, as compared to 2.9 cc in the conformal external beam treatment. The implant also delivered higher doses inside the prostate volume. On average, 47% of the prostate received > or =150% of the prescription dose. The CVT analysis revealed a range of PDG using the HDR brachytherapy boost which depended on the following variables: critical volume (CV), critical volume tolerance dose (CVTD), number of HDR fractions (N), and the dose of external beam radiotherapy (XRT) delivered with brachytherapy boost. The PDG varied from -3.45% to 10.53% for tumor with an alpha-beta ratio of 10 and 7.14% to 64.6% for tumor with an alpha-beta ratio of 1.5 based on the parameters used for calculation in this study.

CONCLUSIONS

HDR brachytherapy can provide better sparing of rectum and bladder while delivering a higher dose to the prostate. Even with the increased late effects of high dose per fraction, there is still a potential for dose escalation beyond external radiotherapy limits using HDR brachytherapy.

Static field intensity modulation to treat a dominant intra-prostatic lesion to 90 Gy compared to seven field 3-dimensional radiotherapy

PURPOSE/OBJECTIVE

Recent studies supported by histopathological correlation suggest that the combined use of endorectal magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) allows differentiation of normal and carcinomatous prostate. The goal of this study was to use static field intensity modulated three-dimensional conformal radiotherapy (SF-IMRT) to treat the entire prostate to a total dose of >70 Gy, while concurrently treating a dominant intraprostatic lesion (DIL) defined by MRI+MRS to 90 Gy while not exceeding normal tissue tolerances.

MATERIALS AND METHODS

For the example chosen, the DIL consisted of a large portion of the peripheral zone of the left lobe of the prostate. University of Michigan (UM-PLAN) three-dimensional treatment planning software was used to design a partially shielded 7 field conformal isodose plan that would treat the entire prostate to >70 Gy at 1.8 Gy per day (80% isodose line), while concurrently treating the DIL to 2.25 Gy per day for a total dose of 90 Gy. Dose volume histograms (DVH) were used to compare the rectal doses to rectum and other adjacent normal tissues using these two techniques.

RESULTS

SF-IMRT as described, allowed a total dose of 90 Gy to encompass the DIL, while the rectal dose was slightly lower than that using the standard 7 field technique to the prostate alone. For example, the dose to 30 cm3 of the rectum was 40 Gy using SF-IMRT and 48 Gy for the standard 7 field technique. Because of differences in the dose per fraction the biologic advantages of the SF-IMRT technique are likely to be even greater.

CONCLUSIONS

This study demonstrates the feasibility of using SF-IMRT to treat a DIL involving a single lobe of the prostate, as defined by MRI/MRS, to 90 Gy, while simultaneously treating the prostate to >70 Gy without increasing the dose to surrounding normal tissues. A similar approach could be used to treat multifocal disease. This method of treatment is an alternative to dynamic intensity modulation. It is less expensive, and can be adapted to any radiation therapy department without the use of an inverse treatment planning programs.

A three-field arc technique (3-FAT) for treating prostate cancer

PURPOSE

We have previously designed two external beam radiotherapy techniques for treating prostate cancer. The seven-field, coplanar fixed beam technique resulted in dose distributions that were superior to other coplanar plans studied. The other technique using bilateral blocked arcs produced slightly higher doses to normal tissues but was far simpler to execute. We combined aspects of both these plans to produce a technique that was less complicated yet resulted in an improved dose distribution, i.e., to improve dose delivery to the clinical target volume (CTV) while minimizing doses to the rectum, bladder, and femoral heads.

METHODS AND MATERIALS

Twenty patients, previously treated at the University of California, San Francisco (UCSF) with radiotherapy for adenocarcinoma of the prostate, were studied. Each patient was treated with an immobilizer, urethrogram, and a preplanning CT scan. A previously employed, seven-field, coplanar, fixed beam technique was compared with a newly designed three-field, arc technique (3-FAT). This 3-FAT was designed using two equally weighted rotational beams, with nonuniform blocks, beginning in the lateral gantry position and spanning anteriorly 35 degrees. The two beams became noncoplanar by turning the table 20 degrees, bringing the patient's feet toward the gantry (inferior oblique arcs). An anterior inferior oblique (AIO), angled 20 degrees to the inferior of anterior was included for 10% of the daily treatment. Dose-volume histograms (DVH) were used to evaluate doses to adjacent critical structures. The dose to each critical structure was averaged and tabulated for the 20 patients. In addition, we compared normalized doses to adjacent structures using 3-FAT and seven-coplanar, fixed beams vs. a technique using four noncoplanar, fixed beams.

RESULTS

The three-field arc technique produced favorable dose distributions for the rectum, bladder, and femoral heads. Compared to the seven-field plan, employing the 3-FAT resulted in a 13% lower dose to 40% of the rectum, and 25% lower dose to 40% of the bladder. Compared to the four-field plan, employing the 3-FAT resulted in a 23% lower dose to 40% of the rectum, and 1% decrease in dose to 40% of the bladder. The three-field arc technique reduced the dose delivered to 40% of the femoral heads by approximately 45% when compared to the other techniques. Compared to other standard treatment techniques, the improvement in dose distribution was even greater.

CONCLUSIONS

The 3-FAT represents a technical improvement in the treatment of cancer of the prostate and seminal vesicles by minimizing the dose delivered to adjacent critical structures. The 3-FAT can incorporate the advances of multileaf collimation and digitally reconstructed radiographs to deliver treatment with cost effectiveness and technological efficiency.

Pulsed low dose rate brachytherapy for pelvic malignancies

PURPOSE

The pulsed low dose rate remote afterloading unit was designed to combine the radiation safety and isodose optimization advantages of high dose rate technology with the radiobiologic advantages of continuous low dose rate brachytherapy. This is the first report of a prospective clinical trial evaluating the relative incidence of acute toxicity and local control in patients with pelvic malignancies who underwent interstitial or intracavitary brachytherapy with the pulsed low dose rate remote afterloader.

METHODS AND MATERIALS

From 5/11/92-6/21/95, 65 patients underwent 77 brachytherapy procedures as part of their treatment regimen for pelvic malignancies. Using the pulsed low dose rate Selectron, equipped with a single cable-driven 0.3-1.0 Ci Ir192 source, target volume doses of 0.40-0.85 Gy per pulse were prescribed to deliver the clinically determined dose. Forty-five intracavitary and 32 interstitial procedures were performed. Fifty-four patients had primary and 11 recurrent disease. Patients were followed closely to assess incidence of Grade 3-5 acute and delayed toxicity, local control, and survival.

RESULTS

With a median follow-up of 16.1 months (range 1-29), 33 patients are NED, 10 alive with disease, 13 dead with disease, 4 dead of intercurrent disease, and 5 lost to follow-up. Local control was maintained until last follow-up or death in 48 cases, local failure occurred in 11, unknown in 5. Grade 3-5 acute toxicities (requiring medical or surgical intervention) occurred in 5 out of 77 procedures (6.5%), delayed complications in 10 patients (15% actuarial incidence at 2 years). In the 52 procedures performed for 42 patients with cervix cancer, the acute toxicity incidence was 5.8%, with a 14% 2-year actuarial incidence of delayed complications. Of 32 interstitial templates performed on 30 patients for pelvic malignancies, there were three incidences of acute toxicity and five delayed toxicities.

CONCLUSION

Using the parameters described for this initial clinical study in patients treated for pelvic malignancies, pulsed low dose rate brachytherapy shows no significant increase in acute toxicity above that seen with the standard continuous low dose rate approach. Using the isodose optimization possible with pulsed brachytherapy, local control is excellent in patients treated at initial presentation, although longer follow-up is required for full assessment of local control and late toxicity. Further trials will need to be carried out to determine if larger doses per pulse and shorter total treatment times have comparable therapeutic ratios.

The use of a partial transmission shield to reduce the optic chiasm doses during radiation therapy treatment of brain tumors

Historically, brain tumors have been treated with lateral opposed beams for 40-45 Gy followed by more conformal reduced fields. Advances in treatment planning computers have led to the implementation of conformal non-axial techniques, allowing for escalation of dose. In patients where total doses exceed 50 Gy, adjacent critical structures can be protected with a partially shielded transmission block over the optic nerves and chiasm. By eliminating the conedown portion of the treatment a more cost and time effective treatment is achieved. Partially shielded blocks can be designed by using cerrobend or multileaf collimation. They can be included in the treatment plan and verified by an irregular field calculation and/ or thermoluminescent dosimeters.

Three dimensional comparison of blocked arcs vs. four and six field conformal treatment of the prostate

PURPOSE

The purpose of this study is to compare five different techniques for treatment the prostate without seminal vesicles. Dose volume histograms and a time survey are the tools that were used for this analysis.

METHODS AND MATERIALS

For this study we compared 3D techniques using four and six field conformal treatments, to open and blocked 8 x 8 cm2 120 degrees bilateral arcs. All the plans were normalized to deliver 100% to the central axis, and full 3D calculations were performed. Blocked arcs were created using the 'average beam's eye view' (A-BEV) technique.

RESULTS

Analysis of the dose volume histograms revealed: (1) Arcs with blocks result in an improved dose distribution compared to standard arcs and four field 3DCRT techniques, (2) The DVH associated with blocked arcs, using block margins of 1.3 cm, resulted in a somewhat lower dose to the rectum but a 'tighter' margin around the prostate compared to the DVH generated using the six field 3DCRT technique.

CONCLUSION

This technique is for treatment of the prostate only, when treatment of the seminal vesicle is not required. The use of blocked arcs significantly improved the dose distribution compared to using standard arcs and 4-field conformal techniques. The DVHs associated with using blocked arcs is comparable to the SFC technique. It is likely to be less expensive, faster to set-up and may allow for safe dose escalation when only the prostate is receiving treatment.

The "critical volume tolerance method" for estimating the limits of dose escalation during three-dimensional conformal radiotherapy for prostate cancer

PURPOSE

To describe the "Critical Volume Tolerance" (CVT) method for defining normal tissue tolerance during 3D-based dose escalation studies for prostate cancer.

METHODS AND MATERIALS

The CVT method predicts the tolerance to radiation for "in series"-type functional units based on the assumption that tolerance depends on a critical threshold "low-volume high-dose region." The data used for describing this model were generated from 3D analysis of randomly selected patients with prostate cancer. Commonly used coplanar four-and six-field conformal (SFC) techniques were chosen as the comparison techniques. For purposes of comparison, rectal tolerance was assumed to be reached following whole pelvic irradiation using a four-field box technique to 50 Gy, followed by a conedown boost to 70 Gy using bilateral 9 x 9 cm 120 degree arcs as popularized by investigators from Stanford University (SUH).

RESULTS

Based on the average dose volume histograms for the patients studied, the maximum safe increase in dose for the SFC technique compared to the SUH technique, would be 10% if 30% of the rectal volume was the critical dose limiting volume (CVT = 30%), 5% if the CVT = 10%, or greater than 20% if the CVT = 40%. Commonly used four-field conformal techniques would not be expected to allow significant escalation of the dose without increasing the risk of complications.

CONCLUSIONS

The CVT method is relatively simple, and data generated based on it can be used to support normal tissue complication probability equations. The CVT method can be verified or modified as partial tolerance data become available. Based on the CVT model, sophisticated treatment techniques should allow a modest increase in the total dose of radiation delivered to the prostate without an increase in late complications.

The impact of isocenter placement errors associated with dose distributions used in irradiating prostate cancer

Historically, four perpendicular treatment fields or bilateral arcs have been used in the treatment of prostate cancer. "New techniques" including four conformal fields, seven conformal fields, 120 degrees bilateral conformal coplanar, and non-coplanar arc'ed beam arrangements, are replacing the "older" approaches. These techniques result in a reduction in doses to adjacent critical structures while covering the clinical target volume (CTV). This study, analyzes the impact of random or systematic isocenter displacement errors (IDE) associated with the delivery of radiotherapy, using the best of these "newer techniques". Dose Volume Histograms (DVH) were used to evaluate the dose to the prostate and surrounding normal tissues with 0.3 cm, 0.5 cm and 0.8 cm IDE. It was determined that IDE associated with fixed coplanar treatment techniques could reduced the prescribed dose to the prostate by 0-8%, the coplanar are technique reduced the prescribed prostate dose by 3-10%, and the noncoplanar conformal arc technique could lower the prescribed prostate dose by 0-5%. Predictably, 0.3 cm IDE found in the posterior and inferior direction increase the dose to the rectum by 5-12% and lowered the dose to the bladder by 4-8%. Errors in the superior and anterior direction increased the dose to the bladder by 4-8% and decreased the dose to the rectum by 8-10%. Errors in the right to left direction slightly increased the dose to the ipsilateral femoral head. Doses to the rectum and bladder associated with 0.5 cm and 0.8 cm IDE are significantly larger. The frequency and magnitude of IDE must be accounted for before higher doses can be delivered safety. Recognizing the impact of IDE on our ability to deliver the prescribed dose to the planning target volumes (PTV) could incorporate the impact of IDE during the planning process.

Four-year experience with swan neck presternal peritoneal dialysis catheter

The swan neck presternal catheter is composed of two flexible (silicon rubber) tubes joined by a titanium connector at the time of implantation. The exit site is located in the presternal or parasternal area. The catheter located on the chest was designed to reduce the incidence of exit site infections compared with peritoneal dialysis catheters with abdominal exit sites. From August 1991 to May 1995, 24 swan neck presternal catheters have been implanted in 24 patients for the following reasons: obesity nine patients, ostomies three patients, a suprapubic catheter one patient, previous problems with abdominal catheters two patients, desire to use a bathtub five patients, need to use a whirlpool one patient, need to wear sweatpants with an elastic waistband one patient, and body image two patients. In the same period, 47 abdominal swan neck catheters were implanted in 44 patients who preferred catheters with the exit on the abdomen. Presternal catheters tended to perform better regarding exit and tunnel infections, even though they were implanted in several patients in whom regular catheters with the exit on the abdomen would be difficult or impossible to implant. Two-year survival probability of presternal catheters was 0.88 +/- 0.14 (+/- SE). Recurrent/refractory peritonitis was the only reason of catheter failure. The differences in results between presternal and abdominal catheters were statistically insignificant; only the use of antibiotics to treat exit site infection was significantly higher with abdominal catheters. Patient acceptance of the exit position was good; at least seven patients preferred presternal catheter for psychological or body image reasons. We conclude that the swan neck presternal catheters provide excellent results comparable to those achieved with swan neck abdominal catheters. The catheter seems suitable for any patient commencing peritoneal dialysis and is particularly useful in extremely obese patients (body mass index > 40 kg/m2) and those with ostomies. The catheter exit location in the chest may be preferred by some patients, both men and women, for psychological or body image reasons. No specific contraindications to the presternal catheter implantation have been identified.

The value of nonuniform margins for six-field conformal irradiation of localized prostate cancer

PURPOSE

Evaluate the hypothesis that by combining nonuniform margins with a technique for limiting the possible extent of posterior motion of the prostate during the delivery of six-field conformal radiotherapy (SFCRT) of the prostate, it is possible to adequately treat the clinical target volume (CTV) and minimize dose to normal structures.

METHODS AND MATERIALS

Serial computed tomography (CT) scans of prostate patients were taken at 0.5 cm intervals for treatment planning purposes. The initial treatment planning scans were performed with the rectum empty and the bladder full. Subsequent scans were taken at the end of the first week of treatment with the bladder full, but with no attempt to empty the rectum, to mimic the typical treatment situation. The gross tumor volume (GTV), consisting of the prostate and seminal vesicles, as well as the CTV, were defined on the CT images with the aid of a urethrogram to define the inferior border (apex) of the prostate. Variable blocking margins were designed around the CTV using the University of Michigan three dimensional (3D) treatment planning system (UM-PLAN). Isodose distributions displayed on axial, sagittal, coronal, and oblique slices were used to evaluate the adequacy of the various margins applied. Nonuniform margins varying from 0.75 cm posteriorly to 2.0 cm anteriorly and inferiorly were compared to uniform margins of 1.0, 1.5, and 2.0 cm for each patient. Dose volume histograms (DVH) were used to compare doses to the GTV, CTV, rectum, and bladder.

RESULTS

In a series of 10 patients scanned with the above protocol, treatment plans with nonuniform margins were compared with uniform margins of 1.0, 1.5, and 2.0 cm. Dose-volume histograms showed that nonuniform and 1.0 cm uniform margins deliver the lowest doses to the rectum and bladder, but the use of 1.0 cm uniform margins resulted in inadequate coverage of the CTV in 40% of the cases. The 1.5 and 2.0 cm uniform margins adequately covered the CTV but resulted in significantly higher doses to the bladder and rectum.

CONCLUSIONS

The use of nonuniform margins, when combined with CT scans performed with the rectum empty and bladder full, can improve tumor control probability while minimizing the risk of morbidity to adjacent critical structures.

3D conformal radiotherapy in the sagittal plane for centrally located thoracic tumors

One hundred patients, recently treated at the University of California, San Francisco (UCSF) with radiotherapy for lung and esophageal cancer, were studied. Three subsets of these patients were defined based on tumor location to test how commonly the use of three-dimensional (3D) conformal radiotherapy (3DCRT) could improve significantly the delivery of high dosages. Comparisons were made between isodose distributions and dose volume histograms (DVHs) of patients' prior computed tomography (CT)-based treatment plans and newly generated 3D-based treatment plans. The use of beam angles outside the conventional horizontal plane did not significantly improve the dose distribution for patients if a peripheral mass was the target volume. Patients with a target volume involving the central thorax represented a subset (> 40% of the patients) who would have benefited the most from the use of nonconventional beam angles. In these patients, sagittal coplanar beams (sagittal Y technique) reduced the dose to 30% of the lung volume from 30% to 5%. Doses to the spinal cord and heart were slightly higher than with conventional techniques but were within the tolerance of normal tissues. The sagittal Y technique allows an escalation in total dose to the tumor without increasing dose to normal tissues beyond their tolerance.

Source localization for template implants with particular reference to stepping-source afterloaders

Source localization from radiographs can be very difficult for template-guided implants if the needle images overlap. At UCSF several techniques to make this task easier have been developed. The techniques include selection of an optimum simulator gantry angle, use of different types of dummies, and differential dummy loading. In addition, several modifications have been made in our brachytherapy planning computer program to facilitate source entry. As a result of these improvements, source localization is now accomplished in much less time with improved accuracy.

Indications for and the significance of seminal vesicle irradiation during 3D conformal radiotherapy for localized prostate cancer

PURPOSE

To evaluate the use of pretreatment prostate specific antigen, Gleason score, and clinical stage as predictors of the risk of seminal vesicle involvement in patients with clinically localized prostatic cancer, and to determine the impact of excluding the seminal vesicles on the dose received by surrounding normal tissues.

METHODS AND MATERIALS

An empirically derived equation combining the preoperative prostate specific antigen and Gleason score was applied to 188 patients treated with radical prostatectomy, for whom pathologic evaluation of the seminal vesicles was available. High and low risk groups for seminal vesicle involvement were defined using this equation. The observed risks of seminal vesicle involvement was compared to the predicted risk using the preoperative prostate specific antigen, Gleason score or clinical stage alone or using the empirical equation. Dose-volume histograms for five patients treated using six-field conformal radiotherapy were compared including and excluding the seminal vesicles.

RESULTS

Using the empirically derived equation, a low risk group of 109 patients was identified with a calculated risk of seminal vesicle involvement of < or = 13% and an observed incidence of 7.3%. Among the high risk group of 79 patients, which included all patients with a calculated risk > 13%, 37% had seminal vesicle involvement (p < 0.001 low vs. high risk). Twenty percent of the rectal volume received on average above 86% of the total dose for the five plans which included the seminal vesicles compared to 68% for the five plans excluding the seminal vesicles. The doses to 40% of the rectal volume were 64% and 37% if the seminal vesicles were included and excluded, respectively. The dose to the bladder and femoral heads was also decreased but to a lesser extent.

CONCLUSION

The empirical formula predicts risk of seminal vesicle involvement with a higher degree of significance for a larger number of patients than either Gleason score, clinical stage, or prostate specific antigen alone. Based on an analysis of our first 100 patients treated with definitive conformal therapy alone, approximately 47% of those patients could have been treated excluding the seminal vesicles. Excluding the seminal vesicles may allow us to go to a higher total dose with less rectal toxicity.

Optimization of the oblique angles in the treatment of prostate cancer during six-field conformal radiotherapy

Historically, four perpendicular treatment fields or bilateral arcs have been used in the treatment of prostate cancer. As new techniques challenge the four-field box technique for their superiority of tumor coverage and adjacent critical structure sparing, oblique beam angles (in addition to right and left laterals) have been introduced as an alternative to anterior (AP) and posterior (PA) beams. Among the most popular of these alternative approaches is a six-field technique. Traditionally 45 degrees angles have been used with this technique. In this study, opposed coplanar oblique beams angled 20, 25, 30, 35, 40, and 45 degrees off the lateral beam position, were compared for their ability to minimize adjacent critical structure doses, while maintaining maximum clinical target volume (CTV) coverage. This analysis compared rectum, bladder, and femoral head dose volume histograms (DVH) for each of these varying oblique gantry angles. As the angle of the posterior oblique beams became more horizontal, it is more difficult to encompass the apex of the prostate in the 95% isodose value. On inferior CT slices near the apex of the prostate, the density of the pelvic bones in the path of the posterior oblique fields causes the beam to be slightly attenuated, thereby underdosing the CTV. The oblique angles most affected by this bone heterogeneity are beams angled from 20 to 30 degrees off the lateral beam position. As the angles approach the vertical direction, rectal and bladder doses increase, while femoral head doses decrease. Oblique gantry angles approaching the horizontal direction result in a decrease in rectal and bladder doses, while femoral head doses increase. Of the oblique angles studied, 35 degrees off the lateral position provides lower rectum and bladder doses than 30, 40, and 45 degrees; lower femoral head doses than 20, 25, and 30 degrees, and the maximum CTV coverage on all CT slices studied.

Applicability of standard irregular field calculations for high-energy photon beams

Irregular-field calculations are usually based on a sector-summation technique. This technique typically involves separating the dose into primary and scatter components. While the conceptual basis for this procedure is valid at lower energies, it becomes suspect at higher energies. However, despite this limitation, a straightforward application of the technique yields good accuracy for 18 MV X-rays if certain limitations are observed. This work describes the calculational formalism used and the approximations made. Computed results are compared with measurements. The levels of uncertainty in both are discussed.

Bilateral arcs using "averaged beam's eye views": a simplified technique for delivering 3-D based conformal radiotherapy

The purpose of this study is to describe a conformal radiotherapy technique for treating only the prostate with bilateral 120 degrees arcs using "averaged beams-eye-views" (A-BEV). For this study a CT scan from a patient with a large prostate but with a low risk for seminal vesicle involvement was chosen for comparing several different treatment techniques. Dose volume histograms (DVHs) of the prostate, femoral heads, bladder, and rectum were compared for plans using "standard" bilateral 120 degree unblocked arcs (8 x 8 and 9 x 9 cm), similar sized arcs with "generic" (small corner) blocks applied, arcs using hand drawn "semi-conformal" blocks added, and arcs using the A-BEV. The A-BEV was generated by averaging the shapes of fixed lateral and oblique BEVs from a six-field plan. These arc techniques were compared to four-field conformal (4-FC) and six-field conformal (6-FC) techniques. The addition of generic corner blocks to a 9 x 9 field resulted in a more favorable dose distribution than using open unblocked 9 x 9 arcs. The technique employing the A-BEV resulted in an improvement in the DVHs compared to other arc techniques and to 4-FC techniques. The dose volume histograms associated with using this technique approached those associated with using a 6-FC technique. Treating only the prostate with blocked arcs generated using an A-BEV results in an improved dose distribution compared to unblocked arcs and 4-FC techniques. This blocked arc technique also results in a DVH that is comparable to using a more complex 6-FC technique. Blocks that are drawn on manually reduce the dose to the surrounding normal tissues but are associated with a greater risk of underdosing the target volume. This problem is diminished when computer generated conformal blocks are used.

Defining treatment margins for six field conformal irradiation of localized prostate cancer

PURPOSE

To define the "ideal margins" to be used for the delivery of six-field conformal radiotherapy for localized prostate cancer.

METHODS AND MATERIALS

For a typical patient, 3-D based 6-field conformal treatment plans were generated using uniform margins ranging from 0.5-2.5 cm (in 0.25 cm increments). In a step-wise fashion the minimum margins required to encompass the gross tumor volume within the 90% isodose shell were identified. Additional margins were then added to account for extracapsular penetration, setup and patients movement error as well as for organ movement. Assumptions about the relative tolerance of surrounding normal tissues were also incorporated into the final decisions regarding margins.

RESULTS

For the various areas of interface, between the prostate and surrounding normal tissues "ideal margins" varied from 0.75-2.25 cm.

CONCLUSION

The use of nonuniform "ideal margins" appears to insure adequate coverage of the tumor, while minimizing the volume of surrounding dose limiting normal tissues irradiated. This approach should in theory improve the tumor control and complication probabilities compared to using conventional treatment techniques and to using a 6-field conformal technique with uniform margins.

Immobilization improves the reproducibility of patient positioning during six-field conformal radiation therapy for prostate carcinoma

PURPOSE

To determine the magnitude of patient positioning errors associated with six field conformal therapy for carcinoma of the prostate, and to assess the impact of alpha-cradle immobilization on these errors.

METHODS AND MATERIALS

The records of 22 patients, treated at two of the treatment facilities within our department, using computed tomography-planned conformal six field therapy for carcinoma of the prostate, were reviewed. At one facility (UCD), patients were routinely treated with immobilization, while at the other (UCSF) no rigid immobilization was used. Portal films of patients treated at both facilities were subsequently reviewed, and the deviation of each portal from the simulation film was determined (simulation-to-treatment variability). In addition, for each patient, the average deviation of each portal film from the average portal film (treatment-to-treatment variability) was determined.

RESULTS

The mean and median simulation-to-treatment variability was 0.4 cm for those patients treated with immobilization, versus 0.6 cm for those treated without immobilization. The 90th percentile of simulation-to-treatment variability was 0.7 cm for those patients treated with immobilization, versus 1.1 cm for those not immobilized. There was a significant reduction in the number of portals observed with errors of > or = 0.50 cm (132/201 vs. 37/87, 66% vs. 43%; p < 0.001), 0.75 cm (184/201 vs. 59/87, 92% vs. 68%; p < 0.001), and 1.0 cm (196/201 vs. 74/87, 98% vs. 85%; p < 0.001) for patients treated with immobilization. There was also a significant reduction in the number of patients with treatment-to-treatment variability > or = 0.5 cm (1/10 vs. 8/12; p = 0.01) for patients treated with immobilization.

CONCLUSION

The use of immobilization devices significantly reduces errors in patient positioning, potentially permitting the use of smaller treatment volumes. Immobilization should be a component of conformal radiation therapy programs for prostate carcinoma.

The role of the urethrogram during simulation for localized prostate cancer

Urethrograms on 89 consecutive patients with localized prostate cancer were evaluated retrospectively, and the inferior border of the treatment field based on this study was compared with the inferior border that would have been defined by using the lower border of the ischial tuberosities. An analysis of the relationship between the margin used and the dose at the inferior border of the prostate supported our policy of requiring a 2 cm margin for optimal coverage of the prostate. Inclusion of at least 1 cm of proximal penile urethra was essential to ensure this 2 cm margin. Based on this assumption, twenty-five percent of patients would have had an inadequate margin if the lower border of the ischial tuberosities had been used instead of the urethrogram to define the inferior border of the treatment field. Assuming that a margin of more than 3 cm inferiorly is excessive, 11% of patients would have had excessive urethral irradiation if the bottom of the ischial tuberosities had been used to define the inferior border. Combining these two extremes, more than one in three patients would have had an inappropriate inferior margin if the bottom of the ischial tuberosities had been used to define the inferior border of the treatment field. There was no apparent increase in morbidity as a result of the urethrograms or an increase in treatment related toxicity in association with using the treatment fields defined by urethrography. Computed tomography was complimentary in defining the apex of the prostate. These data support the routine use of the urethrograms during simulation for localized prostate cancer. The use of the lower border of the ischial tuberosities to define the inferior border of the treatment field is associated with an unacceptable risk of either underdosing the apical portion of the prostate or overdosing the urethra.

Normalization: what does it really mean?

Attempts have been made to standardize normalization techniques, but it remains a confusing issue. As of late there are five different possible normalization tools consisting of normalizing: 1. to the isocenter, 2. to a specific point other than the isocenter, 3. to a minimum target absorbed dose whose are of maximum target absorbed dose is a 2 cm2 area, 4. to an isodose value, 5. to the dmax of the beams. The International Commission on Radiation Units and Measurements has updated its report number 29 to further recommend normalization standards to include planning to a specification point that: 1. is easy to define in an unambiguous way, 2. is chosen in a region where the dose is rather homogeneous and representative of the dose distribution throughout the target volume, 3. defined where the dose can be accurately determined, 4. normalizes to the isocenter of the plan where the maximum absorbed dose is a 1.5 cm2 area, 5. limits the dose variation to 10%. With increasing complexity involving three-dimensional planning, mixed-beam therapy, and heterogeneity compensation, a full understanding of dose determination is necessary to accurately implement treatment plans.

An approach to abutting adjacent fields

Problems arise in designing treatment techniques involving two pair of adjacent opposing fields where machine limitations require the patient to flip from supine to prone positions. Mantle and para-aortic treatments, in particular, can create challenging problems because of changes in patient position, different SSD's between adjacent fields, internal anatomical changes from supine to prone position, as well as field size and other treatment machine limitations. A simulator technique has been developed which takes cognizance of these limitations in specifying the gap between adjacent fields. It employs collinearity of the 50% decrement lines of adjacent-opposed field edges and the intersection of all four edges at an internal mid-plane match point. The technique maintains dose homogeneity and eliminates hot and cold triangles in the area of abutment. Simulation radiographs facilitate identification of collinearity with respect to a specific vertebra in the plane of abutment. In summary, this approach: Verifies abutment of coplanar fields by use of match film, improves isodose uniformity at mid-plane, evaluates dose distributions when abutment occurs at a point anterior or posterior to midline, prevents the possibility of spinal cord complications that might occur due to three field overlap.