Complications from radiotherapy dose escalation in prostate cancer: preliminary results of a randomized trial

Adjuvant androgen deprivation impacts late rectal toxicity after conformal radiotherapy of prostate carcinoma

To evaluate whether androgen deprivation impacts late rectal toxicity in patients with localised prostate carcinoma treated with three-dimensional conformal radiotherapy. One hundred and eighty-two consecutive patients treated with 3DCRT between 1995 and 1999 at our Institution and with at least 12 months follow-up were analysed. three-dimensional conformal radiotherapy consisted in 70-76 Gy delivered with a conformal 3-field arrangement to the prostate+/-seminal vesicles. As part of treatment, 117 patients (64%) received neo-adjuvant and concomitant androgen deprivation while 88 (48.4%) patients were continued on androgen deprivation at the end of three-dimensional conformal radiotherapy as well. Late rectal toxicity was graded according to the RTOG morbidity scoring scale. Median follow up is 25.8 (range: 12-70.2 months). The 2-year actuarial likelihood of grade 2-4 rectal toxicity was 21.8+/-3.2%. A multivariate analysis identified the use of adjuvant androgen deprivation (P=0.0196) along with the dose to the posterior wall of the rectum on the central axis (P=0.0055) and the grade of acute rectal toxicity (P=0.0172) as independent predictors of grade 2-4 late rectal toxicity. The 2-year estimates of grade 2-4 late rectal toxicity for patients receiving or not adjuvant hormonal treatment were 30.3+/-5.2% and 14.1+/-3.8%, respectively. Rectal tolerance is reduced in presence of adjuvant androgen deprivation.

Rectum contouring variability in patients treated for prostate cancer: impact on rectum dose-volume histograms and normal tissue complication probability

BACKGROUND

Recent investigations showed some correlation between three-dimensional (3D) treatment planning dose-volume data (dose-volume histograms: DVH, dose statistics) and rectal toxicity for patients treated for prostate cancer. However, no data are available about the possible impact of inter-institute variability in contouring the rectum, so that the possibility of reliably using information from single-centre studies remains doubtful.

PURPOSE

Within a retrospective three-institutes study on correlation between dose-volume treatment planning data and rectum bleeding in patients treated for prostate cancer, an investigation about the impact of inter- and intra-observer variability in contouring the rectum was performed.

MATERIALS AND METHODS

Ten patients were considered for a dummy run exercise and three observers (one per Institute) contoured the rectum (including filling). An anatomically based definition of rectum extension was previously accepted by the three observers. Six of the ten patients were randomly chosen in the subgroup of patients (large spacing, LS) with a distance between computed tomography (CT) slices (outside the prostate region) equal to 10 mm; for the remaining four patients the distance between CT slices was 5 mm over the whole rectum volume (small spacing, SS). The original 3D treatment planning was recovered on the Cadplan treatment planning system for each patient and rectum dose statistics (mean, median and maximum rectum dose), volume, DVH and NTCP values were calculated for each observer. For DVH analysis, the values of V(50), V(55), V(60), V(65) and V(70) (defined as the % of rectum volume receiving at least 50, 55, 60, 65, 70 Gy) were considered. Normal tissue complication probabilities (NTCPs) were calculated for the original ICRU dose and for a 75.6 Gy ICRU dose (NTCP and NTCP(75.6), respectively). Intra-observer variability was investigated by asking the observers to redraw the same rectum contours 6 months later and comparing the two contouring sessions.

RESULTS

In general, a good agreement was found for most patients and, in particular, for all SS patients. The impact of inter-observer variability was quite significant on dose statistics and DVH in two of six LS patients. Looking at the patient population, some systematic deviations, even if quite small, were demonstrated between institute B and institute C (volume, P = 0.02) and between institute A and institute B (mean/median dose, V(50)-V(65), NTCP(75.6); P < 0.05). Four of six LS patients (0/4 in the SS group) presented a maximum difference among observers at the cranial and/or caudal limit of the rectum equal to 1 cm. For these patients, inter-observer variability was significantly higher than for the others (P < 0.03). When inter-observer variability was expressed in terms of standard deviations (SD), values around 2-3 Gy and 0.5 Gy for LS and SS patients, respectively, were found for mean/median dose; values around 3-4% and 0.5-2% for LS and SS patients, respectively, were found for V(50)-V(70). The average SD for NTCP and NTCP(75.6) were 0.4 and 0.6%, respectively (0.5 and 0.9% for LS patients; 0.2 and 0.3% for SS patients). Intra-observer variability was found to be lower than inter-observer variability even if the impact on dose statistics and DVH was visible.

CONCLUSIONS

Once a robust definition of rectum is assessed, inter- and intra-institute variability in contouring the rectum appear relatively modest. However, the results suggest that the number of LS patients in DVH correlation studies should be as low as possible; the low number of these patients in the multi-centric trial involving our institutions should not have significant impact on the results of the study.

[Conformal radiotherapy in prostate cancer: for whom and how?]

External radiotherapy is one of the modalities used to cure localized prostate carcinoma. Most of localized prostate carcinomas, specially those of the intermediate prognostic group, may benefit from escalated dose above 70 Gy at least as regard biochemical and clinical relapse free survival. 3D-CRT allows a reduction of the dose received by organs at risk and an increase of prostate dose over 70 Gy. It is on the way to become a standard. Intensity modulated radiation therapy increases dose homogeneity and reduces rectal dose. These methods necessitate rigorous procedures in reproducibility, delineation of volumes, dosimetry, daily treatment. They need also technological and human means. It is clear that localized prostate cancer is a good example for evaluation of these new radiotherapy modalities.

Conformal photon-beam radiotherapy of prostate carcinoma

Three-dimensional conformal radiotherapy is the recommended radiation technique for localized or locally advanced prostate cancer. In the past decades, external beam irradiation procedures have evolved in the context of technical developments of radiation and imaging equipment. The article summarizes these developments and gives a definition of new techniques and their potential advantages over conventional irradiation. It is meant to provide urologists and medical and radiation oncologists with a better comprehension of modern radiation treatment of prostate cancer and its possible improvements in the future.

Initial clinical experience with infrared-reflecting skin markers in the positioning of patients treated by conformal radiotherapy for prostate cancer

PURPOSE

To evaluate an infrared (IR) marker-based positioning system in patients receiving conformal radiotherapy for prostate cancer.

METHODS AND MATERIALS

During 553 treatments, the ability of the IR system to automatically position the isocenter was recorded. Setup errors were measured by means of orthogonal verification films and compared to conventional positioning (using skin drawings and lasers) in 184 treatments.

RESULTS

The standard deviation of anteroposterior (AP) and lateral setup errors was significantly reduced with IR marker positioning compared to conventional: 2 vs. 4.8 mm AP (p < 0.01) and 1.6 vs. 3.5 mm laterally (p < 0.01). Longitudinally, the difference was not significant (3.5 vs. 3.0 mm). Systematic errors were on the average smaller AP and laterally for the IR method: 4.1 vs. 7.8 mm AP (p = 0.01) and 3.1 vs. 5.6 mm lateral (p = 0.07). Longitudinally, the IR system resulted in somewhat larger systematic errors: 5.0 vs. 3.4 mm for conventional positioning (p = 0.03). The use of an off-line correction protocol, based on the average deviation measured over the first four fractions, allowed virtual elimination of systematic errors. Inability of the IR system to correctly locate the markers, leading to an executional failure, occurred in 21% of 553 fractions.

CONCLUSION

IR marker-assisted patient positioning significantly improves setup accuracy along the AP and lateral axes. Executional failures need to be reduced.

Trade-off to low-grade toxicity with conformal radiation therapy for prostate cancer on Radiation Therapy Oncology Group 9406

The aim of this study was to evaluate and compare the rates of grade 2 or worse late effects in patients treated for prostate cancer on Radiation Therapy Oncology Group (RTOG) 9406. The authors previously have reported the results of patients treated on the first 2 dose levels of this study with respect to grade 3 or greater late toxicity. This analysis examines the incidence of grade 2 toxicity in this study. From August 1994 to September 1999, 424 patients were entered on this dose escalation trial of 3-dimensional conformal radiation therapy (3D CRT) for localized adenocarcinoma of the prostate at doses of 68.4 Gy (level I) and 73.8 Gy (level II). All radiation prescriptions were a minimum dose to a planning target volume. Patients were stratified according to clinical stage and risk of seminal vesicle invasion based on Gleason score and presenting prostate-specific antigen. Average time at risk after completion of therapy ranged from 33.1 to 40.1 months for patients treated at dose level I and 15.6 to 34.2 months for patients at dose level II. The frequency of late effects > or = grade 2 was compared with a similar group of patients treated on RTOG studies 7506 and 7706 with adjustments made for the interval from completion of therapy. The RTOG toxicity scoring scales for late effects were used. The rate of grade 3 or greater late toxicity continues to be low compared with RTOG historical controls. No grade 4 or 5 late complications were reported in any of the 406 evaluable patients during the period of observation. Interestingly, the incidence of grade 2 late toxicity was increased relative to historical controls in all groups and dose levels. In group 1, level I and group 3, level II, the increase in grade 2 complications was statistically significant; 16 complications were observed in group 1, level I when 9.2 were expected (P =.026) and 22 were observed in group 3, level II when 7.6 were expected (P <.0001). When examining all late effects > or = grade 2, there were no significant differences in the rate of late effects in both groups and both dose levels with the exception of group 1, level II. This, in combination with the statistically significant decrease in late effects > or = grade 3, suggests that in most circumstances there has been a shift of grade 3 complications to grade 2. In group 1, dose level II there was a statistically significant reduction in > or = grade 2 late effects, suggesting there was no shift from grade 3 to grade 2 in these patients. In this circumstance there may have been a global reduction in all complications or a shift to late effects less severe than grade 2. In group 2, dose level II there is a trend (P =.085) toward this same result. It is important to continue to examine late effects closely in patients treated on RTOG 9406. The primary objective of dose escalation without an increase rate of > or = grade 3 complications has been achieved. However, the reduction in grade 3 complications may have resulted in a higher incidence of grade 2 late effects. Because grade 2 late effects may have a significant impact on a patient's quality of life, it is important to reduce these complications as much as possible. Improved conformal treatment delivery with intensity-modulated radiation therapy or the use of radioprotective agents could be considered. Clinical trials should use quality-of-life measures to determine that trade-offs between severity and rates of toxicity are acceptable to patients.

IMRT clinical implementation: prostate and pelvic node irradiation using Helios and a 120-leaf multileaf collimator

Dynamic intensity modulated radiation therapy (IMRT) to treat prostate and pelvic nodes using the Varian 120-leaf Millennium multileaf collimator (MLC) has been implemented in our clinic. This paper describes the procedures that have been undertaken to achieve this, including some of the commissioning aspects of Helios, verification of the dynamic dose delivery, and quality assurance (QA) of the dose delivered to the patient. Commissioning of Helios included measurements of transmission through the 120-leaf MLC, which were found to be 1.7% for 6 mV and 1.8% for 10 MV. The rounded leaf edge effect, known as the dosimetric separation, was also determined using two independent methods. Values of 1.05 and 1.65 mm were obtained for 6 and 10 MV beams. Five test patients were planned for prostate and pelvic node irradiation to 70 and 50 Gy, respectively. Dose and fluence verification were carried out on specially designed phantoms and dose points in the prostate were measured to be within 2.0% (mean 0.9%, s.d. 0.6%) of the calculated dose and in the nodes within 3.0% (mean 1.6%, s.d. 1.1%). Following the results of this commissioning and implementation study, we have started to treat men with a target Volume including the prostate and pelvic nodes using Helios optimized dynamic IMRT delivery in a dose escalation protocol.

Intrarectal application of amifostine for the prevention of radiation-induced rectal injury

Clinically symptomatic late injury to the rectal wall occurs in about one third of patients with prostate cancer treated with external beam irradiation. Reducing the physical dose to the anterior rectal wall without a similar reduction in the posterior peripheral zone is difficult because of the proximity of these structures. Based on our previous observations that intrarectal application of amifostine resulted in very high concentrations of amifostine and its active metabolite WR-1065 in the rectal wall of Copenhagen rats, the authors initiated a phase I clinical trial in 1998. Twenty-nine patients with localized prostate cancer were accrued. Eligibility criteria included histologically confirmed adenocarcinoma, a Karnofsky performance status of > or =70, and no pelvic lymphadenopathy or distant metastases. The total dose to the prostate was 70.2 Gy (20 patients) and 73.8 Gy (9 patients). Therapy was delivered using a 4-field axial technique and 3-dimensional conformal planning. Amifostine was administered intrarectally as an aqueous solution 30 minutes before irradiation on the first 15 days of therapy. Amifostine dose was escalated, in cohorts, from 500 mg to 2,500 mg. Toxicity was evaluated using the Radiation Therapy Oncology Group late morbidity scale. All patients completed therapy with no amifostine-related toxicity at any dose level. The application was feasible and well tolerated. With a median follow-up time of 21 months, 9 patients (33%) had rectal bleeding (8 grade 1, 1 grade 2). Four patients (14%) had symptoms suggestive of radiation injury, which proved to be secondary to nonrelated processes. These included preexisting nonspecific proctitis (1 patient), diverticular disease of the sigmoid colon, rectal polyp (1 patient), and ulcerative colitis (1 patient). Symptoms developed significantly more often in patients receiving 500 to 1,000 mg than in patients receiving 1,500 to 2,500 mg amifostine (7 of 14 [50%] versus 2 of 13 [15%]; P =.0325, 1-sided chi(2) test). Intrarectal application of amifostine is feasible and well tolerated. A complete lack of systemic toxicity obviates the need for close monitoring of patients during and after administration. Rectal symptomatology after external beam radiotherapy to the pelvis cannot be assumed to reflect late radiation damage, because it often is a manifestation of an unrelated pathologic process. The preliminary efficacy data are encouraging and suggest that intrarectal administration of amifostine may reduce radiation damage. Further clinical studies are warranted.

Defining a dose-response relationship with radiotherapy for prostate cancer: is more really better?

PURPOSE

Data were reviewed addressing the association between radiation therapy (RT) dose and treatment outcome for localized prostate cancer to help clarify the existence of a potential dose-response relationship.

METHODS AND MATERIALS

Articles were identified through the MEDLINE database, CancerLit database, and reference lists of relevant articles. Studies were categorized into four groups based upon the endpoint analyzed, including biochemical control (BC), local control (LC), pathologic control (PC), and cause-specific survival (CSS). The impact of increasing RT dose with each endpoint was recorded.

RESULTS

Twenty-two trials involving a total of 11,297 patients were identified. Of the 11 trials addressing the association of RT dose with LC, 9 showed statistically significant improvements. Of the 12 trials that reported BC with RT dose, all showed statistically significant improvements. Two out of 4 studies analyzing PC with increasing dose showed a positive correlation. Finally, 3 out of 9 studies addressing RT dose with CSS showed statistically significant improvements. Despite inconclusive results, patients with poor risk features (e.g., prostate-specific antigen [PSA] > or = 10, Gleason score [GS] > or = 7, or tumor stage > or = T2b) were most likely to benefit from increasing dose with respect to each endpoint. However, the optimal RT dose and the magnitude of benefit of dose escalation could not be identified.

CONCLUSIONS

Although RT dose appears to correlate with various measures of treatment outcome, objective, high-quality data addressing this critical issue are still lacking. At the present time, the absolute improvement in outcome due to dose escalation, the subset of patients benefitting most, and the optimal dose remain to be defined.

A practical method to achieve prostate gland immobilization and target verification for daily treatment

PURPOSE

A practical method to achieve prostate immobilization and daily target localization for external beam radiation treatment is described.

METHODS AND MATERIALS

Ten patients who underwent prostate brachytherapy using permanent radioactive source placement were selected for study. To quantify prostate motion both with and without the presence of a specially designed inflatable intrarectal balloon, the computerized tomography-based coordinates of all intraprostatic radioactive sources were compared over 3 consecutive measurements at 1-min intervals.

RESULTS

The placement and inflation of the intrarectal balloon were well tolerated by all patients. The mean (range) displacement of the prostate gland when the intrarectal balloon was present vs. absent was 1.3 (0-2.2) mm vs. 1.8 (0-9.1) mm (p = 0.03) at 2 min respectively. The maximum displacement in any direction (anterior-posterior, superior-inferior, or right-left) when the intrarectal balloon was inflated vs. absent was reduced to < or =1 mm from 4 mm.

CONCLUSIONS

Both prostate gland immobilization and target verification are possible using a specially designed inflatable intrarectal balloon. Using this device, the posterior margin necessary on the lateral fields to ensure dosimetric coverage of the entire prostate gland could be safely reduced to 5 mm and treatment could be set up and verified using a lateral portal image.

Quantification of late complications after radiation therapy

BACKGROUND

An increasing number of patients survive cancer after having received radiation therapy. Therefore, the occurrence of late normal tissue complications among long-term survivors is of particular concern.

METHODS

Sixty-three patients treated by radical surgery and irradiation for rectal carcinoma were subjected to an unconventional sandwich therapy. Preoperative irradiation was given in four fractions of 5 Gy each applied within 2 or 3 days; postoperative irradiation consisted mostly of 15 x 2 Gy (range, 20-40 Gy). A considerable proportion of these patients developed severe late complications (Radiother Oncol 53 (1999) 177). The data allowed a detailed analysis of complication kinetics, leading to a new model which was tested using data from the literature.

RESULTS

Data on late complications were obtained for eight different organs with a follow-up of up to 10 years. For the various organs, the percentage of patients being free from late complications, plotted as a function of time after start of radiation therapy, was adequately described by exponential regression. From the fit, the parameter p(a) was obtained, which is the percentage of patients at risk in a given year of developing a complication in a given organ during that year. The rate p(a) remained about constant with time. Following sandwich therapy, the annual incidence of complications in the bladder, ileum, lymphatic and soft tissue, and ureters was about the same (p(a)=10-14%/year), whereas complications in bone or dermis occurred at lower rates (4.7 or 7.5%/year, respectively).

DISCUSSION

Numerous data sets collected from published reports were analyzed in the same way. Many of the data sets studied were from patients in a series where there was a high incidence of late effects. Three types of kinetics for the occurrence of late effects after radiotherapy were identified: Type 1, purely exponential kinetics; Type 2, exponential kinetics, the slope of which decreased exponentially with time; Type 3, curves composed of two components, a fast initial decline followed by an exponential decrease. For each kind of kinetics, provided that the dose distribution is not too heterogeneous, the incidence of late effects appears to occur at exponential or approximately exponential kinetics, even many years after treatment. This implies that a random process might be involved in the occurrence of late radiation sequelae.

CONCLUSIONS

There might be a lifelong risk of developing late complications, of which patients and clinicians should be aware. It appears worthwhile to try to identify, in follow-up examinations of patients after radiation therapy, what kind of processes might be involved in triggering subclinical residual injury to develop into a clinically manifest late effect.

The influence of interpatient and intrapatient rectum variation on external beam treatment of prostate cancer

PURPOSE

The rectal dose/volume relationship and inherent variations thereof are fundamental parameters to guide dose escalation in prostate cancer treatment. This study evaluates the effect of rectal dose/volume variation on the risk of rectal complication for different planning target volume (PTV) constructions.

METHODS AND MATERIALS

Thirty prostate patients with multiple daily CT scans obtained during the treatment course were included in this retrospective study. The dose distribution was calculated based on the pretreatment CT image alone. Treatment plans were generated by applying the four-field-box beam arrangement to each of three different PTVs: PTVs with 0.5-cm and 1.0-cm uniform margins, and a patient-specific PTV constructed using treatment imaging feedback. For each of the 30 patients, the rectal wall as manifested on each of multiple CT images was delineated after image bony registration to the pretreatment CT image, and applied to the corresponding treatment plan to obtain the rectal wall dose-volume histogram (DVH). Interpatient and intrapatient rectal dose/volume variations were quantified accordingly. The corresponding uncertainty and sensitivity of the risk of rectal complication to the variations were evaluated for each of the three PTVs. Finally, the efficacy of using multiple CT images to reduce uncertainty in planning evaluation was examined.

RESULTS

Sensitivity of the risk of rectal complication to rectal dose/volume variation strongly depends on the clinical target volume (CTV)-to-PTV margin or prescription dose, or both. Compared to the conventional two-dimensional (2D) prostate cancer treatment, the sensitivity for a conformal treatment can be 3 times higher or more. Due to the interpatient rectal dose/volume variation, the individual normal tissue complication probability (NTCP) was distributed from 10% to 37% when a common prescription dose was applied for all patients. The intrapatient rectal dose/volume variation introduces at least +/- 25% uncertainty in the NTCP calculation for at least 10% or 25% of the patients treated with the PTV of 1.0- or 0.5-cm margin, respectively. These uncertainties were larger for the smaller PTV, with the standard deviation up to 20%. By applying multiple CT image feedback, the NTCP uncertainty could be reduced by a factor of 2.

CONCLUSIONS

Shape and position variation of rectum has less influence on treatment planning in the conventional 2D treatment of prostate cancer. However, this influence is quickly growing with high treatment dose or small CTV-to-PTV margins. To reduce the variation and uncertainties in the treatment planning evaluation associated with the inter- and intrapatient rectal dose/volume variation, the iso-NTCP model and treatment image feedback technique can be applied in dose escalation trials of prostate cancer treatment.

The rush to judgment: Does the evidence support the enthusiasm over three-dimensional conformal radiation therapy and dose escalation in the treatment of prostate cancer?

PURPOSE

To discuss the assumptions behind and current clinical evidence on three-dimensional conformal radiation therapy (3D-CRT) and dose escalation in the treatment of prostate cancer.

METHODS

We first define 3D-CRT in comparison to standard radiation therapy and discuss the assumptions on which the technology of 3D-CRT and dose escalation are based. We then examine the evidence on the benefits and limitations from the current most commonly cited studies on dose-escalation trials to treat prostate cancer.

RESULTS

The assumption that 3D-CRT can provide a tighter margin around the tumor area to allow for dose escalation is not yet proven by studies that show continual difficulty in defining the planning treatment volume because of extrinsic and intrinsic difficulties, such as imaging variabilities and patient and organ movement. Current short-term dose-escalation studies on the use of 3D-CRT to treat prostate cancer are limited in their ability to prove that increasing dose improves survival and does not incur potential long-term complications to normal tissue.

CONCLUSION

Although 3D-CRT is a promising technology that many radiation oncologists and clinics are quickly adopting to treat such tumors as prostate cancer, the long-term evidence on the benefits and limitations of this technology is still lacking. Until we have solid long-term evidence on the true clinical potential of this new technology, let us not rush to judgment, but exercise caution, diligence, and thoughtfulness in using this new technology to treat our patients.

High-dose-rate brachytherapy in the treatment of carcinoma of the prostate

BACKGROUND

Although radical prostatectomy for localized disease is considered as a standard of care, external-beam radiotherapy and brachytherapy are equally effective. We report on the technique and preliminary results of high-dose-rate (HDR) brachytherapy using a temporary iridium-192 implant technique.

METHODS

The authors reviewed the literature on the techniques, treatment protocols, and results of HDR brachytherapy in the treatment of carcinoma of the prostate, and they report their own protocols, technique, and results.

RESULTS

The combination of HDR brachytherapy and external irradiation has been well tolerated by all 200 patients in our series, with less than 3% grade 3 late complications and with 95% PSA relapse-free survival with a median follow-up of 24 months.

CONCLUSIONS

HDR brachytherapy may be the most conformal type of irradiation in the treatment of carcinoma of the prostate regardless of tumor size, anatomical distortion, and organ mobility.

Partial irradiation of the rectum

Data gathered from dose escalation protocols for the treatment of prostate cancers conducted in the past 10 years have shown that rectal toxicity can be controlled by the use of careful conformal techniques. The most severe complications of rectal irradiation (obstruction and fistula requiring colostomy) have been essentially eliminated. The most frequent gastrointestinal complications of conformal radiotherapy of prostate cancer are now rectal bleeding associated with telangiectatic changes to the vasculature of the submucosa, and in severe cases, ulceration requiring cautery procedures and or transfusion. The benefits of 3-dimensional conformal radiotherapy (3D-CRT) are strongly technique dependent, with a strong dose response for single techniques for prescription doses over 70 Gy. Studies of rectal motion show that the anterior wall can move approximately 1 cm during treatment, so portions of the anterior rectal wall will regularly receive the full prescription dose if posterior margin sizes >/= 1 cm are used in designing the planning target volume (PTV). There is strong evidence that increased rectal shielding and posterior PTV margin sizes approximately 0.6 cm reduce rectal complication rates. Despite uncertainties due to rectal motion, studies of dose-volume histograms (DVHs) show that rectal toxicity is strongly influenced by the percent volumes of rectal wall exposed to doses approximately 70 Gy and higher. Recent data suggests that percent volumes of rectal wall exposed doses between 40 to 50 Gy, and the existence of a reserve of unexposed tissue may also play a role in determining rectal bleeding rates.

Development of a treatment planning protocol for prostate treatments using intensity modulated radiotherapy

We have developed a treatment planning protocol for intensity-modulated radiation therapy of the prostate using commercially available inverse planning software. Treatment plans were developed for ten patients using the Corvus version 3.8 planning system, testing various prescription options, including tissue types, dose volume histogram values for the target and normal structures, beam arrangements, and number of intensity levels. All plans were scaled so that 95% of the clinical target volume received 75.6 Gy; mean doses to the prostate were typically 79 Gy. The reproducibility of the inverse planning algorithm was tested by repeating a set of the plans five times. Plans were deemed acceptable if they satisfied predefined dose constraints for the targets and critical organs. Figures of merit for target coverage, target dose uniformity, and organ sparing were used to rank acceptable plans. Certain systematic behaviors of the optimizer were noted: the high dose regions for both targets and critical organs were 5-10 Gy more than prescribed; reducing bladder and rectum tolerance increased the range of doses within the target; increasing the number of fields incrementally improved plan quality. A set of planning parameters was found that usually satisfied the minimum requirements. Repeating the optimization with different beam order produced similar but slightly different dose distributions, which was sometimes useful for finding acceptable solutions for difficult cases. The standard set of parameters serves as a useful starting point for individualized planning.